walsender.c

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/*-------------------------------------------------------------------------
 *
 * walsender.c
 *
 * The WAL sender process (walsender) is new as of Postgres 9.0. It takes
 * care of sending XLOG from the primary server to a single recipient.
 * (Note that there can be more than one walsender process concurrently.)
 * It is started by the postmaster when the walreceiver of a standby server
 * connects to the primary server and requests XLOG streaming replication.
 *
 * A walsender is similar to a regular backend, ie. there is a one-to-one
 * relationship between a connection and a walsender process, but instead
 * of processing SQL queries, it understands a small set of special
 * replication-mode commands. The START_REPLICATION command begins streaming
 * WAL to the client. While streaming, the walsender keeps reading XLOG
 * records from the disk and sends them to the standby server over the
 * COPY protocol, until either side ends the replication by exiting COPY
 * mode (or until the connection is closed).
 *
 * Normal termination is by SIGTERM, which instructs the walsender to
 * close the connection and exit(0) at the next convenient moment. Emergency
 * termination is by SIGQUIT; like any backend, the walsender will simply
 * abort and exit on SIGQUIT. A close of the connection and a FATAL error
 * are treated as not a crash but approximately normal termination;
 * the walsender will exit quickly without sending any more XLOG records.
 *
 * If the server is shut down, checkpointer sends us
 * PROCSIG_WALSND_INIT_STOPPING after all regular backends have exited.  If
 * the backend is idle or runs an SQL query this causes the backend to
 * shutdown, if logical replication is in progress all existing WAL records
 * are processed followed by a shutdown.  Otherwise this causes the walsender
 * to switch to the "stopping" state. In this state, the walsender will reject
 * any further replication commands. The checkpointer begins the shutdown
 * checkpoint once all walsenders are confirmed as stopping. When the shutdown
 * checkpoint finishes, the postmaster sends us SIGUSR2. This instructs
 * walsender to send any outstanding WAL, including the shutdown checkpoint
 * record, wait for it to be replicated to the standby, and then exit.
 *
 *
 * Portions Copyright (c) 2010-2023, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/replication/walsender.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include <signal.h>
#include <unistd.h>
 
#include "access/printtup.h"
#include "access/timeline.h"
#include "access/transam.h"
#include "access/xact.h"
#include "access/xlog_internal.h"
#include "access/xlogreader.h"
#include "access/xlogrecovery.h"
#include "access/xlogutils.h"
#include "backup/basebackup.h"
#include "catalog/pg_authid.h"
#include "catalog/pg_type.h"
#include "commands/dbcommands.h"
#include "commands/defrem.h"
#include "funcapi.h"
#include "libpq/libpq.h"
#include "libpq/pqformat.h"
#include "miscadmin.h"
#include "nodes/replnodes.h"
#include "pgstat.h"
#include "postmaster/interrupt.h"
#include "replication/decode.h"
#include "replication/logical.h"
#include "replication/slot.h"
#include "replication/snapbuild.h"
#include "replication/syncrep.h"
#include "replication/walreceiver.h"
#include "replication/walsender.h"
#include "replication/walsender_private.h"
#include "storage/condition_variable.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/pmsignal.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "tcop/dest.h"
#include "tcop/tcopprot.h"
#include "utils/acl.h"
#include "utils/builtins.h"
#include "utils/guc.h"
#include "utils/memutils.h"
#include "utils/pg_lsn.h"
#include "utils/portal.h"
#include "utils/ps_status.h"
#include "utils/timeout.h"
#include "utils/timestamp.h"
 
/*
 * Maximum data payload in a WAL data message.  Must be >= XLOG_BLCKSZ.
 *
 * We don't have a good idea of what a good value would be; there's some
 * overhead per message in both walsender and walreceiver, but on the other
 * hand sending large batches makes walsender less responsive to signals
 * because signals are checked only between messages.  128kB (with
 * default 8k blocks) seems like a reasonable guess for now.
 */
#define MAX_SEND_SIZE (XLOG_BLCKSZ * 16)
 
/* Array of WalSnds in shared memory */
WalSndCtlData *WalSndCtl = NULL;
 
/* My slot in the shared memory array */
WalSnd     *MyWalSnd = NULL;
 
/* Global state */
bool        am_walsender = false;   /* Am I a walsender process? */
bool        am_cascading_walsender = false; /* Am I cascading WAL to another
                                             * standby? */
bool        am_db_walsender = false;    /* Connected to a database? */
 
/* GUC variables */
int         max_wal_senders = 10;   /* the maximum number of concurrent
                                     * walsenders */
int         wal_sender_timeout = 60 * 1000; /* maximum time to send one WAL
                                             * data message */
bool        log_replication_commands = false;
 
/*
 * State for WalSndWakeupRequest
 */
bool        wake_wal_senders = false;
 
/*
 * xlogreader used for replication.  Note that a WAL sender doing physical
 * replication does not need xlogreader to read WAL, but it needs one to
 * keep a state of its work.
 */
static XLogReaderState *xlogreader = NULL;
 
/*
 * These variables keep track of the state of the timeline we're currently
 * sending. sendTimeLine identifies the timeline. If sendTimeLineIsHistoric,
 * the timeline is not the latest timeline on this server, and the server's
 * history forked off from that timeline at sendTimeLineValidUpto.
 */
static TimeLineID sendTimeLine = 0;
static TimeLineID sendTimeLineNextTLI = 0;
static bool sendTimeLineIsHistoric = false;
static XLogRecPtr sendTimeLineValidUpto = InvalidXLogRecPtr;
 
/*
 * How far have we sent WAL already? This is also advertised in
 * MyWalSnd->sentPtr.  (Actually, this is the next WAL location to send.)
 */
static XLogRecPtr sentPtr = InvalidXLogRecPtr;
 
/* Buffers for constructing outgoing messages and processing reply messages. */
static StringInfoData output_message;
static StringInfoData reply_message;
static StringInfoData tmpbuf;
 
/* Timestamp of last ProcessRepliesIfAny(). */
static TimestampTz last_processing = 0;
 
/*
 * Timestamp of last ProcessRepliesIfAny() that saw a reply from the
 * standby. Set to 0 if wal_sender_timeout doesn't need to be active.
 */
static TimestampTz last_reply_timestamp = 0;
 
/* Have we sent a heartbeat message asking for reply, since last reply? */
static bool waiting_for_ping_response = false;
 
/*
 * While streaming WAL in Copy mode, streamingDoneSending is set to true
 * after we have sent CopyDone. We should not send any more CopyData messages
 * after that. streamingDoneReceiving is set to true when we receive CopyDone
 * from the other end. When both become true, it's time to exit Copy mode.
 */
static bool streamingDoneSending;
static bool streamingDoneReceiving;
 
/* Are we there yet? */
static bool WalSndCaughtUp = false;
 
/* Flags set by signal handlers for later service in main loop */
static volatile sig_atomic_t got_SIGUSR2 = false;
static volatile sig_atomic_t got_STOPPING = false;
 
/*
 * This is set while we are streaming. When not set
 * PROCSIG_WALSND_INIT_STOPPING signal will be handled like SIGTERM. When set,
 * the main loop is responsible for checking got_STOPPING and terminating when
 * it's set (after streaming any remaining WAL).
 */
static volatile sig_atomic_t replication_active = false;
 
static LogicalDecodingContext *logical_decoding_ctx = NULL;
 
/* A sample associating a WAL location with the time it was written. */
typedef struct
{
    XLogRecPtr  lsn;
    TimestampTz time;
} WalTimeSample;
 
/* The size of our buffer of time samples. */
#define LAG_TRACKER_BUFFER_SIZE 8192
 
/* A mechanism for tracking replication lag. */
typedef struct
{
    XLogRecPtr  last_lsn;
    WalTimeSample buffer[LAG_TRACKER_BUFFER_SIZE];
    int         write_head;
    int         read_heads[NUM_SYNC_REP_WAIT_MODE];
    WalTimeSample last_read[NUM_SYNC_REP_WAIT_MODE];
} LagTracker;
 
static LagTracker *lag_tracker;
 
/* Signal handlers */
static void WalSndLastCycleHandler(SIGNAL_ARGS);
 
/* Prototypes for private functions */
typedef void (*WalSndSendDataCallback) (void);
static void WalSndLoop(WalSndSendDataCallback send_data);
static void InitWalSenderSlot(void);
static void WalSndKill(int code, Datum arg);
static void WalSndShutdown(void) pg_attribute_noreturn();
static void XLogSendPhysical(void);
static void XLogSendLogical(void);
static void WalSndDone(WalSndSendDataCallback send_data);
static XLogRecPtr GetStandbyFlushRecPtr(TimeLineID *tli);
static void IdentifySystem(void);
static void ReadReplicationSlot(ReadReplicationSlotCmd *cmd);
static void CreateReplicationSlot(CreateReplicationSlotCmd *cmd);
static void DropReplicationSlot(DropReplicationSlotCmd *cmd);
static void StartReplication(StartReplicationCmd *cmd);
static void StartLogicalReplication(StartReplicationCmd *cmd);
static void ProcessStandbyMessage(void);
static void ProcessStandbyReplyMessage(void);
static void ProcessStandbyHSFeedbackMessage(void);
static void ProcessRepliesIfAny(void);
static void ProcessPendingWrites(void);
static void WalSndKeepalive(bool requestReply, XLogRecPtr writePtr);
static void WalSndKeepaliveIfNecessary(void);
static void WalSndCheckTimeOut(void);
static long WalSndComputeSleeptime(TimestampTz now);
static void WalSndWait(uint32 socket_events, long timeout, uint32 wait_event);
static void WalSndPrepareWrite(LogicalDecodingContext *ctx, XLogRecPtr lsn, TransactionId xid, bool last_write);
static void WalSndWriteData(LogicalDecodingContext *ctx, XLogRecPtr lsn, TransactionId xid, bool last_write);
static void WalSndUpdateProgress(LogicalDecodingContext *ctx, XLogRecPtr lsn, TransactionId xid,
                                 bool skipped_xact);
static XLogRecPtr WalSndWaitForWal(XLogRecPtr loc);
static void LagTrackerWrite(XLogRecPtr lsn, TimestampTz local_flush_time);
static TimeOffset LagTrackerRead(int head, XLogRecPtr lsn, TimestampTz now);
static bool TransactionIdInRecentPast(TransactionId xid, uint32 epoch);
 
static void WalSndSegmentOpen(XLogReaderState *state, XLogSegNo nextSegNo,
                              TimeLineID *tli_p);
 
 
/* Initialize walsender process before entering the main command loop */
void
InitWalSender(void)
{
    am_cascading_walsender = RecoveryInProgress();
 
    /* Create a per-walsender data structure in shared memory */
    InitWalSenderSlot();
 
    /*
     * We don't currently need any ResourceOwner in a walsender process, but
     * if we did, we could call CreateAuxProcessResourceOwner here.
     */
 
    /*
     * Let postmaster know that we're a WAL sender. Once we've declared us as
     * a WAL sender process, postmaster will let us outlive the bgwriter and
     * kill us last in the shutdown sequence, so we get a chance to stream all
     * remaining WAL at shutdown, including the shutdown checkpoint. Note that
     * there's no going back, and we mustn't write any WAL records after this.
     */
    MarkPostmasterChildWalSender();
    SendPostmasterSignal(PMSIGNAL_ADVANCE_STATE_MACHINE);
 
    /*
     * If the client didn't specify a database to connect to, show in PGPROC
     * that our advertised xmin should affect vacuum horizons in all
     * databases.  This allows physical replication clients to send hot
     * standby feedback that will delay vacuum cleanup in all databases.
     */
    if (MyDatabaseId == InvalidOid)
    {
        Assert(MyProc->xmin == InvalidTransactionId);
        LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
        MyProc->statusFlags |= PROC_AFFECTS_ALL_HORIZONS;
        ProcGlobal->statusFlags[MyProc->pgxactoff] = MyProc->statusFlags;
        LWLockRelease(ProcArrayLock);
    }
 
    /* Initialize empty timestamp buffer for lag tracking. */
    lag_tracker = MemoryContextAllocZero(TopMemoryContext, sizeof(LagTracker));
}
 
/*
 * Clean up after an error.
 *
 * WAL sender processes don't use transactions like regular backends do.
 * This function does any cleanup required after an error in a WAL sender
 * process, similar to what transaction abort does in a regular backend.
 */
void
WalSndErrorCleanup(void)
{
    LWLockReleaseAll();
    ConditionVariableCancelSleep();
    pgstat_report_wait_end();
 
    if (xlogreader != NULL && xlogreader->seg.ws_file >= 0)
        wal_segment_close(xlogreader);
 
    if (MyReplicationSlot != NULL)
        ReplicationSlotRelease();
 
    ReplicationSlotCleanup();
 
    replication_active = false;
 
    /*
     * If there is a transaction in progress, it will clean up our
     * ResourceOwner, but if a replication command set up a resource owner
     * without a transaction, we've got to clean that up now.
     */
    if (!IsTransactionOrTransactionBlock())
        WalSndResourceCleanup(false);
 
    if (got_STOPPING || got_SIGUSR2)
        proc_exit(0);
 
    /* Revert back to startup state */
    WalSndSetState(WALSNDSTATE_STARTUP);
}
 
/*
 * Clean up any ResourceOwner we created.
 */
void
WalSndResourceCleanup(bool isCommit)
{
    ResourceOwner resowner;
 
    if (CurrentResourceOwner == NULL)
        return;
 
    /*
     * Deleting CurrentResourceOwner is not allowed, so we must save a pointer
     * in a local variable and clear it first.
     */
    resowner = CurrentResourceOwner;
    CurrentResourceOwner = NULL;
 
    /* Now we can release resources and delete it. */
    ResourceOwnerRelease(resowner,
                         RESOURCE_RELEASE_BEFORE_LOCKS, isCommit, true);
    ResourceOwnerRelease(resowner,
                         RESOURCE_RELEASE_LOCKS, isCommit, true);
    ResourceOwnerRelease(resowner,
                         RESOURCE_RELEASE_AFTER_LOCKS, isCommit, true);
    ResourceOwnerDelete(resowner);
}
 
/*
 * Handle a client's connection abort in an orderly manner.
 */
static void
WalSndShutdown(void)
{
    /*
     * Reset whereToSendOutput to prevent ereport from attempting to send any
     * more messages to the standby.
     */
    if (whereToSendOutput == DestRemote)
        whereToSendOutput = DestNone;
 
    proc_exit(0);
    abort();                    /* keep the compiler quiet */
}
 
/*
 * Handle the IDENTIFY_SYSTEM command.
 */
static void
IdentifySystem(void)
{
    char        sysid[32];
    char        xloc[MAXFNAMELEN];
    XLogRecPtr  logptr;
    char       *dbname = NULL;
    DestReceiver *dest;
    TupOutputState *tstate;
    TupleDesc   tupdesc;
    Datum       values[4];
    bool        nulls[4] = {0};
    TimeLineID  currTLI;
 
    /*
     * Reply with a result set with one row, four columns. First col is system
     * ID, second is timeline ID, third is current xlog location and the
     * fourth contains the database name if we are connected to one.
     */
 
    snprintf(sysid, sizeof(sysid), UINT64_FORMAT,
             GetSystemIdentifier());
 
    am_cascading_walsender = RecoveryInProgress();
    if (am_cascading_walsender)
        logptr = GetStandbyFlushRecPtr(&currTLI);
    else
        logptr = GetFlushRecPtr(&currTLI);
 
    snprintf(xloc, sizeof(xloc), "%X/%X", LSN_FORMAT_ARGS(logptr));
 
    if (MyDatabaseId != InvalidOid)
    {
        MemoryContext cur = CurrentMemoryContext;
 
        /* syscache access needs a transaction env. */
        StartTransactionCommand();
        /* make dbname live outside TX context */
        MemoryContextSwitchTo(cur);
        dbname = get_database_name(MyDatabaseId);
        CommitTransactionCommand();
        /* CommitTransactionCommand switches to TopMemoryContext */
        MemoryContextSwitchTo(cur);
    }
 
    dest = CreateDestReceiver(DestRemoteSimple);
 
    /* need a tuple descriptor representing four columns */
    tupdesc = CreateTemplateTupleDesc(4);
    TupleDescInitBuiltinEntry(tupdesc, (AttrNumber) 1, "systemid",
                              TEXTOID, -1, 0);
    TupleDescInitBuiltinEntry(tupdesc, (AttrNumber) 2, "timeline",
                              INT8OID, -1, 0);
    TupleDescInitBuiltinEntry(tupdesc, (AttrNumber) 3, "xlogpos",
                              TEXTOID, -1, 0);
    TupleDescInitBuiltinEntry(tupdesc, (AttrNumber) 4, "dbname",
                              TEXTOID, -1, 0);
 
    /* prepare for projection of tuples */
    tstate = begin_tup_output_tupdesc(dest, tupdesc, &TTSOpsVirtual);
 
    /* column 1: system identifier */
    values[0] = CStringGetTextDatum(sysid);
 
    /* column 2: timeline */
    values[1] = Int64GetDatum(currTLI);
 
    /* column 3: wal location */
    values[2] = CStringGetTextDatum(xloc);
 
    /* column 4: database name, or NULL if none */
    if (dbname)
        values[3] = CStringGetTextDatum(dbname);
    else
        nulls[3] = true;
 
    /* send it to dest */
    do_tup_output(tstate, values, nulls);
 
    end_tup_output(tstate);
}
 
/* Handle READ_REPLICATION_SLOT command */
static void
ReadReplicationSlot(ReadReplicationSlotCmd *cmd)
{
#define READ_REPLICATION_SLOT_COLS 3
    ReplicationSlot *slot;
    DestReceiver *dest;
    TupOutputState *tstate;
    TupleDesc   tupdesc;
    Datum       values[READ_REPLICATION_SLOT_COLS] = {0};
    bool        nulls[READ_REPLICATION_SLOT_COLS];
 
    tupdesc = CreateTemplateTupleDesc(READ_REPLICATION_SLOT_COLS);
    TupleDescInitBuiltinEntry(tupdesc, (AttrNumber) 1, "slot_type",
                              TEXTOID, -1, 0);
    TupleDescInitBuiltinEntry(tupdesc, (AttrNumber) 2, "restart_lsn",
                              TEXTOID, -1, 0);
    /* TimeLineID is unsigned, so int4 is not wide enough. */
    TupleDescInitBuiltinEntry(tupdesc, (AttrNumber) 3, "restart_tli",
                              INT8OID, -1, 0);
 
    memset(nulls, true, READ_REPLICATION_SLOT_COLS * sizeof(bool));
 
    LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
    slot = SearchNamedReplicationSlot(cmd->slotname, false);
    if (slot == NULL || !slot->in_use)
    {
        LWLockRelease(ReplicationSlotControlLock);
    }
    else
    {
        ReplicationSlot slot_contents;
        int         i = 0;
 
        /* Copy slot contents while holding spinlock */
        SpinLockAcquire(&slot->mutex);
        slot_contents = *slot;
        SpinLockRelease(&slot->mutex);
        LWLockRelease(ReplicationSlotControlLock);
 
        if (OidIsValid(slot_contents.data.database))
            ereport(ERROR,
                    errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                    errmsg("cannot use %s with a logical replication slot",
                           "READ_REPLICATION_SLOT"));
 
        /* slot type */
        values[i] = CStringGetTextDatum("physical");
        nulls[i] = false;
        i++;
 
        /* start LSN */
        if (!XLogRecPtrIsInvalid(slot_contents.data.restart_lsn))
        {
            char        xloc[64];
 
            snprintf(xloc, sizeof(xloc), "%X/%X",
                     LSN_FORMAT_ARGS(slot_contents.data.restart_lsn));
            values[i] = CStringGetTextDatum(xloc);
            nulls[i] = false;
        }
        i++;
 
        /* timeline this WAL was produced on */
        if (!XLogRecPtrIsInvalid(slot_contents.data.restart_lsn))
        {
            TimeLineID  slots_position_timeline;
            TimeLineID  current_timeline;
            List       *timeline_history = NIL;
 
            /*
             * While in recovery, use as timeline the currently-replaying one
             * to get the LSN position's history.
             */
            if (RecoveryInProgress())
                (void) GetXLogReplayRecPtr(&current_timeline);
            else
                current_timeline = GetWALInsertionTimeLine();
 
            timeline_history = readTimeLineHistory(current_timeline);
            slots_position_timeline = tliOfPointInHistory(slot_contents.data.restart_lsn,
                                                          timeline_history);
            values[i] = Int64GetDatum((int64) slots_position_timeline);
            nulls[i] = false;
        }
        i++;
 
        Assert(i == READ_REPLICATION_SLOT_COLS);
    }
 
    dest = CreateDestReceiver(DestRemoteSimple);
    tstate = begin_tup_output_tupdesc(dest, tupdesc, &TTSOpsVirtual);
    do_tup_output(tstate, values, nulls);
    end_tup_output(tstate);
}
 
 
/*
 * Handle TIMELINE_HISTORY command.
 */
static void
SendTimeLineHistory(TimeLineHistoryCmd *cmd)
{
    DestReceiver *dest;
    TupleDesc   tupdesc;
    StringInfoData buf;
    char        histfname[MAXFNAMELEN];
    char        path[MAXPGPATH];
    int         fd;
    off_t       histfilelen;
    off_t       bytesleft;
    Size        len;
 
    dest = CreateDestReceiver(DestRemoteSimple);
 
    /*
     * Reply with a result set with one row, and two columns. The first col is
     * the name of the history file, 2nd is the contents.
     */
    tupdesc = CreateTemplateTupleDesc(2);
    TupleDescInitBuiltinEntry(tupdesc, (AttrNumber) 1, "filename", TEXTOID, -1, 0);
    TupleDescInitBuiltinEntry(tupdesc, (AttrNumber) 2, "content", TEXTOID, -1, 0);
 
    TLHistoryFileName(histfname, cmd->timeline);
    TLHistoryFilePath(path, cmd->timeline);
 
    /* Send a RowDescription message */
    dest->rStartup(dest, CMD_SELECT, tupdesc);
 
    /* Send a DataRow message */
    pq_beginmessage(&buf, 'D');
    pq_sendint16(&buf, 2);      /* # of columns */
    len = strlen(histfname);
    pq_sendint32(&buf, len);    /* col1 len */
    pq_sendbytes(&buf, histfname, len);
 
    fd = OpenTransientFile(path, O_RDONLY | PG_BINARY);
    if (fd < 0)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not open file \"%s\": %m", path)));
 
    /* Determine file length and send it to client */
    histfilelen = lseek(fd, 0, SEEK_END);
    if (histfilelen < 0)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not seek to end of file \"%s\": %m", path)));
    if (lseek(fd, 0, SEEK_SET) != 0)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not seek to beginning of file \"%s\": %m", path)));
 
    pq_sendint32(&buf, histfilelen);    /* col2 len */
 
    bytesleft = histfilelen;
    while (bytesleft > 0)
    {
        PGAlignedBlock rbuf;
        int         nread;
 
        pgstat_report_wait_start(WAIT_EVENT_WALSENDER_TIMELINE_HISTORY_READ);
        nread = read(fd, rbuf.data, sizeof(rbuf));
        pgstat_report_wait_end();
        if (nread < 0)
            ereport(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not read file \"%s\": %m",
                            path)));
        else if (nread == 0)
            ereport(ERROR,
                    (errcode(ERRCODE_DATA_CORRUPTED),
                     errmsg("could not read file \"%s\": read %d of %zu",
                            path, nread, (Size) bytesleft)));
 
        pq_sendbytes(&buf, rbuf.data, nread);
        bytesleft -= nread;
    }
 
    if (CloseTransientFile(fd) != 0)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not close file \"%s\": %m", path)));
 
    pq_endmessage(&buf);
}
 
/*
 * Handle START_REPLICATION command.
 *
 * At the moment, this never returns, but an ereport(ERROR) will take us back
 * to the main loop.
 */
static void
StartReplication(StartReplicationCmd *cmd)
{
    StringInfoData buf;
    XLogRecPtr  FlushPtr;
    TimeLineID  FlushTLI;
 
    /* create xlogreader for physical replication */
    xlogreader =
        XLogReaderAllocate(wal_segment_size, NULL,
                           XL_ROUTINE(.segment_open = WalSndSegmentOpen,
                                      .segment_close = wal_segment_close),
                           NULL);
 
    if (!xlogreader)
        ereport(ERROR,
                (errcode(ERRCODE_OUT_OF_MEMORY),
                 errmsg("out of memory"),
                 errdetail("Failed while allocating a WAL reading processor.")));
 
    /*
     * We assume here that we're logging enough information in the WAL for
     * log-shipping, since this is checked in PostmasterMain().
     *
     * NOTE: wal_level can only change at shutdown, so in most cases it is
     * difficult for there to be WAL data that we can still see that was
     * written at wal_level='minimal'.
     */
 
    if (cmd->slotname)
    {
        ReplicationSlotAcquire(cmd->slotname, true);
        if (SlotIsLogical(MyReplicationSlot))
            ereport(ERROR,
                    (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                     errmsg("cannot use a logical replication slot for physical replication")));
 
        /*
         * We don't need to verify the slot's restart_lsn here; instead we
         * rely on the caller requesting the starting point to use.  If the
         * WAL segment doesn't exist, we'll fail later.
         */
    }
 
    /*
     * Select the timeline. If it was given explicitly by the client, use
     * that. Otherwise use the timeline of the last replayed record.
     */
    am_cascading_walsender = RecoveryInProgress();
    if (am_cascading_walsender)
        FlushPtr = GetStandbyFlushRecPtr(&FlushTLI);
    else
        FlushPtr = GetFlushRecPtr(&FlushTLI);
 
    if (cmd->timeline != 0)
    {
        XLogRecPtr  switchpoint;
 
        sendTimeLine = cmd->timeline;
        if (sendTimeLine == FlushTLI)
        {
            sendTimeLineIsHistoric = false;
            sendTimeLineValidUpto = InvalidXLogRecPtr;
        }
        else
        {
            List       *timeLineHistory;
 
            sendTimeLineIsHistoric = true;
 
            /*
             * Check that the timeline the client requested exists, and the
             * requested start location is on that timeline.
             */
            timeLineHistory = readTimeLineHistory(FlushTLI);
            switchpoint = tliSwitchPoint(cmd->timeline, timeLineHistory,
                                         &sendTimeLineNextTLI);
            list_free_deep(timeLineHistory);
 
            /*
             * Found the requested timeline in the history. Check that
             * requested startpoint is on that timeline in our history.
             *
             * This is quite loose on purpose. We only check that we didn't
             * fork off the requested timeline before the switchpoint. We
             * don't check that we switched *to* it before the requested
             * starting point. This is because the client can legitimately
             * request to start replication from the beginning of the WAL
             * segment that contains switchpoint, but on the new timeline, so
             * that it doesn't end up with a partial segment. If you ask for
             * too old a starting point, you'll get an error later when we
             * fail to find the requested WAL segment in pg_wal.
             *
             * XXX: we could be more strict here and only allow a startpoint
             * that's older than the switchpoint, if it's still in the same
             * WAL segment.
             */
            if (!XLogRecPtrIsInvalid(switchpoint) &&
                switchpoint < cmd->startpoint)
            {
                ereport(ERROR,
                        (errmsg("requested starting point %X/%X on timeline %u is not in this server's history",
                                LSN_FORMAT_ARGS(cmd->startpoint),
                                cmd->timeline),
                         errdetail("This server's history forked from timeline %u at %X/%X.",
                                   cmd->timeline,
                                   LSN_FORMAT_ARGS(switchpoint))));
            }
            sendTimeLineValidUpto = switchpoint;
        }
    }
    else
    {
        sendTimeLine = FlushTLI;
        sendTimeLineValidUpto = InvalidXLogRecPtr;
        sendTimeLineIsHistoric = false;
    }
 
    streamingDoneSending = streamingDoneReceiving = false;
 
    /* If there is nothing to stream, don't even enter COPY mode */
    if (!sendTimeLineIsHistoric || cmd->startpoint < sendTimeLineValidUpto)
    {
        /*
         * When we first start replication the standby will be behind the
         * primary. For some applications, for example synchronous
         * replication, it is important to have a clear state for this initial
         * catchup mode, so we can trigger actions when we change streaming
         * state later. We may stay in this state for a long time, which is
         * exactly why we want to be able to monitor whether or not we are
         * still here.
         */
        WalSndSetState(WALSNDSTATE_CATCHUP);
 
        /* Send a CopyBothResponse message, and start streaming */
        pq_beginmessage(&buf, 'W');
        pq_sendbyte(&buf, 0);
        pq_sendint16(&buf, 0);
        pq_endmessage(&buf);
        pq_flush();
 
        /*
         * Don't allow a request to stream from a future point in WAL that
         * hasn't been flushed to disk in this server yet.
         */
        if (FlushPtr < cmd->startpoint)
        {
            ereport(ERROR,
                    (errmsg("requested starting point %X/%X is ahead of the WAL flush position of this server %X/%X",
                            LSN_FORMAT_ARGS(cmd->startpoint),
                            LSN_FORMAT_ARGS(FlushPtr))));
        }
 
        /* Start streaming from the requested point */
        sentPtr = cmd->startpoint;
 
        /* Initialize shared memory status, too */
        SpinLockAcquire(&MyWalSnd->mutex);
        MyWalSnd->sentPtr = sentPtr;
        SpinLockRelease(&MyWalSnd->mutex);
 
        SyncRepInitConfig();
 
        /* Main loop of walsender */
        replication_active = true;
 
        WalSndLoop(XLogSendPhysical);
 
        replication_active = false;
        if (got_STOPPING)
            proc_exit(0);
        WalSndSetState(WALSNDSTATE_STARTUP);
 
        Assert(streamingDoneSending && streamingDoneReceiving);
    }
 
    if (cmd->slotname)
        ReplicationSlotRelease();
 
    /*
     * Copy is finished now. Send a single-row result set indicating the next
     * timeline.
     */
    if (sendTimeLineIsHistoric)
    {
        char        startpos_str[8 + 1 + 8 + 1];
        DestReceiver *dest;
        TupOutputState *tstate;
        TupleDesc   tupdesc;
        Datum       values[2];
        bool        nulls[2] = {0};
 
        snprintf(startpos_str, sizeof(startpos_str), "%X/%X",
                 LSN_FORMAT_ARGS(sendTimeLineValidUpto));
 
        dest = CreateDestReceiver(DestRemoteSimple);
 
        /*
         * Need a tuple descriptor representing two columns. int8 may seem
         * like a surprising data type for this, but in theory int4 would not
         * be wide enough for this, as TimeLineID is unsigned.
         */
        tupdesc = CreateTemplateTupleDesc(2);
        TupleDescInitBuiltinEntry(tupdesc, (AttrNumber) 1, "next_tli",
                                  INT8OID, -1, 0);
        TupleDescInitBuiltinEntry(tupdesc, (AttrNumber) 2, "next_tli_startpos",
                                  TEXTOID, -1, 0);
 
        /* prepare for projection of tuple */
        tstate = begin_tup_output_tupdesc(dest, tupdesc, &TTSOpsVirtual);
 
        values[0] = Int64GetDatum((int64) sendTimeLineNextTLI);
        values[1] = CStringGetTextDatum(startpos_str);
 
        /* send it to dest */
        do_tup_output(tstate, values, nulls);
 
        end_tup_output(tstate);
    }
 
    /* Send CommandComplete message */
    EndReplicationCommand("START_STREAMING");
}
 
/*
 * XLogReaderRoutine->page_read callback for logical decoding contexts, as a
 * walsender process.
 *
 * Inside the walsender we can do better than read_local_xlog_page,
 * which has to do a plain sleep/busy loop, because the walsender's latch gets
 * set every time WAL is flushed.
 */
static int
logical_read_xlog_page(XLogReaderState *state, XLogRecPtr targetPagePtr, int reqLen,
                       XLogRecPtr targetRecPtr, char *cur_page)
{
    XLogRecPtr  flushptr;
    int         count;
    WALReadError errinfo;
    XLogSegNo   segno;
    TimeLineID  currTLI = GetWALInsertionTimeLine();
 
    /*
     * Since logical decoding is only permitted on a primary server, we know
     * that the current timeline ID can't be changing any more. If we did this
     * on a standby, we'd have to worry about the values we compute here
     * becoming invalid due to a promotion or timeline change.
     */
    XLogReadDetermineTimeline(state, targetPagePtr, reqLen, currTLI);
    sendTimeLineIsHistoric = (state->currTLI != currTLI);
    sendTimeLine = state->currTLI;
    sendTimeLineValidUpto = state->currTLIValidUntil;
    sendTimeLineNextTLI = state->nextTLI;
 
    /* make sure we have enough WAL available */
    flushptr = WalSndWaitForWal(targetPagePtr + reqLen);
 
    /* fail if not (implies we are going to shut down) */
    if (flushptr < targetPagePtr + reqLen)
        return -1;
 
    if (targetPagePtr + XLOG_BLCKSZ <= flushptr)
        count = XLOG_BLCKSZ;    /* more than one block available */
    else
        count = flushptr - targetPagePtr;   /* part of the page available */
 
    /* now actually read the data, we know it's there */
    if (!WALRead(state,
                 cur_page,
                 targetPagePtr,
                 XLOG_BLCKSZ,
                 state->seg.ws_tli, /* Pass the current TLI because only
                                     * WalSndSegmentOpen controls whether new
                                     * TLI is needed. */
                 &errinfo))
        WALReadRaiseError(&errinfo);
 
    /*
     * After reading into the buffer, check that what we read was valid. We do
     * this after reading, because even though the segment was present when we
     * opened it, it might get recycled or removed while we read it. The
     * read() succeeds in that case, but the data we tried to read might
     * already have been overwritten with new WAL records.
     */
    XLByteToSeg(targetPagePtr, segno, state->segcxt.ws_segsize);
    CheckXLogRemoved(segno, state->seg.ws_tli);
 
    return count;
}
 
/*
 * Process extra options given to CREATE_REPLICATION_SLOT.
 */
static void
parseCreateReplSlotOptions(CreateReplicationSlotCmd *cmd,
                           bool *reserve_wal,
                           CRSSnapshotAction *snapshot_action,
                           bool *two_phase)
{
    ListCell   *lc;
    bool        snapshot_action_given = false;
    bool        reserve_wal_given = false;
    bool        two_phase_given = false;
 
    /* Parse options */
    foreach(lc, cmd->options)
    {
        DefElem    *defel = (DefElem *) lfirst(lc);
 
        if (strcmp(defel->defname, "snapshot") == 0)
        {
            char       *action;
 
            if (snapshot_action_given || cmd->kind != REPLICATION_KIND_LOGICAL)
                ereport(ERROR,
                        (errcode(ERRCODE_SYNTAX_ERROR),
                         errmsg("conflicting or redundant options")));
 
            action = defGetString(defel);
            snapshot_action_given = true;
 
            if (strcmp(action, "export") == 0)
                *snapshot_action = CRS_EXPORT_SNAPSHOT;
            else if (strcmp(action, "nothing") == 0)
                *snapshot_action = CRS_NOEXPORT_SNAPSHOT;
            else if (strcmp(action, "use") == 0)
                *snapshot_action = CRS_USE_SNAPSHOT;
            else
                ereport(ERROR,
                        (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                         errmsg("unrecognized value for CREATE_REPLICATION_SLOT option \"%s\": \"%s\"",
                                defel->defname, action)));
        }
        else if (strcmp(defel->defname, "reserve_wal") == 0)
        {
            if (reserve_wal_given || cmd->kind != REPLICATION_KIND_PHYSICAL)
                ereport(ERROR,
                        (errcode(ERRCODE_SYNTAX_ERROR),
                         errmsg("conflicting or redundant options")));
 
            reserve_wal_given = true;
            *reserve_wal = defGetBoolean(defel);
        }
        else if (strcmp(defel->defname, "two_phase") == 0)
        {
            if (two_phase_given || cmd->kind != REPLICATION_KIND_LOGICAL)
                ereport(ERROR,
                        (errcode(ERRCODE_SYNTAX_ERROR),
                         errmsg("conflicting or redundant options")));
            two_phase_given = true;
            *two_phase = defGetBoolean(defel);
        }
        else
            elog(ERROR, "unrecognized option: %s", defel->defname);
    }
}
 
/*
 * Create a new replication slot.
 */
static void
CreateReplicationSlot(CreateReplicationSlotCmd *cmd)
{
    const char *snapshot_name = NULL;
    char        xloc[MAXFNAMELEN];
    char       *slot_name;
    bool        reserve_wal = false;
    bool        two_phase = false;
    CRSSnapshotAction snapshot_action = CRS_EXPORT_SNAPSHOT;
    DestReceiver *dest;
    TupOutputState *tstate;
    TupleDesc   tupdesc;
    Datum       values[4];
    bool        nulls[4] = {0};
 
    Assert(!MyReplicationSlot);
 
    parseCreateReplSlotOptions(cmd, &reserve_wal, &snapshot_action, &two_phase);
 
    if (cmd->kind == REPLICATION_KIND_PHYSICAL)
    {
        ReplicationSlotCreate(cmd->slotname, false,
                              cmd->temporary ? RS_TEMPORARY : RS_PERSISTENT,
                              false);
    }
    else
    {
        CheckLogicalDecodingRequirements();
 
        /*
         * Initially create persistent slot as ephemeral - that allows us to
         * nicely handle errors during initialization because it'll get
         * dropped if this transaction fails. We'll make it persistent at the
         * end. Temporary slots can be created as temporary from beginning as
         * they get dropped on error as well.
         */
        ReplicationSlotCreate(cmd->slotname, true,
                              cmd->temporary ? RS_TEMPORARY : RS_EPHEMERAL,
                              two_phase);
    }
 
    if (cmd->kind == REPLICATION_KIND_LOGICAL)
    {
        LogicalDecodingContext *ctx;
        bool        need_full_snapshot = false;
 
        /*
         * Do options check early so that we can bail before calling the
         * DecodingContextFindStartpoint which can take long time.
         */
        if (snapshot_action == CRS_EXPORT_SNAPSHOT)
        {
            if (IsTransactionBlock())
                ereport(ERROR,
                /*- translator: %s is a CREATE_REPLICATION_SLOT statement */
                        (errmsg("%s must not be called inside a transaction",
                                "CREATE_REPLICATION_SLOT ... (SNAPSHOT 'export')")));
 
            need_full_snapshot = true;
        }
        else if (snapshot_action == CRS_USE_SNAPSHOT)
        {
            if (!IsTransactionBlock())
                ereport(ERROR,
                /*- translator: %s is a CREATE_REPLICATION_SLOT statement */
                        (errmsg("%s must be called inside a transaction",
                                "CREATE_REPLICATION_SLOT ... (SNAPSHOT 'use')")));
 
            if (XactIsoLevel != XACT_REPEATABLE_READ)
                ereport(ERROR,
                /*- translator: %s is a CREATE_REPLICATION_SLOT statement */
                        (errmsg("%s must be called in REPEATABLE READ isolation mode transaction",
                                "CREATE_REPLICATION_SLOT ... (SNAPSHOT 'use')")));
            if (!XactReadOnly)
                ereport(ERROR,
                /*- translator: %s is a CREATE_REPLICATION_SLOT statement */
                        (errmsg("%s must be called in a read only transaction",
                                "CREATE_REPLICATION_SLOT ... (SNAPSHOT 'use')")));
 
            if (FirstSnapshotSet)
                ereport(ERROR,
                /*- translator: %s is a CREATE_REPLICATION_SLOT statement */
                        (errmsg("%s must be called before any query",
                                "CREATE_REPLICATION_SLOT ... (SNAPSHOT 'use')")));
 
            if (IsSubTransaction())
                ereport(ERROR,
                /*- translator: %s is a CREATE_REPLICATION_SLOT statement */
                        (errmsg("%s must not be called in a subtransaction",
                                "CREATE_REPLICATION_SLOT ... (SNAPSHOT 'use')")));
 
            need_full_snapshot = true;
        }
 
        ctx = CreateInitDecodingContext(cmd->plugin, NIL, need_full_snapshot,
                                        InvalidXLogRecPtr,
                                        XL_ROUTINE(.page_read = logical_read_xlog_page,
                                                   .segment_open = WalSndSegmentOpen,
                                                   .segment_close = wal_segment_close),
                                        WalSndPrepareWrite, WalSndWriteData,
                                        WalSndUpdateProgress);
 
        /*
         * Signal that we don't need the timeout mechanism. We're just
         * creating the replication slot and don't yet accept feedback
         * messages or send keepalives. As we possibly need to wait for
         * further WAL the walsender would otherwise possibly be killed too
         * soon.
         */
        last_reply_timestamp = 0;
 
        /* build initial snapshot, might take a while */
        DecodingContextFindStartpoint(ctx);
 
        /*
         * Export or use the snapshot if we've been asked to do so.
         *
         * NB. We will convert the snapbuild.c kind of snapshot to normal
         * snapshot when doing this.
         */
        if (snapshot_action == CRS_EXPORT_SNAPSHOT)
        {
            snapshot_name = SnapBuildExportSnapshot(ctx->snapshot_builder);
        }
        else if (snapshot_action == CRS_USE_SNAPSHOT)
        {
            Snapshot    snap;
 
            snap = SnapBuildInitialSnapshot(ctx->snapshot_builder);
            RestoreTransactionSnapshot(snap, MyProc);
        }
 
        /* don't need the decoding context anymore */
        FreeDecodingContext(ctx);
 
        if (!cmd->temporary)
            ReplicationSlotPersist();
    }
    else if (cmd->kind == REPLICATION_KIND_PHYSICAL && reserve_wal)
    {
        ReplicationSlotReserveWal();
 
        ReplicationSlotMarkDirty();
 
        /* Write this slot to disk if it's a permanent one. */
        if (!cmd->temporary)
            ReplicationSlotSave();
    }
 
    snprintf(xloc, sizeof(xloc), "%X/%X",
             LSN_FORMAT_ARGS(MyReplicationSlot->data.confirmed_flush));
 
    dest = CreateDestReceiver(DestRemoteSimple);
 
    /*----------
     * Need a tuple descriptor representing four columns:
     * - first field: the slot name
     * - second field: LSN at which we became consistent
     * - third field: exported snapshot's name
     * - fourth field: output plugin
     *----------
     */
    tupdesc = CreateTemplateTupleDesc(4);
    TupleDescInitBuiltinEntry(tupdesc, (AttrNumber) 1, "slot_name",
                              TEXTOID, -1, 0);
    TupleDescInitBuiltinEntry(tupdesc, (AttrNumber) 2, "consistent_point",
                              TEXTOID, -1, 0);
    TupleDescInitBuiltinEntry(tupdesc, (AttrNumber) 3, "snapshot_name",
                              TEXTOID, -1, 0);
    TupleDescInitBuiltinEntry(tupdesc, (AttrNumber) 4, "output_plugin",
                              TEXTOID, -1, 0);
 
    /* prepare for projection of tuples */
    tstate = begin_tup_output_tupdesc(dest, tupdesc, &TTSOpsVirtual);
 
    /* slot_name */
    slot_name = NameStr(MyReplicationSlot->data.name);
    values[0] = CStringGetTextDatum(slot_name);
 
    /* consistent wal location */
    values[1] = CStringGetTextDatum(xloc);
 
    /* snapshot name, or NULL if none */
    if (snapshot_name != NULL)
        values[2] = CStringGetTextDatum(snapshot_name);
    else
        nulls[2] = true;
 
    /* plugin, or NULL if none */
    if (cmd->plugin != NULL)
        values[3] = CStringGetTextDatum(cmd->plugin);
    else
        nulls[3] = true;
 
    /* send it to dest */
    do_tup_output(tstate, values, nulls);
    end_tup_output(tstate);
 
    ReplicationSlotRelease();
}
 
/*
 * Get rid of a replication slot that is no longer wanted.
 */
static void
DropReplicationSlot(DropReplicationSlotCmd *cmd)
{
    ReplicationSlotDrop(cmd->slotname, !cmd->wait);
}
 
/*
 * Load previously initiated logical slot and prepare for sending data (via
 * WalSndLoop).
 */
static void
StartLogicalReplication(StartReplicationCmd *cmd)
{
    StringInfoData buf;
    QueryCompletion qc;
 
    /* make sure that our requirements are still fulfilled */
    CheckLogicalDecodingRequirements();
 
    Assert(!MyReplicationSlot);
 
    ReplicationSlotAcquire(cmd->slotname, true);
 
    if (XLogRecPtrIsInvalid(MyReplicationSlot->data.restart_lsn))
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("cannot read from logical replication slot \"%s\"",
                        cmd->slotname),
                 errdetail("This slot has been invalidated because it exceeded the maximum reserved size.")));
 
    /*
     * Force a disconnect, so that the decoding code doesn't need to care
     * about an eventual switch from running in recovery, to running in a
     * normal environment. Client code is expected to handle reconnects.
     */
    if (am_cascading_walsender && !RecoveryInProgress())
    {
        ereport(LOG,
                (errmsg("terminating walsender process after promotion")));
        got_STOPPING = true;
    }
 
    /*
     * Create our decoding context, making it start at the previously ack'ed
     * position.
     *
     * Do this before sending a CopyBothResponse message, so that any errors
     * are reported early.
     */
    logical_decoding_ctx =
        CreateDecodingContext(cmd->startpoint, cmd->options, false,
                              XL_ROUTINE(.page_read = logical_read_xlog_page,
                                         .segment_open = WalSndSegmentOpen,
                                         .segment_close = wal_segment_close),
                              WalSndPrepareWrite, WalSndWriteData,
                              WalSndUpdateProgress);
    xlogreader = logical_decoding_ctx->reader;
 
    WalSndSetState(WALSNDSTATE_CATCHUP);
 
    /* Send a CopyBothResponse message, and start streaming */
    pq_beginmessage(&buf, 'W');
    pq_sendbyte(&buf, 0);
    pq_sendint16(&buf, 0);
    pq_endmessage(&buf);
    pq_flush();
 
    /* Start reading WAL from the oldest required WAL. */
    XLogBeginRead(logical_decoding_ctx->reader,
                  MyReplicationSlot->data.restart_lsn);
 
    /*
     * Report the location after which we'll send out further commits as the
     * current sentPtr.
     */
    sentPtr = MyReplicationSlot->data.confirmed_flush;
 
    /* Also update the sent position status in shared memory */
    SpinLockAcquire(&MyWalSnd->mutex);
    MyWalSnd->sentPtr = MyReplicationSlot->data.restart_lsn;
    SpinLockRelease(&MyWalSnd->mutex);
 
    replication_active = true;
 
    SyncRepInitConfig();
 
    /* Main loop of walsender */
    WalSndLoop(XLogSendLogical);
 
    FreeDecodingContext(logical_decoding_ctx);
    ReplicationSlotRelease();
 
    replication_active = false;
    if (got_STOPPING)
        proc_exit(0);
    WalSndSetState(WALSNDSTATE_STARTUP);
 
    /* Get out of COPY mode (CommandComplete). */
    SetQueryCompletion(&qc, CMDTAG_COPY, 0);
    EndCommand(&qc, DestRemote, false);
}
 
/*
 * LogicalDecodingContext 'prepare_write' callback.
 *
 * Prepare a write into a StringInfo.
 *
 * Don't do anything lasting in here, it's quite possible that nothing will be done
 * with the data.
 */
static void
WalSndPrepareWrite(LogicalDecodingContext *ctx, XLogRecPtr lsn, TransactionId xid, bool last_write)
{
    /* can't have sync rep confused by sending the same LSN several times */
    if (!last_write)
        lsn = InvalidXLogRecPtr;
 
    resetStringInfo(ctx->out);
 
    pq_sendbyte(ctx->out, 'w');
    pq_sendint64(ctx->out, lsn);    /* dataStart */
    pq_sendint64(ctx->out, lsn);    /* walEnd */
 
    /*
     * Fill out the sendtime later, just as it's done in XLogSendPhysical, but
     * reserve space here.
     */
    pq_sendint64(ctx->out, 0);  /* sendtime */
}
 
/*
 * LogicalDecodingContext 'write' callback.
 *
 * Actually write out data previously prepared by WalSndPrepareWrite out to
 * the network. Take as long as needed, but process replies from the other
 * side and check timeouts during that.
 */
static void
WalSndWriteData(LogicalDecodingContext *ctx, XLogRecPtr lsn, TransactionId xid,
                bool last_write)
{
    TimestampTz now;
 
    /*
     * Fill the send timestamp last, so that it is taken as late as possible.
     * This is somewhat ugly, but the protocol is set as it's already used for
     * several releases by streaming physical replication.
     */
    resetStringInfo(&tmpbuf);
    now = GetCurrentTimestamp();
    pq_sendint64(&tmpbuf, now);
    memcpy(&ctx->out->data[1 + sizeof(int64) + sizeof(int64)],
           tmpbuf.data, sizeof(int64));
 
    /* output previously gathered data in a CopyData packet */
    pq_putmessage_noblock('d', ctx->out->data, ctx->out->len);
 
    CHECK_FOR_INTERRUPTS();
 
    /* Try to flush pending output to the client */
    if (pq_flush_if_writable() != 0)
        WalSndShutdown();
 
    /* Try taking fast path unless we get too close to walsender timeout. */
    if (now < TimestampTzPlusMilliseconds(last_reply_timestamp,
                                          wal_sender_timeout / 2) &&
        !pq_is_send_pending())
    {
        return;
    }
 
    /* If we have pending write here, go to slow path */
    ProcessPendingWrites();
}
 
/*
 * Wait until there is no pending write. Also process replies from the other
 * side and check timeouts during that.
 */
static void
ProcessPendingWrites(void)
{
    for (;;)
    {
        long        sleeptime;
 
        /* Check for input from the client */
        ProcessRepliesIfAny();
 
        /* die if timeout was reached */
        WalSndCheckTimeOut();
 
        /* Send keepalive if the time has come */
        WalSndKeepaliveIfNecessary();
 
        if (!pq_is_send_pending())
            break;
 
        sleeptime = WalSndComputeSleeptime(GetCurrentTimestamp());
 
        /* Sleep until something happens or we time out */
        WalSndWait(WL_SOCKET_WRITEABLE | WL_SOCKET_READABLE, sleeptime,
                   WAIT_EVENT_WAL_SENDER_WRITE_DATA);
 
        /* Clear any already-pending wakeups */
        ResetLatch(MyLatch);
 
        CHECK_FOR_INTERRUPTS();
 
        /* Process any requests or signals received recently */
        if (ConfigReloadPending)
        {
            ConfigReloadPending = false;
            ProcessConfigFile(PGC_SIGHUP);
            SyncRepInitConfig();
        }
 
        /* Try to flush pending output to the client */
        if (pq_flush_if_writable() != 0)
            WalSndShutdown();
    }
 
    /* reactivate latch so WalSndLoop knows to continue */
    SetLatch(MyLatch);
}
 
/*
 * LogicalDecodingContext 'update_progress' callback.
 *
 * Write the current position to the lag tracker (see XLogSendPhysical).
 *
 * When skipping empty transactions, send a keepalive message if necessary.
 */
static void
WalSndUpdateProgress(LogicalDecodingContext *ctx, XLogRecPtr lsn, TransactionId xid,
                     bool skipped_xact)
{
    static TimestampTz sendTime = 0;
    TimestampTz now = GetCurrentTimestamp();
    bool        pending_writes = false;
    bool        end_xact = ctx->end_xact;
 
    /*
     * Track lag no more than once per WALSND_LOGICAL_LAG_TRACK_INTERVAL_MS to
     * avoid flooding the lag tracker when we commit frequently.
     *
     * We don't have a mechanism to get the ack for any LSN other than end
     * xact LSN from the downstream. So, we track lag only for end of
     * transaction LSN.
     */
#define WALSND_LOGICAL_LAG_TRACK_INTERVAL_MS    1000
    if (end_xact && TimestampDifferenceExceeds(sendTime, now,
                                               WALSND_LOGICAL_LAG_TRACK_INTERVAL_MS))
    {
        LagTrackerWrite(lsn, now);
        sendTime = now;
    }
 
    /*
     * When skipping empty transactions in synchronous replication, we send a
     * keepalive message to avoid delaying such transactions.
     *
     * It is okay to check sync_standbys_defined flag without lock here as in
     * the worst case we will just send an extra keepalive message when it is
     * really not required.
     */
    if (skipped_xact &&
        SyncRepRequested() &&
        ((volatile WalSndCtlData *) WalSndCtl)->sync_standbys_defined)
    {
        WalSndKeepalive(false, lsn);
 
        /* Try to flush pending output to the client */
        if (pq_flush_if_writable() != 0)
            WalSndShutdown();
 
        /* If we have pending write here, make sure it's actually flushed */
        if (pq_is_send_pending())
            pending_writes = true;
    }
 
    /*
     * Process pending writes if any or try to send a keepalive if required.
     * We don't need to try sending keep alive messages at the transaction end
     * as that will be done at a later point in time. This is required only
     * for large transactions where we don't send any changes to the
     * downstream and the receiver can timeout due to that.
     */
    if (pending_writes || (!end_xact &&
                           now >= TimestampTzPlusMilliseconds(last_reply_timestamp,
                                                              wal_sender_timeout / 2)))
        ProcessPendingWrites();
}
 
/*
 * Wait till WAL < loc is flushed to disk so it can be safely sent to client.
 *
 * Returns end LSN of flushed WAL.  Normally this will be >= loc, but
 * if we detect a shutdown request (either from postmaster or client)
 * we will return early, so caller must always check.
 */
static XLogRecPtr
WalSndWaitForWal(XLogRecPtr loc)
{
    int         wakeEvents;
    static XLogRecPtr RecentFlushPtr = InvalidXLogRecPtr;
 
    /*
     * Fast path to avoid acquiring the spinlock in case we already know we
     * have enough WAL available. This is particularly interesting if we're
     * far behind.
     */
    if (RecentFlushPtr != InvalidXLogRecPtr &&
        loc <= RecentFlushPtr)
        return RecentFlushPtr;
 
    /* Get a more recent flush pointer. */
    if (!RecoveryInProgress())
        RecentFlushPtr = GetFlushRecPtr(NULL);
    else
        RecentFlushPtr = GetXLogReplayRecPtr(NULL);
 
    for (;;)
    {
        long        sleeptime;
 
        /* Clear any already-pending wakeups */
        ResetLatch(MyLatch);
 
        CHECK_FOR_INTERRUPTS();
 
        /* Process any requests or signals received recently */
        if (ConfigReloadPending)
        {
            ConfigReloadPending = false;
            ProcessConfigFile(PGC_SIGHUP);
            SyncRepInitConfig();
        }
 
        /* Check for input from the client */
        ProcessRepliesIfAny();
 
        /*
         * If we're shutting down, trigger pending WAL to be written out,
         * otherwise we'd possibly end up waiting for WAL that never gets
         * written, because walwriter has shut down already.
         */
        if (got_STOPPING)
            XLogBackgroundFlush();
 
        /* Update our idea of the currently flushed position. */
        if (!RecoveryInProgress())
            RecentFlushPtr = GetFlushRecPtr(NULL);
        else
            RecentFlushPtr = GetXLogReplayRecPtr(NULL);
 
        /*
         * If postmaster asked us to stop, don't wait anymore.
         *
         * It's important to do this check after the recomputation of
         * RecentFlushPtr, so we can send all remaining data before shutting
         * down.
         */
        if (got_STOPPING)
            break;
 
        /*
         * We only send regular messages to the client for full decoded
         * transactions, but a synchronous replication and walsender shutdown
         * possibly are waiting for a later location. So, before sleeping, we
         * send a ping containing the flush location. If the receiver is
         * otherwise idle, this keepalive will trigger a reply. Processing the
         * reply will update these MyWalSnd locations.
         */
        if (MyWalSnd->flush < sentPtr &&
            MyWalSnd->write < sentPtr &&
            !waiting_for_ping_response)
            WalSndKeepalive(false, InvalidXLogRecPtr);
 
        /* check whether we're done */
        if (loc <= RecentFlushPtr)
            break;
 
        /* Waiting for new WAL. Since we need to wait, we're now caught up. */
        WalSndCaughtUp = true;
 
        /*
         * Try to flush any pending output to the client.
         */
        if (pq_flush_if_writable() != 0)
            WalSndShutdown();
 
        /*
         * If we have received CopyDone from the client, sent CopyDone
         * ourselves, and the output buffer is empty, it's time to exit
         * streaming, so fail the current WAL fetch request.
         */
        if (streamingDoneReceiving && streamingDoneSending &&
            !pq_is_send_pending())
            break;
 
        /* die if timeout was reached */
        WalSndCheckTimeOut();
 
        /* Send keepalive if the time has come */
        WalSndKeepaliveIfNecessary();
 
        /*
         * Sleep until something happens or we time out.  Also wait for the
         * socket becoming writable, if there's still pending output.
         * Otherwise we might sit on sendable output data while waiting for
         * new WAL to be generated.  (But if we have nothing to send, we don't
         * want to wake on socket-writable.)
         */
        sleeptime = WalSndComputeSleeptime(GetCurrentTimestamp());
 
        wakeEvents = WL_SOCKET_READABLE;
 
        if (pq_is_send_pending())
            wakeEvents |= WL_SOCKET_WRITEABLE;
 
        WalSndWait(wakeEvents, sleeptime, WAIT_EVENT_WAL_SENDER_WAIT_WAL);
    }
 
    /* reactivate latch so WalSndLoop knows to continue */
    SetLatch(MyLatch);
    return RecentFlushPtr;
}
 
/*
 * Execute an incoming replication command.
 *
 * Returns true if the cmd_string was recognized as WalSender command, false
 * if not.
 */
bool
exec_replication_command(const char *cmd_string)
{
    int         parse_rc;
    Node       *cmd_node;
    const char *cmdtag;
    MemoryContext cmd_context;
    MemoryContext old_context;
 
    /*
     * If WAL sender has been told that shutdown is getting close, switch its
     * status accordingly to handle the next replication commands correctly.
     */
    if (got_STOPPING)
        WalSndSetState(WALSNDSTATE_STOPPING);
 
    /*
     * Throw error if in stopping mode.  We need prevent commands that could
     * generate WAL while the shutdown checkpoint is being written.  To be
     * safe, we just prohibit all new commands.
     */
    if (MyWalSnd->state == WALSNDSTATE_STOPPING)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("cannot execute new commands while WAL sender is in stopping mode")));
 
    /*
     * CREATE_REPLICATION_SLOT ... LOGICAL exports a snapshot until the next
     * command arrives. Clean up the old stuff if there's anything.
     */
    SnapBuildClearExportedSnapshot();
 
    CHECK_FOR_INTERRUPTS();
 
    /*
     * Prepare to parse and execute the command.
     */
    cmd_context = AllocSetContextCreate(CurrentMemoryContext,
                                        "Replication command context",
                                        ALLOCSET_DEFAULT_SIZES);
    old_context = MemoryContextSwitchTo(cmd_context);
 
    replication_scanner_init(cmd_string);
 
    /*
     * Is it a WalSender command?
     */
    if (!replication_scanner_is_replication_command())
    {
        /* Nope; clean up and get out. */
        replication_scanner_finish();
 
        MemoryContextSwitchTo(old_context);
        MemoryContextDelete(cmd_context);
 
        /* XXX this is a pretty random place to make this check */
        if (MyDatabaseId == InvalidOid)
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("cannot execute SQL commands in WAL sender for physical replication")));
 
        /* Tell the caller that this wasn't a WalSender command. */
        return false;
    }
 
    /*
     * Looks like a WalSender command, so parse it.
     */
    parse_rc = replication_yyparse();
    if (parse_rc != 0)
        ereport(ERROR,
                (errcode(ERRCODE_SYNTAX_ERROR),
                 errmsg_internal("replication command parser returned %d",
                                 parse_rc)));
    replication_scanner_finish();
 
    cmd_node = replication_parse_result;
 
    /*
     * Report query to various monitoring facilities.  For this purpose, we
     * report replication commands just like SQL commands.
     */
    debug_query_string = cmd_string;
 
    pgstat_report_activity(STATE_RUNNING, cmd_string);
 
    /*
     * Log replication command if log_replication_commands is enabled. Even
     * when it's disabled, log the command with DEBUG1 level for backward
     * compatibility.
     */
    ereport(log_replication_commands ? LOG : DEBUG1,
            (errmsg("received replication command: %s", cmd_string)));
 
    /*
     * Disallow replication commands in aborted transaction blocks.
     */
    if (IsAbortedTransactionBlockState())
        ereport(ERROR,
                (errcode(ERRCODE_IN_FAILED_SQL_TRANSACTION),
                 errmsg("current transaction is aborted, "
                        "commands ignored until end of transaction block")));
 
    CHECK_FOR_INTERRUPTS();
 
    /*
     * Allocate buffers that will be used for each outgoing and incoming
     * message.  We do this just once per command to reduce palloc overhead.
     */
    initStringInfo(&output_message);
    initStringInfo(&reply_message);
    initStringInfo(&tmpbuf);
 
    switch (cmd_node->type)
    {
        case T_IdentifySystemCmd:
            cmdtag = "IDENTIFY_SYSTEM";
            set_ps_display(cmdtag);
            IdentifySystem();
            EndReplicationCommand(cmdtag);
            break;
 
        case T_ReadReplicationSlotCmd:
            cmdtag = "READ_REPLICATION_SLOT";
            set_ps_display(cmdtag);
            ReadReplicationSlot((ReadReplicationSlotCmd *) cmd_node);
            EndReplicationCommand(cmdtag);
            break;
 
        case T_BaseBackupCmd:
            cmdtag = "BASE_BACKUP";
            set_ps_display(cmdtag);
            PreventInTransactionBlock(true, cmdtag);
            SendBaseBackup((BaseBackupCmd *) cmd_node);
            EndReplicationCommand(cmdtag);
            break;
 
        case T_CreateReplicationSlotCmd:
            cmdtag = "CREATE_REPLICATION_SLOT";
            set_ps_display(cmdtag);
            CreateReplicationSlot((CreateReplicationSlotCmd *) cmd_node);
            EndReplicationCommand(cmdtag);
            break;
 
        case T_DropReplicationSlotCmd:
            cmdtag = "DROP_REPLICATION_SLOT";
            set_ps_display(cmdtag);
            DropReplicationSlot((DropReplicationSlotCmd *) cmd_node);
            EndReplicationCommand(cmdtag);
            break;
 
        case T_StartReplicationCmd:
            {
                StartReplicationCmd *cmd = (StartReplicationCmd *) cmd_node;
 
                cmdtag = "START_REPLICATION";
                set_ps_display(cmdtag);
                PreventInTransactionBlock(true, cmdtag);
 
                if (cmd->kind == REPLICATION_KIND_PHYSICAL)
                    StartReplication(cmd);
                else
                    StartLogicalReplication(cmd);
 
                /* dupe, but necessary per libpqrcv_endstreaming */
                EndReplicationCommand(cmdtag);
 
                Assert(xlogreader != NULL);
                break;
            }
 
        case T_TimeLineHistoryCmd:
            cmdtag = "TIMELINE_HISTORY";
            set_ps_display(cmdtag);
            PreventInTransactionBlock(true, cmdtag);
            SendTimeLineHistory((TimeLineHistoryCmd *) cmd_node);
            EndReplicationCommand(cmdtag);
            break;
 
        case T_VariableShowStmt:
            {
                DestReceiver *dest = CreateDestReceiver(DestRemoteSimple);
                VariableShowStmt *n = (VariableShowStmt *) cmd_node;
 
                cmdtag = "SHOW";
                set_ps_display(cmdtag);
 
                /* syscache access needs a transaction environment */
                StartTransactionCommand();
                GetPGVariable(n->name, dest);
                CommitTransactionCommand();
                EndReplicationCommand(cmdtag);
            }
            break;
 
        default:
            elog(ERROR, "unrecognized replication command node tag: %u",
                 cmd_node->type);
    }
 
    /* done */
    MemoryContextSwitchTo(old_context);
    MemoryContextDelete(cmd_context);
 
    /*
     * We need not update ps display or pg_stat_activity, because PostgresMain
     * will reset those to "idle".  But we must reset debug_query_string to
     * ensure it doesn't become a dangling pointer.
     */
    debug_query_string = NULL;
 
    return true;
}
 
/*
 * Process any incoming messages while streaming. Also checks if the remote
 * end has closed the connection.
 */
static void
ProcessRepliesIfAny(void)
{
    unsigned char firstchar;
    int         maxmsglen;
    int         r;
    bool        received = false;
 
    last_processing = GetCurrentTimestamp();
 
    /*
     * If we already received a CopyDone from the frontend, any subsequent
     * message is the beginning of a new command, and should be processed in
     * the main processing loop.
     */
    while (!streamingDoneReceiving)
    {
        pq_startmsgread();
        r = pq_getbyte_if_available(&firstchar);
        if (r < 0)
        {
            /* unexpected error or EOF */
            ereport(COMMERROR,
                    (errcode(ERRCODE_PROTOCOL_VIOLATION),
                     errmsg("unexpected EOF on standby connection")));
            proc_exit(0);
        }
        if (r == 0)
        {
            /* no data available without blocking */
            pq_endmsgread();
            break;
        }
 
        /* Validate message type and set packet size limit */
        switch (firstchar)
        {
            case 'd':
                maxmsglen = PQ_LARGE_MESSAGE_LIMIT;
                break;
            case 'c':
            case 'X':
                maxmsglen = PQ_SMALL_MESSAGE_LIMIT;
                break;
            default:
                ereport(FATAL,
                        (errcode(ERRCODE_PROTOCOL_VIOLATION),
                         errmsg("invalid standby message type \"%c\"",
                                firstchar)));
                maxmsglen = 0;  /* keep compiler quiet */
                break;
        }
 
        /* Read the message contents */
        resetStringInfo(&reply_message);
        if (pq_getmessage(&reply_message, maxmsglen))
        {
            ereport(COMMERROR,
                    (errcode(ERRCODE_PROTOCOL_VIOLATION),
                     errmsg("unexpected EOF on standby connection")));
            proc_exit(0);
        }
 
        /* ... and process it */
        switch (firstchar)
        {
                /*
                 * 'd' means a standby reply wrapped in a CopyData packet.
                 */
            case 'd':
                ProcessStandbyMessage();
                received = true;
                break;
 
                /*
                 * CopyDone means the standby requested to finish streaming.
                 * Reply with CopyDone, if we had not sent that already.
                 */
            case 'c':
                if (!streamingDoneSending)
                {
                    pq_putmessage_noblock('c', NULL, 0);
                    streamingDoneSending = true;
                }
 
                streamingDoneReceiving = true;
                received = true;
                break;
 
                /*
                 * 'X' means that the standby is closing down the socket.
                 */
            case 'X':
                proc_exit(0);
 
            default:
                Assert(false);  /* NOT REACHED */
        }
    }
 
    /*
     * Save the last reply timestamp if we've received at least one reply.
     */
    if (received)
    {
        last_reply_timestamp = last_processing;
        waiting_for_ping_response = false;
    }
}
 
/*
 * Process a status update message received from standby.
 */
static void
ProcessStandbyMessage(void)
{
    char        msgtype;
 
    /*
     * Check message type from the first byte.
     */
    msgtype = pq_getmsgbyte(&reply_message);
 
    switch (msgtype)
    {
        case 'r':
            ProcessStandbyReplyMessage();
            break;
 
        case 'h':
            ProcessStandbyHSFeedbackMessage();
            break;
 
        default:
            ereport(COMMERROR,
                    (errcode(ERRCODE_PROTOCOL_VIOLATION),
                     errmsg("unexpected message type \"%c\"", msgtype)));
            proc_exit(0);
    }
}
 
/*
 * Remember that a walreceiver just confirmed receipt of lsn `lsn`.
 */
static void
PhysicalConfirmReceivedLocation(XLogRecPtr lsn)
{
    bool        changed = false;
    ReplicationSlot *slot = MyReplicationSlot;
 
    Assert(lsn != InvalidXLogRecPtr);
    SpinLockAcquire(&slot->mutex);
    if (slot->data.restart_lsn != lsn)
    {
        changed = true;
        slot->data.restart_lsn = lsn;
    }
    SpinLockRelease(&slot->mutex);
 
    if (changed)
    {
        ReplicationSlotMarkDirty();
        ReplicationSlotsComputeRequiredLSN();
    }
 
    /*
     * One could argue that the slot should be saved to disk now, but that'd
     * be energy wasted - the worst thing lost information could cause here is
     * to give wrong information in a statistics view - we'll just potentially
     * be more conservative in removing files.
     */
}
 
/*
 * Regular reply from standby advising of WAL locations on standby server.
 */
static void
ProcessStandbyReplyMessage(void)
{
    XLogRecPtr  writePtr,
                flushPtr,
                applyPtr;
    bool        replyRequested;
    TimeOffset  writeLag,
                flushLag,
                applyLag;
    bool        clearLagTimes;
    TimestampTz now;
    TimestampTz replyTime;
 
    static bool fullyAppliedLastTime = false;
 
    /* the caller already consumed the msgtype byte */
    writePtr = pq_getmsgint64(&reply_message);
    flushPtr = pq_getmsgint64(&reply_message);
    applyPtr = pq_getmsgint64(&reply_message);
    replyTime = pq_getmsgint64(&reply_message);
    replyRequested = pq_getmsgbyte(&reply_message);
 
    if (message_level_is_interesting(DEBUG2))
    {
        char       *replyTimeStr;
 
        /* Copy because timestamptz_to_str returns a static buffer */
        replyTimeStr = pstrdup(timestamptz_to_str(replyTime));
 
        elog(DEBUG2, "write %X/%X flush %X/%X apply %X/%X%s reply_time %s",
             LSN_FORMAT_ARGS(writePtr),
             LSN_FORMAT_ARGS(flushPtr),
             LSN_FORMAT_ARGS(applyPtr),
             replyRequested ? " (reply requested)" : "",
             replyTimeStr);
 
        pfree(replyTimeStr);
    }
 
    /* See if we can compute the round-trip lag for these positions. */
    now = GetCurrentTimestamp();
    writeLag = LagTrackerRead(SYNC_REP_WAIT_WRITE, writePtr, now);
    flushLag = LagTrackerRead(SYNC_REP_WAIT_FLUSH, flushPtr, now);
    applyLag = LagTrackerRead(SYNC_REP_WAIT_APPLY, applyPtr, now);
 
    /*
     * If the standby reports that it has fully replayed the WAL in two
     * consecutive reply messages, then the second such message must result
     * from wal_receiver_status_interval expiring on the standby.  This is a
     * convenient time to forget the lag times measured when it last
     * wrote/flushed/applied a WAL record, to avoid displaying stale lag data
     * until more WAL traffic arrives.
     */
    clearLagTimes = false;
    if (applyPtr == sentPtr)
    {
        if (fullyAppliedLastTime)
            clearLagTimes = true;
        fullyAppliedLastTime = true;
    }
    else
        fullyAppliedLastTime = false;
 
    /* Send a reply if the standby requested one. */
    if (replyRequested)
        WalSndKeepalive(false, InvalidXLogRecPtr);
 
    /*
     * Update shared state for this WalSender process based on reply data from
     * standby.
     */
    {
        WalSnd     *walsnd = MyWalSnd;
 
        SpinLockAcquire(&walsnd->mutex);
        walsnd->write = writePtr;
        walsnd->flush = flushPtr;
        walsnd->apply = applyPtr;
        if (writeLag != -1 || clearLagTimes)
            walsnd->writeLag = writeLag;
        if (flushLag != -1 || clearLagTimes)
            walsnd->flushLag = flushLag;
        if (applyLag != -1 || clearLagTimes)
            walsnd->applyLag = applyLag;
        walsnd->replyTime = replyTime;
        SpinLockRelease(&walsnd->mutex);
    }
 
    if (!am_cascading_walsender)
        SyncRepReleaseWaiters();
 
    /*
     * Advance our local xmin horizon when the client confirmed a flush.
     */
    if (MyReplicationSlot && flushPtr != InvalidXLogRecPtr)
    {
        if (SlotIsLogical(MyReplicationSlot))
            LogicalConfirmReceivedLocation(flushPtr);
        else
            PhysicalConfirmReceivedLocation(flushPtr);
    }
}
 
/* compute new replication slot xmin horizon if needed */
static void
PhysicalReplicationSlotNewXmin(TransactionId feedbackXmin, TransactionId feedbackCatalogXmin)
{
    bool        changed = false;
    ReplicationSlot *slot = MyReplicationSlot;
 
    SpinLockAcquire(&slot->mutex);
    MyProc->xmin = InvalidTransactionId;
 
    /*
     * For physical replication we don't need the interlock provided by xmin
     * and effective_xmin since the consequences of a missed increase are
     * limited to query cancellations, so set both at once.
     */
    if (!TransactionIdIsNormal(slot->data.xmin) ||
        !TransactionIdIsNormal(feedbackXmin) ||
        TransactionIdPrecedes(slot->data.xmin, feedbackXmin))
    {
        changed = true;
        slot->data.xmin = feedbackXmin;
        slot->effective_xmin = feedbackXmin;
    }
    if (!TransactionIdIsNormal(slot->data.catalog_xmin) ||
        !TransactionIdIsNormal(feedbackCatalogXmin) ||
        TransactionIdPrecedes(slot->data.catalog_xmin, feedbackCatalogXmin))
    {
        changed = true;
        slot->data.catalog_xmin = feedbackCatalogXmin;
        slot->effective_catalog_xmin = feedbackCatalogXmin;
    }
    SpinLockRelease(&slot->mutex);
 
    if (changed)
    {
        ReplicationSlotMarkDirty();
        ReplicationSlotsComputeRequiredXmin(false);
    }
}
 
/*
 * Check that the provided xmin/epoch are sane, that is, not in the future
 * and not so far back as to be already wrapped around.
 *
 * Epoch of nextXid should be same as standby, or if the counter has
 * wrapped, then one greater than standby.
 *
 * This check doesn't care about whether clog exists for these xids
 * at all.
 */
static bool
TransactionIdInRecentPast(TransactionId xid, uint32 epoch)
{
    FullTransactionId nextFullXid;
    TransactionId nextXid;
    uint32      nextEpoch;
 
    nextFullXid = ReadNextFullTransactionId();
    nextXid = XidFromFullTransactionId(nextFullXid);
    nextEpoch = EpochFromFullTransactionId(nextFullXid);
 
    if (xid <= nextXid)
    {
        if (epoch != nextEpoch)
            return false;
    }
    else
    {
        if (epoch + 1 != nextEpoch)
            return false;
    }
 
    if (!TransactionIdPrecedesOrEquals(xid, nextXid))
        return false;           /* epoch OK, but it's wrapped around */
 
    return true;
}
 
/*
 * Hot Standby feedback
 */
static void
ProcessStandbyHSFeedbackMessage(void)
{
    TransactionId feedbackXmin;
    uint32      feedbackEpoch;
    TransactionId feedbackCatalogXmin;
    uint32      feedbackCatalogEpoch;
    TimestampTz replyTime;
 
    /*
     * Decipher the reply message. The caller already consumed the msgtype
     * byte. See XLogWalRcvSendHSFeedback() in walreceiver.c for the creation
     * of this message.
     */
    replyTime = pq_getmsgint64(&reply_message);
    feedbackXmin = pq_getmsgint(&reply_message, 4);
    feedbackEpoch = pq_getmsgint(&reply_message, 4);
    feedbackCatalogXmin = pq_getmsgint(&reply_message, 4);
    feedbackCatalogEpoch = pq_getmsgint(&reply_message, 4);
 
    if (message_level_is_interesting(DEBUG2))
    {
        char       *replyTimeStr;
 
        /* Copy because timestamptz_to_str returns a static buffer */
        replyTimeStr = pstrdup(timestamptz_to_str(replyTime));
 
        elog(DEBUG2, "hot standby feedback xmin %u epoch %u, catalog_xmin %u epoch %u reply_time %s",
             feedbackXmin,
             feedbackEpoch,
             feedbackCatalogXmin,
             feedbackCatalogEpoch,
             replyTimeStr);
 
        pfree(replyTimeStr);
    }
 
    /*
     * Update shared state for this WalSender process based on reply data from
     * standby.
     */
    {
        WalSnd     *walsnd = MyWalSnd;
 
        SpinLockAcquire(&walsnd->mutex);
        walsnd->replyTime = replyTime;
        SpinLockRelease(&walsnd->mutex);
    }
 
    /*
     * Unset WalSender's xmins if the feedback message values are invalid.
     * This happens when the downstream turned hot_standby_feedback off.
     */
    if (!TransactionIdIsNormal(feedbackXmin)
        && !TransactionIdIsNormal(feedbackCatalogXmin))
    {
        MyProc->xmin = InvalidTransactionId;
        if (MyReplicationSlot != NULL)
            PhysicalReplicationSlotNewXmin(feedbackXmin, feedbackCatalogXmin);
        return;
    }
 
    /*
     * Check that the provided xmin/epoch are sane, that is, not in the future
     * and not so far back as to be already wrapped around.  Ignore if not.
     */
    if (TransactionIdIsNormal(feedbackXmin) &&
        !TransactionIdInRecentPast(feedbackXmin, feedbackEpoch))
        return;
 
    if (TransactionIdIsNormal(feedbackCatalogXmin) &&
        !TransactionIdInRecentPast(feedbackCatalogXmin, feedbackCatalogEpoch))
        return;
 
    /*
     * Set the WalSender's xmin equal to the standby's requested xmin, so that
     * the xmin will be taken into account by GetSnapshotData() /
     * ComputeXidHorizons().  This will hold back the removal of dead rows and
     * thereby prevent the generation of cleanup conflicts on the standby
     * server.
     *
     * There is a small window for a race condition here: although we just
     * checked that feedbackXmin precedes nextXid, the nextXid could have
     * gotten advanced between our fetching it and applying the xmin below,
     * perhaps far enough to make feedbackXmin wrap around.  In that case the
     * xmin we set here would be "in the future" and have no effect.  No point
     * in worrying about this since it's too late to save the desired data
     * anyway.  Assuming that the standby sends us an increasing sequence of
     * xmins, this could only happen during the first reply cycle, else our
     * own xmin would prevent nextXid from advancing so far.
     *
     * We don't bother taking the ProcArrayLock here.  Setting the xmin field
     * is assumed atomic, and there's no real need to prevent concurrent
     * horizon determinations.  (If we're moving our xmin forward, this is
     * obviously safe, and if we're moving it backwards, well, the data is at
     * risk already since a VACUUM could already have determined the horizon.)
     *
     * If we're using a replication slot we reserve the xmin via that,
     * otherwise via the walsender's PGPROC entry. We can only track the
     * catalog xmin separately when using a slot, so we store the least of the
     * two provided when not using a slot.
     *
     * XXX: It might make sense to generalize the ephemeral slot concept and
     * always use the slot mechanism to handle the feedback xmin.
     */
    if (MyReplicationSlot != NULL)  /* XXX: persistency configurable? */
        PhysicalReplicationSlotNewXmin(feedbackXmin, feedbackCatalogXmin);
    else
    {
        if (TransactionIdIsNormal(feedbackCatalogXmin)
            && TransactionIdPrecedes(feedbackCatalogXmin, feedbackXmin))
            MyProc->xmin = feedbackCatalogXmin;
        else
            MyProc->xmin = feedbackXmin;
    }
}
 
/*
 * Compute how long send/receive loops should sleep.
 *
 * If wal_sender_timeout is enabled we want to wake up in time to send
 * keepalives and to abort the connection if wal_sender_timeout has been
 * reached.
 */
static long
WalSndComputeSleeptime(TimestampTz now)
{
    long        sleeptime = 10000;  /* 10 s */
 
    if (wal_sender_timeout > 0 && last_reply_timestamp > 0)
    {
        TimestampTz wakeup_time;
 
        /*
         * At the latest stop sleeping once wal_sender_timeout has been
         * reached.
         */
        wakeup_time = TimestampTzPlusMilliseconds(last_reply_timestamp,
                                                  wal_sender_timeout);
 
        /*
         * If no ping has been sent yet, wakeup when it's time to do so.
         * WalSndKeepaliveIfNecessary() wants to send a keepalive once half of
         * the timeout passed without a response.
         */
        if (!waiting_for_ping_response)
            wakeup_time = TimestampTzPlusMilliseconds(last_reply_timestamp,
                                                      wal_sender_timeout / 2);
 
        /* Compute relative time until wakeup. */
        sleeptime = TimestampDifferenceMilliseconds(now, wakeup_time);
    }
 
    return sleeptime;
}
 
/*
 * Check whether there have been responses by the client within
 * wal_sender_timeout and shutdown if not.  Using last_processing as the
 * reference point avoids counting server-side stalls against the client.
 * However, a long server-side stall can make WalSndKeepaliveIfNecessary()
 * postdate last_processing by more than wal_sender_timeout.  If that happens,
 * the client must reply almost immediately to avoid a timeout.  This rarely
 * affects the default configuration, under which clients spontaneously send a
 * message every standby_message_timeout = wal_sender_timeout/6 = 10s.  We
 * could eliminate that problem by recognizing timeout expiration at
 * wal_sender_timeout/2 after the keepalive.
 */
static void
WalSndCheckTimeOut(void)
{
    TimestampTz timeout;
 
    /* don't bail out if we're doing something that doesn't require timeouts */
    if (last_reply_timestamp <= 0)
        return;
 
    timeout = TimestampTzPlusMilliseconds(last_reply_timestamp,
                                          wal_sender_timeout);
 
    if (wal_sender_timeout > 0 && last_processing >= timeout)
    {
        /*
         * Since typically expiration of replication timeout means
         * communication problem, we don't send the error message to the
         * standby.
         */
        ereport(COMMERROR,
                (errmsg("terminating walsender process due to replication timeout")));
 
        WalSndShutdown();
    }
}
 
/* Main loop of walsender process that streams the WAL over Copy messages. */
static void
WalSndLoop(WalSndSendDataCallback send_data)
{
    /*
     * Initialize the last reply timestamp. That enables timeout processing
     * from hereon.
     */
    last_reply_timestamp = GetCurrentTimestamp();
    waiting_for_ping_response = false;
 
    /*
     * Loop until we reach the end of this timeline or the client requests to
     * stop streaming.
     */
    for (;;)
    {
        /* Clear any already-pending wakeups */
        ResetLatch(MyLatch);
 
        CHECK_FOR_INTERRUPTS();
 
        /* Process any requests or signals received recently */
        if (ConfigReloadPending)
        {
            ConfigReloadPending = false;
            ProcessConfigFile(PGC_SIGHUP);
            SyncRepInitConfig();
        }
 
        /* Check for input from the client */
        ProcessRepliesIfAny();
 
        /*
         * If we have received CopyDone from the client, sent CopyDone
         * ourselves, and the output buffer is empty, it's time to exit
         * streaming.
         */
        if (streamingDoneReceiving && streamingDoneSending &&
            !pq_is_send_pending())
            break;
 
        /*
         * If we don't have any pending data in the output buffer, try to send
         * some more.  If there is some, we don't bother to call send_data
         * again until we've flushed it ... but we'd better assume we are not
         * caught up.
         */
        if (!pq_is_send_pending())
            send_data();
        else
            WalSndCaughtUp = false;
 
        /* Try to flush pending output to the client */
        if (pq_flush_if_writable() != 0)
            WalSndShutdown();
 
        /* If nothing remains to be sent right now ... */
        if (WalSndCaughtUp && !pq_is_send_pending())
        {
            /*
             * If we're in catchup state, move to streaming.  This is an
             * important state change for users to know about, since before
             * this point data loss might occur if the primary dies and we
             * need to failover to the standby. The state change is also
             * important for synchronous replication, since commits that
             * started to wait at that point might wait for some time.
             */
            if (MyWalSnd->state == WALSNDSTATE_CATCHUP)
            {
                ereport(DEBUG1,
                        (errmsg_internal("\"%s\" has now caught up with upstream server",
                                         application_name)));
                WalSndSetState(WALSNDSTATE_STREAMING);
            }
 
            /*
             * When SIGUSR2 arrives, we send any outstanding logs up to the
             * shutdown checkpoint record (i.e., the latest record), wait for
             * them to be replicated to the standby, and exit. This may be a
             * normal termination at shutdown, or a promotion, the walsender
             * is not sure which.
             */
            if (got_SIGUSR2)
                WalSndDone(send_data);
        }
 
        /* Check for replication timeout. */
        WalSndCheckTimeOut();
 
        /* Send keepalive if the time has come */
        WalSndKeepaliveIfNecessary();
 
        /*
         * Block if we have unsent data.  XXX For logical replication, let
         * WalSndWaitForWal() handle any other blocking; idle receivers need
         * its additional actions.  For physical replication, also block if
         * caught up; its send_data does not block.
         */
        if ((WalSndCaughtUp && send_data != XLogSendLogical &&
             !streamingDoneSending) ||
            pq_is_send_pending())
        {
            long        sleeptime;
            int         wakeEvents;
 
            if (!streamingDoneReceiving)
                wakeEvents = WL_SOCKET_READABLE;
            else
                wakeEvents = 0;
 
            /*
             * Use fresh timestamp, not last_processing, to reduce the chance
             * of reaching wal_sender_timeout before sending a keepalive.
             */
            sleeptime = WalSndComputeSleeptime(GetCurrentTimestamp());
 
            if (pq_is_send_pending())
                wakeEvents |= WL_SOCKET_WRITEABLE;
 
            /* Sleep until something happens or we time out */
            WalSndWait(wakeEvents, sleeptime, WAIT_EVENT_WAL_SENDER_MAIN);
        }
    }
}
 
/* Initialize a per-walsender data structure for this walsender process */
static void
InitWalSenderSlot(void)
{
    int         i;
 
    /*
     * WalSndCtl should be set up already (we inherit this by fork() or
     * EXEC_BACKEND mechanism from the postmaster).
     */
    Assert(WalSndCtl != NULL);
    Assert(MyWalSnd == NULL);
 
    /*
     * Find a free walsender slot and reserve it. This must not fail due to
     * the prior check for free WAL senders in InitProcess().
     */
    for (i = 0; i < max_wal_senders; i++)
    {
        WalSnd     *walsnd = &WalSndCtl->walsnds[i];
 
        SpinLockAcquire(&walsnd->mutex);
 
        if (walsnd->pid != 0)
        {
            SpinLockRelease(&walsnd->mutex);
            continue;
        }
        else
        {
            /*
             * Found a free slot. Reserve it for us.
             */
            walsnd->pid = MyProcPid;
            walsnd->state = WALSNDSTATE_STARTUP;
            walsnd->sentPtr = InvalidXLogRecPtr;
            walsnd->needreload = false;
            walsnd->write = InvalidXLogRecPtr;
            walsnd->flush = InvalidXLogRecPtr;
            walsnd->apply = InvalidXLogRecPtr;
            walsnd->writeLag = -1;
            walsnd->flushLag = -1;
            walsnd->applyLag = -1;
            walsnd->sync_standby_priority = 0;
            walsnd->latch = &MyProc->procLatch;
            walsnd->replyTime = 0;
            SpinLockRelease(&walsnd->mutex);
            /* don't need the lock anymore */
            MyWalSnd = (WalSnd *) walsnd;
 
            break;
        }
    }
 
    Assert(MyWalSnd != NULL);
 
    /* Arrange to clean up at walsender exit */
    on_shmem_exit(WalSndKill, 0);
}
 
/* Destroy the per-walsender data structure for this walsender process */
static void
WalSndKill(int code, Datum arg)
{
    WalSnd     *walsnd = MyWalSnd;
 
    Assert(walsnd != NULL);
 
    MyWalSnd = NULL;
 
    SpinLockAcquire(&walsnd->mutex);
    /* clear latch while holding the spinlock, so it can safely be read */
    walsnd->latch = NULL;
    /* Mark WalSnd struct as no longer being in use. */
    walsnd->pid = 0;
    SpinLockRelease(&walsnd->mutex);
}
 
/* XLogReaderRoutine->segment_open callback */
static void
WalSndSegmentOpen(XLogReaderState *state, XLogSegNo nextSegNo,
                  TimeLineID *tli_p)
{
    char        path[MAXPGPATH];
 
    /*-------
     * When reading from a historic timeline, and there is a timeline switch
     * within this segment, read from the WAL segment belonging to the new
     * timeline.
     *
     * For example, imagine that this server is currently on timeline 5, and
     * we're streaming timeline 4. The switch from timeline 4 to 5 happened at
     * 0/13002088. In pg_wal, we have these files:
     *
     * ...
     * 000000040000000000000012
     * 000000040000000000000013
     * 000000050000000000000013
     * 000000050000000000000014
     * ...
     *
     * In this situation, when requested to send the WAL from segment 0x13, on
     * timeline 4, we read the WAL from file 000000050000000000000013. Archive
     * recovery prefers files from newer timelines, so if the segment was
     * restored from the archive on this server, the file belonging to the old
     * timeline, 000000040000000000000013, might not exist. Their contents are
     * equal up to the switchpoint, because at a timeline switch, the used
     * portion of the old segment is copied to the new file.  -------
     */
    *tli_p = sendTimeLine;
    if (sendTimeLineIsHistoric)
    {
        XLogSegNo   endSegNo;
 
        XLByteToSeg(sendTimeLineValidUpto, endSegNo, state->segcxt.ws_segsize);
        if (nextSegNo == endSegNo)
            *tli_p = sendTimeLineNextTLI;
    }
 
    XLogFilePath(path, *tli_p, nextSegNo, state->segcxt.ws_segsize);
    state->seg.ws_file = BasicOpenFile(path, O_RDONLY | PG_BINARY);
    if (state->seg.ws_file >= 0)
        return;
 
    /*
     * If the file is not found, assume it's because the standby asked for a
     * too old WAL segment that has already been removed or recycled.
     */
    if (errno == ENOENT)
    {
        char        xlogfname[MAXFNAMELEN];
        int         save_errno = errno;
 
        XLogFileName(xlogfname, *tli_p, nextSegNo, wal_segment_size);
        errno = save_errno;
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("requested WAL segment %s has already been removed",
                        xlogfname)));
    }
    else
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not open file \"%s\": %m",
                        path)));
}
 
/*
 * Send out the WAL in its normal physical/stored form.
 *
 * Read up to MAX_SEND_SIZE bytes of WAL that's been flushed to disk,
 * but not yet sent to the client, and buffer it in the libpq output
 * buffer.
 *
 * If there is no unsent WAL remaining, WalSndCaughtUp is set to true,
 * otherwise WalSndCaughtUp is set to false.
 */
static void
XLogSendPhysical(void)
{
    XLogRecPtr  SendRqstPtr;
    XLogRecPtr  startptr;
    XLogRecPtr  endptr;
    Size        nbytes;
    XLogSegNo   segno;
    WALReadError errinfo;
 
    /* If requested switch the WAL sender to the stopping state. */
    if (got_STOPPING)
        WalSndSetState(WALSNDSTATE_STOPPING);
 
    if (streamingDoneSending)
    {
        WalSndCaughtUp = true;
        return;
    }
 
    /* Figure out how far we can safely send the WAL. */
    if (sendTimeLineIsHistoric)
    {
        /*
         * Streaming an old timeline that's in this server's history, but is
         * not the one we're currently inserting or replaying. It can be
         * streamed up to the point where we switched off that timeline.
         */
        SendRqstPtr = sendTimeLineValidUpto;
    }
    else if (am_cascading_walsender)
    {
        TimeLineID  SendRqstTLI;
 
        /*
         * Streaming the latest timeline on a standby.
         *
         * Attempt to send all WAL that has already been replayed, so that we
         * know it's valid. If we're receiving WAL through streaming
         * replication, it's also OK to send any WAL that has been received
         * but not replayed.
         *
         * The timeline we're recovering from can change, or we can be
         * promoted. In either case, the current timeline becomes historic. We
         * need to detect that so that we don't try to stream past the point
         * where we switched to another timeline. We check for promotion or
         * timeline switch after calculating FlushPtr, to avoid a race
         * condition: if the timeline becomes historic just after we checked
         * that it was still current, it's still be OK to stream it up to the
         * FlushPtr that was calculated before it became historic.
         */
        bool        becameHistoric = false;
 
        SendRqstPtr = GetStandbyFlushRecPtr(&SendRqstTLI);
 
        if (!RecoveryInProgress())
        {
            /* We have been promoted. */
            SendRqstTLI = GetWALInsertionTimeLine();
            am_cascading_walsender = false;
            becameHistoric = true;
        }
        else
        {
            /*
             * Still a cascading standby. But is the timeline we're sending
             * still the one recovery is recovering from?
             */
            if (sendTimeLine != SendRqstTLI)
                becameHistoric = true;
        }
 
        if (becameHistoric)
        {
            /*
             * The timeline we were sending has become historic. Read the
             * timeline history file of the new timeline to see where exactly
             * we forked off from the timeline we were sending.
             */
            List       *history;
 
            history = readTimeLineHistory(SendRqstTLI);
            sendTimeLineValidUpto = tliSwitchPoint(sendTimeLine, history, &sendTimeLineNextTLI);
 
            Assert(sendTimeLine < sendTimeLineNextTLI);
            list_free_deep(history);
 
            sendTimeLineIsHistoric = true;
 
            SendRqstPtr = sendTimeLineValidUpto;
        }
    }
    else
    {
        /*
         * Streaming the current timeline on a primary.
         *
         * Attempt to send all data that's already been written out and
         * fsync'd to disk.  We cannot go further than what's been written out
         * given the current implementation of WALRead().  And in any case
         * it's unsafe to send WAL that is not securely down to disk on the
         * primary: if the primary subsequently crashes and restarts, standbys
         * must not have applied any WAL that got lost on the primary.
         */
        SendRqstPtr = GetFlushRecPtr(NULL);
    }
 
    /*
     * Record the current system time as an approximation of the time at which
     * this WAL location was written for the purposes of lag tracking.
     *
     * In theory we could make XLogFlush() record a time in shmem whenever WAL
     * is flushed and we could get that time as well as the LSN when we call
     * GetFlushRecPtr() above (and likewise for the cascading standby
     * equivalent), but rather than putting any new code into the hot WAL path
     * it seems good enough to capture the time here.  We should reach this
     * after XLogFlush() runs WalSndWakeupProcessRequests(), and although that
     * may take some time, we read the WAL flush pointer and take the time
     * very close to together here so that we'll get a later position if it is
     * still moving.
     *
     * Because LagTrackerWrite ignores samples when the LSN hasn't advanced,
     * this gives us a cheap approximation for the WAL flush time for this
     * LSN.
     *
     * Note that the LSN is not necessarily the LSN for the data contained in
     * the present message; it's the end of the WAL, which might be further
     * ahead.  All the lag tracking machinery cares about is finding out when
     * that arbitrary LSN is eventually reported as written, flushed and
     * applied, so that it can measure the elapsed time.
     */
    LagTrackerWrite(SendRqstPtr, GetCurrentTimestamp());
 
    /*
     * If this is a historic timeline and we've reached the point where we
     * forked to the next timeline, stop streaming.
     *
     * Note: We might already have sent WAL > sendTimeLineValidUpto. The
     * startup process will normally replay all WAL that has been received
     * from the primary, before promoting, but if the WAL streaming is
     * terminated at a WAL page boundary, the valid portion of the timeline
     * might end in the middle of a WAL record. We might've already sent the
     * first half of that partial WAL record to the cascading standby, so that
     * sentPtr > sendTimeLineValidUpto. That's OK; the cascading standby can't
     * replay the partial WAL record either, so it can still follow our
     * timeline switch.
     */
    if (sendTimeLineIsHistoric && sendTimeLineValidUpto <= sentPtr)
    {
        /* close the current file. */
        if (xlogreader->seg.ws_file >= 0)
            wal_segment_close(xlogreader);
 
        /* Send CopyDone */
        pq_putmessage_noblock('c', NULL, 0);
        streamingDoneSending = true;
 
        WalSndCaughtUp = true;
 
        elog(DEBUG1, "walsender reached end of timeline at %X/%X (sent up to %X/%X)",
             LSN_FORMAT_ARGS(sendTimeLineValidUpto),
             LSN_FORMAT_ARGS(sentPtr));
        return;
    }
 
    /* Do we have any work to do? */
    Assert(sentPtr <= SendRqstPtr);
    if (SendRqstPtr <= sentPtr)
    {
        WalSndCaughtUp = true;
        return;
    }
 
    /*
     * Figure out how much to send in one message. If there's no more than
     * MAX_SEND_SIZE bytes to send, send everything. Otherwise send
     * MAX_SEND_SIZE bytes, but round back to logfile or page boundary.
     *
     * The rounding is not only for performance reasons. Walreceiver relies on
     * the fact that we never split a WAL record across two messages. Since a
     * long WAL record is split at page boundary into continuation records,
     * page boundary is always a safe cut-off point. We also assume that
     * SendRqstPtr never points to the middle of a WAL record.
     */
    startptr = sentPtr;
    endptr = startptr;
    endptr += MAX_SEND_SIZE;
 
    /* if we went beyond SendRqstPtr, back off */
    if (SendRqstPtr <= endptr)
    {
        endptr = SendRqstPtr;
        if (sendTimeLineIsHistoric)
            WalSndCaughtUp = false;
        else
            WalSndCaughtUp = true;
    }
    else
    {
        /* round down to page boundary. */
        endptr -= (endptr % XLOG_BLCKSZ);
        WalSndCaughtUp = false;
    }
 
    nbytes = endptr - startptr;
    Assert(nbytes <= MAX_SEND_SIZE);
 
    /*
     * OK to read and send the slice.
     */
    resetStringInfo(&output_message);
    pq_sendbyte(&output_message, 'w');
 
    pq_sendint64(&output_message, startptr);    /* dataStart */
    pq_sendint64(&output_message, SendRqstPtr); /* walEnd */
    pq_sendint64(&output_message, 0);   /* sendtime, filled in last */
 
    /*
     * Read the log directly into the output buffer to avoid extra memcpy
     * calls.
     */
    enlargeStringInfo(&output_message, nbytes);
 
retry:
    if (!WALRead(xlogreader,
                 &output_message.data[output_message.len],
                 startptr,
                 nbytes,
                 xlogreader->seg.ws_tli,    /* Pass the current TLI because
                                             * only WalSndSegmentOpen controls
                                             * whether new TLI is needed. */
                 &errinfo))
        WALReadRaiseError(&errinfo);
 
    /* See logical_read_xlog_page(). */
    XLByteToSeg(startptr, segno, xlogreader->segcxt.ws_segsize);
    CheckXLogRemoved(segno, xlogreader->seg.ws_tli);
 
    /*
     * During recovery, the currently-open WAL file might be replaced with the
     * file of the same name retrieved from archive. So we always need to
     * check what we read was valid after reading into the buffer. If it's
     * invalid, we try to open and read the file again.
     */
    if (am_cascading_walsender)
    {
        WalSnd     *walsnd = MyWalSnd;
        bool        reload;
 
        SpinLockAcquire(&walsnd->mutex);
        reload = walsnd->needreload;
        walsnd->needreload = false;
        SpinLockRelease(&walsnd->mutex);
 
        if (reload && xlogreader->seg.ws_file >= 0)
        {
            wal_segment_close(xlogreader);
 
            goto retry;
        }
    }
 
    output_message.len += nbytes;
    output_message.data[output_message.len] = '\0';
 
    /*
     * Fill the send timestamp last, so that it is taken as late as possible.
     */
    resetStringInfo(&tmpbuf);
    pq_sendint64(&tmpbuf, GetCurrentTimestamp());
    memcpy(&output_message.data[1 + sizeof(int64) + sizeof(int64)],
           tmpbuf.data, sizeof(int64));
 
    pq_putmessage_noblock('d', output_message.data, output_message.len);
 
    sentPtr = endptr;
 
    /* Update shared memory status */
    {
        WalSnd     *walsnd = MyWalSnd;
 
        SpinLockAcquire(&walsnd->mutex);
        walsnd->sentPtr = sentPtr;
        SpinLockRelease(&walsnd->mutex);
    }
 
    /* Report progress of XLOG streaming in PS display */
    if (update_process_title)
    {
        char        activitymsg[50];
 
        snprintf(activitymsg, sizeof(activitymsg), "streaming %X/%X",
                 LSN_FORMAT_ARGS(sentPtr));
        set_ps_display(activitymsg);
    }
}
 
/*
 * Stream out logically decoded data.
 */
static void
XLogSendLogical(void)
{
    XLogRecord *record;
    char       *errm;
 
    /*
     * We'll use the current flush point to determine whether we've caught up.
     * This variable is static in order to cache it across calls.  Caching is
     * helpful because GetFlushRecPtr() needs to acquire a heavily-contended
     * spinlock.
     */
    static XLogRecPtr flushPtr = InvalidXLogRecPtr;
 
    /*
     * Don't know whether we've caught up yet. We'll set WalSndCaughtUp to
     * true in WalSndWaitForWal, if we're actually waiting. We also set to
     * true if XLogReadRecord() had to stop reading but WalSndWaitForWal
     * didn't wait - i.e. when we're shutting down.
     */
    WalSndCaughtUp = false;
 
    record = XLogReadRecord(logical_decoding_ctx->reader, &errm);
 
    /* xlog record was invalid */
    if (errm != NULL)
        elog(ERROR, "could not find record while sending logically-decoded data: %s",
             errm);
 
    if (record != NULL)
    {
        /*
         * Note the lack of any call to LagTrackerWrite() which is handled by
         * WalSndUpdateProgress which is called by output plugin through
         * logical decoding write api.
         */
        LogicalDecodingProcessRecord(logical_decoding_ctx, logical_decoding_ctx->reader);
 
        sentPtr = logical_decoding_ctx->reader->EndRecPtr;
    }
 
    /*
     * If first time through in this session, initialize flushPtr.  Otherwise,
     * we only need to update flushPtr if EndRecPtr is past it.
     */
    if (flushPtr == InvalidXLogRecPtr)
        flushPtr = GetFlushRecPtr(NULL);
    else if (logical_decoding_ctx->reader->EndRecPtr >= flushPtr)
        flushPtr = GetFlushRecPtr(NULL);
 
    /* If EndRecPtr is still past our flushPtr, it means we caught up. */
    if (logical_decoding_ctx->reader->EndRecPtr >= flushPtr)
        WalSndCaughtUp = true;
 
    /*
     * If we're caught up and have been requested to stop, have WalSndLoop()
     * terminate the connection in an orderly manner, after writing out all
     * the pending data.
     */
    if (WalSndCaughtUp && got_STOPPING)
        got_SIGUSR2 = true;
 
    /* Update shared memory status */
    {
        WalSnd     *walsnd = MyWalSnd;
 
        SpinLockAcquire(&walsnd->mutex);
        walsnd->sentPtr = sentPtr;
        SpinLockRelease(&walsnd->mutex);
    }
}
 
/*
 * Shutdown if the sender is caught up.
 *
 * NB: This should only be called when the shutdown signal has been received
 * from postmaster.
 *
 * Note that if we determine that there's still more data to send, this
 * function will return control to the caller.
 */
static void
WalSndDone(WalSndSendDataCallback send_data)
{
    XLogRecPtr  replicatedPtr;
 
    /* ... let's just be real sure we're caught up ... */
    send_data();
 
    /*
     * To figure out whether all WAL has successfully been replicated, check
     * flush location if valid, write otherwise. Tools like pg_receivewal will
     * usually (unless in synchronous mode) return an invalid flush location.
     */
    replicatedPtr = XLogRecPtrIsInvalid(MyWalSnd->flush) ?
        MyWalSnd->write : MyWalSnd->flush;
 
    if (WalSndCaughtUp && sentPtr == replicatedPtr &&
        !pq_is_send_pending())
    {
        QueryCompletion qc;
 
        /* Inform the standby that XLOG streaming is done */
        SetQueryCompletion(&qc, CMDTAG_COPY, 0);
        EndCommand(&qc, DestRemote, false);
        pq_flush();
 
        proc_exit(0);
    }
    if (!waiting_for_ping_response)
        WalSndKeepalive(true, InvalidXLogRecPtr);
}
 
/*
 * Returns the latest point in WAL that has been safely flushed to disk, and
 * can be sent to the standby. This should only be called when in recovery,
 * ie. we're streaming to a cascaded standby.
 *
 * As a side-effect, *tli is updated to the TLI of the last
 * replayed WAL record.
 */
static XLogRecPtr
GetStandbyFlushRecPtr(TimeLineID *tli)
{
    XLogRecPtr  replayPtr;
    TimeLineID  replayTLI;
    XLogRecPtr  receivePtr;
    TimeLineID  receiveTLI;
    XLogRecPtr  result;
 
    /*
     * We can safely send what's already been replayed. Also, if walreceiver
     * is streaming WAL from the same timeline, we can send anything that it
     * has streamed, but hasn't been replayed yet.
     */
 
    receivePtr = GetWalRcvFlushRecPtr(NULL, &receiveTLI);
    replayPtr = GetXLogReplayRecPtr(&replayTLI);
 
    *tli = replayTLI;
 
    result = replayPtr;
    if (receiveTLI == replayTLI && receivePtr > replayPtr)
        result = receivePtr;
 
    return result;
}
 
/*
 * Request walsenders to reload the currently-open WAL file
 */
void
WalSndRqstFileReload(void)
{
    int         i;
 
    for (i = 0; i < max_wal_senders; i++)
    {
        WalSnd     *walsnd = &WalSndCtl->walsnds[i];
 
        SpinLockAcquire(&walsnd->mutex);
        if (walsnd->pid == 0)
        {
            SpinLockRelease(&walsnd->mutex);
            continue;
        }
        walsnd->needreload = true;
        SpinLockRelease(&walsnd->mutex);
    }
}
 
/*
 * Handle PROCSIG_WALSND_INIT_STOPPING signal.
 */
void
HandleWalSndInitStopping(void)
{
    Assert(am_walsender);
 
    /*
     * If replication has not yet started, die like with SIGTERM. If
     * replication is active, only set a flag and wake up the main loop. It
     * will send any outstanding WAL, wait for it to be replicated to the
     * standby, and then exit gracefully.
     */
    if (!replication_active)
        kill(MyProcPid, SIGTERM);
    else
        got_STOPPING = true;
}
 
/*
 * SIGUSR2: set flag to do a last cycle and shut down afterwards. The WAL
 * sender should already have been switched to WALSNDSTATE_STOPPING at
 * this point.
 */
static void
WalSndLastCycleHandler(SIGNAL_ARGS)
{
    int         save_errno = errno;
 
    got_SIGUSR2 = true;
    SetLatch(MyLatch);
 
    errno = save_errno;
}
 
/* Set up signal handlers */
void
WalSndSignals(void)
{
    /* Set up signal handlers */
    pqsignal(SIGHUP, SignalHandlerForConfigReload);
    pqsignal(SIGINT, StatementCancelHandler);   /* query cancel */
    pqsignal(SIGTERM, die);     /* request shutdown */
    /* SIGQUIT handler was already set up by InitPostmasterChild */
    InitializeTimeouts();       /* establishes SIGALRM handler */
    pqsignal(SIGPIPE, SIG_IGN);
    pqsignal(SIGUSR1, procsignal_sigusr1_handler);
    pqsignal(SIGUSR2, WalSndLastCycleHandler);  /* request a last cycle and
                                                 * shutdown */
 
    /* Reset some signals that are accepted by postmaster but not here */
    pqsignal(SIGCHLD, SIG_DFL);
}
 
/* Report shared-memory space needed by WalSndShmemInit */
Size
WalSndShmemSize(void)
{
    Size        size = 0;
 
    size = offsetof(WalSndCtlData, walsnds);
    size = add_size(size, mul_size(max_wal_senders, sizeof(WalSnd)));
 
    return size;
}
 
/* Allocate and initialize walsender-related shared memory */
void
WalSndShmemInit(void)
{
    bool        found;
    int         i;
 
    WalSndCtl = (WalSndCtlData *)
        ShmemInitStruct("Wal Sender Ctl", WalSndShmemSize(), &found);
 
    if (!found)
    {
        /* First time through, so initialize */
        MemSet(WalSndCtl, 0, WalSndShmemSize());
 
        for (i = 0; i < NUM_SYNC_REP_WAIT_MODE; i++)
            dlist_init(&(WalSndCtl->SyncRepQueue[i]));
 
        for (i = 0; i < max_wal_senders; i++)
        {
            WalSnd     *walsnd = &WalSndCtl->walsnds[i];
 
            SpinLockInit(&walsnd->mutex);
        }
    }
}
 
/*
 * Wake up all walsenders
 *
 * This will be called inside critical sections, so throwing an error is not
 * advisable.
 */
void
WalSndWakeup(void)
{
    int         i;
 
    for (i = 0; i < max_wal_senders; i++)
    {
        Latch      *latch;
        WalSnd     *walsnd = &WalSndCtl->walsnds[i];
 
        /*
         * Get latch pointer with spinlock held, for the unlikely case that
         * pointer reads aren't atomic (as they're 8 bytes).
         */
        SpinLockAcquire(&walsnd->mutex);
        latch = walsnd->latch;
        SpinLockRelease(&walsnd->mutex);
 
        if (latch != NULL)
            SetLatch(latch);
    }
}
 
/*
 * Wait for readiness on the FeBe socket, or a timeout.  The mask should be
 * composed of optional WL_SOCKET_WRITEABLE and WL_SOCKET_READABLE flags.  Exit
 * on postmaster death.
 */
static void
WalSndWait(uint32 socket_events, long timeout, uint32 wait_event)
{
    WaitEvent   event;
 
    ModifyWaitEvent(FeBeWaitSet, FeBeWaitSetSocketPos, socket_events, NULL);
    if (WaitEventSetWait(FeBeWaitSet, timeout, &event, 1, wait_event) == 1 &&
        (event.events & WL_POSTMASTER_DEATH))
        proc_exit(1);
}
 
/*
 * Signal all walsenders to move to stopping state.
 *
 * This will trigger walsenders to move to a state where no further WAL can be
 * generated. See this file's header for details.
 */
void
WalSndInitStopping(void)
{
    int         i;
 
    for (i = 0; i < max_wal_senders; i++)
    {
        WalSnd     *walsnd = &WalSndCtl->walsnds[i];
        pid_t       pid;
 
        SpinLockAcquire(&walsnd->mutex);
        pid = walsnd->pid;
        SpinLockRelease(&walsnd->mutex);
 
        if (pid == 0)
            continue;
 
        SendProcSignal(pid, PROCSIG_WALSND_INIT_STOPPING, InvalidBackendId);
    }
}
 
/*
 * Wait that all the WAL senders have quit or reached the stopping state. This
 * is used by the checkpointer to control when the shutdown checkpoint can
 * safely be performed.
 */
void
WalSndWaitStopping(void)
{
    for (;;)
    {
        int         i;
        bool        all_stopped = true;
 
        for (i = 0; i < max_wal_senders; i++)
        {
            WalSnd     *walsnd = &WalSndCtl->walsnds[i];
 
            SpinLockAcquire(&walsnd->mutex);
 
            if (walsnd->pid == 0)
            {
                SpinLockRelease(&walsnd->mutex);
                continue;
            }
 
            if (walsnd->state != WALSNDSTATE_STOPPING)
            {
                all_stopped = false;
                SpinLockRelease(&walsnd->mutex);
                break;
            }
            SpinLockRelease(&walsnd->mutex);
        }
 
        /* safe to leave if confirmation is done for all WAL senders */
        if (all_stopped)
            return;
 
        pg_usleep(10000L);      /* wait for 10 msec */
    }
}
 
/* Set state for current walsender (only called in walsender) */
void
WalSndSetState(WalSndState state)
{
    WalSnd     *walsnd = MyWalSnd;
 
    Assert(am_walsender);
 
    if (walsnd->state == state)
        return;
 
    SpinLockAcquire(&walsnd->mutex);
    walsnd->state = state;
    SpinLockRelease(&walsnd->mutex);
}
 
/*
 * Return a string constant representing the state. This is used
 * in system views, and should *not* be translated.
 */
static const char *
WalSndGetStateString(WalSndState state)
{
    switch (state)
    {
        case WALSNDSTATE_STARTUP:
            return "startup";
        case WALSNDSTATE_BACKUP:
            return "backup";
        case WALSNDSTATE_CATCHUP:
            return "catchup";
        case WALSNDSTATE_STREAMING:
            return "streaming";
        case WALSNDSTATE_STOPPING:
            return "stopping";
    }
    return "UNKNOWN";
}
 
static Interval *
offset_to_interval(TimeOffset offset)
{
    Interval   *result = palloc(sizeof(Interval));
 
    result->month = 0;
    result->day = 0;
    result->time = offset;
 
    return result;
}
 
/*
 * Returns activity of walsenders, including pids and xlog locations sent to
 * standby servers.
 */
Datum
pg_stat_get_wal_senders(PG_FUNCTION_ARGS)
{
#define PG_STAT_GET_WAL_SENDERS_COLS    12
    ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
    SyncRepStandbyData *sync_standbys;
    int         num_standbys;
    int         i;
 
    InitMaterializedSRF(fcinfo, 0);
 
    /*
     * Get the currently active synchronous standbys.  This could be out of
     * date before we're done, but we'll use the data anyway.
     */
    num_standbys = SyncRepGetCandidateStandbys(&sync_standbys);
 
    for (i = 0; i < max_wal_senders; i++)
    {
        WalSnd     *walsnd = &WalSndCtl->walsnds[i];
        XLogRecPtr  sentPtr;
        XLogRecPtr  write;
        XLogRecPtr  flush;
        XLogRecPtr  apply;
        TimeOffset  writeLag;
        TimeOffset  flushLag;
        TimeOffset  applyLag;
        int         priority;
        int         pid;
        WalSndState state;
        TimestampTz replyTime;
        bool        is_sync_standby;
        Datum       values[PG_STAT_GET_WAL_SENDERS_COLS];
        bool        nulls[PG_STAT_GET_WAL_SENDERS_COLS] = {0};
        int         j;
 
        /* Collect data from shared memory */
        SpinLockAcquire(&walsnd->mutex);
        if (walsnd->pid == 0)
        {
            SpinLockRelease(&walsnd->mutex);
            continue;
        }
        pid = walsnd->pid;
        sentPtr = walsnd->sentPtr;
        state = walsnd->state;
        write = walsnd->write;
        flush = walsnd->flush;
        apply = walsnd->apply;
        writeLag = walsnd->writeLag;
        flushLag = walsnd->flushLag;
        applyLag = walsnd->applyLag;
        priority = walsnd->sync_standby_priority;
        replyTime = walsnd->replyTime;
        SpinLockRelease(&walsnd->mutex);
 
        /*
         * Detect whether walsender is/was considered synchronous.  We can
         * provide some protection against stale data by checking the PID
         * along with walsnd_index.
         */
        is_sync_standby = false;
        for (j = 0; j < num_standbys; j++)
        {
            if (sync_standbys[j].walsnd_index == i &&
                sync_standbys[j].pid == pid)
            {
                is_sync_standby = true;
                break;
            }
        }
 
        values[0] = Int32GetDatum(pid);
 
        if (!has_privs_of_role(GetUserId(), ROLE_PG_READ_ALL_STATS))
        {
            /*
             * Only superusers and roles with privileges of pg_read_all_stats
             * can see details. Other users only get the pid value to know
             * it's a walsender, but no details.
             */
            MemSet(&nulls[1], true, PG_STAT_GET_WAL_SENDERS_COLS - 1);
        }
        else
        {
            values[1] = CStringGetTextDatum(WalSndGetStateString(state));
 
            if (XLogRecPtrIsInvalid(sentPtr))
                nulls[2] = true;
            values[2] = LSNGetDatum(sentPtr);
 
            if (XLogRecPtrIsInvalid(write))
                nulls[3] = true;
            values[3] = LSNGetDatum(write);
 
            if (XLogRecPtrIsInvalid(flush))
                nulls[4] = true;
            values[4] = LSNGetDatum(flush);
 
            if (XLogRecPtrIsInvalid(apply))
                nulls[5] = true;
            values[5] = LSNGetDatum(apply);
 
            /*
             * Treat a standby such as a pg_basebackup background process
             * which always returns an invalid flush location, as an
             * asynchronous standby.
             */
            priority = XLogRecPtrIsInvalid(flush) ? 0 : priority;
 
            if (writeLag < 0)
                nulls[6] = true;
            else
                values[6] = IntervalPGetDatum(offset_to_interval(writeLag));
 
            if (flushLag < 0)
                nulls[7] = true;
            else
                values[7] = IntervalPGetDatum(offset_to_interval(flushLag));
 
            if (applyLag < 0)
                nulls[8] = true;
            else
                values[8] = IntervalPGetDatum(offset_to_interval(applyLag));
 
            values[9] = Int32GetDatum(priority);
 
            /*
             * More easily understood version of standby state. This is purely
             * informational.
             *
             * In quorum-based sync replication, the role of each standby
             * listed in synchronous_standby_names can be changing very
             * frequently. Any standbys considered as "sync" at one moment can
             * be switched to "potential" ones at the next moment. So, it's
             * basically useless to report "sync" or "potential" as their sync
             * states. We report just "quorum" for them.
             */
            if (priority == 0)
                values[10] = CStringGetTextDatum("async");
            else if (is_sync_standby)
                values[10] = SyncRepConfig->syncrep_method == SYNC_REP_PRIORITY ?
                    CStringGetTextDatum("sync") : CStringGetTextDatum("quorum");
            else
                values[10] = CStringGetTextDatum("potential");
 
            if (replyTime == 0)
                nulls[11] = true;
            else
                values[11] = TimestampTzGetDatum(replyTime);
        }
 
        tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc,
                             values, nulls);
    }
 
    return (Datum) 0;
}
 
/*
 * Send a keepalive message to standby.
 *
 * If requestReply is set, the message requests the other party to send
 * a message back to us, for heartbeat purposes.  We also set a flag to
 * let nearby code know that we're waiting for that response, to avoid
 * repeated requests.
 *
 * writePtr is the location up to which the WAL is sent. It is essentially
 * the same as sentPtr but in some cases, we need to send keep alive before
 * sentPtr is updated like when skipping empty transactions.
 */
static void
WalSndKeepalive(bool requestReply, XLogRecPtr writePtr)
{
    elog(DEBUG2, "sending replication keepalive");
 
    /* construct the message... */
    resetStringInfo(&output_message);
    pq_sendbyte(&output_message, 'k');
    pq_sendint64(&output_message, XLogRecPtrIsInvalid(writePtr) ? sentPtr : writePtr);
    pq_sendint64(&output_message, GetCurrentTimestamp());
    pq_sendbyte(&output_message, requestReply ? 1 : 0);
 
    /* ... and send it wrapped in CopyData */
    pq_putmessage_noblock('d', output_message.data, output_message.len);
 
    /* Set local flag */
    if (requestReply)
        waiting_for_ping_response = true;
}
 
/*
 * Send keepalive message if too much time has elapsed.
 */
static void
WalSndKeepaliveIfNecessary(void)
{
    TimestampTz ping_time;
 
    /*
     * Don't send keepalive messages if timeouts are globally disabled or
     * we're doing something not partaking in timeouts.
     */
    if (wal_sender_timeout <= 0 || last_reply_timestamp <= 0)
        return;
 
    if (waiting_for_ping_response)
        return;
 
    /*
     * If half of wal_sender_timeout has lapsed without receiving any reply
     * from the standby, send a keep-alive message to the standby requesting
     * an immediate reply.
     */
    ping_time = TimestampTzPlusMilliseconds(last_reply_timestamp,
                                            wal_sender_timeout / 2);
    if (last_processing >= ping_time)
    {
        WalSndKeepalive(true, InvalidXLogRecPtr);
 
        /* Try to flush pending output to the client */
        if (pq_flush_if_writable() != 0)
            WalSndShutdown();
    }
}
 
/*
 * Record the end of the WAL and the time it was flushed locally, so that
 * LagTrackerRead can compute the elapsed time (lag) when this WAL location is
 * eventually reported to have been written, flushed and applied by the
 * standby in a reply message.
 */
static void
LagTrackerWrite(XLogRecPtr lsn, TimestampTz local_flush_time)
{
    bool        buffer_full;
    int         new_write_head;
    int         i;
 
    if (!am_walsender)
        return;
 
    /*
     * If the lsn hasn't advanced since last time, then do nothing.  This way
     * we only record a new sample when new WAL has been written.
     */
    if (lag_tracker->last_lsn == lsn)
        return;
    lag_tracker->last_lsn = lsn;
 
    /*
     * If advancing the write head of the circular buffer would crash into any
     * of the read heads, then the buffer is full.  In other words, the
     * slowest reader (presumably apply) is the one that controls the release
     * of space.
     */
    new_write_head = (lag_tracker->write_head + 1) % LAG_TRACKER_BUFFER_SIZE;
    buffer_full = false;
    for (i = 0; i < NUM_SYNC_REP_WAIT_MODE; ++i)
    {
        if (new_write_head == lag_tracker->read_heads[i])
            buffer_full = true;
    }
 
    /*
     * If the buffer is full, for now we just rewind by one slot and overwrite
     * the last sample, as a simple (if somewhat uneven) way to lower the
     * sampling rate.  There may be better adaptive compaction algorithms.
     */
    if (buffer_full)
    {
        new_write_head = lag_tracker->write_head;
        if (lag_tracker->write_head > 0)
            lag_tracker->write_head--;
        else
            lag_tracker->write_head = LAG_TRACKER_BUFFER_SIZE - 1;
    }
 
    /* Store a sample at the current write head position. */
    lag_tracker->buffer[lag_tracker->write_head].lsn = lsn;
    lag_tracker->buffer[lag_tracker->write_head].time = local_flush_time;
    lag_tracker->write_head = new_write_head;
}
 
/*
 * Find out how much time has elapsed between the moment WAL location 'lsn'
 * (or the highest known earlier LSN) was flushed locally and the time 'now'.
 * We have a separate read head for each of the reported LSN locations we
 * receive in replies from standby; 'head' controls which read head is
 * used.  Whenever a read head crosses an LSN which was written into the
 * lag buffer with LagTrackerWrite, we can use the associated timestamp to
 * find out the time this LSN (or an earlier one) was flushed locally, and
 * therefore compute the lag.
 *
 * Return -1 if no new sample data is available, and otherwise the elapsed
 * time in microseconds.
 */
static TimeOffset
LagTrackerRead(int head, XLogRecPtr lsn, TimestampTz now)
{
    TimestampTz time = 0;
 
    /* Read all unread samples up to this LSN or end of buffer. */
    while (lag_tracker->read_heads[head] != lag_tracker->write_head &&
           lag_tracker->buffer[lag_tracker->read_heads[head]].lsn <= lsn)
    {
        time = lag_tracker->buffer[lag_tracker->read_heads[head]].time;
        lag_tracker->last_read[head] =
            lag_tracker->buffer[lag_tracker->read_heads[head]];
        lag_tracker->read_heads[head] =
            (lag_tracker->read_heads[head] + 1) % LAG_TRACKER_BUFFER_SIZE;
    }
 
    /*
     * If the lag tracker is empty, that means the standby has processed
     * everything we've ever sent so we should now clear 'last_read'.  If we
     * didn't do that, we'd risk using a stale and irrelevant sample for
     * interpolation at the beginning of the next burst of WAL after a period
     * of idleness.
     */
    if (lag_tracker->read_heads[head] == lag_tracker->write_head)
        lag_tracker->last_read[head].time = 0;
 
    if (time > now)
    {
        /* If the clock somehow went backwards, treat as not found. */
        return -1;
    }
    else if (time == 0)
    {
        /*
         * We didn't cross a time.  If there is a future sample that we
         * haven't reached yet, and we've already reached at least one sample,
         * let's interpolate the local flushed time.  This is mainly useful
         * for reporting a completely stuck apply position as having
         * increasing lag, since otherwise we'd have to wait for it to
         * eventually start moving again and cross one of our samples before
         * we can show the lag increasing.
         */
        if (lag_tracker->read_heads[head] == lag_tracker->write_head)
        {
            /* There are no future samples, so we can't interpolate. */
            return -1;
        }
        else if (lag_tracker->last_read[head].time != 0)
        {
            /* We can interpolate between last_read and the next sample. */
            double      fraction;
            WalTimeSample prev = lag_tracker->last_read[head];
            WalTimeSample next = lag_tracker->buffer[lag_tracker->read_heads[head]];
 
            if (lsn < prev.lsn)
            {
                /*
                 * Reported LSNs shouldn't normally go backwards, but it's
                 * possible when there is a timeline change.  Treat as not
                 * found.
                 */
                return -1;
            }
 
            Assert(prev.lsn < next.lsn);
 
            if (prev.time > next.time)
            {
                /* If the clock somehow went backwards, treat as not found. */
                return -1;
            }
 
            /* See how far we are between the previous and next samples. */
            fraction =
                (double) (lsn - prev.lsn) / (double) (next.lsn - prev.lsn);
 
            /* Scale the local flush time proportionally. */
            time = (TimestampTz)
                ((double) prev.time + (next.time - prev.time) * fraction);
        }
        else
        {
            /*
             * We have only a future sample, implying that we were entirely
             * caught up but and now there is a new burst of WAL and the
             * standby hasn't processed the first sample yet.  Until the
             * standby reaches the future sample the best we can do is report
             * the hypothetical lag if that sample were to be replayed now.
             */
            time = lag_tracker->buffer[lag_tracker->read_heads[head]].time;
        }
    }
 
    /* Return the elapsed time since local flush time in microseconds. */
    Assert(time != 0);
    return now - time;
}