walreceiver.c

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/*-------------------------------------------------------------------------
 *
 * walreceiver.c
 *
 * The WAL receiver process (walreceiver) is new as of Postgres 9.0. It
 * is the process in the standby server that takes charge of receiving
 * XLOG records from a primary server during streaming replication.
 *
 * When the startup process determines that it's time to start streaming,
 * it instructs postmaster to start walreceiver. Walreceiver first connects
 * to the primary server (it will be served by a walsender process
 * in the primary server), and then keeps receiving XLOG records and
 * writing them to the disk as long as the connection is alive. As XLOG
 * records are received and flushed to disk, it updates the
 * WalRcv->flushedUpto variable in shared memory, to inform the startup
 * process of how far it can proceed with XLOG replay.
 *
 * A WAL receiver cannot directly load GUC parameters used when establishing
 * its connection to the primary. Instead it relies on parameter values
 * that are passed down by the startup process when streaming is requested.
 * This applies, for example, to the replication slot and the connection
 * string to be used for the connection with the primary.
 *
 * If the primary server ends streaming, but doesn't disconnect, walreceiver
 * goes into "waiting" mode, and waits for the startup process to give new
 * instructions. The startup process will treat that the same as
 * disconnection, and will rescan the archive/pg_wal directory. But when the
 * startup process wants to try streaming replication again, it will just
 * nudge the existing walreceiver process that's waiting, instead of launching
 * a new one.
 *
 * Normal termination is by SIGTERM, which instructs the walreceiver to
 * exit(0). Emergency termination is by SIGQUIT; like any postmaster child
 * process, the walreceiver will simply abort and exit on SIGQUIT. A close
 * of the connection and a FATAL error are treated not as a crash but as
 * normal operation.
 *
 * This file contains the server-facing parts of walreceiver. The libpq-
 * specific parts are in the libpqwalreceiver module. It's loaded
 * dynamically to avoid linking the server with libpq.
 *
 * Portions Copyright (c) 2010-2021, PostgreSQL Global Development Group
 *
 *
 * IDENTIFICATION
 *    src/backend/replication/walreceiver.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <unistd.h>

#include "access/htup_details.h"
#include "access/timeline.h"
#include "access/transam.h"
#include "access/xlog_internal.h"
#include "access/xlogarchive.h"
#include "catalog/pg_authid.h"
#include "catalog/pg_type.h"
#include "common/ip.h"
#include "funcapi.h"
#include "libpq/pqformat.h"
#include "libpq/pqsignal.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/interrupt.h"
#include "replication/walreceiver.h"
#include "replication/walsender.h"
#include "storage/ipc.h"
#include "storage/pmsignal.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/procsignal.h"
#include "utils/acl.h"
#include "utils/builtins.h"
#include "utils/guc.h"
#include "utils/pg_lsn.h"
#include "utils/ps_status.h"
#include "utils/resowner.h"
#include "utils/timestamp.h"


/*
 * GUC variables.  (Other variables that affect walreceiver are in xlog.c
 * because they're passed down from the startup process, for better
 * synchronization.)
 */
int         wal_receiver_status_interval;
int         wal_receiver_timeout;
bool        hot_standby_feedback;

/* libpqwalreceiver connection */
static WalReceiverConn *wrconn = NULL;
WalReceiverFunctionsType *WalReceiverFunctions = NULL;

#define NAPTIME_PER_CYCLE 100   /* max sleep time between cycles (100ms) */

/*
 * These variables are used similarly to openLogFile/SegNo,
 * but for walreceiver to write the XLOG. recvFileTLI is the TimeLineID
 * corresponding the filename of recvFile.
 */
static int  recvFile = -1;
static TimeLineID recvFileTLI = 0;
static XLogSegNo recvSegNo = 0;

/*
 * LogstreamResult indicates the byte positions that we have already
 * written/fsynced.
 */
static struct
{
    XLogRecPtr  Write;          /* last byte + 1 written out in the standby */
    XLogRecPtr  Flush;          /* last byte + 1 flushed in the standby */
}           LogstreamResult;

static StringInfoData reply_message;
static StringInfoData incoming_message;

/* Prototypes for private functions */
static void WalRcvFetchTimeLineHistoryFiles(TimeLineID first, TimeLineID last);
static void WalRcvWaitForStartPosition(XLogRecPtr *startpoint, TimeLineID *startpointTLI);
static void WalRcvDie(int code, Datum arg);
static void XLogWalRcvProcessMsg(unsigned char type, char *buf, Size len);
static void XLogWalRcvWrite(char *buf, Size nbytes, XLogRecPtr recptr);
static void XLogWalRcvFlush(bool dying);
static void XLogWalRcvClose(XLogRecPtr recptr);
static void XLogWalRcvSendReply(bool force, bool requestReply);
static void XLogWalRcvSendHSFeedback(bool immed);
static void ProcessWalSndrMessage(XLogRecPtr walEnd, TimestampTz sendTime);

/*
 * Process any interrupts the walreceiver process may have received.
 * This should be called any time the process's latch has become set.
 *
 * Currently, only SIGTERM is of interest.  We can't just exit(1) within the
 * SIGTERM signal handler, because the signal might arrive in the middle of
 * some critical operation, like while we're holding a spinlock.  Instead, the
 * signal handler sets a flag variable as well as setting the process's latch.
 * We must check the flag (by calling ProcessWalRcvInterrupts) anytime the
 * latch has become set.  Operations that could block for a long time, such as
 * reading from a remote server, must pay attention to the latch too; see
 * libpqrcv_PQgetResult for example.
 */
void
ProcessWalRcvInterrupts(void)
{
    /*
     * Although walreceiver interrupt handling doesn't use the same scheme as
     * regular backends, call CHECK_FOR_INTERRUPTS() to make sure we receive
     * any incoming signals on Win32, and also to make sure we process any
     * barrier events.
     */
    CHECK_FOR_INTERRUPTS();

    if (ShutdownRequestPending)
    {
        ereport(FATAL,
                (errcode(ERRCODE_ADMIN_SHUTDOWN),
                 errmsg("terminating walreceiver process due to administrator command")));
    }
}


/* Main entry point for walreceiver process */
void
WalReceiverMain(void)
{
    char        conninfo[MAXCONNINFO];
    char       *tmp_conninfo;
    char        slotname[NAMEDATALEN];
    bool        is_temp_slot;
    XLogRecPtr  startpoint;
    TimeLineID  startpointTLI;
    TimeLineID  primaryTLI;
    bool        first_stream;
    WalRcvData *walrcv = WalRcv;
    TimestampTz last_recv_timestamp;
    TimestampTz now;
    bool        ping_sent;
    char       *err;
    char       *sender_host = NULL;
    int         sender_port = 0;

    /*
     * WalRcv should be set up already (if we are a backend, we inherit this
     * by fork() or EXEC_BACKEND mechanism from the postmaster).
     */
    Assert(walrcv != NULL);

    now = GetCurrentTimestamp();

    /*
     * Mark walreceiver as running in shared memory.
     *
     * Do this as early as possible, so that if we fail later on, we'll set
     * state to STOPPED. If we die before this, the startup process will keep
     * waiting for us to start up, until it times out.
     */
    SpinLockAcquire(&walrcv->mutex);
    Assert(walrcv->pid == 0);
    switch (walrcv->walRcvState)
    {
        case WALRCV_STOPPING:
            /* If we've already been requested to stop, don't start up. */
            walrcv->walRcvState = WALRCV_STOPPED;
            /* fall through */

        case WALRCV_STOPPED:
            SpinLockRelease(&walrcv->mutex);
            ConditionVariableBroadcast(&walrcv->walRcvStoppedCV);
            proc_exit(1);
            break;

        case WALRCV_STARTING:
            /* The usual case */
            break;

        case WALRCV_WAITING:
        case WALRCV_STREAMING:
        case WALRCV_RESTARTING:
        default:
            /* Shouldn't happen */
            SpinLockRelease(&walrcv->mutex);
            elog(PANIC, "walreceiver still running according to shared memory state");
    }
    /* Advertise our PID so that the startup process can kill us */
    walrcv->pid = MyProcPid;
    walrcv->walRcvState = WALRCV_STREAMING;

    /* Fetch information required to start streaming */
    walrcv->ready_to_display = false;
    strlcpy(conninfo, (char *) walrcv->conninfo, MAXCONNINFO);
    strlcpy(slotname, (char *) walrcv->slotname, NAMEDATALEN);
    is_temp_slot = walrcv->is_temp_slot;
    startpoint = walrcv->receiveStart;
    startpointTLI = walrcv->receiveStartTLI;

    /*
     * At most one of is_temp_slot and slotname can be set; otherwise,
     * RequestXLogStreaming messed up.
     */
    Assert(!is_temp_slot || (slotname[0] == '\0'));

    /* Initialise to a sanish value */
    walrcv->lastMsgSendTime =
        walrcv->lastMsgReceiptTime = walrcv->latestWalEndTime = now;

    /* Report the latch to use to awaken this process */
    walrcv->latch = &MyProc->procLatch;

    SpinLockRelease(&walrcv->mutex);

    pg_atomic_write_u64(&WalRcv->writtenUpto, 0);

    /* Arrange to clean up at walreceiver exit */
    on_shmem_exit(WalRcvDie, 0);

    /* Properly accept or ignore signals the postmaster might send us */
    pqsignal(SIGHUP, SignalHandlerForConfigReload); /* set flag to read config
                                                     * file */
    pqsignal(SIGINT, SIG_IGN);
    pqsignal(SIGTERM, SignalHandlerForShutdownRequest); /* request shutdown */
    /* SIGQUIT handler was already set up by InitPostmasterChild */
    pqsignal(SIGALRM, SIG_IGN);
    pqsignal(SIGPIPE, SIG_IGN);
    pqsignal(SIGUSR1, procsignal_sigusr1_handler);
    pqsignal(SIGUSR2, SIG_IGN);

    /* Reset some signals that are accepted by postmaster but not here */
    pqsignal(SIGCHLD, SIG_DFL);

    /* Load the libpq-specific functions */
    load_file("libpqwalreceiver", false);
    if (WalReceiverFunctions == NULL)
        elog(ERROR, "libpqwalreceiver didn't initialize correctly");

    /* Unblock signals (they were blocked when the postmaster forked us) */
    PG_SETMASK(&UnBlockSig);

    /* Establish the connection to the primary for XLOG streaming */
    wrconn = walrcv_connect(conninfo, false,
                            cluster_name[0] ? cluster_name : "walreceiver",
                            &err);
    if (!wrconn)
        ereport(ERROR,
                (errcode(ERRCODE_CONNECTION_FAILURE),
                 errmsg("could not connect to the primary server: %s", err)));

    /*
     * Save user-visible connection string.  This clobbers the original
     * conninfo, for security. Also save host and port of the sender server
     * this walreceiver is connected to.
     */
    tmp_conninfo = walrcv_get_conninfo(wrconn);
    walrcv_get_senderinfo(wrconn, &sender_host, &sender_port);
    SpinLockAcquire(&walrcv->mutex);
    memset(walrcv->conninfo, 0, MAXCONNINFO);
    if (tmp_conninfo)
        strlcpy((char *) walrcv->conninfo, tmp_conninfo, MAXCONNINFO);

    memset(walrcv->sender_host, 0, NI_MAXHOST);
    if (sender_host)
        strlcpy((char *) walrcv->sender_host, sender_host, NI_MAXHOST);

    walrcv->sender_port = sender_port;
    walrcv->ready_to_display = true;
    SpinLockRelease(&walrcv->mutex);

    if (tmp_conninfo)
        pfree(tmp_conninfo);

    if (sender_host)
        pfree(sender_host);

    first_stream = true;
    for (;;)
    {
        char       *primary_sysid;
        char        standby_sysid[32];
        WalRcvStreamOptions options;

        /*
         * Check that we're connected to a valid server using the
         * IDENTIFY_SYSTEM replication command.
         */
        primary_sysid = walrcv_identify_system(wrconn, &primaryTLI);

        snprintf(standby_sysid, sizeof(standby_sysid), UINT64_FORMAT,
                 GetSystemIdentifier());
        if (strcmp(primary_sysid, standby_sysid) != 0)
        {
            ereport(ERROR,
                    (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                     errmsg("database system identifier differs between the primary and standby"),
                     errdetail("The primary's identifier is %s, the standby's identifier is %s.",
                               primary_sysid, standby_sysid)));
        }

        /*
         * Confirm that the current timeline of the primary is the same or
         * ahead of ours.
         */
        if (primaryTLI < startpointTLI)
            ereport(ERROR,
                    (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                     errmsg("highest timeline %u of the primary is behind recovery timeline %u",
                            primaryTLI, startpointTLI)));

        /*
         * Get any missing history files. We do this always, even when we're
         * not interested in that timeline, so that if we're promoted to
         * become the primary later on, we don't select the same timeline that
         * was already used in the current primary. This isn't bullet-proof -
         * you'll need some external software to manage your cluster if you
         * need to ensure that a unique timeline id is chosen in every case,
         * but let's avoid the confusion of timeline id collisions where we
         * can.
         */
        WalRcvFetchTimeLineHistoryFiles(startpointTLI, primaryTLI);

        /*
         * Create temporary replication slot if requested, and update slot
         * name in shared memory.  (Note the slot name cannot already be set
         * in this case.)
         */
        if (is_temp_slot)
        {
            snprintf(slotname, sizeof(slotname),
                     "pg_walreceiver_%lld",
                     (long long int) walrcv_get_backend_pid(wrconn));

            walrcv_create_slot(wrconn, slotname, true, false, 0, NULL);

            SpinLockAcquire(&walrcv->mutex);
            strlcpy(walrcv->slotname, slotname, NAMEDATALEN);
            SpinLockRelease(&walrcv->mutex);
        }

        /*
         * Start streaming.
         *
         * We'll try to start at the requested starting point and timeline,
         * even if it's different from the server's latest timeline. In case
         * we've already reached the end of the old timeline, the server will
         * finish the streaming immediately, and we will go back to await
         * orders from the startup process. If recovery_target_timeline is
         * 'latest', the startup process will scan pg_wal and find the new
         * history file, bump recovery target timeline, and ask us to restart
         * on the new timeline.
         */
        options.logical = false;
        options.startpoint = startpoint;
        options.slotname = slotname[0] != '\0' ? slotname : NULL;
        options.proto.physical.startpointTLI = startpointTLI;
        ThisTimeLineID = startpointTLI;
        if (walrcv_startstreaming(wrconn, &options))
        {
            if (first_stream)
                ereport(LOG,
                        (errmsg("started streaming WAL from primary at %X/%X on timeline %u",
                                LSN_FORMAT_ARGS(startpoint), startpointTLI)));
            else
                ereport(LOG,
                        (errmsg("restarted WAL streaming at %X/%X on timeline %u",
                                LSN_FORMAT_ARGS(startpoint), startpointTLI)));
            first_stream = false;

            /* Initialize LogstreamResult and buffers for processing messages */
            LogstreamResult.Write = LogstreamResult.Flush = GetXLogReplayRecPtr(NULL);
            initStringInfo(&reply_message);
            initStringInfo(&incoming_message);

            /* Initialize the last recv timestamp */
            last_recv_timestamp = GetCurrentTimestamp();
            ping_sent = false;

            /* Loop until end-of-streaming or error */
            for (;;)
            {
                char       *buf;
                int         len;
                bool        endofwal = false;
                pgsocket    wait_fd = PGINVALID_SOCKET;
                int         rc;

                /*
                 * Exit walreceiver if we're not in recovery. This should not
                 * happen, but cross-check the status here.
                 */
                if (!RecoveryInProgress())
                    ereport(FATAL,
                            (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                             errmsg("cannot continue WAL streaming, recovery has already ended")));

                /* Process any requests or signals received recently */
                ProcessWalRcvInterrupts();

                if (ConfigReloadPending)
                {
                    ConfigReloadPending = false;
                    ProcessConfigFile(PGC_SIGHUP);
                    XLogWalRcvSendHSFeedback(true);
                }

                /* See if we can read data immediately */
                len = walrcv_receive(wrconn, &buf, &wait_fd);
                if (len != 0)
                {
                    /*
                     * Process the received data, and any subsequent data we
                     * can read without blocking.
                     */
                    for (;;)
                    {
                        if (len > 0)
                        {
                            /*
                             * Something was received from primary, so reset
                             * timeout
                             */
                            last_recv_timestamp = GetCurrentTimestamp();
                            ping_sent = false;
                            XLogWalRcvProcessMsg(buf[0], &buf[1], len - 1);
                        }
                        else if (len == 0)
                            break;
                        else if (len < 0)
                        {
                            ereport(LOG,
                                    (errmsg("replication terminated by primary server"),
                                     errdetail("End of WAL reached on timeline %u at %X/%X.",
                                               startpointTLI,
                                               LSN_FORMAT_ARGS(LogstreamResult.Write))));
                            endofwal = true;
                            break;
                        }
                        len = walrcv_receive(wrconn, &buf, &wait_fd);
                    }

                    /* Let the primary know that we received some data. */
                    XLogWalRcvSendReply(false, false);

                    /*
                     * If we've written some records, flush them to disk and
                     * let the startup process and primary server know about
                     * them.
                     */
                    XLogWalRcvFlush(false);
                }

                /* Check if we need to exit the streaming loop. */
                if (endofwal)
                    break;

                /*
                 * Ideally we would reuse a WaitEventSet object repeatedly
                 * here to avoid the overheads of WaitLatchOrSocket on epoll
                 * systems, but we can't be sure that libpq (or any other
                 * walreceiver implementation) has the same socket (even if
                 * the fd is the same number, it may have been closed and
                 * reopened since the last time).  In future, if there is a
                 * function for removing sockets from WaitEventSet, then we
                 * could add and remove just the socket each time, potentially
                 * avoiding some system calls.
                 */
                Assert(wait_fd != PGINVALID_SOCKET);
                rc = WaitLatchOrSocket(MyLatch,
                                       WL_EXIT_ON_PM_DEATH | WL_SOCKET_READABLE |
                                       WL_TIMEOUT | WL_LATCH_SET,
                                       wait_fd,
                                       NAPTIME_PER_CYCLE,
                                       WAIT_EVENT_WAL_RECEIVER_MAIN);
                if (rc & WL_LATCH_SET)
                {
                    ResetLatch(MyLatch);
                    ProcessWalRcvInterrupts();

                    if (walrcv->force_reply)
                    {
                        /*
                         * The recovery process has asked us to send apply
                         * feedback now.  Make sure the flag is really set to
                         * false in shared memory before sending the reply, so
                         * we don't miss a new request for a reply.
                         */
                        walrcv->force_reply = false;
                        pg_memory_barrier();
                        XLogWalRcvSendReply(true, false);
                    }
                }
                if (rc & WL_TIMEOUT)
                {
                    /*
                     * We didn't receive anything new. If we haven't heard
                     * anything from the server for more than
                     * wal_receiver_timeout / 2, ping the server. Also, if
                     * it's been longer than wal_receiver_status_interval
                     * since the last update we sent, send a status update to
                     * the primary anyway, to report any progress in applying
                     * WAL.
                     */
                    bool        requestReply = false;

                    /*
                     * Check if time since last receive from primary has
                     * reached the configured limit.
                     */
                    if (wal_receiver_timeout > 0)
                    {
                        TimestampTz now = GetCurrentTimestamp();
                        TimestampTz timeout;

                        timeout =
                            TimestampTzPlusMilliseconds(last_recv_timestamp,
                                                        wal_receiver_timeout);

                        if (now >= timeout)
                            ereport(ERROR,
                                    (errcode(ERRCODE_CONNECTION_FAILURE),
                                     errmsg("terminating walreceiver due to timeout")));

                        /*
                         * We didn't receive anything new, for half of
                         * receiver replication timeout. Ping the server.
                         */
                        if (!ping_sent)
                        {
                            timeout = TimestampTzPlusMilliseconds(last_recv_timestamp,
                                                                  (wal_receiver_timeout / 2));
                            if (now >= timeout)
                            {
                                requestReply = true;
                                ping_sent = true;
                            }
                        }
                    }

                    XLogWalRcvSendReply(requestReply, requestReply);
                    XLogWalRcvSendHSFeedback(false);
                }
            }

            /*
             * The backend finished streaming. Exit streaming COPY-mode from
             * our side, too.
             */
            walrcv_endstreaming(wrconn, &primaryTLI);

            /*
             * If the server had switched to a new timeline that we didn't
             * know about when we began streaming, fetch its timeline history
             * file now.
             */
            WalRcvFetchTimeLineHistoryFiles(startpointTLI, primaryTLI);
        }
        else
            ereport(LOG,
                    (errmsg("primary server contains no more WAL on requested timeline %u",
                            startpointTLI)));

        /*
         * End of WAL reached on the requested timeline. Close the last
         * segment, and await for new orders from the startup process.
         */
        if (recvFile >= 0)
        {
            char        xlogfname[MAXFNAMELEN];

            XLogWalRcvFlush(false);
            XLogFileName(xlogfname, recvFileTLI, recvSegNo, wal_segment_size);
            if (close(recvFile) != 0)
                ereport(PANIC,
                        (errcode_for_file_access(),
                         errmsg("could not close log segment %s: %m",
                                xlogfname)));

            /*
             * Create .done file forcibly to prevent the streamed segment from
             * being archived later.
             */
            if (XLogArchiveMode != ARCHIVE_MODE_ALWAYS)
                XLogArchiveForceDone(xlogfname);
            else
                XLogArchiveNotify(xlogfname);
        }
        recvFile = -1;

        elog(DEBUG1, "walreceiver ended streaming and awaits new instructions");
        WalRcvWaitForStartPosition(&startpoint, &startpointTLI);
    }
    /* not reached */
}

/*
 * Wait for startup process to set receiveStart and receiveStartTLI.
 */
static void
WalRcvWaitForStartPosition(XLogRecPtr *startpoint, TimeLineID *startpointTLI)
{
    WalRcvData *walrcv = WalRcv;
    int         state;

    SpinLockAcquire(&walrcv->mutex);
    state = walrcv->walRcvState;
    if (state != WALRCV_STREAMING)
    {
        SpinLockRelease(&walrcv->mutex);
        if (state == WALRCV_STOPPING)
            proc_exit(0);
        else
            elog(FATAL, "unexpected walreceiver state");
    }
    walrcv->walRcvState = WALRCV_WAITING;
    walrcv->receiveStart = InvalidXLogRecPtr;
    walrcv->receiveStartTLI = 0;
    SpinLockRelease(&walrcv->mutex);

    set_ps_display("idle");

    /*
     * nudge startup process to notice that we've stopped streaming and are
     * now waiting for instructions.
     */
    WakeupRecovery();
    for (;;)
    {
        ResetLatch(MyLatch);

        ProcessWalRcvInterrupts();

        SpinLockAcquire(&walrcv->mutex);
        Assert(walrcv->walRcvState == WALRCV_RESTARTING ||
               walrcv->walRcvState == WALRCV_WAITING ||
               walrcv->walRcvState == WALRCV_STOPPING);
        if (walrcv->walRcvState == WALRCV_RESTARTING)
        {
            /*
             * No need to handle changes in primary_conninfo or
             * primary_slotname here. Startup process will signal us to
             * terminate in case those change.
             */
            *startpoint = walrcv->receiveStart;
            *startpointTLI = walrcv->receiveStartTLI;
            walrcv->walRcvState = WALRCV_STREAMING;
            SpinLockRelease(&walrcv->mutex);
            break;
        }
        if (walrcv->walRcvState == WALRCV_STOPPING)
        {
            /*
             * We should've received SIGTERM if the startup process wants us
             * to die, but might as well check it here too.
             */
            SpinLockRelease(&walrcv->mutex);
            exit(1);
        }
        SpinLockRelease(&walrcv->mutex);

        (void) WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, 0,
                         WAIT_EVENT_WAL_RECEIVER_WAIT_START);
    }

    if (update_process_title)
    {
        char        activitymsg[50];

        snprintf(activitymsg, sizeof(activitymsg), "restarting at %X/%X",
                 LSN_FORMAT_ARGS(*startpoint));
        set_ps_display(activitymsg);
    }
}

/*
 * Fetch any missing timeline history files between 'first' and 'last'
 * (inclusive) from the server.
 */
static void
WalRcvFetchTimeLineHistoryFiles(TimeLineID first, TimeLineID last)
{
    TimeLineID  tli;

    for (tli = first; tli <= last; tli++)
    {
        /* there's no history file for timeline 1 */
        if (tli != 1 && !existsTimeLineHistory(tli))
        {
            char       *fname;
            char       *content;
            int         len;
            char        expectedfname[MAXFNAMELEN];

            ereport(LOG,
                    (errmsg("fetching timeline history file for timeline %u from primary server",
                            tli)));

            walrcv_readtimelinehistoryfile(wrconn, tli, &fname, &content, &len);

            /*
             * Check that the filename on the primary matches what we
             * calculated ourselves. This is just a sanity check, it should
             * always match.
             */
            TLHistoryFileName(expectedfname, tli);
            if (strcmp(fname, expectedfname) != 0)
                ereport(ERROR,
                        (errcode(ERRCODE_PROTOCOL_VIOLATION),
                         errmsg_internal("primary reported unexpected file name for timeline history file of timeline %u",
                                         tli)));

            /*
             * Write the file to pg_wal.
             */
            writeTimeLineHistoryFile(tli, content, len);

            /*
             * Mark the streamed history file as ready for archiving if
             * archive_mode is always.
             */
            if (XLogArchiveMode != ARCHIVE_MODE_ALWAYS)
                XLogArchiveForceDone(fname);
            else
                XLogArchiveNotify(fname);

            pfree(fname);
            pfree(content);
        }
    }
}

/*
 * Mark us as STOPPED in shared memory at exit.
 */
static void
WalRcvDie(int code, Datum arg)
{
    WalRcvData *walrcv = WalRcv;

    /* Ensure that all WAL records received are flushed to disk */
    XLogWalRcvFlush(true);

    /* Mark ourselves inactive in shared memory */
    SpinLockAcquire(&walrcv->mutex);
    Assert(walrcv->walRcvState == WALRCV_STREAMING ||
           walrcv->walRcvState == WALRCV_RESTARTING ||
           walrcv->walRcvState == WALRCV_STARTING ||
           walrcv->walRcvState == WALRCV_WAITING ||
           walrcv->walRcvState == WALRCV_STOPPING);
    Assert(walrcv->pid == MyProcPid);
    walrcv->walRcvState = WALRCV_STOPPED;
    walrcv->pid = 0;
    walrcv->ready_to_display = false;
    walrcv->latch = NULL;
    SpinLockRelease(&walrcv->mutex);

    ConditionVariableBroadcast(&walrcv->walRcvStoppedCV);

    /* Terminate the connection gracefully. */
    if (wrconn != NULL)
        walrcv_disconnect(wrconn);

    /* Wake up the startup process to notice promptly that we're gone */
    WakeupRecovery();
}

/*
 * Accept the message from XLOG stream, and process it.
 */
static void
XLogWalRcvProcessMsg(unsigned char type, char *buf, Size len)
{
    int         hdrlen;
    XLogRecPtr  dataStart;
    XLogRecPtr  walEnd;
    TimestampTz sendTime;
    bool        replyRequested;

    resetStringInfo(&incoming_message);

    switch (type)
    {
        case 'w':               /* WAL records */
            {
                /* copy message to StringInfo */
                hdrlen = sizeof(int64) + sizeof(int64) + sizeof(int64);
                if (len < hdrlen)
                    ereport(ERROR,
                            (errcode(ERRCODE_PROTOCOL_VIOLATION),
                             errmsg_internal("invalid WAL message received from primary")));
                appendBinaryStringInfo(&incoming_message, buf, hdrlen);

                /* read the fields */
                dataStart = pq_getmsgint64(&incoming_message);
                walEnd = pq_getmsgint64(&incoming_message);
                sendTime = pq_getmsgint64(&incoming_message);
                ProcessWalSndrMessage(walEnd, sendTime);

                buf += hdrlen;
                len -= hdrlen;
                XLogWalRcvWrite(buf, len, dataStart);
                break;
            }
        case 'k':               /* Keepalive */
            {
                /* copy message to StringInfo */
                hdrlen = sizeof(int64) + sizeof(int64) + sizeof(char);
                if (len != hdrlen)
                    ereport(ERROR,
                            (errcode(ERRCODE_PROTOCOL_VIOLATION),
                             errmsg_internal("invalid keepalive message received from primary")));
                appendBinaryStringInfo(&incoming_message, buf, hdrlen);

                /* read the fields */
                walEnd = pq_getmsgint64(&incoming_message);
                sendTime = pq_getmsgint64(&incoming_message);
                replyRequested = pq_getmsgbyte(&incoming_message);

                ProcessWalSndrMessage(walEnd, sendTime);

                /* If the primary requested a reply, send one immediately */
                if (replyRequested)
                    XLogWalRcvSendReply(true, false);
                break;
            }
        default:
            ereport(ERROR,
                    (errcode(ERRCODE_PROTOCOL_VIOLATION),
                     errmsg_internal("invalid replication message type %d",
                                     type)));
    }
}

/*
 * Write XLOG data to disk.
 */
static void
XLogWalRcvWrite(char *buf, Size nbytes, XLogRecPtr recptr)
{
    int         startoff;
    int         byteswritten;

    while (nbytes > 0)
    {
        int         segbytes;

        /* Close the current segment if it's completed */
        if (recvFile >= 0 && !XLByteInSeg(recptr, recvSegNo, wal_segment_size))
            XLogWalRcvClose(recptr);

        if (recvFile < 0)
        {
            /* Create/use new log file */
            XLByteToSeg(recptr, recvSegNo, wal_segment_size);
            recvFile = XLogFileInit(recvSegNo);
            recvFileTLI = ThisTimeLineID;
        }

        /* Calculate the start offset of the received logs */
        startoff = XLogSegmentOffset(recptr, wal_segment_size);

        if (startoff + nbytes > wal_segment_size)
            segbytes = wal_segment_size - startoff;
        else
            segbytes = nbytes;

        /* OK to write the logs */
        errno = 0;

        byteswritten = pg_pwrite(recvFile, buf, segbytes, (off_t) startoff);
        if (byteswritten <= 0)
        {
            char        xlogfname[MAXFNAMELEN];
            int         save_errno;

            /* if write didn't set errno, assume no disk space */
            if (errno == 0)
                errno = ENOSPC;

            save_errno = errno;
            XLogFileName(xlogfname, recvFileTLI, recvSegNo, wal_segment_size);
            errno = save_errno;
            ereport(PANIC,
                    (errcode_for_file_access(),
                     errmsg("could not write to log segment %s "
                            "at offset %u, length %lu: %m",
                            xlogfname, startoff, (unsigned long) segbytes)));
        }

        /* Update state for write */
        recptr += byteswritten;

        nbytes -= byteswritten;
        buf += byteswritten;

        LogstreamResult.Write = recptr;
    }

    /* Update shared-memory status */
    pg_atomic_write_u64(&WalRcv->writtenUpto, LogstreamResult.Write);

    /*
     * Close the current segment if it's fully written up in the last cycle of
     * the loop, to create its archive notification file soon. Otherwise WAL
     * archiving of the segment will be delayed until any data in the next
     * segment is received and written.
     */
    if (recvFile >= 0 && !XLByteInSeg(recptr, recvSegNo, wal_segment_size))
        XLogWalRcvClose(recptr);
}

/*
 * Flush the log to disk.
 *
 * If we're in the midst of dying, it's unwise to do anything that might throw
 * an error, so we skip sending a reply in that case.
 */
static void
XLogWalRcvFlush(bool dying)
{
    if (LogstreamResult.Flush < LogstreamResult.Write)
    {
        WalRcvData *walrcv = WalRcv;

        issue_xlog_fsync(recvFile, recvSegNo);

        LogstreamResult.Flush = LogstreamResult.Write;

        /* Update shared-memory status */
        SpinLockAcquire(&walrcv->mutex);
        if (walrcv->flushedUpto < LogstreamResult.Flush)
        {
            walrcv->latestChunkStart = walrcv->flushedUpto;
            walrcv->flushedUpto = LogstreamResult.Flush;
            walrcv->receivedTLI = ThisTimeLineID;
        }
        SpinLockRelease(&walrcv->mutex);

        /* Signal the startup process and walsender that new WAL has arrived */
        WakeupRecovery();
        if (AllowCascadeReplication())
            WalSndWakeup();

        /* Report XLOG streaming progress in PS display */
        if (update_process_title)
        {
            char        activitymsg[50];

            snprintf(activitymsg, sizeof(activitymsg), "streaming %X/%X",
                     LSN_FORMAT_ARGS(LogstreamResult.Write));
            set_ps_display(activitymsg);
        }

        /* Also let the primary know that we made some progress */
        if (!dying)
        {
            XLogWalRcvSendReply(false, false);
            XLogWalRcvSendHSFeedback(false);
        }
    }
}

/*
 * Close the current segment.
 *
 * Flush the segment to disk before closing it. Otherwise we have to
 * reopen and fsync it later.
 *
 * Create an archive notification file since the segment is known completed.
 */
static void
XLogWalRcvClose(XLogRecPtr recptr)
{
    char        xlogfname[MAXFNAMELEN];

    Assert(recvFile >= 0 && !XLByteInSeg(recptr, recvSegNo, wal_segment_size));

    /*
     * fsync() and close current file before we switch to next one. We would
     * otherwise have to reopen this file to fsync it later
     */
    XLogWalRcvFlush(false);

    XLogFileName(xlogfname, recvFileTLI, recvSegNo, wal_segment_size);

    /*
     * XLOG segment files will be re-read by recovery in startup process soon,
     * so we don't advise the OS to release cache pages associated with the
     * file like XLogFileClose() does.
     */
    if (close(recvFile) != 0)
        ereport(PANIC,
                (errcode_for_file_access(),
                 errmsg("could not close log segment %s: %m",
                        xlogfname)));

    /*
     * Create .done file forcibly to prevent the streamed segment from being
     * archived later.
     */
    if (XLogArchiveMode != ARCHIVE_MODE_ALWAYS)
        XLogArchiveForceDone(xlogfname);
    else
        XLogArchiveNotify(xlogfname);

    recvFile = -1;
}

/*
 * Send reply message to primary, indicating our current WAL locations, oldest
 * xmin and the current time.
 *
 * If 'force' is not set, the message is only sent if enough time has
 * passed since last status update to reach wal_receiver_status_interval.
 * If wal_receiver_status_interval is disabled altogether and 'force' is
 * false, this is a no-op.
 *
 * If 'requestReply' is true, requests the server to reply immediately upon
 * receiving this message. This is used for heartbeats, when approaching
 * wal_receiver_timeout.
 */
static void
XLogWalRcvSendReply(bool force, bool requestReply)
{
    static XLogRecPtr writePtr = 0;
    static XLogRecPtr flushPtr = 0;
    XLogRecPtr  applyPtr;
    static TimestampTz sendTime = 0;
    TimestampTz now;

    /*
     * If the user doesn't want status to be reported to the primary, be sure
     * to exit before doing anything at all.
     */
    if (!force && wal_receiver_status_interval <= 0)
        return;

    /* Get current timestamp. */
    now = GetCurrentTimestamp();

    /*
     * We can compare the write and flush positions to the last message we
     * sent without taking any lock, but the apply position requires a spin
     * lock, so we don't check that unless something else has changed or 10
     * seconds have passed.  This means that the apply WAL location will
     * appear, from the primary's point of view, to lag slightly, but since
     * this is only for reporting purposes and only on idle systems, that's
     * probably OK.
     */
    if (!force
        && writePtr == LogstreamResult.Write
        && flushPtr == LogstreamResult.Flush
        && !TimestampDifferenceExceeds(sendTime, now,
                                       wal_receiver_status_interval * 1000))
        return;
    sendTime = now;

    /* Construct a new message */
    writePtr = LogstreamResult.Write;
    flushPtr = LogstreamResult.Flush;
    applyPtr = GetXLogReplayRecPtr(NULL);

    resetStringInfo(&reply_message);
    pq_sendbyte(&reply_message, 'r');
    pq_sendint64(&reply_message, writePtr);
    pq_sendint64(&reply_message, flushPtr);
    pq_sendint64(&reply_message, applyPtr);
    pq_sendint64(&reply_message, GetCurrentTimestamp());
    pq_sendbyte(&reply_message, requestReply ? 1 : 0);

    /* Send it */
    elog(DEBUG2, "sending write %X/%X flush %X/%X apply %X/%X%s",
         LSN_FORMAT_ARGS(writePtr),
         LSN_FORMAT_ARGS(flushPtr),
         LSN_FORMAT_ARGS(applyPtr),
         requestReply ? " (reply requested)" : "");

    walrcv_send(wrconn, reply_message.data, reply_message.len);
}

/*
 * Send hot standby feedback message to primary, plus the current time,
 * in case they don't have a watch.
 *
 * If the user disables feedback, send one final message to tell sender
 * to forget about the xmin on this standby. We also send this message
 * on first connect because a previous connection might have set xmin
 * on a replication slot. (If we're not using a slot it's harmless to
 * send a feedback message explicitly setting InvalidTransactionId).
 */
static void
XLogWalRcvSendHSFeedback(bool immed)
{
    TimestampTz now;
    FullTransactionId nextFullXid;
    TransactionId nextXid;
    uint32      xmin_epoch,
                catalog_xmin_epoch;
    TransactionId xmin,
                catalog_xmin;
    static TimestampTz sendTime = 0;

    /* initially true so we always send at least one feedback message */
    static bool primary_has_standby_xmin = true;

    /*
     * If the user doesn't want status to be reported to the primary, be sure
     * to exit before doing anything at all.
     */
    if ((wal_receiver_status_interval <= 0 || !hot_standby_feedback) &&
        !primary_has_standby_xmin)
        return;

    /* Get current timestamp. */
    now = GetCurrentTimestamp();

    if (!immed)
    {
        /*
         * Send feedback at most once per wal_receiver_status_interval.
         */
        if (!TimestampDifferenceExceeds(sendTime, now,
                                        wal_receiver_status_interval * 1000))
            return;
        sendTime = now;
    }

    /*
     * If Hot Standby is not yet accepting connections there is nothing to
     * send. Check this after the interval has expired to reduce number of
     * calls.
     *
     * Bailing out here also ensures that we don't send feedback until we've
     * read our own replication slot state, so we don't tell the primary to
     * discard needed xmin or catalog_xmin from any slots that may exist on
     * this replica.
     */
    if (!HotStandbyActive())
        return;

    /*
     * Make the expensive call to get the oldest xmin once we are certain
     * everything else has been checked.
     */
    if (hot_standby_feedback)
    {
        GetReplicationHorizons(&xmin, &catalog_xmin);
    }
    else
    {
        xmin = InvalidTransactionId;
        catalog_xmin = InvalidTransactionId;
    }

    /*
     * Get epoch and adjust if nextXid and oldestXmin are different sides of
     * the epoch boundary.
     */
    nextFullXid = ReadNextFullTransactionId();
    nextXid = XidFromFullTransactionId(nextFullXid);
    xmin_epoch = EpochFromFullTransactionId(nextFullXid);
    catalog_xmin_epoch = xmin_epoch;
    if (nextXid < xmin)
        xmin_epoch--;
    if (nextXid < catalog_xmin)
        catalog_xmin_epoch--;

    elog(DEBUG2, "sending hot standby feedback xmin %u epoch %u catalog_xmin %u catalog_xmin_epoch %u",
         xmin, xmin_epoch, catalog_xmin, catalog_xmin_epoch);

    /* Construct the message and send it. */
    resetStringInfo(&reply_message);
    pq_sendbyte(&reply_message, 'h');
    pq_sendint64(&reply_message, GetCurrentTimestamp());
    pq_sendint32(&reply_message, xmin);
    pq_sendint32(&reply_message, xmin_epoch);
    pq_sendint32(&reply_message, catalog_xmin);
    pq_sendint32(&reply_message, catalog_xmin_epoch);
    walrcv_send(wrconn, reply_message.data, reply_message.len);
    if (TransactionIdIsValid(xmin) || TransactionIdIsValid(catalog_xmin))
        primary_has_standby_xmin = true;
    else
        primary_has_standby_xmin = false;
}

/*
 * Update shared memory status upon receiving a message from primary.
 *
 * 'walEnd' and 'sendTime' are the end-of-WAL and timestamp of the latest
 * message, reported by primary.
 */
static void
ProcessWalSndrMessage(XLogRecPtr walEnd, TimestampTz sendTime)
{
    WalRcvData *walrcv = WalRcv;

    TimestampTz lastMsgReceiptTime = GetCurrentTimestamp();

    /* Update shared-memory status */
    SpinLockAcquire(&walrcv->mutex);
    if (walrcv->latestWalEnd < walEnd)
        walrcv->latestWalEndTime = sendTime;
    walrcv->latestWalEnd = walEnd;
    walrcv->lastMsgSendTime = sendTime;
    walrcv->lastMsgReceiptTime = lastMsgReceiptTime;
    SpinLockRelease(&walrcv->mutex);

    if (message_level_is_interesting(DEBUG2))
    {
        char       *sendtime;
        char       *receipttime;
        int         applyDelay;

        /* Copy because timestamptz_to_str returns a static buffer */
        sendtime = pstrdup(timestamptz_to_str(sendTime));
        receipttime = pstrdup(timestamptz_to_str(lastMsgReceiptTime));
        applyDelay = GetReplicationApplyDelay();

        /* apply delay is not available */
        if (applyDelay == -1)
            elog(DEBUG2, "sendtime %s receipttime %s replication apply delay (N/A) transfer latency %d ms",
                 sendtime,
                 receipttime,
                 GetReplicationTransferLatency());
        else
            elog(DEBUG2, "sendtime %s receipttime %s replication apply delay %d ms transfer latency %d ms",
                 sendtime,
                 receipttime,
                 applyDelay,
                 GetReplicationTransferLatency());

        pfree(sendtime);
        pfree(receipttime);
    }
}

/*
 * Wake up the walreceiver main loop.
 *
 * This is called by the startup process whenever interesting xlog records
 * are applied, so that walreceiver can check if it needs to send an apply
 * notification back to the primary which may be waiting in a COMMIT with
 * synchronous_commit = remote_apply.
 */
void
WalRcvForceReply(void)
{
    Latch      *latch;

    WalRcv->force_reply = true;
    /* fetching the latch pointer might not be atomic, so use spinlock */
    SpinLockAcquire(&WalRcv->mutex);
    latch = WalRcv->latch;
    SpinLockRelease(&WalRcv->mutex);
    if (latch)
        SetLatch(latch);
}

/*
 * Return a string constant representing the state. This is used
 * in system functions and views, and should *not* be translated.
 */
static const char *
WalRcvGetStateString(WalRcvState state)
{
    switch (state)
    {
        case WALRCV_STOPPED:
            return "stopped";
        case WALRCV_STARTING:
            return "starting";
        case WALRCV_STREAMING:
            return "streaming";
        case WALRCV_WAITING:
            return "waiting";
        case WALRCV_RESTARTING:
            return "restarting";
        case WALRCV_STOPPING:
            return "stopping";
    }
    return "UNKNOWN";
}

/*
 * Returns activity of WAL receiver, including pid, state and xlog locations
 * received from the WAL sender of another server.
 */
Datum
pg_stat_get_wal_receiver(PG_FUNCTION_ARGS)
{
    TupleDesc   tupdesc;
    Datum      *values;
    bool       *nulls;
    int         pid;
    bool        ready_to_display;
    WalRcvState state;
    XLogRecPtr  receive_start_lsn;
    TimeLineID  receive_start_tli;
    XLogRecPtr  written_lsn;
    XLogRecPtr  flushed_lsn;
    TimeLineID  received_tli;
    TimestampTz last_send_time;
    TimestampTz last_receipt_time;
    XLogRecPtr  latest_end_lsn;
    TimestampTz latest_end_time;
    char        sender_host[NI_MAXHOST];
    int         sender_port = 0;
    char        slotname[NAMEDATALEN];
    char        conninfo[MAXCONNINFO];

    /* Take a lock to ensure value consistency */
    SpinLockAcquire(&WalRcv->mutex);
    pid = (int) WalRcv->pid;
    ready_to_display = WalRcv->ready_to_display;
    state = WalRcv->walRcvState;
    receive_start_lsn = WalRcv->receiveStart;
    receive_start_tli = WalRcv->receiveStartTLI;
    flushed_lsn = WalRcv->flushedUpto;
    received_tli = WalRcv->receivedTLI;
    last_send_time = WalRcv->lastMsgSendTime;
    last_receipt_time = WalRcv->lastMsgReceiptTime;
    latest_end_lsn = WalRcv->latestWalEnd;
    latest_end_time = WalRcv->latestWalEndTime;
    strlcpy(slotname, (char *) WalRcv->slotname, sizeof(slotname));
    strlcpy(sender_host, (char *) WalRcv->sender_host, sizeof(sender_host));
    sender_port = WalRcv->sender_port;
    strlcpy(conninfo, (char *) WalRcv->conninfo, sizeof(conninfo));
    SpinLockRelease(&WalRcv->mutex);

    /*
     * No WAL receiver (or not ready yet), just return a tuple with NULL
     * values
     */
    if (pid == 0 || !ready_to_display)
        PG_RETURN_NULL();

    /*
     * Read "writtenUpto" without holding a spinlock.  Note that it may not be
     * consistent with the other shared variables of the WAL receiver
     * protected by a spinlock, but this should not be used for data integrity
     * checks.
     */
    written_lsn = pg_atomic_read_u64(&WalRcv->writtenUpto);

    /* determine result type */
    if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
        elog(ERROR, "return type must be a row type");

    values = palloc0(sizeof(Datum) * tupdesc->natts);
    nulls = palloc0(sizeof(bool) * tupdesc->natts);

    /* Fetch values */
    values[0] = Int32GetDatum(pid);

    if (!is_member_of_role(GetUserId(), ROLE_PG_READ_ALL_STATS))
    {
        /*
         * Only superusers and members of pg_read_all_stats can see details.
         * Other users only get the pid value to know whether it is a WAL
         * receiver, but no details.
         */
        MemSet(&nulls[1], true, sizeof(bool) * (tupdesc->natts - 1));
    }
    else
    {
        values[1] = CStringGetTextDatum(WalRcvGetStateString(state));

        if (XLogRecPtrIsInvalid(receive_start_lsn))
            nulls[2] = true;
        else
            values[2] = LSNGetDatum(receive_start_lsn);
        values[3] = Int32GetDatum(receive_start_tli);
        if (XLogRecPtrIsInvalid(written_lsn))
            nulls[4] = true;
        else
            values[4] = LSNGetDatum(written_lsn);
        if (XLogRecPtrIsInvalid(flushed_lsn))
            nulls[5] = true;
        else
            values[5] = LSNGetDatum(flushed_lsn);
        values[6] = Int32GetDatum(received_tli);
        if (last_send_time == 0)
            nulls[7] = true;
        else
            values[7] = TimestampTzGetDatum(last_send_time);
        if (last_receipt_time == 0)
            nulls[8] = true;
        else
            values[8] = TimestampTzGetDatum(last_receipt_time);
        if (XLogRecPtrIsInvalid(latest_end_lsn))
            nulls[9] = true;
        else
            values[9] = LSNGetDatum(latest_end_lsn);
        if (latest_end_time == 0)
            nulls[10] = true;
        else
            values[10] = TimestampTzGetDatum(latest_end_time);
        if (*slotname == '\0')
            nulls[11] = true;
        else
            values[11] = CStringGetTextDatum(slotname);
        if (*sender_host == '\0')
            nulls[12] = true;
        else
            values[12] = CStringGetTextDatum(sender_host);
        if (sender_port == 0)
            nulls[13] = true;
        else
            values[13] = Int32GetDatum(sender_port);
        if (*conninfo == '\0')
            nulls[14] = true;
        else
            values[14] = CStringGetTextDatum(conninfo);
    }

    /* Returns the record as Datum */
    PG_RETURN_DATUM(HeapTupleGetDatum(heap_form_tuple(tupdesc, values, nulls)));
}