slotsync.c

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/*-------------------------------------------------------------------------
 * slotsync.c
 *     Functionality for synchronizing slots to a standby server from the
 *         primary server.
 *
 * Copyright (c) 2024-2026, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/replication/logical/slotsync.c
 *
 * This file contains the code for slot synchronization on a physical standby
 * to fetch logical failover slots information from the primary server, create
 * the slots on the standby and synchronize them periodically.
 *
 * Slot synchronization can be performed either automatically by enabling slot
 * sync worker or manually by calling SQL function pg_sync_replication_slots().
 *
 * If the WAL corresponding to the remote's restart_lsn is not available on the
 * physical standby or the remote's catalog_xmin precedes the oldest xid for
 * which it is guaranteed that rows wouldn't have been removed then we cannot
 * create the local standby slot because that would mean moving the local slot
 * backward and decoding won't be possible via such a slot. In this case, the
 * slot will be marked as RS_TEMPORARY. Once the primary server catches up,
 * the slot will be marked as RS_PERSISTENT (which means sync-ready) after
 * which slot sync worker can perform the sync periodically or user can call
 * pg_sync_replication_slots() periodically to perform the syncs.
 *
 * If synchronized slots fail to build a consistent snapshot from the
 * restart_lsn before reaching confirmed_flush_lsn, they would become
 * unreliable after promotion due to potential data loss from changes
 * before reaching a consistent point. This can happen because the slots can
 * be synced at some random time and we may not reach the consistent point
 * at the same WAL location as the primary. So, we mark such slots as
 * RS_TEMPORARY. Once the decoding from corresponding LSNs can reach a
 * consistent point, they will be marked as RS_PERSISTENT.
 *
 * The slot sync worker waits for some time before the next synchronization,
 * with the duration varying based on whether any slots were updated during
 * the last cycle. Refer to the comments above wait_for_slot_activity() for
 * more details.
 *
 * If the SQL function pg_sync_replication_slots() is used to sync the slots,
 * and if the slots are not ready to be synced and are marked as RS_TEMPORARY
 * because of any of the reasons mentioned above, then the SQL function also
 * waits and retries until the slots are marked as RS_PERSISTENT (which means
 * sync-ready). Refer to the comments in SyncReplicationSlots() for more
 * details.
 *
 * Any standby synchronized slots will be dropped if they no longer need
 * to be synchronized. See comment atop drop_local_obsolete_slots() for more
 * details.
 *---------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include <time.h>
 
#include "access/xlog_internal.h"
#include "access/xlogrecovery.h"
#include "catalog/pg_database.h"
#include "libpq/pqsignal.h"
#include "pgstat.h"
#include "postmaster/interrupt.h"
#include "replication/logical.h"
#include "replication/slotsync.h"
#include "replication/snapbuild.h"
#include "storage/ipc.h"
#include "storage/lmgr.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "tcop/tcopprot.h"
#include "utils/builtins.h"
#include "utils/memutils.h"
#include "utils/pg_lsn.h"
#include "utils/ps_status.h"
#include "utils/timeout.h"
 
/*
 * Struct for sharing information to control slot synchronization.
 *
 * The 'pid' is either the slot sync worker's pid or the backend's pid running
 * the SQL function pg_sync_replication_slots(). When the startup process sets
 * 'stopSignaled' during promotion, it uses this 'pid' to wake up the currently
 * synchronizing process so that the process can immediately stop its
 * synchronizing work on seeing 'stopSignaled' set.
 * Setting 'stopSignaled' is also used to handle the race condition when the
 * postmaster has not noticed the promotion yet and thus may end up restarting
 * the slot sync worker. If 'stopSignaled' is set, the worker will exit in such a
 * case. The SQL function pg_sync_replication_slots() will also error out if
 * this flag is set. Note that we don't need to reset this variable as after
 * promotion the slot sync worker won't be restarted because the pmState
 * changes to PM_RUN from PM_HOT_STANDBY and we don't support demoting
 * primary without restarting the server. See LaunchMissingBackgroundProcesses.
 *
 * The 'syncing' flag is needed to prevent concurrent slot syncs to avoid slot
 * overwrites.
 *
 * The 'last_start_time' is needed by postmaster to start the slot sync worker
 * once per SLOTSYNC_RESTART_INTERVAL_SEC. In cases where an immediate restart
 * is expected (e.g., slot sync GUCs change), slot sync worker will reset
 * last_start_time before exiting, so that postmaster can start the worker
 * without waiting for SLOTSYNC_RESTART_INTERVAL_SEC.
 */
typedef struct SlotSyncCtxStruct
{
    pid_t       pid;
    bool        stopSignaled;
    bool        syncing;
    time_t      last_start_time;
    slock_t     mutex;
} SlotSyncCtxStruct;
 
static SlotSyncCtxStruct *SlotSyncCtx = NULL;
 
/* GUC variable */
bool        sync_replication_slots = false;
 
/*
 * The sleep time (ms) between slot-sync cycles varies dynamically
 * (within a MIN/MAX range) according to slot activity. See
 * wait_for_slot_activity() for details.
 */
#define MIN_SLOTSYNC_WORKER_NAPTIME_MS  200
#define MAX_SLOTSYNC_WORKER_NAPTIME_MS  30000   /* 30s */
 
static long sleep_ms = MIN_SLOTSYNC_WORKER_NAPTIME_MS;
 
/* The restart interval for slot sync work used by postmaster */
#define SLOTSYNC_RESTART_INTERVAL_SEC 10
 
/*
 * Flag to tell if we are syncing replication slots. Unlike the 'syncing' flag
 * in SlotSyncCtxStruct, this flag is true only if the current process is
 * performing slot synchronization.
 */
static bool syncing_slots = false;
 
/*
 * Structure to hold information fetched from the primary server about a logical
 * replication slot.
 */
typedef struct RemoteSlot
{
    char       *name;
    char       *plugin;
    char       *database;
    bool        two_phase;
    bool        failover;
    XLogRecPtr  restart_lsn;
    XLogRecPtr  confirmed_lsn;
    XLogRecPtr  two_phase_at;
    TransactionId catalog_xmin;
 
    /* RS_INVAL_NONE if valid, or the reason of invalidation */
    ReplicationSlotInvalidationCause invalidated;
} RemoteSlot;
 
static void slotsync_failure_callback(int code, Datum arg);
static void update_synced_slots_inactive_since(void);
 
/*
 * Update slot sync skip stats. This function requires the caller to acquire
 * the slot.
 */
static void
update_slotsync_skip_stats(SlotSyncSkipReason skip_reason)
{
    ReplicationSlot *slot;
 
    Assert(MyReplicationSlot);
 
    slot = MyReplicationSlot;
 
    /*
     * Update the slot sync related stats in pg_stat_replication_slots when a
     * slot sync is skipped
     */
    if (skip_reason != SS_SKIP_NONE)
        pgstat_report_replslotsync(slot);
 
    /* Update the slot sync skip reason */
    if (slot->slotsync_skip_reason != skip_reason)
    {
        SpinLockAcquire(&slot->mutex);
        slot->slotsync_skip_reason = skip_reason;
        SpinLockRelease(&slot->mutex);
    }
}
 
/*
 * If necessary, update the local synced slot's metadata based on the data
 * from the remote slot.
 *
 * If no update was needed (the data of the remote slot is the same as the
 * local slot) return false, otherwise true.
 *
 * *found_consistent_snapshot will be true iff the remote slot's LSN or xmin is
 * modified, and decoding from the corresponding LSN's can reach a
 * consistent snapshot.
 *
 * *remote_slot_precedes will be true if the remote slot's LSN or xmin
 * precedes locally reserved position.
 */
static bool
update_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
                         bool *found_consistent_snapshot,
                         bool *remote_slot_precedes)
{
    ReplicationSlot *slot = MyReplicationSlot;
    bool        updated_xmin_or_lsn = false;
    bool        updated_config = false;
    SlotSyncSkipReason skip_reason = SS_SKIP_NONE;
 
    Assert(slot->data.invalidated == RS_INVAL_NONE);
 
    if (found_consistent_snapshot)
        *found_consistent_snapshot = false;
 
    if (remote_slot_precedes)
        *remote_slot_precedes = false;
 
    /*
     * Don't overwrite if we already have a newer catalog_xmin and
     * restart_lsn.
     */
    if (remote_slot->restart_lsn < slot->data.restart_lsn ||
        TransactionIdPrecedes(remote_slot->catalog_xmin,
                              slot->data.catalog_xmin))
    {
        /* Update slot sync skip stats */
        update_slotsync_skip_stats(SS_SKIP_WAL_OR_ROWS_REMOVED);
 
        /*
         * This can happen in following situations:
         *
         * If the slot is temporary, it means either the initial WAL location
         * reserved for the local slot is ahead of the remote slot's
         * restart_lsn or the initial xmin_horizon computed for the local slot
         * is ahead of the remote slot.
         *
         * If the slot is persistent, both restart_lsn and catalog_xmin of the
         * synced slot could still be ahead of the remote slot. Since we use
         * slot advance functionality to keep snapbuild/slot updated, it is
         * possible that the restart_lsn and catalog_xmin are advanced to a
         * later position than it has on the primary. This can happen when
         * slot advancing machinery finds running xacts record after reaching
         * the consistent state at a later point than the primary where it
         * serializes the snapshot and updates the restart_lsn.
         *
         * We LOG the message if the slot is temporary as it can help the user
         * to understand why the slot is not sync-ready. In the case of a
         * persistent slot, it would be a more common case and won't directly
         * impact the users, so we used DEBUG1 level to log the message.
         */
        ereport(slot->data.persistency == RS_TEMPORARY ? LOG : DEBUG1,
                errmsg("could not synchronize replication slot \"%s\"",
                       remote_slot->name),
                errdetail("Synchronization could lead to data loss, because the remote slot needs WAL at LSN %X/%08X and catalog xmin %u, but the standby has LSN %X/%08X and catalog xmin %u.",
                          LSN_FORMAT_ARGS(remote_slot->restart_lsn),
                          remote_slot->catalog_xmin,
                          LSN_FORMAT_ARGS(slot->data.restart_lsn),
                          slot->data.catalog_xmin));
 
        if (remote_slot_precedes)
            *remote_slot_precedes = true;
 
        /*
         * Skip updating the configuration. This is required to avoid syncing
         * two_phase_at without syncing confirmed_lsn. Otherwise, the prepared
         * transaction between old confirmed_lsn and two_phase_at will
         * unexpectedly get decoded and sent to the downstream after
         * promotion. See comments in ReorderBufferFinishPrepared.
         */
        return false;
    }
 
    /*
     * Attempt to sync LSNs and xmins only if remote slot is ahead of local
     * slot.
     */
    if (remote_slot->confirmed_lsn > slot->data.confirmed_flush ||
        remote_slot->restart_lsn > slot->data.restart_lsn ||
        TransactionIdFollows(remote_slot->catalog_xmin,
                             slot->data.catalog_xmin))
    {
        /*
         * We can't directly copy the remote slot's LSN or xmin unless there
         * exists a consistent snapshot at that point. Otherwise, after
         * promotion, the slots may not reach a consistent point before the
         * confirmed_flush_lsn which can lead to a data loss. To avoid data
         * loss, we let slot machinery advance the slot which ensures that
         * snapbuilder/slot statuses are updated properly.
         */
        if (SnapBuildSnapshotExists(remote_slot->restart_lsn))
        {
            /*
             * Update the slot info directly if there is a serialized snapshot
             * at the restart_lsn, as the slot can quickly reach consistency
             * at restart_lsn by restoring the snapshot.
             */
            SpinLockAcquire(&slot->mutex);
            slot->data.restart_lsn = remote_slot->restart_lsn;
            slot->data.confirmed_flush = remote_slot->confirmed_lsn;
            slot->data.catalog_xmin = remote_slot->catalog_xmin;
            SpinLockRelease(&slot->mutex);
 
            if (found_consistent_snapshot)
                *found_consistent_snapshot = true;
        }
        else
        {
            LogicalSlotAdvanceAndCheckSnapState(remote_slot->confirmed_lsn,
                                                found_consistent_snapshot);
 
            /* Sanity check */
            if (slot->data.confirmed_flush != remote_slot->confirmed_lsn)
                ereport(ERROR,
                        errmsg_internal("synchronized confirmed_flush for slot \"%s\" differs from remote slot",
                                        remote_slot->name),
                        errdetail_internal("Remote slot has LSN %X/%08X but local slot has LSN %X/%08X.",
                                           LSN_FORMAT_ARGS(remote_slot->confirmed_lsn),
                                           LSN_FORMAT_ARGS(slot->data.confirmed_flush)));
 
            /*
             * If we can't reach a consistent snapshot, the slot won't be
             * persisted. See update_and_persist_local_synced_slot().
             */
            if (found_consistent_snapshot && !(*found_consistent_snapshot))
                skip_reason = SS_SKIP_NO_CONSISTENT_SNAPSHOT;
        }
 
        updated_xmin_or_lsn = true;
    }
 
    /* Update slot sync skip stats */
    update_slotsync_skip_stats(skip_reason);
 
    if (remote_dbid != slot->data.database ||
        remote_slot->two_phase != slot->data.two_phase ||
        remote_slot->failover != slot->data.failover ||
        strcmp(remote_slot->plugin, NameStr(slot->data.plugin)) != 0 ||
        remote_slot->two_phase_at != slot->data.two_phase_at)
    {
        NameData    plugin_name;
 
        /* Avoid expensive operations while holding a spinlock. */
        namestrcpy(&plugin_name, remote_slot->plugin);
 
        SpinLockAcquire(&slot->mutex);
        slot->data.plugin = plugin_name;
        slot->data.database = remote_dbid;
        slot->data.two_phase = remote_slot->two_phase;
        slot->data.two_phase_at = remote_slot->two_phase_at;
        slot->data.failover = remote_slot->failover;
        SpinLockRelease(&slot->mutex);
 
        updated_config = true;
 
        /*
         * Ensure that there is no risk of sending prepared transactions
         * unexpectedly after the promotion.
         */
        Assert(slot->data.two_phase_at <= slot->data.confirmed_flush);
    }
 
    /*
     * We have to write the changed xmin to disk *before* we change the
     * in-memory value, otherwise after a crash we wouldn't know that some
     * catalog tuples might have been removed already.
     */
    if (updated_config || updated_xmin_or_lsn)
    {
        ReplicationSlotMarkDirty();
        ReplicationSlotSave();
    }
 
    /*
     * Now the new xmin is safely on disk, we can let the global value
     * advance. We do not take ProcArrayLock or similar since we only advance
     * xmin here and there's not much harm done by a concurrent computation
     * missing that.
     */
    if (updated_xmin_or_lsn)
    {
        SpinLockAcquire(&slot->mutex);
        slot->effective_catalog_xmin = remote_slot->catalog_xmin;
        SpinLockRelease(&slot->mutex);
 
        ReplicationSlotsComputeRequiredXmin(false);
        ReplicationSlotsComputeRequiredLSN();
    }
 
    return updated_config || updated_xmin_or_lsn;
}
 
/*
 * Get the list of local logical slots that are synchronized from the
 * primary server.
 */
static List *
get_local_synced_slots(void)
{
    List       *local_slots = NIL;
 
    LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
 
    for (int i = 0; i < max_replication_slots; i++)
    {
        ReplicationSlot *s = &ReplicationSlotCtl->replication_slots[i];
 
        /* Check if it is a synchronized slot */
        if (s->in_use && s->data.synced)
        {
            Assert(SlotIsLogical(s));
            local_slots = lappend(local_slots, s);
        }
    }
 
    LWLockRelease(ReplicationSlotControlLock);
 
    return local_slots;
}
 
/*
 * Helper function to check if local_slot is required to be retained.
 *
 * Return false either if local_slot does not exist in the remote_slots list
 * or is invalidated while the corresponding remote slot is still valid,
 * otherwise true.
 */
static bool
local_sync_slot_required(ReplicationSlot *local_slot, List *remote_slots)
{
    bool        remote_exists = false;
    bool        locally_invalidated = false;
 
    foreach_ptr(RemoteSlot, remote_slot, remote_slots)
    {
        if (strcmp(remote_slot->name, NameStr(local_slot->data.name)) == 0)
        {
            remote_exists = true;
 
            /*
             * If remote slot is not invalidated but local slot is marked as
             * invalidated, then set locally_invalidated flag.
             */
            SpinLockAcquire(&local_slot->mutex);
            locally_invalidated =
                (remote_slot->invalidated == RS_INVAL_NONE) &&
                (local_slot->data.invalidated != RS_INVAL_NONE);
            SpinLockRelease(&local_slot->mutex);
 
            break;
        }
    }
 
    return (remote_exists && !locally_invalidated);
}
 
/*
 * Drop local obsolete slots.
 *
 * Drop the local slots that no longer need to be synced i.e. these either do
 * not exist on the primary or are no longer enabled for failover.
 *
 * Additionally, drop any slots that are valid on the primary but got
 * invalidated on the standby. This situation may occur due to the following
 * reasons:
 * - The 'max_slot_wal_keep_size' on the standby is insufficient to retain WAL
 *   records from the restart_lsn of the slot.
 * - 'primary_slot_name' is temporarily reset to null and the physical slot is
 *   removed.
 * These dropped slots will get recreated in next sync-cycle and it is okay to
 * drop and recreate such slots as long as these are not consumable on the
 * standby (which is the case currently).
 *
 * Note: Change of 'wal_level' on the primary server to a level lower than
 * logical may also result in slot invalidation and removal on the standby.
 * This is because such 'wal_level' change is only possible if the logical
 * slots are removed on the primary server, so it's expected to see the
 * slots being invalidated and removed on the standby too (and re-created
 * if they are re-created on the primary server).
 */
static void
drop_local_obsolete_slots(List *remote_slot_list)
{
    List       *local_slots = get_local_synced_slots();
 
    foreach_ptr(ReplicationSlot, local_slot, local_slots)
    {
        /* Drop the local slot if it is not required to be retained. */
        if (!local_sync_slot_required(local_slot, remote_slot_list))
        {
            bool        synced_slot;
 
            /*
             * Use shared lock to prevent a conflict with
             * ReplicationSlotsDropDBSlots(), trying to drop the same slot
             * during a drop-database operation.
             */
            LockSharedObject(DatabaseRelationId, local_slot->data.database,
                             0, AccessShareLock);
 
            /*
             * In the small window between getting the slot to drop and
             * locking the database, there is a possibility of a parallel
             * database drop by the startup process and the creation of a new
             * slot by the user. This new user-created slot may end up using
             * the same shared memory as that of 'local_slot'. Thus check if
             * local_slot is still the synced one before performing actual
             * drop.
             */
            SpinLockAcquire(&local_slot->mutex);
            synced_slot = local_slot->in_use && local_slot->data.synced;
            SpinLockRelease(&local_slot->mutex);
 
            if (synced_slot)
            {
                ReplicationSlotAcquire(NameStr(local_slot->data.name), true, false);
                ReplicationSlotDropAcquired();
            }
 
            UnlockSharedObject(DatabaseRelationId, local_slot->data.database,
                               0, AccessShareLock);
 
            ereport(LOG,
                    errmsg("dropped replication slot \"%s\" of database with OID %u",
                           NameStr(local_slot->data.name),
                           local_slot->data.database));
        }
    }
}
 
/*
 * Reserve WAL for the currently active local slot using the specified WAL
 * location (restart_lsn).
 *
 * If the given WAL location has been removed, reserve WAL using the oldest
 * existing WAL segment.
 */
static void
reserve_wal_for_local_slot(XLogRecPtr restart_lsn)
{
    XLogSegNo   oldest_segno;
    XLogSegNo   segno;
    ReplicationSlot *slot = MyReplicationSlot;
 
    Assert(slot != NULL);
    Assert(!XLogRecPtrIsValid(slot->data.restart_lsn));
 
    while (true)
    {
        SpinLockAcquire(&slot->mutex);
        slot->data.restart_lsn = restart_lsn;
        SpinLockRelease(&slot->mutex);
 
        /* Prevent WAL removal as fast as possible */
        ReplicationSlotsComputeRequiredLSN();
 
        XLByteToSeg(slot->data.restart_lsn, segno, wal_segment_size);
 
        /*
         * Find the oldest existing WAL segment file.
         *
         * Normally, we can determine it by using the last removed segment
         * number. However, if no WAL segment files have been removed by a
         * checkpoint since startup, we need to search for the oldest segment
         * file from the current timeline existing in XLOGDIR.
         *
         * XXX: Currently, we are searching for the oldest segment in the
         * current timeline as there is less chance of the slot's restart_lsn
         * from being some prior timeline, and even if it happens, in the
         * worst case, we will wait to sync till the slot's restart_lsn moved
         * to the current timeline.
         */
        oldest_segno = XLogGetLastRemovedSegno() + 1;
 
        if (oldest_segno == 1)
        {
            TimeLineID  cur_timeline;
 
            GetWalRcvFlushRecPtr(NULL, &cur_timeline);
            oldest_segno = XLogGetOldestSegno(cur_timeline);
        }
 
        elog(DEBUG1, "segno: " UINT64_FORMAT " of purposed restart_lsn for the synced slot, oldest_segno: " UINT64_FORMAT " available",
             segno, oldest_segno);
 
        /*
         * If all required WAL is still there, great, otherwise retry. The
         * slot should prevent further removal of WAL, unless there's a
         * concurrent ReplicationSlotsComputeRequiredLSN() after we've written
         * the new restart_lsn above, so normally we should never need to loop
         * more than twice.
         */
        if (segno >= oldest_segno)
            break;
 
        /* Retry using the location of the oldest wal segment */
        XLogSegNoOffsetToRecPtr(oldest_segno, 0, wal_segment_size, restart_lsn);
    }
}
 
/*
 * If the remote restart_lsn and catalog_xmin have caught up with the
 * local ones, then update the LSNs and persist the local synced slot for
 * future synchronization; otherwise, do nothing.
 *
 * *slot_persistence_pending is set to true if any of the slots fail to
 * persist.
 *
 * Return true if the slot is marked as RS_PERSISTENT (sync-ready), otherwise
 * false.
 */
static bool
update_and_persist_local_synced_slot(RemoteSlot *remote_slot, Oid remote_dbid,
                                     bool *slot_persistence_pending)
{
    ReplicationSlot *slot = MyReplicationSlot;
    bool        found_consistent_snapshot = false;
    bool        remote_slot_precedes = false;
 
    /* Slotsync skip stats are handled in function update_local_synced_slot() */
    (void) update_local_synced_slot(remote_slot, remote_dbid,
                                    &found_consistent_snapshot,
                                    &remote_slot_precedes);
 
    /*
     * Check if the primary server has caught up. Refer to the comment atop
     * the file for details on this check.
     */
    if (remote_slot_precedes)
    {
        /*
         * The remote slot didn't catch up to locally reserved position.
         *
         * We do not drop the slot because the restart_lsn can be ahead of the
         * current location when recreating the slot in the next cycle. It may
         * take more time to create such a slot. Therefore, we keep this slot
         * and attempt the synchronization in the next cycle.
         *
         * We also update the slot_persistence_pending parameter, so the SQL
         * function can retry.
         */
        if (slot_persistence_pending)
            *slot_persistence_pending = true;
 
        return false;
    }
 
    /*
     * Don't persist the slot if it cannot reach the consistent point from the
     * restart_lsn. See comments atop this file.
     */
    if (!found_consistent_snapshot)
    {
        ereport(LOG,
                errmsg("could not synchronize replication slot \"%s\"", remote_slot->name),
                errdetail("Synchronization could lead to data loss, because the standby could not build a consistent snapshot to decode WALs at LSN %X/%08X.",
                          LSN_FORMAT_ARGS(slot->data.restart_lsn)));
 
        /* Set this, so that SQL function can retry */
        if (slot_persistence_pending)
            *slot_persistence_pending = true;
 
        return false;
    }
 
    ReplicationSlotPersist();
 
    ereport(LOG,
            errmsg("newly created replication slot \"%s\" is sync-ready now",
                   remote_slot->name));
 
    return true;
}
 
/*
 * Synchronize a single slot to the given position.
 *
 * This creates a new slot if there is no existing one and updates the
 * metadata of the slot as per the data received from the primary server.
 *
 * The slot is created as a temporary slot and stays in the same state until the
 * remote_slot catches up with locally reserved position and local slot is
 * updated. The slot is then persisted and is considered as sync-ready for
 * periodic syncs.
 *
 * *slot_persistence_pending is set to true if any of the slots fail to
 * persist.
 *
 * Returns TRUE if the local slot is updated.
 */
static bool
synchronize_one_slot(RemoteSlot *remote_slot, Oid remote_dbid,
                     bool *slot_persistence_pending)
{
    ReplicationSlot *slot;
    XLogRecPtr  latestFlushPtr = GetStandbyFlushRecPtr(NULL);
    bool        slot_updated = false;
 
    /* Search for the named slot */
    if ((slot = SearchNamedReplicationSlot(remote_slot->name, true)))
    {
        bool        synced;
 
        SpinLockAcquire(&slot->mutex);
        synced = slot->data.synced;
        SpinLockRelease(&slot->mutex);
 
        /* User-created slot with the same name exists, raise ERROR. */
        if (!synced)
            ereport(ERROR,
                    errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                    errmsg("exiting from slot synchronization because same"
                           " name slot \"%s\" already exists on the standby",
                           remote_slot->name));
 
        /*
         * The slot has been synchronized before.
         *
         * It is important to acquire the slot here before checking
         * invalidation. If we don't acquire the slot first, there could be a
         * race condition that the local slot could be invalidated just after
         * checking the 'invalidated' flag here and we could end up
         * overwriting 'invalidated' flag to remote_slot's value. See
         * InvalidatePossiblyObsoleteSlot() where it invalidates slot directly
         * if the slot is not acquired by other processes.
         *
         * XXX: If it ever turns out that slot acquire/release is costly for
         * cases when none of the slot properties is changed then we can do a
         * pre-check to ensure that at least one of the slot properties is
         * changed before acquiring the slot.
         */
        ReplicationSlotAcquire(remote_slot->name, true, false);
 
        Assert(slot == MyReplicationSlot);
 
        /*
         * Copy the invalidation cause from remote only if local slot is not
         * invalidated locally, we don't want to overwrite existing one.
         */
        if (slot->data.invalidated == RS_INVAL_NONE &&
            remote_slot->invalidated != RS_INVAL_NONE)
        {
            SpinLockAcquire(&slot->mutex);
            slot->data.invalidated = remote_slot->invalidated;
            SpinLockRelease(&slot->mutex);
 
            /* Make sure the invalidated state persists across server restart */
            ReplicationSlotMarkDirty();
            ReplicationSlotSave();
 
            slot_updated = true;
        }
 
        /* Skip the sync of an invalidated slot */
        if (slot->data.invalidated != RS_INVAL_NONE)
        {
            update_slotsync_skip_stats(SS_SKIP_INVALID);
 
            ReplicationSlotRelease();
            return slot_updated;
        }
 
        /*
         * Make sure that concerned WAL is received and flushed before syncing
         * slot to target lsn received from the primary server.
         *
         * Report statistics only after the slot has been acquired, ensuring
         * it cannot be dropped during the reporting process.
         */
        if (remote_slot->confirmed_lsn > latestFlushPtr)
        {
            update_slotsync_skip_stats(SS_SKIP_WAL_NOT_FLUSHED);
 
            /*
             * Can get here only if GUC 'synchronized_standby_slots' on the
             * primary server was not configured correctly.
             */
            ereport(AmLogicalSlotSyncWorkerProcess() ? LOG : ERROR,
                    errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                    errmsg("skipping slot synchronization because the received slot sync"
                           " LSN %X/%08X for slot \"%s\" is ahead of the standby position %X/%08X",
                           LSN_FORMAT_ARGS(remote_slot->confirmed_lsn),
                           remote_slot->name,
                           LSN_FORMAT_ARGS(latestFlushPtr)));
 
            ReplicationSlotRelease();
 
            return slot_updated;
        }
 
        /* Slot not ready yet, let's attempt to make it sync-ready now. */
        if (slot->data.persistency == RS_TEMPORARY)
        {
            slot_updated = update_and_persist_local_synced_slot(remote_slot,
                                                                remote_dbid,
                                                                slot_persistence_pending);
        }
 
        /* Slot ready for sync, so sync it. */
        else
        {
            /*
             * Sanity check: As long as the invalidations are handled
             * appropriately as above, this should never happen.
             *
             * We don't need to check restart_lsn here. See the comments in
             * update_local_synced_slot() for details.
             */
            if (remote_slot->confirmed_lsn < slot->data.confirmed_flush)
                ereport(ERROR,
                        errmsg_internal("cannot synchronize local slot \"%s\"",
                                        remote_slot->name),
                        errdetail_internal("Local slot's start streaming location LSN(%X/%08X) is ahead of remote slot's LSN(%X/%08X).",
                                           LSN_FORMAT_ARGS(slot->data.confirmed_flush),
                                           LSN_FORMAT_ARGS(remote_slot->confirmed_lsn)));
 
            slot_updated = update_local_synced_slot(remote_slot, remote_dbid,
                                                    NULL, NULL);
        }
    }
    /* Otherwise create the slot first. */
    else
    {
        NameData    plugin_name;
        TransactionId xmin_horizon = InvalidTransactionId;
 
        /* Skip creating the local slot if remote_slot is invalidated already */
        if (remote_slot->invalidated != RS_INVAL_NONE)
            return false;
 
        /*
         * We create temporary slots instead of ephemeral slots here because
         * we want the slots to survive after releasing them. This is done to
         * avoid dropping and re-creating the slots in each synchronization
         * cycle if the restart_lsn or catalog_xmin of the remote slot has not
         * caught up.
         */
        ReplicationSlotCreate(remote_slot->name, true, RS_TEMPORARY,
                              remote_slot->two_phase,
                              remote_slot->failover,
                              true);
 
        /* For shorter lines. */
        slot = MyReplicationSlot;
 
        /* Avoid expensive operations while holding a spinlock. */
        namestrcpy(&plugin_name, remote_slot->plugin);
 
        SpinLockAcquire(&slot->mutex);
        slot->data.database = remote_dbid;
        slot->data.plugin = plugin_name;
        SpinLockRelease(&slot->mutex);
 
        reserve_wal_for_local_slot(remote_slot->restart_lsn);
 
        LWLockAcquire(ReplicationSlotControlLock, LW_EXCLUSIVE);
        LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
        xmin_horizon = GetOldestSafeDecodingTransactionId(true);
        SpinLockAcquire(&slot->mutex);
        slot->effective_catalog_xmin = xmin_horizon;
        slot->data.catalog_xmin = xmin_horizon;
        SpinLockRelease(&slot->mutex);
        ReplicationSlotsComputeRequiredXmin(true);
        LWLockRelease(ProcArrayLock);
        LWLockRelease(ReplicationSlotControlLock);
 
        /*
         * Make sure that concerned WAL is received and flushed before syncing
         * slot to target lsn received from the primary server.
         *
         * Report statistics only after the slot has been acquired, ensuring
         * it cannot be dropped during the reporting process.
         */
        if (remote_slot->confirmed_lsn > latestFlushPtr)
        {
            update_slotsync_skip_stats(SS_SKIP_WAL_NOT_FLUSHED);
 
            /*
             * Can get here only if GUC 'synchronized_standby_slots' on the
             * primary server was not configured correctly.
             */
            ereport(AmLogicalSlotSyncWorkerProcess() ? LOG : ERROR,
                    errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                    errmsg("skipping slot synchronization because the received slot sync"
                           " LSN %X/%08X for slot \"%s\" is ahead of the standby position %X/%08X",
                           LSN_FORMAT_ARGS(remote_slot->confirmed_lsn),
                           remote_slot->name,
                           LSN_FORMAT_ARGS(latestFlushPtr)));
 
            ReplicationSlotRelease();
 
            return false;
        }
 
        update_and_persist_local_synced_slot(remote_slot, remote_dbid,
                                             slot_persistence_pending);
 
        slot_updated = true;
    }
 
    ReplicationSlotRelease();
 
    return slot_updated;
}
 
/*
 * Fetch remote slots.
 *
 * If slot_names is NIL, fetches all failover logical slots from the
 * primary server, otherwise fetches only the ones with names in slot_names.
 *
 * Returns a list of remote slot information structures, or NIL if none
 * are found.
 */
static List *
fetch_remote_slots(WalReceiverConn *wrconn, List *slot_names)
{
#define SLOTSYNC_COLUMN_COUNT 10
    Oid         slotRow[SLOTSYNC_COLUMN_COUNT] = {TEXTOID, TEXTOID, LSNOID,
    LSNOID, XIDOID, BOOLOID, LSNOID, BOOLOID, TEXTOID, TEXTOID};
 
    WalRcvExecResult *res;
    TupleTableSlot *tupslot;
    List       *remote_slot_list = NIL;
    StringInfoData query;
 
    initStringInfo(&query);
    appendStringInfoString(&query,
                           "SELECT slot_name, plugin, confirmed_flush_lsn,"
                           " restart_lsn, catalog_xmin, two_phase,"
                           " two_phase_at, failover,"
                           " database, invalidation_reason"
                           " FROM pg_catalog.pg_replication_slots"
                           " WHERE failover and NOT temporary");
 
    if (slot_names != NIL)
    {
        bool        first_slot = true;
 
        /*
         * Construct the query to fetch only the specified slots
         */
        appendStringInfoString(&query, " AND slot_name IN (");
 
        foreach_ptr(char, slot_name, slot_names)
        {
            if (!first_slot)
                appendStringInfoString(&query, ", ");
 
            appendStringInfo(&query, "%s", quote_literal_cstr(slot_name));
            first_slot = false;
        }
        appendStringInfoChar(&query, ')');
    }
 
    /* Execute the query */
    res = walrcv_exec(wrconn, query.data, SLOTSYNC_COLUMN_COUNT, slotRow);
    pfree(query.data);
    if (res->status != WALRCV_OK_TUPLES)
        ereport(ERROR,
                errmsg("could not fetch failover logical slots info from the primary server: %s",
                       res->err));
 
    tupslot = MakeSingleTupleTableSlot(res->tupledesc, &TTSOpsMinimalTuple);
    while (tuplestore_gettupleslot(res->tuplestore, true, false, tupslot))
    {
        bool        isnull;
        RemoteSlot *remote_slot = palloc0_object(RemoteSlot);
        Datum       d;
        int         col = 0;
 
        remote_slot->name = TextDatumGetCString(slot_getattr(tupslot, ++col,
                                                             &isnull));
        Assert(!isnull);
 
        remote_slot->plugin = TextDatumGetCString(slot_getattr(tupslot, ++col,
                                                               &isnull));
        Assert(!isnull);
 
        /*
         * It is possible to get null values for LSN and Xmin if slot is
         * invalidated on the primary server, so handle accordingly.
         */
        d = slot_getattr(tupslot, ++col, &isnull);
        remote_slot->confirmed_lsn = isnull ? InvalidXLogRecPtr :
            DatumGetLSN(d);
 
        d = slot_getattr(tupslot, ++col, &isnull);
        remote_slot->restart_lsn = isnull ? InvalidXLogRecPtr : DatumGetLSN(d);
 
        d = slot_getattr(tupslot, ++col, &isnull);
        remote_slot->catalog_xmin = isnull ? InvalidTransactionId :
            DatumGetTransactionId(d);
 
        remote_slot->two_phase = DatumGetBool(slot_getattr(tupslot, ++col,
                                                           &isnull));
        Assert(!isnull);
 
        d = slot_getattr(tupslot, ++col, &isnull);
        remote_slot->two_phase_at = isnull ? InvalidXLogRecPtr : DatumGetLSN(d);
 
        remote_slot->failover = DatumGetBool(slot_getattr(tupslot, ++col,
                                                          &isnull));
        Assert(!isnull);
 
        remote_slot->database = TextDatumGetCString(slot_getattr(tupslot,
                                                                 ++col, &isnull));
        Assert(!isnull);
 
        d = slot_getattr(tupslot, ++col, &isnull);
        remote_slot->invalidated = isnull ? RS_INVAL_NONE :
            GetSlotInvalidationCause(TextDatumGetCString(d));
 
        /* Sanity check */
        Assert(col == SLOTSYNC_COLUMN_COUNT);
 
        /*
         * If restart_lsn, confirmed_lsn or catalog_xmin is invalid but the
         * slot is valid, that means we have fetched the remote_slot in its
         * RS_EPHEMERAL state. In such a case, don't sync it; we can always
         * sync it in the next sync cycle when the remote_slot is persisted
         * and has valid lsn(s) and xmin values.
         *
         * XXX: In future, if we plan to expose 'slot->data.persistency' in
         * pg_replication_slots view, then we can avoid fetching RS_EPHEMERAL
         * slots in the first place.
         */
        if ((!XLogRecPtrIsValid(remote_slot->restart_lsn) ||
             !XLogRecPtrIsValid(remote_slot->confirmed_lsn) ||
             !TransactionIdIsValid(remote_slot->catalog_xmin)) &&
            remote_slot->invalidated == RS_INVAL_NONE)
            pfree(remote_slot);
        else
            /* Create list of remote slots */
            remote_slot_list = lappend(remote_slot_list, remote_slot);
 
        ExecClearTuple(tupslot);
    }
 
    walrcv_clear_result(res);
 
    return remote_slot_list;
}
 
/*
 * Synchronize slots.
 *
 * This function takes a list of remote slots and synchronizes them locally. It
 * creates the slots if not present on the standby and updates existing ones.
 *
 * If slot_persistence_pending is not NULL, it will be set to true if one or
 * more slots could not be persisted. This allows callers such as
 * SyncReplicationSlots() to retry those slots.
 *
 * Returns TRUE if any of the slots gets updated in this sync-cycle.
 */
static bool
synchronize_slots(WalReceiverConn *wrconn, List *remote_slot_list,
                  bool *slot_persistence_pending)
{
    bool        some_slot_updated = false;
 
    /* Drop local slots that no longer need to be synced. */
    drop_local_obsolete_slots(remote_slot_list);
 
    /* Now sync the slots locally */
    foreach_ptr(RemoteSlot, remote_slot, remote_slot_list)
    {
        Oid         remote_dbid = get_database_oid(remote_slot->database, false);
 
        /*
         * Use shared lock to prevent a conflict with
         * ReplicationSlotsDropDBSlots(), trying to drop the same slot during
         * a drop-database operation.
         */
        LockSharedObject(DatabaseRelationId, remote_dbid, 0, AccessShareLock);
 
        some_slot_updated |= synchronize_one_slot(remote_slot, remote_dbid,
                                                  slot_persistence_pending);
 
        UnlockSharedObject(DatabaseRelationId, remote_dbid, 0, AccessShareLock);
    }
 
    return some_slot_updated;
}
 
/*
 * Checks the remote server info.
 *
 * We ensure that the 'primary_slot_name' exists on the remote server and the
 * remote server is not a standby node.
 */
static void
validate_remote_info(WalReceiverConn *wrconn)
{
#define PRIMARY_INFO_OUTPUT_COL_COUNT 2
    WalRcvExecResult *res;
    Oid         slotRow[PRIMARY_INFO_OUTPUT_COL_COUNT] = {BOOLOID, BOOLOID};
    StringInfoData cmd;
    bool        isnull;
    TupleTableSlot *tupslot;
    bool        remote_in_recovery;
    bool        primary_slot_valid;
    bool        started_tx = false;
 
    initStringInfo(&cmd);
    appendStringInfo(&cmd,
                     "SELECT pg_is_in_recovery(), count(*) = 1"
                     " FROM pg_catalog.pg_replication_slots"
                     " WHERE slot_type='physical' AND slot_name=%s",
                     quote_literal_cstr(PrimarySlotName));
 
    /* The syscache access in walrcv_exec() needs a transaction env. */
    if (!IsTransactionState())
    {
        StartTransactionCommand();
        started_tx = true;
    }
 
    res = walrcv_exec(wrconn, cmd.data, PRIMARY_INFO_OUTPUT_COL_COUNT, slotRow);
    pfree(cmd.data);
 
    if (res->status != WALRCV_OK_TUPLES)
        ereport(ERROR,
                errmsg("could not fetch primary slot name \"%s\" info from the primary server: %s",
                       PrimarySlotName, res->err),
                errhint("Check if \"primary_slot_name\" is configured correctly."));
 
    tupslot = MakeSingleTupleTableSlot(res->tupledesc, &TTSOpsMinimalTuple);
    if (!tuplestore_gettupleslot(res->tuplestore, true, false, tupslot))
        elog(ERROR,
             "failed to fetch tuple for the primary server slot specified by \"primary_slot_name\"");
 
    remote_in_recovery = DatumGetBool(slot_getattr(tupslot, 1, &isnull));
    Assert(!isnull);
 
    /*
     * Slot sync is currently not supported on a cascading standby. This is
     * because if we allow it, the primary server needs to wait for all the
     * cascading standbys, otherwise, logical subscribers can still be ahead
     * of one of the cascading standbys which we plan to promote. Thus, to
     * avoid this additional complexity, we restrict it for the time being.
     */
    if (remote_in_recovery)
        ereport(ERROR,
                errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                errmsg("cannot synchronize replication slots from a standby server"));
 
    primary_slot_valid = DatumGetBool(slot_getattr(tupslot, 2, &isnull));
    Assert(!isnull);
 
    if (!primary_slot_valid)
        ereport(ERROR,
                errcode(ERRCODE_INVALID_PARAMETER_VALUE),
        /* translator: second %s is a GUC variable name */
                errmsg("replication slot \"%s\" specified by \"%s\" does not exist on primary server",
                       PrimarySlotName, "primary_slot_name"));
 
    ExecClearTuple(tupslot);
    walrcv_clear_result(res);
 
    if (started_tx)
        CommitTransactionCommand();
}
 
/*
 * Checks if dbname is specified in 'primary_conninfo'.
 *
 * Error out if not specified otherwise return it.
 */
char *
CheckAndGetDbnameFromConninfo(void)
{
    char       *dbname;
 
    /*
     * The slot synchronization needs a database connection for walrcv_exec to
     * work.
     */
    dbname = walrcv_get_dbname_from_conninfo(PrimaryConnInfo);
    if (dbname == NULL)
        ereport(ERROR,
                errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 
        /*
         * translator: first %s is a connection option; second %s is a GUC
         * variable name
         */
                errmsg("replication slot synchronization requires \"%s\" to be specified in \"%s\"",
                       "dbname", "primary_conninfo"));
    return dbname;
}
 
/*
 * Return true if all necessary GUCs for slot synchronization are set
 * appropriately, otherwise, return false.
 */
bool
ValidateSlotSyncParams(int elevel)
{
    /*
     * Logical slot sync/creation requires logical decoding to be enabled.
     */
    if (!IsLogicalDecodingEnabled())
    {
        ereport(elevel,
                errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                errmsg("replication slot synchronization requires \"effective_wal_level\" >= \"logical\" on the primary"),
                errhint("To enable logical decoding on primary, set \"wal_level\" >= \"logical\" or create at least one logical slot when \"wal_level\" = \"replica\"."));
 
        return false;
    }
 
    /*
     * A physical replication slot(primary_slot_name) is required on the
     * primary to ensure that the rows needed by the standby are not removed
     * after restarting, so that the synchronized slot on the standby will not
     * be invalidated.
     */
    if (PrimarySlotName == NULL || *PrimarySlotName == '\0')
    {
        ereport(elevel,
                errcode(ERRCODE_INVALID_PARAMETER_VALUE),
        /* translator: %s is a GUC variable name */
                errmsg("replication slot synchronization requires \"%s\" to be set", "primary_slot_name"));
        return false;
    }
 
    /*
     * hot_standby_feedback must be enabled to cooperate with the physical
     * replication slot, which allows informing the primary about the xmin and
     * catalog_xmin values on the standby.
     */
    if (!hot_standby_feedback)
    {
        ereport(elevel,
                errcode(ERRCODE_INVALID_PARAMETER_VALUE),
        /* translator: %s is a GUC variable name */
                errmsg("replication slot synchronization requires \"%s\" to be enabled",
                       "hot_standby_feedback"));
        return false;
    }
 
    /*
     * The primary_conninfo is required to make connection to primary for
     * getting slots information.
     */
    if (PrimaryConnInfo == NULL || *PrimaryConnInfo == '\0')
    {
        ereport(elevel,
                errcode(ERRCODE_INVALID_PARAMETER_VALUE),
        /* translator: %s is a GUC variable name */
                errmsg("replication slot synchronization requires \"%s\" to be set",
                       "primary_conninfo"));
        return false;
    }
 
    return true;
}
 
/*
 * Re-read the config file for slot synchronization.
 *
 * Exit or throw error if relevant GUCs have changed depending on whether
 * called from slot sync worker or from the SQL function pg_sync_replication_slots()
 */
static void
slotsync_reread_config(void)
{
    char       *old_primary_conninfo = pstrdup(PrimaryConnInfo);
    char       *old_primary_slotname = pstrdup(PrimarySlotName);
    bool        old_sync_replication_slots = sync_replication_slots;
    bool        old_hot_standby_feedback = hot_standby_feedback;
    bool        conninfo_changed;
    bool        primary_slotname_changed;
    bool        is_slotsync_worker = AmLogicalSlotSyncWorkerProcess();
    bool        parameter_changed = false;
 
    if (is_slotsync_worker)
        Assert(sync_replication_slots);
 
    ConfigReloadPending = false;
    ProcessConfigFile(PGC_SIGHUP);
 
    conninfo_changed = strcmp(old_primary_conninfo, PrimaryConnInfo) != 0;
    primary_slotname_changed = strcmp(old_primary_slotname, PrimarySlotName) != 0;
    pfree(old_primary_conninfo);
    pfree(old_primary_slotname);
 
    if (old_sync_replication_slots != sync_replication_slots)
    {
        if (is_slotsync_worker)
        {
            ereport(LOG,
            /* translator: %s is a GUC variable name */
                    errmsg("replication slot synchronization worker will stop because \"%s\" is disabled",
                           "sync_replication_slots"));
 
            proc_exit(0);
        }
 
        parameter_changed = true;
    }
    else
    {
        if (conninfo_changed ||
            primary_slotname_changed ||
            (old_hot_standby_feedback != hot_standby_feedback))
        {
 
            if (is_slotsync_worker)
            {
                ereport(LOG,
                        errmsg("replication slot synchronization worker will restart because of a parameter change"));
 
                /*
                 * Reset the last-start time for this worker so that the
                 * postmaster can restart it without waiting for
                 * SLOTSYNC_RESTART_INTERVAL_SEC.
                 */
                SlotSyncCtx->last_start_time = 0;
 
                proc_exit(0);
            }
 
            parameter_changed = true;
        }
    }
 
    /*
     * If we have reached here with a parameter change, we must be running in
     * SQL function, emit error in such a case.
     */
    if (parameter_changed)
    {
        Assert(!is_slotsync_worker);
        ereport(ERROR,
                errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                errmsg("replication slot synchronization will stop because of a parameter change"));
    }
 
}
 
/*
 * Interrupt handler for process performing slot synchronization.
 */
static void
ProcessSlotSyncInterrupts(void)
{
    CHECK_FOR_INTERRUPTS();
 
    if (SlotSyncCtx->stopSignaled)
    {
        if (AmLogicalSlotSyncWorkerProcess())
        {
            ereport(LOG,
                    errmsg("replication slot synchronization worker will stop because promotion is triggered"));
 
            proc_exit(0);
        }
        else
        {
            /*
             * For the backend executing SQL function
             * pg_sync_replication_slots().
             */
            ereport(ERROR,
                    errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                    errmsg("replication slot synchronization will stop because promotion is triggered"));
        }
    }
 
    if (ConfigReloadPending)
        slotsync_reread_config();
}
 
/*
 * Connection cleanup function for slotsync worker.
 *
 * Called on slotsync worker exit.
 */
static void
slotsync_worker_disconnect(int code, Datum arg)
{
    WalReceiverConn *wrconn = (WalReceiverConn *) DatumGetPointer(arg);
 
    walrcv_disconnect(wrconn);
}
 
/*
 * Cleanup function for slotsync worker.
 *
 * Called on slotsync worker exit.
 */
static void
slotsync_worker_onexit(int code, Datum arg)
{
    /*
     * We need to do slots cleanup here just like WalSndErrorCleanup() does.
     *
     * The startup process during promotion invokes ShutDownSlotSync() which
     * waits for slot sync to finish and it does that by checking the
     * 'syncing' flag. Thus the slot sync worker must be done with slots'
     * release and cleanup to avoid any dangling temporary slots or active
     * slots before it marks itself as finished syncing.
     */
 
    /* Make sure active replication slots are released */
    if (MyReplicationSlot != NULL)
        ReplicationSlotRelease();
 
    /* Also cleanup the temporary slots. */
    ReplicationSlotCleanup(false);
 
    SpinLockAcquire(&SlotSyncCtx->mutex);
 
    SlotSyncCtx->pid = InvalidPid;
 
    /*
     * If syncing_slots is true, it indicates that the process errored out
     * without resetting the flag. So, we need to clean up shared memory and
     * reset the flag here.
     */
    if (syncing_slots)
    {
        SlotSyncCtx->syncing = false;
        syncing_slots = false;
    }
 
    SpinLockRelease(&SlotSyncCtx->mutex);
}
 
/*
 * Sleep for long enough that we believe it's likely that the slots on primary
 * get updated.
 *
 * If there is no slot activity the wait time between sync-cycles will double
 * (to a maximum of 30s). If there is some slot activity the wait time between
 * sync-cycles is reset to the minimum (200ms).
 */
static void
wait_for_slot_activity(bool some_slot_updated)
{
    int         rc;
 
    if (!some_slot_updated)
    {
        /*
         * No slots were updated, so double the sleep time, but not beyond the
         * maximum allowable value.
         */
        sleep_ms = Min(sleep_ms * 2, MAX_SLOTSYNC_WORKER_NAPTIME_MS);
    }
    else
    {
        /*
         * Some slots were updated since the last sleep, so reset the sleep
         * time.
         */
        sleep_ms = MIN_SLOTSYNC_WORKER_NAPTIME_MS;
    }
 
    rc = WaitLatch(MyLatch,
                   WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
                   sleep_ms,
                   WAIT_EVENT_REPLICATION_SLOTSYNC_MAIN);
 
    if (rc & WL_LATCH_SET)
        ResetLatch(MyLatch);
}
 
/*
 * Emit an error if a concurrent sync call is in progress.
 * Otherwise, advertise that a sync is in progress.
 */
static void
check_and_set_sync_info(pid_t sync_process_pid)
{
    SpinLockAcquire(&SlotSyncCtx->mutex);
 
    if (SlotSyncCtx->syncing)
    {
        SpinLockRelease(&SlotSyncCtx->mutex);
        ereport(ERROR,
                errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                errmsg("cannot synchronize replication slots concurrently"));
    }
 
    /* The pid must not be already assigned in SlotSyncCtx */
    Assert(SlotSyncCtx->pid == InvalidPid);
 
    SlotSyncCtx->syncing = true;
 
    /*
     * Advertise the required PID so that the startup process can kill the
     * slot sync process on promotion.
     */
    SlotSyncCtx->pid = sync_process_pid;
 
    SpinLockRelease(&SlotSyncCtx->mutex);
 
    syncing_slots = true;
}
 
/*
 * Reset syncing flag.
 */
static void
reset_syncing_flag(void)
{
    SpinLockAcquire(&SlotSyncCtx->mutex);
    SlotSyncCtx->syncing = false;
    SlotSyncCtx->pid = InvalidPid;
    SpinLockRelease(&SlotSyncCtx->mutex);
 
    syncing_slots = false;
}
 
/*
 * The main loop of our worker process.
 *
 * It connects to the primary server, fetches logical failover slots
 * information periodically in order to create and sync the slots.
 *
 * Note: If any changes are made here, check if the corresponding SQL
 * function logic in SyncReplicationSlots() also needs to be changed.
 */
void
ReplSlotSyncWorkerMain(const void *startup_data, size_t startup_data_len)
{
    WalReceiverConn *wrconn = NULL;
    char       *dbname;
    char       *err;
    sigjmp_buf  local_sigjmp_buf;
    StringInfoData app_name;
 
    Assert(startup_data_len == 0);
 
    MyBackendType = B_SLOTSYNC_WORKER;
 
    init_ps_display(NULL);
 
    Assert(GetProcessingMode() == InitProcessing);
 
    /*
     * Create a per-backend PGPROC struct in shared memory.  We must do this
     * before we access any shared memory.
     */
    InitProcess();
 
    /*
     * Early initialization.
     */
    BaseInit();
 
    Assert(SlotSyncCtx != NULL);
 
    /*
     * If an exception is encountered, processing resumes here.
     *
     * We just need to clean up, report the error, and go away.
     *
     * If we do not have this handling here, then since this worker process
     * operates at the bottom of the exception stack, ERRORs turn into FATALs.
     * Therefore, we create our own exception handler to catch ERRORs.
     */
    if (sigsetjmp(local_sigjmp_buf, 1) != 0)
    {
        /* since not using PG_TRY, must reset error stack by hand */
        error_context_stack = NULL;
 
        /* Prevents interrupts while cleaning up */
        HOLD_INTERRUPTS();
 
        /* Report the error to the server log */
        EmitErrorReport();
 
        /*
         * We can now go away.  Note that because we called InitProcess, a
         * callback was registered to do ProcKill, which will clean up
         * necessary state.
         */
        proc_exit(0);
    }
 
    /* We can now handle ereport(ERROR) */
    PG_exception_stack = &local_sigjmp_buf;
 
    /* Setup signal handling */
    pqsignal(SIGHUP, SignalHandlerForConfigReload);
    pqsignal(SIGINT, StatementCancelHandler);
    pqsignal(SIGTERM, die);
    pqsignal(SIGFPE, FloatExceptionHandler);
    pqsignal(SIGUSR1, procsignal_sigusr1_handler);
    pqsignal(SIGUSR2, SIG_IGN);
    pqsignal(SIGPIPE, SIG_IGN);
    pqsignal(SIGCHLD, SIG_DFL);
 
    check_and_set_sync_info(MyProcPid);
 
    ereport(LOG, errmsg("slot sync worker started"));
 
    /* Register it as soon as SlotSyncCtx->pid is initialized. */
    before_shmem_exit(slotsync_worker_onexit, (Datum) 0);
 
    /*
     * Establishes SIGALRM handler and initialize timeout module. It is needed
     * by InitPostgres to register different timeouts.
     */
    InitializeTimeouts();
 
    /* Load the libpq-specific functions */
    load_file("libpqwalreceiver", false);
 
    /*
     * Unblock signals (they were blocked when the postmaster forked us)
     */
    sigprocmask(SIG_SETMASK, &UnBlockSig, NULL);
 
    /*
     * Set always-secure search path, so malicious users can't redirect user
     * code (e.g. operators).
     *
     * It's not strictly necessary since we won't be scanning or writing to
     * any user table locally, but it's good to retain it here for added
     * precaution.
     */
    SetConfigOption("search_path", "", PGC_SUSET, PGC_S_OVERRIDE);
 
    dbname = CheckAndGetDbnameFromConninfo();
 
    /*
     * Connect to the database specified by the user in primary_conninfo. We
     * need a database connection for walrcv_exec to work which we use to
     * fetch slot information from the remote node. See comments atop
     * libpqrcv_exec.
     *
     * We do not specify a specific user here since the slot sync worker will
     * operate as a superuser. This is safe because the slot sync worker does
     * not interact with user tables, eliminating the risk of executing
     * arbitrary code within triggers.
     */
    InitPostgres(dbname, InvalidOid, NULL, InvalidOid, 0, NULL);
 
    SetProcessingMode(NormalProcessing);
 
    initStringInfo(&app_name);
    if (cluster_name[0])
        appendStringInfo(&app_name, "%s_%s", cluster_name, "slotsync worker");
    else
        appendStringInfoString(&app_name, "slotsync worker");
 
    /*
     * Establish the connection to the primary server for slot
     * synchronization.
     */
    wrconn = walrcv_connect(PrimaryConnInfo, false, false, false,
                            app_name.data, &err);
 
    if (!wrconn)
        ereport(ERROR,
                errcode(ERRCODE_CONNECTION_FAILURE),
                errmsg("synchronization worker \"%s\" could not connect to the primary server: %s",
                       app_name.data, err));
 
    pfree(app_name.data);
 
    /*
     * Register the disconnection callback.
     *
     * XXX: This can be combined with previous cleanup registration of
     * slotsync_worker_onexit() but that will need the connection to be made
     * global and we want to avoid introducing global for this purpose.
     */
    before_shmem_exit(slotsync_worker_disconnect, PointerGetDatum(wrconn));
 
    /*
     * Using the specified primary server connection, check that we are not a
     * cascading standby and slot configured in 'primary_slot_name' exists on
     * the primary server.
     */
    validate_remote_info(wrconn);
 
    /* Main loop to synchronize slots */
    for (;;)
    {
        bool        some_slot_updated = false;
        bool        started_tx = false;
        List       *remote_slots;
 
        ProcessSlotSyncInterrupts();
 
        /*
         * The syscache access in fetch_remote_slots() needs a transaction
         * env.
         */
        if (!IsTransactionState())
        {
            StartTransactionCommand();
            started_tx = true;
        }
 
        remote_slots = fetch_remote_slots(wrconn, NIL);
        some_slot_updated = synchronize_slots(wrconn, remote_slots, NULL);
        list_free_deep(remote_slots);
 
        if (started_tx)
            CommitTransactionCommand();
 
        wait_for_slot_activity(some_slot_updated);
    }
 
    /*
     * The slot sync worker can't get here because it will only stop when it
     * receives a stop request from the startup process, or when there is an
     * error.
     */
    Assert(false);
}
 
/*
 * Update the inactive_since property for synced slots.
 *
 * Note that this function is currently called when we shutdown the slot
 * sync machinery.
 */
static void
update_synced_slots_inactive_since(void)
{
    TimestampTz now = 0;
 
    /*
     * We need to update inactive_since only when we are promoting standby to
     * correctly interpret the inactive_since if the standby gets promoted
     * without a restart. We don't want the slots to appear inactive for a
     * long time after promotion if they haven't been synchronized recently.
     * Whoever acquires the slot, i.e., makes the slot active, will reset it.
     */
    if (!StandbyMode)
        return;
 
    /* The slot sync worker or the SQL function mustn't be running by now */
    Assert((SlotSyncCtx->pid == InvalidPid) && !SlotSyncCtx->syncing);
 
    LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
 
    for (int i = 0; i < max_replication_slots; i++)
    {
        ReplicationSlot *s = &ReplicationSlotCtl->replication_slots[i];
 
        /* Check if it is a synchronized slot */
        if (s->in_use && s->data.synced)
        {
            Assert(SlotIsLogical(s));
 
            /* The slot must not be acquired by any process */
            Assert(s->active_pid == 0);
 
            /* Use the same inactive_since time for all the slots. */
            if (now == 0)
                now = GetCurrentTimestamp();
 
            ReplicationSlotSetInactiveSince(s, now, true);
        }
    }
 
    LWLockRelease(ReplicationSlotControlLock);
}
 
/*
 * Shut down slot synchronization.
 *
 * This function sets stopSignaled=true and wakes up the slot sync process
 * (either worker or backend running the SQL function pg_sync_replication_slots())
 * so that worker can exit or the SQL function pg_sync_replication_slots() can
 * finish. It also waits till the slot sync worker has exited or
 * pg_sync_replication_slots() has finished.
 */
void
ShutDownSlotSync(void)
{
    pid_t       sync_process_pid;
 
    SpinLockAcquire(&SlotSyncCtx->mutex);
 
    SlotSyncCtx->stopSignaled = true;
 
    /*
     * Return if neither the slot sync worker is running nor the function
     * pg_sync_replication_slots() is executing.
     */
    if (!SlotSyncCtx->syncing)
    {
        SpinLockRelease(&SlotSyncCtx->mutex);
        update_synced_slots_inactive_since();
        return;
    }
 
    sync_process_pid = SlotSyncCtx->pid;
 
    SpinLockRelease(&SlotSyncCtx->mutex);
 
    /*
     * Signal process doing slotsync, if any. The process will stop upon
     * detecting that the stopSignaled flag is set to true.
     */
    if (sync_process_pid != InvalidPid)
        kill(sync_process_pid, SIGUSR1);
 
    /* Wait for slot sync to end */
    for (;;)
    {
        int         rc;
 
        /* Wait a bit, we don't expect to have to wait long */
        rc = WaitLatch(MyLatch,
                       WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
                       10L, WAIT_EVENT_REPLICATION_SLOTSYNC_SHUTDOWN);
 
        if (rc & WL_LATCH_SET)
        {
            ResetLatch(MyLatch);
            CHECK_FOR_INTERRUPTS();
        }
 
        SpinLockAcquire(&SlotSyncCtx->mutex);
 
        /* Ensure that no process is syncing the slots. */
        if (!SlotSyncCtx->syncing)
            break;
 
        SpinLockRelease(&SlotSyncCtx->mutex);
    }
 
    SpinLockRelease(&SlotSyncCtx->mutex);
 
    update_synced_slots_inactive_since();
}
 
/*
 * SlotSyncWorkerCanRestart
 *
 * Return true, indicating worker is allowed to restart, if enough time has
 * passed since it was last launched to reach SLOTSYNC_RESTART_INTERVAL_SEC.
 * Otherwise return false.
 *
 * This is a safety valve to protect against continuous respawn attempts if the
 * worker is dying immediately at launch. Note that since we will retry to
 * launch the worker from the postmaster main loop, we will get another
 * chance later.
 */
bool
SlotSyncWorkerCanRestart(void)
{
    time_t      curtime = time(NULL);
 
    /*
     * If first time through, or time somehow went backwards, always update
     * last_start_time to match the current clock and allow worker start.
     * Otherwise allow it only once enough time has elapsed.
     */
    if (SlotSyncCtx->last_start_time == 0 ||
        curtime < SlotSyncCtx->last_start_time ||
        curtime - SlotSyncCtx->last_start_time >= SLOTSYNC_RESTART_INTERVAL_SEC)
    {
        SlotSyncCtx->last_start_time = curtime;
        return true;
    }
    return false;
}
 
/*
 * Is current process syncing replication slots?
 *
 * Could be either backend executing SQL function or slot sync worker.
 */
bool
IsSyncingReplicationSlots(void)
{
    return syncing_slots;
}
 
/*
 * Amount of shared memory required for slot synchronization.
 */
Size
SlotSyncShmemSize(void)
{
    return sizeof(SlotSyncCtxStruct);
}
 
/*
 * Allocate and initialize the shared memory of slot synchronization.
 */
void
SlotSyncShmemInit(void)
{
    Size        size = SlotSyncShmemSize();
    bool        found;
 
    SlotSyncCtx = (SlotSyncCtxStruct *)
        ShmemInitStruct("Slot Sync Data", size, &found);
 
    if (!found)
    {
        memset(SlotSyncCtx, 0, size);
        SlotSyncCtx->pid = InvalidPid;
        SpinLockInit(&SlotSyncCtx->mutex);
    }
}
 
/*
 * Error cleanup callback for slot sync SQL function.
 */
static void
slotsync_failure_callback(int code, Datum arg)
{
    WalReceiverConn *wrconn = (WalReceiverConn *) DatumGetPointer(arg);
 
    /*
     * We need to do slots cleanup here just like WalSndErrorCleanup() does.
     *
     * The startup process during promotion invokes ShutDownSlotSync() which
     * waits for slot sync to finish and it does that by checking the
     * 'syncing' flag. Thus the SQL function must be done with slots' release
     * and cleanup to avoid any dangling temporary slots or active slots
     * before it marks itself as finished syncing.
     */
 
    /* Make sure active replication slots are released */
    if (MyReplicationSlot != NULL)
        ReplicationSlotRelease();
 
    /* Also cleanup the synced temporary slots. */
    ReplicationSlotCleanup(true);
 
    /*
     * The set syncing_slots indicates that the process errored out without
     * resetting the flag. So, we need to clean up shared memory and reset the
     * flag here.
     */
    if (syncing_slots)
        reset_syncing_flag();
 
    walrcv_disconnect(wrconn);
}
 
/*
 * Helper function to extract slot names from a list of remote slots
 */
static List *
extract_slot_names(List *remote_slots)
{
    List       *slot_names = NIL;
 
    foreach_ptr(RemoteSlot, remote_slot, remote_slots)
    {
        char       *slot_name;
 
        slot_name = pstrdup(remote_slot->name);
        slot_names = lappend(slot_names, slot_name);
    }
 
    return slot_names;
}
 
/*
 * Synchronize the failover enabled replication slots using the specified
 * primary server connection.
 *
 * Repeatedly fetches and updates replication slot information from the
 * primary until all slots are at least "sync ready".
 *
 * Exits early if promotion is triggered or certain critical
 * configuration parameters have changed.
 */
void
SyncReplicationSlots(WalReceiverConn *wrconn)
{
    PG_ENSURE_ERROR_CLEANUP(slotsync_failure_callback, PointerGetDatum(wrconn));
    {
        List       *remote_slots = NIL;
        List       *slot_names = NIL;   /* List of slot names to track */
 
        check_and_set_sync_info(MyProcPid);
 
        /* Check for interrupts and config changes */
        ProcessSlotSyncInterrupts();
 
        validate_remote_info(wrconn);
 
        /* Retry until all the slots are sync-ready */
        for (;;)
        {
            bool        slot_persistence_pending = false;
            bool        some_slot_updated = false;
 
            /* Check for interrupts and config changes */
            ProcessSlotSyncInterrupts();
 
            /* We must be in a valid transaction state */
            Assert(IsTransactionState());
 
            /*
             * Fetch remote slot info for the given slot_names. If slot_names
             * is NIL, fetch all failover-enabled slots. Note that we reuse
             * slot_names from the first iteration; re-fetching all failover
             * slots each time could cause an endless loop. Instead of
             * reprocessing only the pending slots in each iteration, it's
             * better to process all the slots received in the first
             * iteration. This ensures that by the time we're done, all slots
             * reflect the latest values.
             */
            remote_slots = fetch_remote_slots(wrconn, slot_names);
 
            /* Attempt to synchronize slots */
            some_slot_updated = synchronize_slots(wrconn, remote_slots,
                                                  &slot_persistence_pending);
 
            /*
             * If slot_persistence_pending is true, extract slot names for
             * future iterations (only needed if we haven't done it yet)
             */
            if (slot_names == NIL && slot_persistence_pending)
                slot_names = extract_slot_names(remote_slots);
 
            /* Free the current remote_slots list */
            list_free_deep(remote_slots);
 
            /* Done if all slots are persisted i.e are sync-ready */
            if (!slot_persistence_pending)
                break;
 
            /* wait before retrying again */
            wait_for_slot_activity(some_slot_updated);
        }
 
        if (slot_names)
            list_free_deep(slot_names);
 
        /* Cleanup the synced temporary slots */
        ReplicationSlotCleanup(true);
 
        /* We are done with sync, so reset sync flag */
        reset_syncing_flag();
    }
    PG_END_ENSURE_ERROR_CLEANUP(slotsync_failure_callback, PointerGetDatum(wrconn));
}