launcher.c

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/*-------------------------------------------------------------------------
 * launcher.c
 *     PostgreSQL logical replication worker launcher process
 *
 * Copyright (c) 2016-2021, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/replication/logical/launcher.c
 *
 * NOTES
 *    This module contains the logical replication worker launcher which
 *    uses the background worker infrastructure to start the logical
 *    replication workers for every enabled subscription.
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include "access/heapam.h"
#include "access/htup.h"
#include "access/htup_details.h"
#include "access/tableam.h"
#include "access/xact.h"
#include "catalog/pg_subscription.h"
#include "catalog/pg_subscription_rel.h"
#include "funcapi.h"
#include "libpq/pqsignal.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/bgworker.h"
#include "postmaster/fork_process.h"
#include "postmaster/interrupt.h"
#include "postmaster/postmaster.h"
#include "replication/logicallauncher.h"
#include "replication/logicalworker.h"
#include "replication/slot.h"
#include "replication/walreceiver.h"
#include "replication/worker_internal.h"
#include "storage/ipc.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/procsignal.h"
#include "tcop/tcopprot.h"
#include "utils/memutils.h"
#include "utils/pg_lsn.h"
#include "utils/ps_status.h"
#include "utils/snapmgr.h"
#include "utils/timeout.h"

/* max sleep time between cycles (3min) */
#define DEFAULT_NAPTIME_PER_CYCLE 180000L

int         max_logical_replication_workers = 4;
int         max_sync_workers_per_subscription = 2;

LogicalRepWorker *MyLogicalRepWorker = NULL;

typedef struct LogicalRepCtxStruct
{
    /* Supervisor process. */
    pid_t       launcher_pid;

    /* Background workers. */
    LogicalRepWorker workers[FLEXIBLE_ARRAY_MEMBER];
} LogicalRepCtxStruct;

LogicalRepCtxStruct *LogicalRepCtx;

typedef struct LogicalRepWorkerId
{
    Oid         subid;
    Oid         relid;
} LogicalRepWorkerId;

typedef struct StopWorkersData
{
    int         nestDepth;      /* Sub-transaction nest level */
    List       *workers;        /* List of LogicalRepWorkerId */
    struct StopWorkersData *parent; /* This need not be an immediate
                                     * subtransaction parent */
} StopWorkersData;

/*
 * Stack of StopWorkersData elements. Each stack element contains the workers
 * to be stopped for that subtransaction.
 */
static StopWorkersData *on_commit_stop_workers = NULL;

static void ApplyLauncherWakeup(void);
static void logicalrep_launcher_onexit(int code, Datum arg);
static void logicalrep_worker_onexit(int code, Datum arg);
static void logicalrep_worker_detach(void);
static void logicalrep_worker_cleanup(LogicalRepWorker *worker);

static bool on_commit_launcher_wakeup = false;

Datum       pg_stat_get_subscription(PG_FUNCTION_ARGS);


/*
 * Load the list of subscriptions.
 *
 * Only the fields interesting for worker start/stop functions are filled for
 * each subscription.
 */
static List *
get_subscription_list(void)
{
    List       *res = NIL;
    Relation    rel;
    TableScanDesc scan;
    HeapTuple   tup;
    MemoryContext resultcxt;

    /* This is the context that we will allocate our output data in */
    resultcxt = CurrentMemoryContext;

    /*
     * Start a transaction so we can access pg_database, and get a snapshot.
     * We don't have a use for the snapshot itself, but we're interested in
     * the secondary effect that it sets RecentGlobalXmin.  (This is critical
     * for anything that reads heap pages, because HOT may decide to prune
     * them even if the process doesn't attempt to modify any tuples.)
     *
     * FIXME: This comment is inaccurate / the code buggy. A snapshot that is
     * not pushed/active does not reliably prevent HOT pruning (->xmin could
     * e.g. be cleared when cache invalidations are processed).
     */
    StartTransactionCommand();
    (void) GetTransactionSnapshot();

    rel = table_open(SubscriptionRelationId, AccessShareLock);
    scan = table_beginscan_catalog(rel, 0, NULL);

    while (HeapTupleIsValid(tup = heap_getnext(scan, ForwardScanDirection)))
    {
        Form_pg_subscription subform = (Form_pg_subscription) GETSTRUCT(tup);
        Subscription *sub;
        MemoryContext oldcxt;

        /*
         * Allocate our results in the caller's context, not the
         * transaction's. We do this inside the loop, and restore the original
         * context at the end, so that leaky things like heap_getnext() are
         * not called in a potentially long-lived context.
         */
        oldcxt = MemoryContextSwitchTo(resultcxt);

        sub = (Subscription *) palloc0(sizeof(Subscription));
        sub->oid = subform->oid;
        sub->dbid = subform->subdbid;
        sub->owner = subform->subowner;
        sub->enabled = subform->subenabled;
        sub->name = pstrdup(NameStr(subform->subname));
        /* We don't fill fields we are not interested in. */

        res = lappend(res, sub);
        MemoryContextSwitchTo(oldcxt);
    }

    table_endscan(scan);
    table_close(rel, AccessShareLock);

    CommitTransactionCommand();

    return res;
}

/*
 * Wait for a background worker to start up and attach to the shmem context.
 *
 * This is only needed for cleaning up the shared memory in case the worker
 * fails to attach.
 */
static void
WaitForReplicationWorkerAttach(LogicalRepWorker *worker,
                               uint16 generation,
                               BackgroundWorkerHandle *handle)
{
    BgwHandleStatus status;
    int         rc;

    for (;;)
    {
        pid_t       pid;

        CHECK_FOR_INTERRUPTS();

        LWLockAcquire(LogicalRepWorkerLock, LW_SHARED);

        /* Worker either died or has started; no need to do anything. */
        if (!worker->in_use || worker->proc)
        {
            LWLockRelease(LogicalRepWorkerLock);
            return;
        }

        LWLockRelease(LogicalRepWorkerLock);

        /* Check if worker has died before attaching, and clean up after it. */
        status = GetBackgroundWorkerPid(handle, &pid);

        if (status == BGWH_STOPPED)
        {
            LWLockAcquire(LogicalRepWorkerLock, LW_EXCLUSIVE);
            /* Ensure that this was indeed the worker we waited for. */
            if (generation == worker->generation)
                logicalrep_worker_cleanup(worker);
            LWLockRelease(LogicalRepWorkerLock);
            return;
        }

        /*
         * We need timeout because we generally don't get notified via latch
         * about the worker attach.  But 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_BGWORKER_STARTUP);

        if (rc & WL_LATCH_SET)
        {
            ResetLatch(MyLatch);
            CHECK_FOR_INTERRUPTS();
        }
    }
}

/*
 * Walks the workers array and searches for one that matches given
 * subscription id and relid.
 */
LogicalRepWorker *
logicalrep_worker_find(Oid subid, Oid relid, bool only_running)
{
    int         i;
    LogicalRepWorker *res = NULL;

    Assert(LWLockHeldByMe(LogicalRepWorkerLock));

    /* Search for attached worker for a given subscription id. */
    for (i = 0; i < max_logical_replication_workers; i++)
    {
        LogicalRepWorker *w = &LogicalRepCtx->workers[i];

        if (w->in_use && w->subid == subid && w->relid == relid &&
            (!only_running || w->proc))
        {
            res = w;
            break;
        }
    }

    return res;
}

/*
 * Similar to logicalrep_worker_find(), but returns list of all workers for
 * the subscription, instead just one.
 */
List *
logicalrep_workers_find(Oid subid, bool only_running)
{
    int         i;
    List       *res = NIL;

    Assert(LWLockHeldByMe(LogicalRepWorkerLock));

    /* Search for attached worker for a given subscription id. */
    for (i = 0; i < max_logical_replication_workers; i++)
    {
        LogicalRepWorker *w = &LogicalRepCtx->workers[i];

        if (w->in_use && w->subid == subid && (!only_running || w->proc))
            res = lappend(res, w);
    }

    return res;
}

/*
 * Start new apply background worker, if possible.
 */
void
logicalrep_worker_launch(Oid dbid, Oid subid, const char *subname, Oid userid,
                         Oid relid)
{
    BackgroundWorker bgw;
    BackgroundWorkerHandle *bgw_handle;
    uint16      generation;
    int         i;
    int         slot = 0;
    LogicalRepWorker *worker = NULL;
    int         nsyncworkers;
    TimestampTz now;

    ereport(DEBUG1,
            (errmsg("starting logical replication worker for subscription \"%s\"",
                    subname)));

    /* Report this after the initial starting message for consistency. */
    if (max_replication_slots == 0)
        ereport(ERROR,
                (errcode(ERRCODE_CONFIGURATION_LIMIT_EXCEEDED),
                 errmsg("cannot start logical replication workers when max_replication_slots = 0")));

    /*
     * We need to do the modification of the shared memory under lock so that
     * we have consistent view.
     */
    LWLockAcquire(LogicalRepWorkerLock, LW_EXCLUSIVE);

retry:
    /* Find unused worker slot. */
    for (i = 0; i < max_logical_replication_workers; i++)
    {
        LogicalRepWorker *w = &LogicalRepCtx->workers[i];

        if (!w->in_use)
        {
            worker = w;
            slot = i;
            break;
        }
    }

    nsyncworkers = logicalrep_sync_worker_count(subid);

    now = GetCurrentTimestamp();

    /*
     * If we didn't find a free slot, try to do garbage collection.  The
     * reason we do this is because if some worker failed to start up and its
     * parent has crashed while waiting, the in_use state was never cleared.
     */
    if (worker == NULL || nsyncworkers >= max_sync_workers_per_subscription)
    {
        bool        did_cleanup = false;

        for (i = 0; i < max_logical_replication_workers; i++)
        {
            LogicalRepWorker *w = &LogicalRepCtx->workers[i];

            /*
             * If the worker was marked in use but didn't manage to attach in
             * time, clean it up.
             */
            if (w->in_use && !w->proc &&
                TimestampDifferenceExceeds(w->launch_time, now,
                                           wal_receiver_timeout))
            {
                elog(WARNING,
                     "logical replication worker for subscription %u took too long to start; canceled",
                     w->subid);

                logicalrep_worker_cleanup(w);
                did_cleanup = true;
            }
        }

        if (did_cleanup)
            goto retry;
    }

    /*
     * If we reached the sync worker limit per subscription, just exit
     * silently as we might get here because of an otherwise harmless race
     * condition.
     */
    if (nsyncworkers >= max_sync_workers_per_subscription)
    {
        LWLockRelease(LogicalRepWorkerLock);
        return;
    }

    /*
     * However if there are no more free worker slots, inform user about it
     * before exiting.
     */
    if (worker == NULL)
    {
        LWLockRelease(LogicalRepWorkerLock);
        ereport(WARNING,
                (errcode(ERRCODE_CONFIGURATION_LIMIT_EXCEEDED),
                 errmsg("out of logical replication worker slots"),
                 errhint("You might need to increase max_logical_replication_workers.")));
        return;
    }

    /* Prepare the worker slot. */
    worker->launch_time = now;
    worker->in_use = true;
    worker->generation++;
    worker->proc = NULL;
    worker->dbid = dbid;
    worker->userid = userid;
    worker->subid = subid;
    worker->relid = relid;
    worker->relstate = SUBREL_STATE_UNKNOWN;
    worker->relstate_lsn = InvalidXLogRecPtr;
    worker->last_lsn = InvalidXLogRecPtr;
    TIMESTAMP_NOBEGIN(worker->last_send_time);
    TIMESTAMP_NOBEGIN(worker->last_recv_time);
    worker->reply_lsn = InvalidXLogRecPtr;
    TIMESTAMP_NOBEGIN(worker->reply_time);

    /* Before releasing lock, remember generation for future identification. */
    generation = worker->generation;

    LWLockRelease(LogicalRepWorkerLock);

    /* Register the new dynamic worker. */
    memset(&bgw, 0, sizeof(bgw));
    bgw.bgw_flags = BGWORKER_SHMEM_ACCESS |
        BGWORKER_BACKEND_DATABASE_CONNECTION;
    bgw.bgw_start_time = BgWorkerStart_RecoveryFinished;
    snprintf(bgw.bgw_library_name, BGW_MAXLEN, "postgres");
    snprintf(bgw.bgw_function_name, BGW_MAXLEN, "ApplyWorkerMain");
    if (OidIsValid(relid))
        snprintf(bgw.bgw_name, BGW_MAXLEN,
                 "logical replication worker for subscription %u sync %u", subid, relid);
    else
        snprintf(bgw.bgw_name, BGW_MAXLEN,
                 "logical replication worker for subscription %u", subid);
    snprintf(bgw.bgw_type, BGW_MAXLEN, "logical replication worker");

    bgw.bgw_restart_time = BGW_NEVER_RESTART;
    bgw.bgw_notify_pid = MyProcPid;
    bgw.bgw_main_arg = Int32GetDatum(slot);

    if (!RegisterDynamicBackgroundWorker(&bgw, &bgw_handle))
    {
        /* Failed to start worker, so clean up the worker slot. */
        LWLockAcquire(LogicalRepWorkerLock, LW_EXCLUSIVE);
        Assert(generation == worker->generation);
        logicalrep_worker_cleanup(worker);
        LWLockRelease(LogicalRepWorkerLock);

        ereport(WARNING,
                (errcode(ERRCODE_CONFIGURATION_LIMIT_EXCEEDED),
                 errmsg("out of background worker slots"),
                 errhint("You might need to increase max_worker_processes.")));
        return;
    }

    /* Now wait until it attaches. */
    WaitForReplicationWorkerAttach(worker, generation, bgw_handle);
}

/*
 * Stop the logical replication worker for subid/relid, if any, and wait until
 * it detaches from the slot.
 */
void
logicalrep_worker_stop(Oid subid, Oid relid)
{
    LogicalRepWorker *worker;
    uint16      generation;

    LWLockAcquire(LogicalRepWorkerLock, LW_SHARED);

    worker = logicalrep_worker_find(subid, relid, false);

    /* No worker, nothing to do. */
    if (!worker)
    {
        LWLockRelease(LogicalRepWorkerLock);
        return;
    }

    /*
     * Remember which generation was our worker so we can check if what we see
     * is still the same one.
     */
    generation = worker->generation;

    /*
     * If we found a worker but it does not have proc set then it is still
     * starting up; wait for it to finish starting and then kill it.
     */
    while (worker->in_use && !worker->proc)
    {
        int         rc;

        LWLockRelease(LogicalRepWorkerLock);

        /* 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_BGWORKER_STARTUP);

        if (rc & WL_LATCH_SET)
        {
            ResetLatch(MyLatch);
            CHECK_FOR_INTERRUPTS();
        }

        /* Recheck worker status. */
        LWLockAcquire(LogicalRepWorkerLock, LW_SHARED);

        /*
         * Check whether the worker slot is no longer used, which would mean
         * that the worker has exited, or whether the worker generation is
         * different, meaning that a different worker has taken the slot.
         */
        if (!worker->in_use || worker->generation != generation)
        {
            LWLockRelease(LogicalRepWorkerLock);
            return;
        }

        /* Worker has assigned proc, so it has started. */
        if (worker->proc)
            break;
    }

    /* Now terminate the worker ... */
    kill(worker->proc->pid, SIGTERM);

    /* ... and wait for it to die. */
    for (;;)
    {
        int         rc;

        /* is it gone? */
        if (!worker->proc || worker->generation != generation)
            break;

        LWLockRelease(LogicalRepWorkerLock);

        /* 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_BGWORKER_SHUTDOWN);

        if (rc & WL_LATCH_SET)
        {
            ResetLatch(MyLatch);
            CHECK_FOR_INTERRUPTS();
        }

        LWLockAcquire(LogicalRepWorkerLock, LW_SHARED);
    }

    LWLockRelease(LogicalRepWorkerLock);
}

/*
 * Request worker for specified sub/rel to be stopped on commit.
 */
void
logicalrep_worker_stop_at_commit(Oid subid, Oid relid)
{
    int         nestDepth = GetCurrentTransactionNestLevel();
    LogicalRepWorkerId *wid;
    MemoryContext oldctx;

    /* Make sure we store the info in context that survives until commit. */
    oldctx = MemoryContextSwitchTo(TopTransactionContext);

    /* Check that previous transactions were properly cleaned up. */
    Assert(on_commit_stop_workers == NULL ||
           nestDepth >= on_commit_stop_workers->nestDepth);

    /*
     * Push a new stack element if we don't already have one for the current
     * nestDepth.
     */
    if (on_commit_stop_workers == NULL ||
        nestDepth > on_commit_stop_workers->nestDepth)
    {
        StopWorkersData *newdata = palloc(sizeof(StopWorkersData));

        newdata->nestDepth = nestDepth;
        newdata->workers = NIL;
        newdata->parent = on_commit_stop_workers;
        on_commit_stop_workers = newdata;
    }

    /*
     * Finally add a new worker into the worker list of the current
     * subtransaction.
     */
    wid = palloc(sizeof(LogicalRepWorkerId));
    wid->subid = subid;
    wid->relid = relid;
    on_commit_stop_workers->workers =
        lappend(on_commit_stop_workers->workers, wid);

    MemoryContextSwitchTo(oldctx);
}

/*
 * Wake up (using latch) any logical replication worker for specified sub/rel.
 */
void
logicalrep_worker_wakeup(Oid subid, Oid relid)
{
    LogicalRepWorker *worker;

    LWLockAcquire(LogicalRepWorkerLock, LW_SHARED);

    worker = logicalrep_worker_find(subid, relid, true);

    if (worker)
        logicalrep_worker_wakeup_ptr(worker);

    LWLockRelease(LogicalRepWorkerLock);
}

/*
 * Wake up (using latch) the specified logical replication worker.
 *
 * Caller must hold lock, else worker->proc could change under us.
 */
void
logicalrep_worker_wakeup_ptr(LogicalRepWorker *worker)
{
    Assert(LWLockHeldByMe(LogicalRepWorkerLock));

    SetLatch(&worker->proc->procLatch);
}

/*
 * Attach to a slot.
 */
void
logicalrep_worker_attach(int slot)
{
    /* Block concurrent access. */
    LWLockAcquire(LogicalRepWorkerLock, LW_EXCLUSIVE);

    Assert(slot >= 0 && slot < max_logical_replication_workers);
    MyLogicalRepWorker = &LogicalRepCtx->workers[slot];

    if (!MyLogicalRepWorker->in_use)
    {
        LWLockRelease(LogicalRepWorkerLock);
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("logical replication worker slot %d is empty, cannot attach",
                        slot)));
    }

    if (MyLogicalRepWorker->proc)
    {
        LWLockRelease(LogicalRepWorkerLock);
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("logical replication worker slot %d is already used by "
                        "another worker, cannot attach", slot)));
    }

    MyLogicalRepWorker->proc = MyProc;
    before_shmem_exit(logicalrep_worker_onexit, (Datum) 0);

    LWLockRelease(LogicalRepWorkerLock);
}

/*
 * Detach the worker (cleans up the worker info).
 */
static void
logicalrep_worker_detach(void)
{
    /* Block concurrent access. */
    LWLockAcquire(LogicalRepWorkerLock, LW_EXCLUSIVE);

    logicalrep_worker_cleanup(MyLogicalRepWorker);

    LWLockRelease(LogicalRepWorkerLock);
}

/*
 * Clean up worker info.
 */
static void
logicalrep_worker_cleanup(LogicalRepWorker *worker)
{
    Assert(LWLockHeldByMeInMode(LogicalRepWorkerLock, LW_EXCLUSIVE));

    worker->in_use = false;
    worker->proc = NULL;
    worker->dbid = InvalidOid;
    worker->userid = InvalidOid;
    worker->subid = InvalidOid;
    worker->relid = InvalidOid;
}

/*
 * Cleanup function for logical replication launcher.
 *
 * Called on logical replication launcher exit.
 */
static void
logicalrep_launcher_onexit(int code, Datum arg)
{
    LogicalRepCtx->launcher_pid = 0;
}

/*
 * Cleanup function.
 *
 * Called on logical replication worker exit.
 */
static void
logicalrep_worker_onexit(int code, Datum arg)
{
    /* Disconnect gracefully from the remote side. */
    if (wrconn)
        walrcv_disconnect(wrconn);

    logicalrep_worker_detach();

    ApplyLauncherWakeup();
}

/*
 * Count the number of registered (not necessarily running) sync workers
 * for a subscription.
 */
int
logicalrep_sync_worker_count(Oid subid)
{
    int         i;
    int         res = 0;

    Assert(LWLockHeldByMe(LogicalRepWorkerLock));

    /* Search for attached worker for a given subscription id. */
    for (i = 0; i < max_logical_replication_workers; i++)
    {
        LogicalRepWorker *w = &LogicalRepCtx->workers[i];

        if (w->subid == subid && OidIsValid(w->relid))
            res++;
    }

    return res;
}

/*
 * ApplyLauncherShmemSize
 *      Compute space needed for replication launcher shared memory
 */
Size
ApplyLauncherShmemSize(void)
{
    Size        size;

    /*
     * Need the fixed struct and the array of LogicalRepWorker.
     */
    size = sizeof(LogicalRepCtxStruct);
    size = MAXALIGN(size);
    size = add_size(size, mul_size(max_logical_replication_workers,
                                   sizeof(LogicalRepWorker)));
    return size;
}

/*
 * ApplyLauncherRegister
 *      Register a background worker running the logical replication launcher.
 */
void
ApplyLauncherRegister(void)
{
    BackgroundWorker bgw;

    if (max_logical_replication_workers == 0)
        return;

    memset(&bgw, 0, sizeof(bgw));
    bgw.bgw_flags = BGWORKER_SHMEM_ACCESS |
        BGWORKER_BACKEND_DATABASE_CONNECTION;
    bgw.bgw_start_time = BgWorkerStart_RecoveryFinished;
    snprintf(bgw.bgw_library_name, BGW_MAXLEN, "postgres");
    snprintf(bgw.bgw_function_name, BGW_MAXLEN, "ApplyLauncherMain");
    snprintf(bgw.bgw_name, BGW_MAXLEN,
             "logical replication launcher");
    snprintf(bgw.bgw_type, BGW_MAXLEN,
             "logical replication launcher");
    bgw.bgw_restart_time = 5;
    bgw.bgw_notify_pid = 0;
    bgw.bgw_main_arg = (Datum) 0;

    RegisterBackgroundWorker(&bgw);
}

/*
 * ApplyLauncherShmemInit
 *      Allocate and initialize replication launcher shared memory
 */
void
ApplyLauncherShmemInit(void)
{
    bool        found;

    LogicalRepCtx = (LogicalRepCtxStruct *)
        ShmemInitStruct("Logical Replication Launcher Data",
                        ApplyLauncherShmemSize(),
                        &found);

    if (!found)
    {
        int         slot;

        memset(LogicalRepCtx, 0, ApplyLauncherShmemSize());

        /* Initialize memory and spin locks for each worker slot. */
        for (slot = 0; slot < max_logical_replication_workers; slot++)
        {
            LogicalRepWorker *worker = &LogicalRepCtx->workers[slot];

            memset(worker, 0, sizeof(LogicalRepWorker));
            SpinLockInit(&worker->relmutex);
        }
    }
}

/*
 * Check whether current transaction has manipulated logical replication
 * workers.
 */
bool
XactManipulatesLogicalReplicationWorkers(void)
{
    return (on_commit_stop_workers != NULL);
}

/*
 * Wakeup the launcher on commit if requested.
 */
void
AtEOXact_ApplyLauncher(bool isCommit)
{

    Assert(on_commit_stop_workers == NULL ||
           (on_commit_stop_workers->nestDepth == 1 &&
            on_commit_stop_workers->parent == NULL));

    if (isCommit)
    {
        ListCell   *lc;

        if (on_commit_stop_workers != NULL)
        {
            List       *workers = on_commit_stop_workers->workers;

            foreach(lc, workers)
            {
                LogicalRepWorkerId *wid = lfirst(lc);

                logicalrep_worker_stop(wid->subid, wid->relid);
            }
        }

        if (on_commit_launcher_wakeup)
            ApplyLauncherWakeup();
    }

    /*
     * No need to pfree on_commit_stop_workers.  It was allocated in
     * transaction memory context, which is going to be cleaned soon.
     */
    on_commit_stop_workers = NULL;
    on_commit_launcher_wakeup = false;
}

/*
 * On commit, merge the current on_commit_stop_workers list into the
 * immediate parent, if present.
 * On rollback, discard the current on_commit_stop_workers list.
 * Pop out the stack.
 */
void
AtEOSubXact_ApplyLauncher(bool isCommit, int nestDepth)
{
    StopWorkersData *parent;

    /* Exit immediately if there's no work to do at this level. */
    if (on_commit_stop_workers == NULL ||
        on_commit_stop_workers->nestDepth < nestDepth)
        return;

    Assert(on_commit_stop_workers->nestDepth == nestDepth);

    parent = on_commit_stop_workers->parent;

    if (isCommit)
    {
        /*
         * If the upper stack element is not an immediate parent
         * subtransaction, just decrement the notional nesting depth without
         * doing any real work.  Else, we need to merge the current workers
         * list into the parent.
         */
        if (!parent || parent->nestDepth < nestDepth - 1)
        {
            on_commit_stop_workers->nestDepth--;
            return;
        }

        parent->workers =
            list_concat(parent->workers, on_commit_stop_workers->workers);
    }
    else
    {
        /*
         * Abandon everything that was done at this nesting level.  Explicitly
         * free memory to avoid a transaction-lifespan leak.
         */
        list_free_deep(on_commit_stop_workers->workers);
    }

    /*
     * We have taken care of the current subtransaction workers list for both
     * abort or commit. So we are ready to pop the stack.
     */
    pfree(on_commit_stop_workers);
    on_commit_stop_workers = parent;
}

/*
 * Request wakeup of the launcher on commit of the transaction.
 *
 * This is used to send launcher signal to stop sleeping and process the
 * subscriptions when current transaction commits. Should be used when new
 * tuple was added to the pg_subscription catalog.
*/
void
ApplyLauncherWakeupAtCommit(void)
{
    if (!on_commit_launcher_wakeup)
        on_commit_launcher_wakeup = true;
}

static void
ApplyLauncherWakeup(void)
{
    if (LogicalRepCtx->launcher_pid != 0)
        kill(LogicalRepCtx->launcher_pid, SIGUSR1);
}

/*
 * Main loop for the apply launcher process.
 */
void
ApplyLauncherMain(Datum main_arg)
{
    TimestampTz last_start_time = 0;

    ereport(DEBUG1,
            (errmsg("logical replication launcher started")));

    before_shmem_exit(logicalrep_launcher_onexit, (Datum) 0);

    Assert(LogicalRepCtx->launcher_pid == 0);
    LogicalRepCtx->launcher_pid = MyProcPid;

    /* Establish signal handlers. */
    pqsignal(SIGHUP, SignalHandlerForConfigReload);
    pqsignal(SIGTERM, die);
    BackgroundWorkerUnblockSignals();

    /*
     * Establish connection to nailed catalogs (we only ever access
     * pg_subscription).
     */
    BackgroundWorkerInitializeConnection(NULL, NULL, 0);

    /* Enter main loop */
    for (;;)
    {
        int         rc;
        List       *sublist;
        ListCell   *lc;
        MemoryContext subctx;
        MemoryContext oldctx;
        TimestampTz now;
        long        wait_time = DEFAULT_NAPTIME_PER_CYCLE;

        CHECK_FOR_INTERRUPTS();

        now = GetCurrentTimestamp();

        /* Limit the start retry to once a wal_retrieve_retry_interval */
        if (TimestampDifferenceExceeds(last_start_time, now,
                                       wal_retrieve_retry_interval))
        {
            /* Use temporary context for the database list and worker info. */
            subctx = AllocSetContextCreate(TopMemoryContext,
                                           "Logical Replication Launcher sublist",
                                           ALLOCSET_DEFAULT_SIZES);
            oldctx = MemoryContextSwitchTo(subctx);

            /* search for subscriptions to start or stop. */
            sublist = get_subscription_list();

            /* Start the missing workers for enabled subscriptions. */
            foreach(lc, sublist)
            {
                Subscription *sub = (Subscription *) lfirst(lc);
                LogicalRepWorker *w;

                if (!sub->enabled)
                    continue;

                LWLockAcquire(LogicalRepWorkerLock, LW_SHARED);
                w = logicalrep_worker_find(sub->oid, InvalidOid, false);
                LWLockRelease(LogicalRepWorkerLock);

                if (w == NULL)
                {
                    last_start_time = now;
                    wait_time = wal_retrieve_retry_interval;

                    logicalrep_worker_launch(sub->dbid, sub->oid, sub->name,
                                             sub->owner, InvalidOid);
                }
            }

            /* Switch back to original memory context. */
            MemoryContextSwitchTo(oldctx);
            /* Clean the temporary memory. */
            MemoryContextDelete(subctx);
        }
        else
        {
            /*
             * The wait in previous cycle was interrupted in less than
             * wal_retrieve_retry_interval since last worker was started, this
             * usually means crash of the worker, so we should retry in
             * wal_retrieve_retry_interval again.
             */
            wait_time = wal_retrieve_retry_interval;
        }

        /* Wait for more work. */
        rc = WaitLatch(MyLatch,
                       WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
                       wait_time,
                       WAIT_EVENT_LOGICAL_LAUNCHER_MAIN);

        if (rc & WL_LATCH_SET)
        {
            ResetLatch(MyLatch);
            CHECK_FOR_INTERRUPTS();
        }

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

    /* Not reachable */
}

/*
 * Is current process the logical replication launcher?
 */
bool
IsLogicalLauncher(void)
{
    return LogicalRepCtx->launcher_pid == MyProcPid;
}

/*
 * Returns state of the subscriptions.
 */
Datum
pg_stat_get_subscription(PG_FUNCTION_ARGS)
{
#define PG_STAT_GET_SUBSCRIPTION_COLS   8
    Oid         subid = PG_ARGISNULL(0) ? InvalidOid : PG_GETARG_OID(0);
    int         i;
    ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
    TupleDesc   tupdesc;
    Tuplestorestate *tupstore;
    MemoryContext per_query_ctx;
    MemoryContext oldcontext;

    /* check to see if caller supports us returning a tuplestore */
    if (rsinfo == NULL || !IsA(rsinfo, ReturnSetInfo))
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("set-valued function called in context that cannot accept a set")));
    if (!(rsinfo->allowedModes & SFRM_Materialize))
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("materialize mode required, but it is not allowed in this context")));

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

    per_query_ctx = rsinfo->econtext->ecxt_per_query_memory;
    oldcontext = MemoryContextSwitchTo(per_query_ctx);

    tupstore = tuplestore_begin_heap(true, false, work_mem);
    rsinfo->returnMode = SFRM_Materialize;
    rsinfo->setResult = tupstore;
    rsinfo->setDesc = tupdesc;

    MemoryContextSwitchTo(oldcontext);

    /* Make sure we get consistent view of the workers. */
    LWLockAcquire(LogicalRepWorkerLock, LW_SHARED);

    for (i = 0; i <= max_logical_replication_workers; i++)
    {
        /* for each row */
        Datum       values[PG_STAT_GET_SUBSCRIPTION_COLS];
        bool        nulls[PG_STAT_GET_SUBSCRIPTION_COLS];
        int         worker_pid;
        LogicalRepWorker worker;

        memcpy(&worker, &LogicalRepCtx->workers[i],
               sizeof(LogicalRepWorker));
        if (!worker.proc || !IsBackendPid(worker.proc->pid))
            continue;

        if (OidIsValid(subid) && worker.subid != subid)
            continue;

        worker_pid = worker.proc->pid;

        MemSet(values, 0, sizeof(values));
        MemSet(nulls, 0, sizeof(nulls));

        values[0] = ObjectIdGetDatum(worker.subid);
        if (OidIsValid(worker.relid))
            values[1] = ObjectIdGetDatum(worker.relid);
        else
            nulls[1] = true;
        values[2] = Int32GetDatum(worker_pid);
        if (XLogRecPtrIsInvalid(worker.last_lsn))
            nulls[3] = true;
        else
            values[3] = LSNGetDatum(worker.last_lsn);
        if (worker.last_send_time == 0)
            nulls[4] = true;
        else
            values[4] = TimestampTzGetDatum(worker.last_send_time);
        if (worker.last_recv_time == 0)
            nulls[5] = true;
        else
            values[5] = TimestampTzGetDatum(worker.last_recv_time);
        if (XLogRecPtrIsInvalid(worker.reply_lsn))
            nulls[6] = true;
        else
            values[6] = LSNGetDatum(worker.reply_lsn);
        if (worker.reply_time == 0)
            nulls[7] = true;
        else
            values[7] = TimestampTzGetDatum(worker.reply_time);

        tuplestore_putvalues(tupstore, tupdesc, values, nulls);

        /*
         * If only a single subscription was requested, and we found it,
         * break.
         */
        if (OidIsValid(subid))
            break;
    }

    LWLockRelease(LogicalRepWorkerLock);

    /* clean up and return the tuplestore */
    tuplestore_donestoring(tupstore);

    return (Datum) 0;
}