bgworker.c

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/*--------------------------------------------------------------------
 * bgworker.c
 *      POSTGRES pluggable background workers implementation
 *
 * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *    src/backend/postmaster/bgworker.c
 *
 *-------------------------------------------------------------------------
 */
 
#include "postgres.h"
 
#include "access/parallel.h"
#include "libpq/pqsignal.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "port/atomics.h"
#include "postmaster/bgworker_internals.h"
#include "postmaster/postmaster.h"
#include "replication/logicallauncher.h"
#include "replication/logicalworker.h"
#include "storage/ipc.h"
#include "storage/latch.h"
#include "storage/lwlock.h"
#include "storage/pmsignal.h"
#include "storage/proc.h"
#include "storage/procsignal.h"
#include "storage/shmem.h"
#include "tcop/tcopprot.h"
#include "utils/ascii.h"
#include "utils/memutils.h"
#include "utils/ps_status.h"
#include "utils/timeout.h"
 
/*
 * The postmaster's list of registered background workers, in private memory.
 */
slist_head  BackgroundWorkerList = SLIST_STATIC_INIT(BackgroundWorkerList);
 
/*
 * BackgroundWorkerSlots exist in shared memory and can be accessed (via
 * the BackgroundWorkerArray) by both the postmaster and by regular backends.
 * However, the postmaster cannot take locks, even spinlocks, because this
 * might allow it to crash or become wedged if shared memory gets corrupted.
 * Such an outcome is intolerable.  Therefore, we need a lockless protocol
 * for coordinating access to this data.
 *
 * The 'in_use' flag is used to hand off responsibility for the slot between
 * the postmaster and the rest of the system.  When 'in_use' is false,
 * the postmaster will ignore the slot entirely, except for the 'in_use' flag
 * itself, which it may read.  In this state, regular backends may modify the
 * slot.  Once a backend sets 'in_use' to true, the slot becomes the
 * responsibility of the postmaster.  Regular backends may no longer modify it,
 * but the postmaster may examine it.  Thus, a backend initializing a slot
 * must fully initialize the slot - and insert a write memory barrier - before
 * marking it as in use.
 *
 * As an exception, however, even when the slot is in use, regular backends
 * may set the 'terminate' flag for a slot, telling the postmaster not
 * to restart it.  Once the background worker is no longer running, the slot
 * will be released for reuse.
 *
 * In addition to coordinating with the postmaster, backends modifying this
 * data structure must coordinate with each other.  Since they can take locks,
 * this is straightforward: any backend wishing to manipulate a slot must
 * take BackgroundWorkerLock in exclusive mode.  Backends wishing to read
 * data that might get concurrently modified by other backends should take
 * this lock in shared mode.  No matter what, backends reading this data
 * structure must be able to tolerate concurrent modifications by the
 * postmaster.
 */
typedef struct BackgroundWorkerSlot
{
    bool        in_use;
    bool        terminate;
    pid_t       pid;            /* InvalidPid = not started yet; 0 = dead */
    uint64      generation;     /* incremented when slot is recycled */
    BackgroundWorker worker;
} BackgroundWorkerSlot;
 
/*
 * In order to limit the total number of parallel workers (according to
 * max_parallel_workers GUC), we maintain the number of active parallel
 * workers.  Since the postmaster cannot take locks, two variables are used for
 * this purpose: the number of registered parallel workers (modified by the
 * backends, protected by BackgroundWorkerLock) and the number of terminated
 * parallel workers (modified only by the postmaster, lockless).  The active
 * number of parallel workers is the number of registered workers minus the
 * terminated ones.  These counters can of course overflow, but it's not
 * important here since the subtraction will still give the right number.
 */
typedef struct BackgroundWorkerArray
{
    int         total_slots;
    uint32      parallel_register_count;
    uint32      parallel_terminate_count;
    BackgroundWorkerSlot slot[FLEXIBLE_ARRAY_MEMBER];
} BackgroundWorkerArray;
 
struct BackgroundWorkerHandle
{
    int         slot;
    uint64      generation;
};
 
static BackgroundWorkerArray *BackgroundWorkerData;
 
/*
 * List of internal background worker entry points.  We need this for
 * reasons explained in LookupBackgroundWorkerFunction(), below.
 */
static const struct
{
    const char *fn_name;
    bgworker_main_type fn_addr;
}           InternalBGWorkers[] =
 
{
    {
        "ParallelWorkerMain", ParallelWorkerMain
    },
    {
        "ApplyLauncherMain", ApplyLauncherMain
    },
    {
        "ApplyWorkerMain", ApplyWorkerMain
    },
    {
        "ParallelApplyWorkerMain", ParallelApplyWorkerMain
    },
    {
        "TablesyncWorkerMain", TablesyncWorkerMain
    }
};
 
/* Private functions. */
static bgworker_main_type LookupBackgroundWorkerFunction(const char *libraryname, const char *funcname);
 
 
/*
 * Calculate shared memory needed.
 */
Size
BackgroundWorkerShmemSize(void)
{
    Size        size;
 
    /* Array of workers is variably sized. */
    size = offsetof(BackgroundWorkerArray, slot);
    size = add_size(size, mul_size(max_worker_processes,
                                   sizeof(BackgroundWorkerSlot)));
 
    return size;
}
 
/*
 * Initialize shared memory.
 */
void
BackgroundWorkerShmemInit(void)
{
    bool        found;
 
    BackgroundWorkerData = ShmemInitStruct("Background Worker Data",
                                           BackgroundWorkerShmemSize(),
                                           &found);
    if (!IsUnderPostmaster)
    {
        slist_iter  siter;
        int         slotno = 0;
 
        BackgroundWorkerData->total_slots = max_worker_processes;
        BackgroundWorkerData->parallel_register_count = 0;
        BackgroundWorkerData->parallel_terminate_count = 0;
 
        /*
         * Copy contents of worker list into shared memory.  Record the shared
         * memory slot assigned to each worker.  This ensures a 1-to-1
         * correspondence between the postmaster's private list and the array
         * in shared memory.
         */
        slist_foreach(siter, &BackgroundWorkerList)
        {
            BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
            RegisteredBgWorker *rw;
 
            rw = slist_container(RegisteredBgWorker, rw_lnode, siter.cur);
            Assert(slotno < max_worker_processes);
            slot->in_use = true;
            slot->terminate = false;
            slot->pid = InvalidPid;
            slot->generation = 0;
            rw->rw_shmem_slot = slotno;
            rw->rw_worker.bgw_notify_pid = 0;   /* might be reinit after crash */
            memcpy(&slot->worker, &rw->rw_worker, sizeof(BackgroundWorker));
            ++slotno;
        }
 
        /*
         * Mark any remaining slots as not in use.
         */
        while (slotno < max_worker_processes)
        {
            BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
 
            slot->in_use = false;
            ++slotno;
        }
    }
    else
        Assert(found);
}
 
/*
 * Search the postmaster's backend-private list of RegisteredBgWorker objects
 * for the one that maps to the given slot number.
 */
static RegisteredBgWorker *
FindRegisteredWorkerBySlotNumber(int slotno)
{
    slist_iter  siter;
 
    slist_foreach(siter, &BackgroundWorkerList)
    {
        RegisteredBgWorker *rw;
 
        rw = slist_container(RegisteredBgWorker, rw_lnode, siter.cur);
        if (rw->rw_shmem_slot == slotno)
            return rw;
    }
 
    return NULL;
}
 
/*
 * Notice changes to shared memory made by other backends.
 * Accept new worker requests only if allow_new_workers is true.
 *
 * This code runs in the postmaster, so we must be very careful not to assume
 * that shared memory contents are sane.  Otherwise, a rogue backend could
 * take out the postmaster.
 */
void
BackgroundWorkerStateChange(bool allow_new_workers)
{
    int         slotno;
 
    /*
     * The total number of slots stored in shared memory should match our
     * notion of max_worker_processes.  If it does not, something is very
     * wrong.  Further down, we always refer to this value as
     * max_worker_processes, in case shared memory gets corrupted while we're
     * looping.
     */
    if (max_worker_processes != BackgroundWorkerData->total_slots)
    {
        ereport(LOG,
                (errmsg("inconsistent background worker state (max_worker_processes=%d, total_slots=%d)",
                        max_worker_processes,
                        BackgroundWorkerData->total_slots)));
        return;
    }
 
    /*
     * Iterate through slots, looking for newly-registered workers or workers
     * who must die.
     */
    for (slotno = 0; slotno < max_worker_processes; ++slotno)
    {
        BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
        RegisteredBgWorker *rw;
 
        if (!slot->in_use)
            continue;
 
        /*
         * Make sure we don't see the in_use flag before the updated slot
         * contents.
         */
        pg_read_barrier();
 
        /* See whether we already know about this worker. */
        rw = FindRegisteredWorkerBySlotNumber(slotno);
        if (rw != NULL)
        {
            /*
             * In general, the worker data can't change after it's initially
             * registered.  However, someone can set the terminate flag.
             */
            if (slot->terminate && !rw->rw_terminate)
            {
                rw->rw_terminate = true;
                if (rw->rw_pid != 0)
                    kill(rw->rw_pid, SIGTERM);
                else
                {
                    /* Report never-started, now-terminated worker as dead. */
                    ReportBackgroundWorkerPID(rw);
                }
            }
            continue;
        }
 
        /*
         * If we aren't allowing new workers, then immediately mark it for
         * termination; the next stanza will take care of cleaning it up.
         * Doing this ensures that any process waiting for the worker will get
         * awoken, even though the worker will never be allowed to run.
         */
        if (!allow_new_workers)
            slot->terminate = true;
 
        /*
         * If the worker is marked for termination, we don't need to add it to
         * the registered workers list; we can just free the slot. However, if
         * bgw_notify_pid is set, the process that registered the worker may
         * need to know that we've processed the terminate request, so be sure
         * to signal it.
         */
        if (slot->terminate)
        {
            int         notify_pid;
 
            /*
             * We need a memory barrier here to make sure that the load of
             * bgw_notify_pid and the update of parallel_terminate_count
             * complete before the store to in_use.
             */
            notify_pid = slot->worker.bgw_notify_pid;
            if ((slot->worker.bgw_flags & BGWORKER_CLASS_PARALLEL) != 0)
                BackgroundWorkerData->parallel_terminate_count++;
            slot->pid = 0;
 
            pg_memory_barrier();
            slot->in_use = false;
 
            if (notify_pid != 0)
                kill(notify_pid, SIGUSR1);
 
            continue;
        }
 
        /*
         * Copy the registration data into the registered workers list.
         */
        rw = MemoryContextAllocExtended(PostmasterContext,
                                        sizeof(RegisteredBgWorker),
                                        MCXT_ALLOC_NO_OOM | MCXT_ALLOC_ZERO);
        if (rw == NULL)
        {
            ereport(LOG,
                    (errcode(ERRCODE_OUT_OF_MEMORY),
                     errmsg("out of memory")));
            return;
        }
 
        /*
         * Copy strings in a paranoid way.  If shared memory is corrupted, the
         * source data might not even be NUL-terminated.
         */
        ascii_safe_strlcpy(rw->rw_worker.bgw_name,
                           slot->worker.bgw_name, BGW_MAXLEN);
        ascii_safe_strlcpy(rw->rw_worker.bgw_type,
                           slot->worker.bgw_type, BGW_MAXLEN);
        ascii_safe_strlcpy(rw->rw_worker.bgw_library_name,
                           slot->worker.bgw_library_name, MAXPGPATH);
        ascii_safe_strlcpy(rw->rw_worker.bgw_function_name,
                           slot->worker.bgw_function_name, BGW_MAXLEN);
 
        /*
         * Copy various fixed-size fields.
         *
         * flags, start_time, and restart_time are examined by the postmaster,
         * but nothing too bad will happen if they are corrupted.  The
         * remaining fields will only be examined by the child process.  It
         * might crash, but we won't.
         */
        rw->rw_worker.bgw_flags = slot->worker.bgw_flags;
        rw->rw_worker.bgw_start_time = slot->worker.bgw_start_time;
        rw->rw_worker.bgw_restart_time = slot->worker.bgw_restart_time;
        rw->rw_worker.bgw_main_arg = slot->worker.bgw_main_arg;
        memcpy(rw->rw_worker.bgw_extra, slot->worker.bgw_extra, BGW_EXTRALEN);
 
        /*
         * Copy the PID to be notified about state changes, but only if the
         * postmaster knows about a backend with that PID.  It isn't an error
         * if the postmaster doesn't know about the PID, because the backend
         * that requested the worker could have died (or been killed) just
         * after doing so.  Nonetheless, at least until we get some experience
         * with how this plays out in the wild, log a message at a relative
         * high debug level.
         */
        rw->rw_worker.bgw_notify_pid = slot->worker.bgw_notify_pid;
        if (!PostmasterMarkPIDForWorkerNotify(rw->rw_worker.bgw_notify_pid))
        {
            elog(DEBUG1, "worker notification PID %d is not valid",
                 (int) rw->rw_worker.bgw_notify_pid);
            rw->rw_worker.bgw_notify_pid = 0;
        }
 
        /* Initialize postmaster bookkeeping. */
        rw->rw_backend = NULL;
        rw->rw_pid = 0;
        rw->rw_child_slot = 0;
        rw->rw_crashed_at = 0;
        rw->rw_shmem_slot = slotno;
        rw->rw_terminate = false;
 
        /* Log it! */
        ereport(DEBUG1,
                (errmsg_internal("registering background worker \"%s\"",
                                 rw->rw_worker.bgw_name)));
 
        slist_push_head(&BackgroundWorkerList, &rw->rw_lnode);
    }
}
 
/*
 * Forget about a background worker that's no longer needed.
 *
 * The worker must be identified by passing an slist_mutable_iter that
 * points to it.  This convention allows deletion of workers during
 * searches of the worker list, and saves having to search the list again.
 *
 * Caller is responsible for notifying bgw_notify_pid, if appropriate.
 *
 * This function must be invoked only in the postmaster.
 */
void
ForgetBackgroundWorker(slist_mutable_iter *cur)
{
    RegisteredBgWorker *rw;
    BackgroundWorkerSlot *slot;
 
    rw = slist_container(RegisteredBgWorker, rw_lnode, cur->cur);
 
    Assert(rw->rw_shmem_slot < max_worker_processes);
    slot = &BackgroundWorkerData->slot[rw->rw_shmem_slot];
    Assert(slot->in_use);
 
    /*
     * We need a memory barrier here to make sure that the update of
     * parallel_terminate_count completes before the store to in_use.
     */
    if ((rw->rw_worker.bgw_flags & BGWORKER_CLASS_PARALLEL) != 0)
        BackgroundWorkerData->parallel_terminate_count++;
 
    pg_memory_barrier();
    slot->in_use = false;
 
    ereport(DEBUG1,
            (errmsg_internal("unregistering background worker \"%s\"",
                             rw->rw_worker.bgw_name)));
 
    slist_delete_current(cur);
    pfree(rw);
}
 
/*
 * Report the PID of a newly-launched background worker in shared memory.
 *
 * This function should only be called from the postmaster.
 */
void
ReportBackgroundWorkerPID(RegisteredBgWorker *rw)
{
    BackgroundWorkerSlot *slot;
 
    Assert(rw->rw_shmem_slot < max_worker_processes);
    slot = &BackgroundWorkerData->slot[rw->rw_shmem_slot];
    slot->pid = rw->rw_pid;
 
    if (rw->rw_worker.bgw_notify_pid != 0)
        kill(rw->rw_worker.bgw_notify_pid, SIGUSR1);
}
 
/*
 * Report that the PID of a background worker is now zero because a
 * previously-running background worker has exited.
 *
 * This function should only be called from the postmaster.
 */
void
ReportBackgroundWorkerExit(slist_mutable_iter *cur)
{
    RegisteredBgWorker *rw;
    BackgroundWorkerSlot *slot;
    int         notify_pid;
 
    rw = slist_container(RegisteredBgWorker, rw_lnode, cur->cur);
 
    Assert(rw->rw_shmem_slot < max_worker_processes);
    slot = &BackgroundWorkerData->slot[rw->rw_shmem_slot];
    slot->pid = rw->rw_pid;
    notify_pid = rw->rw_worker.bgw_notify_pid;
 
    /*
     * If this worker is slated for deregistration, do that before notifying
     * the process which started it.  Otherwise, if that process tries to
     * reuse the slot immediately, it might not be available yet.  In theory
     * that could happen anyway if the process checks slot->pid at just the
     * wrong moment, but this makes the window narrower.
     */
    if (rw->rw_terminate ||
        rw->rw_worker.bgw_restart_time == BGW_NEVER_RESTART)
        ForgetBackgroundWorker(cur);
 
    if (notify_pid != 0)
        kill(notify_pid, SIGUSR1);
}
 
/*
 * Cancel SIGUSR1 notifications for a PID belonging to an exiting backend.
 *
 * This function should only be called from the postmaster.
 */
void
BackgroundWorkerStopNotifications(pid_t pid)
{
    slist_iter  siter;
 
    slist_foreach(siter, &BackgroundWorkerList)
    {
        RegisteredBgWorker *rw;
 
        rw = slist_container(RegisteredBgWorker, rw_lnode, siter.cur);
        if (rw->rw_worker.bgw_notify_pid == pid)
            rw->rw_worker.bgw_notify_pid = 0;
    }
}
 
/*
 * Cancel any not-yet-started worker requests that have waiting processes.
 *
 * This is called during a normal ("smart" or "fast") database shutdown.
 * After this point, no new background workers will be started, so anything
 * that might be waiting for them needs to be kicked off its wait.  We do
 * that by canceling the bgworker registration entirely, which is perhaps
 * overkill, but since we're shutting down it does not matter whether the
 * registration record sticks around.
 *
 * This function should only be called from the postmaster.
 */
void
ForgetUnstartedBackgroundWorkers(void)
{
    slist_mutable_iter iter;
 
    slist_foreach_modify(iter, &BackgroundWorkerList)
    {
        RegisteredBgWorker *rw;
        BackgroundWorkerSlot *slot;
 
        rw = slist_container(RegisteredBgWorker, rw_lnode, iter.cur);
        Assert(rw->rw_shmem_slot < max_worker_processes);
        slot = &BackgroundWorkerData->slot[rw->rw_shmem_slot];
 
        /* If it's not yet started, and there's someone waiting ... */
        if (slot->pid == InvalidPid &&
            rw->rw_worker.bgw_notify_pid != 0)
        {
            /* ... then zap it, and notify the waiter */
            int         notify_pid = rw->rw_worker.bgw_notify_pid;
 
            ForgetBackgroundWorker(&iter);
            if (notify_pid != 0)
                kill(notify_pid, SIGUSR1);
        }
    }
}
 
/*
 * Reset background worker crash state.
 *
 * We assume that, after a crash-and-restart cycle, background workers without
 * the never-restart flag should be restarted immediately, instead of waiting
 * for bgw_restart_time to elapse.  On the other hand, workers with that flag
 * should be forgotten immediately, since we won't ever restart them.
 *
 * This function should only be called from the postmaster.
 */
void
ResetBackgroundWorkerCrashTimes(void)
{
    slist_mutable_iter iter;
 
    slist_foreach_modify(iter, &BackgroundWorkerList)
    {
        RegisteredBgWorker *rw;
 
        rw = slist_container(RegisteredBgWorker, rw_lnode, iter.cur);
 
        if (rw->rw_worker.bgw_restart_time == BGW_NEVER_RESTART)
        {
            /*
             * Workers marked BGW_NEVER_RESTART shouldn't get relaunched after
             * the crash, so forget about them.  (If we wait until after the
             * crash to forget about them, and they are parallel workers,
             * parallel_terminate_count will get incremented after we've
             * already zeroed parallel_register_count, which would be bad.)
             */
            ForgetBackgroundWorker(&iter);
        }
        else
        {
            /*
             * The accounting which we do via parallel_register_count and
             * parallel_terminate_count would get messed up if a worker marked
             * parallel could survive a crash and restart cycle. All such
             * workers should be marked BGW_NEVER_RESTART, and thus control
             * should never reach this branch.
             */
            Assert((rw->rw_worker.bgw_flags & BGWORKER_CLASS_PARALLEL) == 0);
 
            /*
             * Allow this worker to be restarted immediately after we finish
             * resetting.
             */
            rw->rw_crashed_at = 0;
 
            /*
             * If there was anyone waiting for it, they're history.
             */
            rw->rw_worker.bgw_notify_pid = 0;
        }
    }
}
 
/*
 * Complain about the BackgroundWorker definition using error level elevel.
 * Return true if it looks ok, false if not (unless elevel >= ERROR, in
 * which case we won't return at all in the not-OK case).
 */
static bool
SanityCheckBackgroundWorker(BackgroundWorker *worker, int elevel)
{
    /* sanity check for flags */
 
    /*
     * We used to support workers not connected to shared memory, but don't
     * anymore. Thus this is a required flag now. We're not removing the flag
     * for compatibility reasons and because the flag still provides some
     * signal when reading code.
     */
    if (!(worker->bgw_flags & BGWORKER_SHMEM_ACCESS))
    {
        ereport(elevel,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("background worker \"%s\": background workers without shared memory access are not supported",
                        worker->bgw_name)));
        return false;
    }
 
    if (worker->bgw_flags & BGWORKER_BACKEND_DATABASE_CONNECTION)
    {
        if (worker->bgw_start_time == BgWorkerStart_PostmasterStart)
        {
            ereport(elevel,
                    (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                     errmsg("background worker \"%s\": cannot request database access if starting at postmaster start",
                            worker->bgw_name)));
            return false;
        }
 
        /* XXX other checks? */
    }
 
    if ((worker->bgw_restart_time < 0 &&
         worker->bgw_restart_time != BGW_NEVER_RESTART) ||
        (worker->bgw_restart_time > USECS_PER_DAY / 1000))
    {
        ereport(elevel,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("background worker \"%s\": invalid restart interval",
                        worker->bgw_name)));
        return false;
    }
 
    /*
     * Parallel workers may not be configured for restart, because the
     * parallel_register_count/parallel_terminate_count accounting can't
     * handle parallel workers lasting through a crash-and-restart cycle.
     */
    if (worker->bgw_restart_time != BGW_NEVER_RESTART &&
        (worker->bgw_flags & BGWORKER_CLASS_PARALLEL) != 0)
    {
        ereport(elevel,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("background worker \"%s\": parallel workers may not be configured for restart",
                        worker->bgw_name)));
        return false;
    }
 
    /*
     * If bgw_type is not filled in, use bgw_name.
     */
    if (strcmp(worker->bgw_type, "") == 0)
        strcpy(worker->bgw_type, worker->bgw_name);
 
    return true;
}
 
/*
 * Standard SIGTERM handler for background workers
 */
static void
bgworker_die(SIGNAL_ARGS)
{
    sigprocmask(SIG_SETMASK, &BlockSig, NULL);
 
    ereport(FATAL,
            (errcode(ERRCODE_ADMIN_SHUTDOWN),
             errmsg("terminating background worker \"%s\" due to administrator command",
                    MyBgworkerEntry->bgw_type)));
}
 
/*
 * Main entry point for background worker processes.
 */
void
BackgroundWorkerMain(char *startup_data, size_t startup_data_len)
{
    sigjmp_buf  local_sigjmp_buf;
    BackgroundWorker *worker;
    bgworker_main_type entrypt;
 
    if (startup_data == NULL)
        elog(FATAL, "unable to find bgworker entry");
    Assert(startup_data_len == sizeof(BackgroundWorker));
    worker = MemoryContextAlloc(TopMemoryContext, sizeof(BackgroundWorker));
    memcpy(worker, startup_data, sizeof(BackgroundWorker));
 
    /*
     * Now that we're done reading the startup data, release postmaster's
     * working memory context.
     */
    if (PostmasterContext)
    {
        MemoryContextDelete(PostmasterContext);
        PostmasterContext = NULL;
    }
 
    MyBgworkerEntry = worker;
    MyBackendType = B_BG_WORKER;
    init_ps_display(worker->bgw_name);
 
    SetProcessingMode(InitProcessing);
 
    /* Apply PostAuthDelay */
    if (PostAuthDelay > 0)
        pg_usleep(PostAuthDelay * 1000000L);
 
    /*
     * Set up signal handlers.
     */
    if (worker->bgw_flags & BGWORKER_BACKEND_DATABASE_CONNECTION)
    {
        /*
         * SIGINT is used to signal canceling the current action
         */
        pqsignal(SIGINT, StatementCancelHandler);
        pqsignal(SIGUSR1, procsignal_sigusr1_handler);
        pqsignal(SIGFPE, FloatExceptionHandler);
 
        /* XXX Any other handlers needed here? */
    }
    else
    {
        pqsignal(SIGINT, SIG_IGN);
        pqsignal(SIGUSR1, SIG_IGN);
        pqsignal(SIGFPE, SIG_IGN);
    }
    pqsignal(SIGTERM, bgworker_die);
    /* SIGQUIT handler was already set up by InitPostmasterChild */
    pqsignal(SIGHUP, SIG_IGN);
 
    InitializeTimeouts();       /* establishes SIGALRM handler */
 
    pqsignal(SIGPIPE, SIG_IGN);
    pqsignal(SIGUSR2, SIG_IGN);
    pqsignal(SIGCHLD, SIG_DFL);
 
    /*
     * If an exception is encountered, processing resumes here.
     *
     * We just need to clean up, report the error, and go away.
     */
    if (sigsetjmp(local_sigjmp_buf, 1) != 0)
    {
        /* Since not using PG_TRY, must reset error stack by hand */
        error_context_stack = NULL;
 
        /* Prevent interrupts while cleaning up */
        HOLD_INTERRUPTS();
 
        /*
         * sigsetjmp will have blocked all signals, but we may need to accept
         * signals while communicating with our parallel leader.  Once we've
         * done HOLD_INTERRUPTS() it should be safe to unblock signals.
         */
        BackgroundWorkerUnblockSignals();
 
        /* Report the error to the parallel leader and the server log */
        EmitErrorReport();
 
        /*
         * Do we need more cleanup here?  For shmem-connected bgworkers, we
         * will call InitProcess below, which will install ProcKill as exit
         * callback.  That will take care of releasing locks, etc.
         */
 
        /* and go away */
        proc_exit(1);
    }
 
    /* We can now handle ereport(ERROR) */
    PG_exception_stack = &local_sigjmp_buf;
 
    /*
     * Create a per-backend PGPROC struct in shared memory.  We must do this
     * before we can use LWLocks or access any shared memory.
     */
    InitProcess();
 
    /*
     * Early initialization.
     */
    BaseInit();
 
    /*
     * Look up the entry point function, loading its library if necessary.
     */
    entrypt = LookupBackgroundWorkerFunction(worker->bgw_library_name,
                                             worker->bgw_function_name);
 
    /*
     * Note that in normal processes, we would call InitPostgres here.  For a
     * worker, however, we don't know what database to connect to, yet; so we
     * need to wait until the user code does it via
     * BackgroundWorkerInitializeConnection().
     */
 
    /*
     * Now invoke the user-defined worker code
     */
    entrypt(worker->bgw_main_arg);
 
    /* ... and if it returns, we're done */
    proc_exit(0);
}
 
/*
 * Register a new static background worker.
 *
 * This can only be called directly from postmaster or in the _PG_init
 * function of a module library that's loaded by shared_preload_libraries;
 * otherwise it will have no effect.
 */
void
RegisterBackgroundWorker(BackgroundWorker *worker)
{
    RegisteredBgWorker *rw;
    static int  numworkers = 0;
 
    /*
     * Static background workers can only be registered in the postmaster
     * process.
     */
    if (IsUnderPostmaster || !IsPostmasterEnvironment)
    {
        /*
         * In EXEC_BACKEND or single-user mode, we process
         * shared_preload_libraries in backend processes too.  We cannot
         * register static background workers at that stage, but many
         * libraries' _PG_init() functions don't distinguish whether they're
         * being loaded in the postmaster or in a backend, they just check
         * process_shared_preload_libraries_in_progress.  It's a bit sloppy,
         * but for historical reasons we tolerate it.  In EXEC_BACKEND mode,
         * the background workers should already have been registered when the
         * library was loaded in postmaster.
         */
        if (process_shared_preload_libraries_in_progress)
            return;
        ereport(LOG,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("background worker \"%s\": must be registered in shared_preload_libraries",
                        worker->bgw_name)));
        return;
    }
 
    /*
     * Cannot register static background workers after calling
     * BackgroundWorkerShmemInit().
     */
    if (BackgroundWorkerData != NULL)
        elog(ERROR, "cannot register background worker \"%s\" after shmem init",
             worker->bgw_name);
 
    ereport(DEBUG1,
            (errmsg_internal("registering background worker \"%s\"", worker->bgw_name)));
 
    if (!SanityCheckBackgroundWorker(worker, LOG))
        return;
 
    if (worker->bgw_notify_pid != 0)
    {
        ereport(LOG,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("background worker \"%s\": only dynamic background workers can request notification",
                        worker->bgw_name)));
        return;
    }
 
    /*
     * Enforce maximum number of workers.  Note this is overly restrictive: we
     * could allow more non-shmem-connected workers, because these don't count
     * towards the MAX_BACKENDS limit elsewhere.  For now, it doesn't seem
     * important to relax this restriction.
     */
    if (++numworkers > max_worker_processes)
    {
        ereport(LOG,
                (errcode(ERRCODE_CONFIGURATION_LIMIT_EXCEEDED),
                 errmsg("too many background workers"),
                 errdetail_plural("Up to %d background worker can be registered with the current settings.",
                                  "Up to %d background workers can be registered with the current settings.",
                                  max_worker_processes,
                                  max_worker_processes),
                 errhint("Consider increasing the configuration parameter max_worker_processes.")));
        return;
    }
 
    /*
     * Copy the registration data into the registered workers list.
     */
    rw = MemoryContextAllocExtended(PostmasterContext,
                                    sizeof(RegisteredBgWorker),
                                    MCXT_ALLOC_NO_OOM);
    if (rw == NULL)
    {
        ereport(LOG,
                (errcode(ERRCODE_OUT_OF_MEMORY),
                 errmsg("out of memory")));
        return;
    }
 
    rw->rw_worker = *worker;
    rw->rw_backend = NULL;
    rw->rw_pid = 0;
    rw->rw_child_slot = 0;
    rw->rw_crashed_at = 0;
    rw->rw_terminate = false;
 
    slist_push_head(&BackgroundWorkerList, &rw->rw_lnode);
}
 
/*
 * Register a new background worker from a regular backend.
 *
 * Returns true on success and false on failure.  Failure typically indicates
 * that no background worker slots are currently available.
 *
 * If handle != NULL, we'll set *handle to a pointer that can subsequently
 * be used as an argument to GetBackgroundWorkerPid().  The caller can
 * free this pointer using pfree(), if desired.
 */
bool
RegisterDynamicBackgroundWorker(BackgroundWorker *worker,
                                BackgroundWorkerHandle **handle)
{
    int         slotno;
    bool        success = false;
    bool        parallel;
    uint64      generation = 0;
 
    /*
     * We can't register dynamic background workers from the postmaster. If
     * this is a standalone backend, we're the only process and can't start
     * any more.  In a multi-process environment, it might be theoretically
     * possible, but we don't currently support it due to locking
     * considerations; see comments on the BackgroundWorkerSlot data
     * structure.
     */
    if (!IsUnderPostmaster)
        return false;
 
    if (!SanityCheckBackgroundWorker(worker, ERROR))
        return false;
 
    parallel = (worker->bgw_flags & BGWORKER_CLASS_PARALLEL) != 0;
 
    LWLockAcquire(BackgroundWorkerLock, LW_EXCLUSIVE);
 
    /*
     * If this is a parallel worker, check whether there are already too many
     * parallel workers; if so, don't register another one.  Our view of
     * parallel_terminate_count may be slightly stale, but that doesn't really
     * matter: we would have gotten the same result if we'd arrived here
     * slightly earlier anyway.  There's no help for it, either, since the
     * postmaster must not take locks; a memory barrier wouldn't guarantee
     * anything useful.
     */
    if (parallel && (BackgroundWorkerData->parallel_register_count -
                     BackgroundWorkerData->parallel_terminate_count) >=
        max_parallel_workers)
    {
        Assert(BackgroundWorkerData->parallel_register_count -
               BackgroundWorkerData->parallel_terminate_count <=
               MAX_PARALLEL_WORKER_LIMIT);
        LWLockRelease(BackgroundWorkerLock);
        return false;
    }
 
    /*
     * Look for an unused slot.  If we find one, grab it.
     */
    for (slotno = 0; slotno < BackgroundWorkerData->total_slots; ++slotno)
    {
        BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
 
        if (!slot->in_use)
        {
            memcpy(&slot->worker, worker, sizeof(BackgroundWorker));
            slot->pid = InvalidPid; /* indicates not started yet */
            slot->generation++;
            slot->terminate = false;
            generation = slot->generation;
            if (parallel)
                BackgroundWorkerData->parallel_register_count++;
 
            /*
             * Make sure postmaster doesn't see the slot as in use before it
             * sees the new contents.
             */
            pg_write_barrier();
 
            slot->in_use = true;
            success = true;
            break;
        }
    }
 
    LWLockRelease(BackgroundWorkerLock);
 
    /* If we found a slot, tell the postmaster to notice the change. */
    if (success)
        SendPostmasterSignal(PMSIGNAL_BACKGROUND_WORKER_CHANGE);
 
    /*
     * If we found a slot and the user has provided a handle, initialize it.
     */
    if (success && handle)
    {
        *handle = palloc(sizeof(BackgroundWorkerHandle));
        (*handle)->slot = slotno;
        (*handle)->generation = generation;
    }
 
    return success;
}
 
/*
 * Get the PID of a dynamically-registered background worker.
 *
 * If the worker is determined to be running, the return value will be
 * BGWH_STARTED and *pidp will get the PID of the worker process.  If the
 * postmaster has not yet attempted to start the worker, the return value will
 * be BGWH_NOT_YET_STARTED.  Otherwise, the return value is BGWH_STOPPED.
 *
 * BGWH_STOPPED can indicate either that the worker is temporarily stopped
 * (because it is configured for automatic restart and exited non-zero),
 * or that the worker is permanently stopped (because it exited with exit
 * code 0, or was not configured for automatic restart), or even that the
 * worker was unregistered without ever starting (either because startup
 * failed and the worker is not configured for automatic restart, or because
 * TerminateBackgroundWorker was used before the worker was successfully
 * started).
 */
BgwHandleStatus
GetBackgroundWorkerPid(BackgroundWorkerHandle *handle, pid_t *pidp)
{
    BackgroundWorkerSlot *slot;
    pid_t       pid;
 
    Assert(handle->slot < max_worker_processes);
    slot = &BackgroundWorkerData->slot[handle->slot];
 
    /*
     * We could probably arrange to synchronize access to data using memory
     * barriers only, but for now, let's just keep it simple and grab the
     * lock.  It seems unlikely that there will be enough traffic here to
     * result in meaningful contention.
     */
    LWLockAcquire(BackgroundWorkerLock, LW_SHARED);
 
    /*
     * The generation number can't be concurrently changed while we hold the
     * lock.  The pid, which is updated by the postmaster, can change at any
     * time, but we assume such changes are atomic.  So the value we read
     * won't be garbage, but it might be out of date by the time the caller
     * examines it (but that's unavoidable anyway).
     *
     * The in_use flag could be in the process of changing from true to false,
     * but if it is already false then it can't change further.
     */
    if (handle->generation != slot->generation || !slot->in_use)
        pid = 0;
    else
        pid = slot->pid;
 
    /* All done. */
    LWLockRelease(BackgroundWorkerLock);
 
    if (pid == 0)
        return BGWH_STOPPED;
    else if (pid == InvalidPid)
        return BGWH_NOT_YET_STARTED;
    *pidp = pid;
    return BGWH_STARTED;
}
 
/*
 * Wait for a background worker to start up.
 *
 * This is like GetBackgroundWorkerPid(), except that if the worker has not
 * yet started, we wait for it to do so; thus, BGWH_NOT_YET_STARTED is never
 * returned.  However, if the postmaster has died, we give up and return
 * BGWH_POSTMASTER_DIED, since it that case we know that startup will not
 * take place.
 *
 * The caller *must* have set our PID as the worker's bgw_notify_pid,
 * else we will not be awoken promptly when the worker's state changes.
 */
BgwHandleStatus
WaitForBackgroundWorkerStartup(BackgroundWorkerHandle *handle, pid_t *pidp)
{
    BgwHandleStatus status;
    int         rc;
 
    for (;;)
    {
        pid_t       pid;
 
        CHECK_FOR_INTERRUPTS();
 
        status = GetBackgroundWorkerPid(handle, &pid);
        if (status == BGWH_STARTED)
            *pidp = pid;
        if (status != BGWH_NOT_YET_STARTED)
            break;
 
        rc = WaitLatch(MyLatch,
                       WL_LATCH_SET | WL_POSTMASTER_DEATH, 0,
                       WAIT_EVENT_BGWORKER_STARTUP);
 
        if (rc & WL_POSTMASTER_DEATH)
        {
            status = BGWH_POSTMASTER_DIED;
            break;
        }
 
        ResetLatch(MyLatch);
    }
 
    return status;
}
 
/*
 * Wait for a background worker to stop.
 *
 * If the worker hasn't yet started, or is running, we wait for it to stop
 * and then return BGWH_STOPPED.  However, if the postmaster has died, we give
 * up and return BGWH_POSTMASTER_DIED, because it's the postmaster that
 * notifies us when a worker's state changes.
 *
 * The caller *must* have set our PID as the worker's bgw_notify_pid,
 * else we will not be awoken promptly when the worker's state changes.
 */
BgwHandleStatus
WaitForBackgroundWorkerShutdown(BackgroundWorkerHandle *handle)
{
    BgwHandleStatus status;
    int         rc;
 
    for (;;)
    {
        pid_t       pid;
 
        CHECK_FOR_INTERRUPTS();
 
        status = GetBackgroundWorkerPid(handle, &pid);
        if (status == BGWH_STOPPED)
            break;
 
        rc = WaitLatch(MyLatch,
                       WL_LATCH_SET | WL_POSTMASTER_DEATH, 0,
                       WAIT_EVENT_BGWORKER_SHUTDOWN);
 
        if (rc & WL_POSTMASTER_DEATH)
        {
            status = BGWH_POSTMASTER_DIED;
            break;
        }
 
        ResetLatch(MyLatch);
    }
 
    return status;
}
 
/*
 * Instruct the postmaster to terminate a background worker.
 *
 * Note that it's safe to do this without regard to whether the worker is
 * still running, or even if the worker may already have exited and been
 * unregistered.
 */
void
TerminateBackgroundWorker(BackgroundWorkerHandle *handle)
{
    BackgroundWorkerSlot *slot;
    bool        signal_postmaster = false;
 
    Assert(handle->slot < max_worker_processes);
    slot = &BackgroundWorkerData->slot[handle->slot];
 
    /* Set terminate flag in shared memory, unless slot has been reused. */
    LWLockAcquire(BackgroundWorkerLock, LW_EXCLUSIVE);
    if (handle->generation == slot->generation)
    {
        slot->terminate = true;
        signal_postmaster = true;
    }
    LWLockRelease(BackgroundWorkerLock);
 
    /* Make sure the postmaster notices the change to shared memory. */
    if (signal_postmaster)
        SendPostmasterSignal(PMSIGNAL_BACKGROUND_WORKER_CHANGE);
}
 
/*
 * Look up (and possibly load) a bgworker entry point function.
 *
 * For functions contained in the core code, we use library name "postgres"
 * and consult the InternalBGWorkers array.  External functions are
 * looked up, and loaded if necessary, using load_external_function().
 *
 * The point of this is to pass function names as strings across process
 * boundaries.  We can't pass actual function addresses because of the
 * possibility that the function has been loaded at a different address
 * in a different process.  This is obviously a hazard for functions in
 * loadable libraries, but it can happen even for functions in the core code
 * on platforms using EXEC_BACKEND (e.g., Windows).
 *
 * At some point it might be worthwhile to get rid of InternalBGWorkers[]
 * in favor of applying load_external_function() for core functions too;
 * but that raises portability issues that are not worth addressing now.
 */
static bgworker_main_type
LookupBackgroundWorkerFunction(const char *libraryname, const char *funcname)
{
    /*
     * If the function is to be loaded from postgres itself, search the
     * InternalBGWorkers array.
     */
    if (strcmp(libraryname, "postgres") == 0)
    {
        int         i;
 
        for (i = 0; i < lengthof(InternalBGWorkers); i++)
        {
            if (strcmp(InternalBGWorkers[i].fn_name, funcname) == 0)
                return InternalBGWorkers[i].fn_addr;
        }
 
        /* We can only reach this by programming error. */
        elog(ERROR, "internal function \"%s\" not found", funcname);
    }
 
    /* Otherwise load from external library. */
    return (bgworker_main_type)
        load_external_function(libraryname, funcname, true, NULL);
}
 
/*
 * Given a PID, get the bgw_type of the background worker.  Returns NULL if
 * not a valid background worker.
 *
 * The return value is in static memory belonging to this function, so it has
 * to be used before calling this function again.  This is so that the caller
 * doesn't have to worry about the background worker locking protocol.
 */
const char *
GetBackgroundWorkerTypeByPid(pid_t pid)
{
    int         slotno;
    bool        found = false;
    static char result[BGW_MAXLEN];
 
    LWLockAcquire(BackgroundWorkerLock, LW_SHARED);
 
    for (slotno = 0; slotno < BackgroundWorkerData->total_slots; slotno++)
    {
        BackgroundWorkerSlot *slot = &BackgroundWorkerData->slot[slotno];
 
        if (slot->pid > 0 && slot->pid == pid)
        {
            strcpy(result, slot->worker.bgw_type);
            found = true;
            break;
        }
    }
 
    LWLockRelease(BackgroundWorkerLock);
 
    if (!found)
        return NULL;
 
    return result;
}