launcher.c

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/*-------------------------------------------------------------------------
 * launcher.c
 *     PostgreSQL logical replication worker launcher process
 *
 * Copyright (c) 2016-2025, 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 "lib/dshash.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/bgworker.h"
#include "postmaster/interrupt.h"
#include "replication/logicallauncher.h"
#include "replication/origin.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 "tcop/tcopprot.h"
#include "utils/builtins.h"
#include "utils/memutils.h"
#include "utils/pg_lsn.h"
#include "utils/snapmgr.h"
#include "utils/syscache.h"
 
/* max sleep time between cycles (3min) */
#define DEFAULT_NAPTIME_PER_CYCLE 180000L
 
/* GUC variables */
int         max_logical_replication_workers = 4;
int         max_sync_workers_per_subscription = 2;
int         max_parallel_apply_workers_per_subscription = 2;
 
LogicalRepWorker *MyLogicalRepWorker = NULL;
 
typedef struct LogicalRepCtxStruct
{
    /* Supervisor process. */
    pid_t       launcher_pid;
 
    /* Hash table holding last start times of subscriptions' apply workers. */
    dsa_handle  last_start_dsa;
    dshash_table_handle last_start_dsh;
 
    /* Background workers. */
    LogicalRepWorker workers[FLEXIBLE_ARRAY_MEMBER];
} LogicalRepCtxStruct;
 
static LogicalRepCtxStruct *LogicalRepCtx;
 
/* an entry in the last-start-times shared hash table */
typedef struct LauncherLastStartTimesEntry
{
    Oid         subid;          /* OID of logrep subscription (hash key) */
    TimestampTz last_start_time;    /* last time its apply worker was started */
} LauncherLastStartTimesEntry;
 
/* parameters for the last-start-times shared hash table */
static const dshash_parameters dsh_params = {
    sizeof(Oid),
    sizeof(LauncherLastStartTimesEntry),
    dshash_memcmp,
    dshash_memhash,
    dshash_memcpy,
    LWTRANCHE_LAUNCHER_HASH
};
 
static dsa_area *last_start_times_dsa = NULL;
static dshash_table *last_start_times = NULL;
 
static bool on_commit_launcher_wakeup = false;
 
 
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 int  logicalrep_pa_worker_count(Oid subid);
static void logicalrep_launcher_attach_dshmem(void);
static void ApplyLauncherSetWorkerStartTime(Oid subid, TimestampTz start_time);
static TimestampTz ApplyLauncherGetWorkerStartTime(Oid subid);
static void compute_min_nonremovable_xid(LogicalRepWorker *worker, TransactionId *xmin);
static bool acquire_conflict_slot_if_exists(void);
static void update_conflict_slot_xmin(TransactionId new_xmin);
static void init_conflict_slot_xmin(void);
 
 
/*
 * 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_subscription.
     */
    StartTransactionCommand();
 
    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));
        sub->retaindeadtuples = subform->subretaindeadtuples;
        sub->retentionactive = subform->subretentionactive;
        /* 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.
 *
 * Returns whether the attach was successful.
 */
static bool
WaitForReplicationWorkerAttach(LogicalRepWorker *worker,
                               uint16 generation,
                               BackgroundWorkerHandle *handle)
{
    bool        result = false;
    bool        dropped_latch = false;
 
    for (;;)
    {
        BgwHandleStatus status;
        pid_t       pid;
        int         rc;
 
        CHECK_FOR_INTERRUPTS();
 
        LWLockAcquire(LogicalRepWorkerLock, LW_SHARED);
 
        /* Worker either died or has started. Return false if died. */
        if (!worker->in_use || worker->proc)
        {
            result = worker->in_use;
            LWLockRelease(LogicalRepWorkerLock);
            break;
        }
 
        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);
            break;              /* result is already false */
        }
 
        /*
         * 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();
            dropped_latch = true;
        }
    }
 
    /*
     * If we had to clear a latch event in order to wait, be sure to restore
     * it before exiting.  Otherwise caller may miss events.
     */
    if (dropped_latch)
        SetLatch(MyLatch);
 
    return result;
}
 
/*
 * Walks the workers array and searches for one that matches given
 * subscription id and relid.
 *
 * We are only interested in the leader apply worker or table sync worker.
 */
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];
 
        /* Skip parallel apply workers. */
        if (isParallelApplyWorker(w))
            continue;
 
        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 a list of all workers for
 * the subscription, instead of just one.
 */
List *
logicalrep_workers_find(Oid subid, bool only_running, bool acquire_lock)
{
    int         i;
    List       *res = NIL;
 
    if (acquire_lock)
        LWLockAcquire(LogicalRepWorkerLock, LW_SHARED);
 
    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);
    }
 
    if (acquire_lock)
        LWLockRelease(LogicalRepWorkerLock);
 
    return res;
}
 
/*
 * Start new logical replication background worker, if possible.
 *
 * Returns true on success, false on failure.
 */
bool
logicalrep_worker_launch(LogicalRepWorkerType wtype,
                         Oid dbid, Oid subid, const char *subname, Oid userid,
                         Oid relid, dsm_handle subworker_dsm,
                         bool retain_dead_tuples)
{
    BackgroundWorker bgw;
    BackgroundWorkerHandle *bgw_handle;
    uint16      generation;
    int         i;
    int         slot = 0;
    LogicalRepWorker *worker = NULL;
    int         nsyncworkers;
    int         nparallelapplyworkers;
    TimestampTz now;
    bool        is_tablesync_worker = (wtype == WORKERTYPE_TABLESYNC);
    bool        is_parallel_apply_worker = (wtype == WORKERTYPE_PARALLEL_APPLY);
 
    /*----------
     * Sanity checks:
     * - must be valid worker type
     * - tablesync workers are only ones to have relid
     * - parallel apply worker is the only kind of subworker
     * - The replication slot used in conflict detection is created when
     *   retain_dead_tuples is enabled
     */
    Assert(wtype != WORKERTYPE_UNKNOWN);
    Assert(is_tablesync_worker == OidIsValid(relid));
    Assert(is_parallel_apply_worker == (subworker_dsm != DSM_HANDLE_INVALID));
    Assert(!retain_dead_tuples || MyReplicationSlot);
 
    ereport(DEBUG1,
            (errmsg_internal("starting logical replication worker for subscription \"%s\"",
                             subname)));
 
    /* Report this after the initial starting message for consistency. */
    if (max_active_replication_origins == 0)
        ereport(ERROR,
                (errcode(ERRCODE_CONFIGURATION_LIMIT_EXCEEDED),
                 errmsg("cannot start logical replication workers when \"max_active_replication_origins\" is 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;
    }
 
    /*
     * We don't allow to invoke more sync workers once we have reached the
     * sync worker limit per subscription. So, just return silently as we
     * might get here because of an otherwise harmless race condition.
     */
    if (is_tablesync_worker && nsyncworkers >= max_sync_workers_per_subscription)
    {
        LWLockRelease(LogicalRepWorkerLock);
        return false;
    }
 
    nparallelapplyworkers = logicalrep_pa_worker_count(subid);
 
    /*
     * Return false if the number of parallel apply workers reached the limit
     * per subscription.
     */
    if (is_parallel_apply_worker &&
        nparallelapplyworkers >= max_parallel_apply_workers_per_subscription)
    {
        LWLockRelease(LogicalRepWorkerLock);
        return false;
    }
 
    /*
     * 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 \"%s\".", "max_logical_replication_workers")));
        return false;
    }
 
    /* Prepare the worker slot. */
    worker->type = wtype;
    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->stream_fileset = NULL;
    worker->leader_pid = is_parallel_apply_worker ? MyProcPid : InvalidPid;
    worker->parallel_apply = is_parallel_apply_worker;
    worker->oldest_nonremovable_xid = retain_dead_tuples
        ? MyReplicationSlot->data.xmin
        : InvalidTransactionId;
    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, MAXPGPATH, "postgres");
 
    switch (worker->type)
    {
        case WORKERTYPE_APPLY:
            snprintf(bgw.bgw_function_name, BGW_MAXLEN, "ApplyWorkerMain");
            snprintf(bgw.bgw_name, BGW_MAXLEN,
                     "logical replication apply worker for subscription %u",
                     subid);
            snprintf(bgw.bgw_type, BGW_MAXLEN, "logical replication apply worker");
            break;
 
        case WORKERTYPE_PARALLEL_APPLY:
            snprintf(bgw.bgw_function_name, BGW_MAXLEN, "ParallelApplyWorkerMain");
            snprintf(bgw.bgw_name, BGW_MAXLEN,
                     "logical replication parallel apply worker for subscription %u",
                     subid);
            snprintf(bgw.bgw_type, BGW_MAXLEN, "logical replication parallel worker");
 
            memcpy(bgw.bgw_extra, &subworker_dsm, sizeof(dsm_handle));
            break;
 
        case WORKERTYPE_TABLESYNC:
            snprintf(bgw.bgw_function_name, BGW_MAXLEN, "TablesyncWorkerMain");
            snprintf(bgw.bgw_name, BGW_MAXLEN,
                     "logical replication tablesync worker for subscription %u sync %u",
                     subid,
                     relid);
            snprintf(bgw.bgw_type, BGW_MAXLEN, "logical replication tablesync worker");
            break;
 
        case WORKERTYPE_UNKNOWN:
            /* Should never happen. */
            elog(ERROR, "unknown worker type");
    }
 
    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 \"%s\".", "max_worker_processes")));
        return false;
    }
 
    /* Now wait until it attaches. */
    return WaitForReplicationWorkerAttach(worker, generation, bgw_handle);
}
 
/*
 * Internal function to stop the worker and wait until it detaches from the
 * slot.
 */
static void
logicalrep_worker_stop_internal(LogicalRepWorker *worker, int signo)
{
    uint16      generation;
 
    Assert(LWLockHeldByMeInMode(LogicalRepWorkerLock, LW_SHARED));
 
    /*
     * 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)
            return;
 
        /* Worker has assigned proc, so it has started. */
        if (worker->proc)
            break;
    }
 
    /* Now terminate the worker ... */
    kill(worker->proc->pid, signo);
 
    /* ... 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);
    }
}
 
/*
 * Stop the logical replication worker for subid/relid, if any.
 */
void
logicalrep_worker_stop(Oid subid, Oid relid)
{
    LogicalRepWorker *worker;
 
    LWLockAcquire(LogicalRepWorkerLock, LW_SHARED);
 
    worker = logicalrep_worker_find(subid, relid, false);
 
    if (worker)
    {
        Assert(!isParallelApplyWorker(worker));
        logicalrep_worker_stop_internal(worker, SIGTERM);
    }
 
    LWLockRelease(LogicalRepWorkerLock);
}
 
/*
 * Stop the given logical replication parallel apply worker.
 *
 * Node that the function sends SIGUSR2 instead of SIGTERM to the parallel apply
 * worker so that the worker exits cleanly.
 */
void
logicalrep_pa_worker_stop(ParallelApplyWorkerInfo *winfo)
{
    int         slot_no;
    uint16      generation;
    LogicalRepWorker *worker;
 
    SpinLockAcquire(&winfo->shared->mutex);
    generation = winfo->shared->logicalrep_worker_generation;
    slot_no = winfo->shared->logicalrep_worker_slot_no;
    SpinLockRelease(&winfo->shared->mutex);
 
    Assert(slot_no >= 0 && slot_no < max_logical_replication_workers);
 
    /*
     * Detach from the error_mq_handle for the parallel apply worker before
     * stopping it. This prevents the leader apply worker from trying to
     * receive the message from the error queue that might already be detached
     * by the parallel apply worker.
     */
    if (winfo->error_mq_handle)
    {
        shm_mq_detach(winfo->error_mq_handle);
        winfo->error_mq_handle = NULL;
    }
 
    LWLockAcquire(LogicalRepWorkerLock, LW_SHARED);
 
    worker = &LogicalRepCtx->workers[slot_no];
    Assert(isParallelApplyWorker(worker));
 
    /*
     * Only stop the worker if the generation matches and the worker is alive.
     */
    if (worker->generation == generation && worker->proc)
        logicalrep_worker_stop_internal(worker, SIGUSR2);
 
    LWLockRelease(LogicalRepWorkerLock);
}
 
/*
 * 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);
}
 
/*
 * Stop the parallel apply workers if any, and detach the leader apply worker
 * (cleans up the worker info).
 */
static void
logicalrep_worker_detach(void)
{
    /* Stop the parallel apply workers. */
    if (am_leader_apply_worker())
    {
        List       *workers;
        ListCell   *lc;
 
        /*
         * Detach from the error_mq_handle for all parallel apply workers
         * before terminating them. This prevents the leader apply worker from
         * receiving the worker termination message and sending it to logs
         * when the same is already done by the parallel worker.
         */
        pa_detach_all_error_mq();
 
        LWLockAcquire(LogicalRepWorkerLock, LW_SHARED);
 
        workers = logicalrep_workers_find(MyLogicalRepWorker->subid, true, false);
        foreach(lc, workers)
        {
            LogicalRepWorker *w = (LogicalRepWorker *) lfirst(lc);
 
            if (isParallelApplyWorker(w))
                logicalrep_worker_stop_internal(w, SIGTERM);
        }
 
        LWLockRelease(LogicalRepWorkerLock);
 
        list_free(workers);
    }
 
    /* 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->type = WORKERTYPE_UNKNOWN;
    worker->in_use = false;
    worker->proc = NULL;
    worker->dbid = InvalidOid;
    worker->userid = InvalidOid;
    worker->subid = InvalidOid;
    worker->relid = InvalidOid;
    worker->leader_pid = InvalidPid;
    worker->parallel_apply = false;
}
 
/*
 * 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 (LogRepWorkerWalRcvConn)
        walrcv_disconnect(LogRepWorkerWalRcvConn);
 
    logicalrep_worker_detach();
 
    /* Cleanup fileset used for streaming transactions. */
    if (MyLogicalRepWorker->stream_fileset != NULL)
        FileSetDeleteAll(MyLogicalRepWorker->stream_fileset);
 
    /*
     * Session level locks may be acquired outside of a transaction in
     * parallel apply mode and will not be released when the worker
     * terminates, so manually release all locks before the worker exits.
     *
     * The locks will be acquired once the worker is initialized.
     */
    if (!InitializingApplyWorker)
        LockReleaseAll(DEFAULT_LOCKMETHOD, true);
 
    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 (isTablesyncWorker(w) && w->subid == subid)
            res++;
    }
 
    return res;
}
 
/*
 * Count the number of registered (but not necessarily running) parallel apply
 * workers for a subscription.
 */
static int
logicalrep_pa_worker_count(Oid subid)
{
    int         i;
    int         res = 0;
 
    Assert(LWLockHeldByMe(LogicalRepWorkerLock));
 
    /*
     * Scan all attached parallel apply workers, only counting those which
     * have the given subscription id.
     */
    for (i = 0; i < max_logical_replication_workers; i++)
    {
        LogicalRepWorker *w = &LogicalRepCtx->workers[i];
 
        if (isParallelApplyWorker(w) && w->subid == subid)
            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;
 
    /*
     * The logical replication launcher is disabled during binary upgrades, to
     * prevent logical replication workers from running on the source cluster.
     * That could cause replication origins to move forward after having been
     * copied to the target cluster, potentially creating conflicts with the
     * copied data files.
     */
    if (max_logical_replication_workers == 0 || IsBinaryUpgrade)
        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, MAXPGPATH, "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());
 
        LogicalRepCtx->last_start_dsa = DSA_HANDLE_INVALID;
        LogicalRepCtx->last_start_dsh = DSHASH_HANDLE_INVALID;
 
        /* 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);
        }
    }
}
 
/*
 * Initialize or attach to the dynamic shared hash table that stores the
 * last-start times, if not already done.
 * This must be called before accessing the table.
 */
static void
logicalrep_launcher_attach_dshmem(void)
{
    MemoryContext oldcontext;
 
    /* Quick exit if we already did this. */
    if (LogicalRepCtx->last_start_dsh != DSHASH_HANDLE_INVALID &&
        last_start_times != NULL)
        return;
 
    /* Otherwise, use a lock to ensure only one process creates the table. */
    LWLockAcquire(LogicalRepWorkerLock, LW_EXCLUSIVE);
 
    /* Be sure any local memory allocated by DSA routines is persistent. */
    oldcontext = MemoryContextSwitchTo(TopMemoryContext);
 
    if (LogicalRepCtx->last_start_dsh == DSHASH_HANDLE_INVALID)
    {
        /* Initialize dynamic shared hash table for last-start times. */
        last_start_times_dsa = dsa_create(LWTRANCHE_LAUNCHER_DSA);
        dsa_pin(last_start_times_dsa);
        dsa_pin_mapping(last_start_times_dsa);
        last_start_times = dshash_create(last_start_times_dsa, &dsh_params, NULL);
 
        /* Store handles in shared memory for other backends to use. */
        LogicalRepCtx->last_start_dsa = dsa_get_handle(last_start_times_dsa);
        LogicalRepCtx->last_start_dsh = dshash_get_hash_table_handle(last_start_times);
    }
    else if (!last_start_times)
    {
        /* Attach to existing dynamic shared hash table. */
        last_start_times_dsa = dsa_attach(LogicalRepCtx->last_start_dsa);
        dsa_pin_mapping(last_start_times_dsa);
        last_start_times = dshash_attach(last_start_times_dsa, &dsh_params,
                                         LogicalRepCtx->last_start_dsh, NULL);
    }
 
    MemoryContextSwitchTo(oldcontext);
    LWLockRelease(LogicalRepWorkerLock);
}
 
/*
 * Set the last-start time for the subscription.
 */
static void
ApplyLauncherSetWorkerStartTime(Oid subid, TimestampTz start_time)
{
    LauncherLastStartTimesEntry *entry;
    bool        found;
 
    logicalrep_launcher_attach_dshmem();
 
    entry = dshash_find_or_insert(last_start_times, &subid, &found);
    entry->last_start_time = start_time;
    dshash_release_lock(last_start_times, entry);
}
 
/*
 * Return the last-start time for the subscription, or 0 if there isn't one.
 */
static TimestampTz
ApplyLauncherGetWorkerStartTime(Oid subid)
{
    LauncherLastStartTimesEntry *entry;
    TimestampTz ret;
 
    logicalrep_launcher_attach_dshmem();
 
    entry = dshash_find(last_start_times, &subid, false);
    if (entry == NULL)
        return 0;
 
    ret = entry->last_start_time;
    dshash_release_lock(last_start_times, entry);
 
    return ret;
}
 
/*
 * Remove the last-start-time entry for the subscription, if one exists.
 *
 * This has two use-cases: to remove the entry related to a subscription
 * that's been deleted or disabled (just to avoid leaking shared memory),
 * and to allow immediate restart of an apply worker that has exited
 * due to subscription parameter changes.
 */
void
ApplyLauncherForgetWorkerStartTime(Oid subid)
{
    logicalrep_launcher_attach_dshmem();
 
    (void) dshash_delete_key(last_start_times, &subid);
}
 
/*
 * Wakeup the launcher on commit if requested.
 */
void
AtEOXact_ApplyLauncher(bool isCommit)
{
    if (isCommit)
    {
        if (on_commit_launcher_wakeup)
            ApplyLauncherWakeup();
    }
 
    on_commit_launcher_wakeup = false;
}
 
/*
 * 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;
}
 
/*
 * Wakeup the launcher immediately.
 */
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)
{
    ereport(DEBUG1,
            (errmsg_internal("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);
 
    /*
     * Acquire the conflict detection slot at startup to ensure it can be
     * dropped if no longer needed after a restart.
     */
    acquire_conflict_slot_if_exists();
 
    /* Enter main loop */
    for (;;)
    {
        int         rc;
        List       *sublist;
        ListCell   *lc;
        MemoryContext subctx;
        MemoryContext oldctx;
        long        wait_time = DEFAULT_NAPTIME_PER_CYCLE;
        bool        can_update_xmin = true;
        bool        retain_dead_tuples = false;
        TransactionId xmin = InvalidTransactionId;
 
        CHECK_FOR_INTERRUPTS();
 
        /* Use temporary context to avoid leaking memory across cycles. */
        subctx = AllocSetContextCreate(TopMemoryContext,
                                       "Logical Replication Launcher sublist",
                                       ALLOCSET_DEFAULT_SIZES);
        oldctx = MemoryContextSwitchTo(subctx);
 
        /*
         * Start any missing workers for enabled subscriptions.
         *
         * Also, during the iteration through all subscriptions, we compute
         * the minimum XID required to protect deleted tuples for conflict
         * detection if one of the subscription enables retain_dead_tuples
         * option.
         */
        sublist = get_subscription_list();
        foreach(lc, sublist)
        {
            Subscription *sub = (Subscription *) lfirst(lc);
            LogicalRepWorker *w;
            TimestampTz last_start;
            TimestampTz now;
            long        elapsed;
 
            if (sub->retaindeadtuples)
            {
                retain_dead_tuples = true;
 
                /*
                 * Create a replication slot to retain information necessary
                 * for conflict detection such as dead tuples, commit
                 * timestamps, and origins.
                 *
                 * The slot is created before starting the apply worker to
                 * prevent it from unnecessarily maintaining its
                 * oldest_nonremovable_xid.
                 *
                 * The slot is created even for a disabled subscription to
                 * ensure that conflict-related information is available when
                 * applying remote changes that occurred before the
                 * subscription was enabled.
                 */
                CreateConflictDetectionSlot();
 
                if (sub->retentionactive)
                {
                    /*
                     * Can't advance xmin of the slot unless all the
                     * subscriptions actively retaining dead tuples are
                     * enabled. This is required to ensure that we don't
                     * advance the xmin of CONFLICT_DETECTION_SLOT if one of
                     * the subscriptions is not enabled. Otherwise, we won't
                     * be able to detect conflicts reliably for such a
                     * subscription even though it has set the
                     * retain_dead_tuples option.
                     */
                    can_update_xmin &= sub->enabled;
 
                    /*
                     * Initialize the slot once the subscription activiates
                     * retention.
                     */
                    if (!TransactionIdIsValid(MyReplicationSlot->data.xmin))
                        init_conflict_slot_xmin();
                }
            }
 
            if (!sub->enabled)
                continue;
 
            LWLockAcquire(LogicalRepWorkerLock, LW_SHARED);
            w = logicalrep_worker_find(sub->oid, InvalidOid, false);
 
            if (w != NULL)
            {
                /*
                 * Compute the minimum xmin required to protect dead tuples
                 * required for conflict detection among all running apply
                 * workers. This computation is performed while holding
                 * LogicalRepWorkerLock to prevent accessing invalid worker
                 * data, in scenarios where a worker might exit and reset its
                 * state concurrently.
                 */
                if (sub->retaindeadtuples &&
                    sub->retentionactive &&
                    can_update_xmin)
                    compute_min_nonremovable_xid(w, &xmin);
 
                LWLockRelease(LogicalRepWorkerLock);
 
                /* worker is running already */
                continue;
            }
 
            LWLockRelease(LogicalRepWorkerLock);
 
            /*
             * Can't advance xmin of the slot unless all the workers
             * corresponding to subscriptions actively retaining dead tuples
             * are running, disabling the further computation of the minimum
             * nonremovable xid.
             */
            if (sub->retaindeadtuples && sub->retentionactive)
                can_update_xmin = false;
 
            /*
             * If the worker is eligible to start now, launch it.  Otherwise,
             * adjust wait_time so that we'll wake up as soon as it can be
             * started.
             *
             * Each subscription's apply worker can only be restarted once per
             * wal_retrieve_retry_interval, so that errors do not cause us to
             * repeatedly restart the worker as fast as possible.  In cases
             * where a restart is expected (e.g., subscription parameter
             * changes), another process should remove the last-start entry
             * for the subscription so that the worker can be restarted
             * without waiting for wal_retrieve_retry_interval to elapse.
             */
            last_start = ApplyLauncherGetWorkerStartTime(sub->oid);
            now = GetCurrentTimestamp();
            if (last_start == 0 ||
                (elapsed = TimestampDifferenceMilliseconds(last_start, now)) >= wal_retrieve_retry_interval)
            {
                ApplyLauncherSetWorkerStartTime(sub->oid, now);
                if (!logicalrep_worker_launch(WORKERTYPE_APPLY,
                                              sub->dbid, sub->oid, sub->name,
                                              sub->owner, InvalidOid,
                                              DSM_HANDLE_INVALID,
                                              sub->retaindeadtuples &&
                                              sub->retentionactive))
                {
                    /*
                     * We get here either if we failed to launch a worker
                     * (perhaps for resource-exhaustion reasons) or if we
                     * launched one but it immediately quit.  Either way, it
                     * seems appropriate to try again after
                     * wal_retrieve_retry_interval.
                     */
                    wait_time = Min(wait_time,
                                    wal_retrieve_retry_interval);
                }
            }
            else
            {
                wait_time = Min(wait_time,
                                wal_retrieve_retry_interval - elapsed);
            }
        }
 
        /*
         * Drop the CONFLICT_DETECTION_SLOT slot if there is no subscription
         * that requires us to retain dead tuples. Otherwise, if required,
         * advance the slot's xmin to protect dead tuples required for the
         * conflict detection.
         *
         * Additionally, if all apply workers for subscriptions with
         * retain_dead_tuples enabled have requested to stop retention, the
         * slot's xmin will be set to InvalidTransactionId allowing the
         * removal of dead tuples.
         */
        if (MyReplicationSlot)
        {
            if (!retain_dead_tuples)
                ReplicationSlotDropAcquired();
            else if (can_update_xmin)
                update_conflict_slot_xmin(xmin);
        }
 
        /* Switch back to original memory context. */
        MemoryContextSwitchTo(oldctx);
        /* Clean the temporary memory. */
        MemoryContextDelete(subctx);
 
        /* 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 */
}
 
/*
 * Determine the minimum non-removable transaction ID across all apply workers
 * for subscriptions that have retain_dead_tuples enabled. Store the result
 * in *xmin.
 */
static void
compute_min_nonremovable_xid(LogicalRepWorker *worker, TransactionId *xmin)
{
    TransactionId nonremovable_xid;
 
    Assert(worker != NULL);
 
    /*
     * The replication slot for conflict detection must be created before the
     * worker starts.
     */
    Assert(MyReplicationSlot);
 
    SpinLockAcquire(&worker->relmutex);
    nonremovable_xid = worker->oldest_nonremovable_xid;
    SpinLockRelease(&worker->relmutex);
 
    /*
     * Return if the apply worker has stopped retention concurrently.
     *
     * Although this function is invoked only when retentionactive is true,
     * the apply worker might stop retention after the launcher fetches the
     * retentionactive flag.
     */
    if (!TransactionIdIsValid(nonremovable_xid))
        return;
 
    if (!TransactionIdIsValid(*xmin) ||
        TransactionIdPrecedes(nonremovable_xid, *xmin))
        *xmin = nonremovable_xid;
}
 
/*
 * Acquire the replication slot used to retain information for conflict
 * detection, if it exists.
 *
 * Return true if successfully acquired, otherwise return false.
 */
static bool
acquire_conflict_slot_if_exists(void)
{
    if (!SearchNamedReplicationSlot(CONFLICT_DETECTION_SLOT, true))
        return false;
 
    ReplicationSlotAcquire(CONFLICT_DETECTION_SLOT, true, false);
    return true;
}
 
/*
 * Update the xmin the replication slot used to retain information required
 * for conflict detection.
 */
static void
update_conflict_slot_xmin(TransactionId new_xmin)
{
    Assert(MyReplicationSlot);
    Assert(!TransactionIdIsValid(new_xmin) ||
           TransactionIdPrecedesOrEquals(MyReplicationSlot->data.xmin, new_xmin));
 
    /* Return if the xmin value of the slot cannot be updated */
    if (TransactionIdEquals(MyReplicationSlot->data.xmin, new_xmin))
        return;
 
    SpinLockAcquire(&MyReplicationSlot->mutex);
    MyReplicationSlot->effective_xmin = new_xmin;
    MyReplicationSlot->data.xmin = new_xmin;
    SpinLockRelease(&MyReplicationSlot->mutex);
 
    elog(DEBUG1, "updated xmin: %u", MyReplicationSlot->data.xmin);
 
    ReplicationSlotMarkDirty();
    ReplicationSlotsComputeRequiredXmin(false);
 
    /*
     * Like PhysicalConfirmReceivedLocation(), do not save slot information
     * each time. This is acceptable because all concurrent transactions on
     * the publisher that require the data preceding the slot's xmin should
     * have already been applied and flushed on the subscriber before the xmin
     * is advanced. So, even if the slot's xmin regresses after a restart, it
     * will be advanced again in the next cycle. Therefore, no data required
     * for conflict detection will be prematurely removed.
     */
    return;
}
 
/*
 * Initialize the xmin for the conflict detection slot.
 */
static void
init_conflict_slot_xmin(void)
{
    TransactionId xmin_horizon;
 
    /* Replication slot must exist but shouldn't be initialized. */
    Assert(MyReplicationSlot &&
           !TransactionIdIsValid(MyReplicationSlot->data.xmin));
 
    LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
 
    xmin_horizon = GetOldestSafeDecodingTransactionId(false);
 
    SpinLockAcquire(&MyReplicationSlot->mutex);
    MyReplicationSlot->effective_xmin = xmin_horizon;
    MyReplicationSlot->data.xmin = xmin_horizon;
    SpinLockRelease(&MyReplicationSlot->mutex);
 
    ReplicationSlotsComputeRequiredXmin(true);
 
    LWLockRelease(ProcArrayLock);
 
    /* Write this slot to disk */
    ReplicationSlotMarkDirty();
    ReplicationSlotSave();
}
 
/*
 * Create and acquire the replication slot used to retain information for
 * conflict detection, if not yet.
 */
void
CreateConflictDetectionSlot(void)
{
    /* Exit early, if the replication slot is already created and acquired */
    if (MyReplicationSlot)
        return;
 
    ereport(LOG,
            errmsg("creating replication conflict detection slot"));
 
    ReplicationSlotCreate(CONFLICT_DETECTION_SLOT, false, RS_PERSISTENT, false,
                          false, false);
 
    init_conflict_slot_xmin();
}
 
/*
 * Is current process the logical replication launcher?
 */
bool
IsLogicalLauncher(void)
{
    return LogicalRepCtx->launcher_pid == MyProcPid;
}
 
/*
 * Return the pid of the leader apply worker if the given pid is the pid of a
 * parallel apply worker, otherwise, return InvalidPid.
 */
pid_t
GetLeaderApplyWorkerPid(pid_t pid)
{
    int         leader_pid = InvalidPid;
    int         i;
 
    LWLockAcquire(LogicalRepWorkerLock, LW_SHARED);
 
    for (i = 0; i < max_logical_replication_workers; i++)
    {
        LogicalRepWorker *w = &LogicalRepCtx->workers[i];
 
        if (isParallelApplyWorker(w) && w->proc && pid == w->proc->pid)
        {
            leader_pid = w->leader_pid;
            break;
        }
    }
 
    LWLockRelease(LogicalRepWorkerLock);
 
    return leader_pid;
}
 
/*
 * Returns state of the subscriptions.
 */
Datum
pg_stat_get_subscription(PG_FUNCTION_ARGS)
{
#define PG_STAT_GET_SUBSCRIPTION_COLS   10
    Oid         subid = PG_ARGISNULL(0) ? InvalidOid : PG_GETARG_OID(0);
    int         i;
    ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
 
    InitMaterializedSRF(fcinfo, 0);
 
    /* 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] = {0};
        bool        nulls[PG_STAT_GET_SUBSCRIPTION_COLS] = {0};
        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;
 
        values[0] = ObjectIdGetDatum(worker.subid);
        if (isTablesyncWorker(&worker))
            values[1] = ObjectIdGetDatum(worker.relid);
        else
            nulls[1] = true;
        values[2] = Int32GetDatum(worker_pid);
 
        if (isParallelApplyWorker(&worker))
            values[3] = Int32GetDatum(worker.leader_pid);
        else
            nulls[3] = true;
 
        if (XLogRecPtrIsInvalid(worker.last_lsn))
            nulls[4] = true;
        else
            values[4] = LSNGetDatum(worker.last_lsn);
        if (worker.last_send_time == 0)
            nulls[5] = true;
        else
            values[5] = TimestampTzGetDatum(worker.last_send_time);
        if (worker.last_recv_time == 0)
            nulls[6] = true;
        else
            values[6] = TimestampTzGetDatum(worker.last_recv_time);
        if (XLogRecPtrIsInvalid(worker.reply_lsn))
            nulls[7] = true;
        else
            values[7] = LSNGetDatum(worker.reply_lsn);
        if (worker.reply_time == 0)
            nulls[8] = true;
        else
            values[8] = TimestampTzGetDatum(worker.reply_time);
 
        switch (worker.type)
        {
            case WORKERTYPE_APPLY:
                values[9] = CStringGetTextDatum("apply");
                break;
            case WORKERTYPE_PARALLEL_APPLY:
                values[9] = CStringGetTextDatum("parallel apply");
                break;
            case WORKERTYPE_TABLESYNC:
                values[9] = CStringGetTextDatum("table synchronization");
                break;
            case WORKERTYPE_UNKNOWN:
                /* Should never happen. */
                elog(ERROR, "unknown worker type");
        }
 
        tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc,
                             values, nulls);
 
        /*
         * If only a single subscription was requested, and we found it,
         * break.
         */
        if (OidIsValid(subid))
            break;
    }
 
    LWLockRelease(LogicalRepWorkerLock);
 
    return (Datum) 0;
}