autoprewarm.c

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/*-------------------------------------------------------------------------
 *
 * autoprewarm.c
 *      Periodically dump information about the blocks present in
 *      shared_buffers, and reload them on server restart.
 *
 *      Due to locking considerations, we can't actually begin prewarming
 *      until the server reaches a consistent state.  We need the catalogs
 *      to be consistent so that we can figure out which relation to lock,
 *      and we need to lock the relations so that we don't try to prewarm
 *      pages from a relation that is in the process of being dropped.
 *
 *      While prewarming, autoprewarm will use two workers.  There's a
 *      leader worker that reads and sorts the list of blocks to be
 *      prewarmed and then launches a per-database worker for each
 *      relevant database in turn.  The former keeps running after the
 *      initial prewarm is complete to update the dump file periodically.
 *
 *  Copyright (c) 2016-2021, PostgreSQL Global Development Group
 *
 *  IDENTIFICATION
 *      contrib/pg_prewarm/autoprewarm.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <unistd.h>

#include "access/relation.h"
#include "access/xact.h"
#include "catalog/pg_class.h"
#include "catalog/pg_type.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/bgworker.h"
#include "postmaster/interrupt.h"
#include "storage/buf_internals.h"
#include "storage/dsm.h"
#include "storage/ipc.h"
#include "storage/latch.h"
#include "storage/lwlock.h"
#include "storage/proc.h"
#include "storage/procsignal.h"
#include "storage/shmem.h"
#include "storage/smgr.h"
#include "tcop/tcopprot.h"
#include "utils/acl.h"
#include "utils/datetime.h"
#include "utils/guc.h"
#include "utils/memutils.h"
#include "utils/rel.h"
#include "utils/relfilenodemap.h"
#include "utils/resowner.h"

#define AUTOPREWARM_FILE "autoprewarm.blocks"

/* Metadata for each block we dump. */
typedef struct BlockInfoRecord
{
    Oid         database;
    Oid         tablespace;
    Oid         filenode;
    ForkNumber  forknum;
    BlockNumber blocknum;
} BlockInfoRecord;

/* Shared state information for autoprewarm bgworker. */
typedef struct AutoPrewarmSharedState
{
    LWLock      lock;           /* mutual exclusion */
    pid_t       bgworker_pid;   /* for main bgworker */
    pid_t       pid_using_dumpfile; /* for autoprewarm or block dump */

    /* Following items are for communication with per-database worker */
    dsm_handle  block_info_handle;
    Oid         database;
    int         prewarm_start_idx;
    int         prewarm_stop_idx;
    int         prewarmed_blocks;
} AutoPrewarmSharedState;

void        _PG_init(void);
void        autoprewarm_main(Datum main_arg);
void        autoprewarm_database_main(Datum main_arg);

PG_FUNCTION_INFO_V1(autoprewarm_start_worker);
PG_FUNCTION_INFO_V1(autoprewarm_dump_now);

static void apw_load_buffers(void);
static int  apw_dump_now(bool is_bgworker, bool dump_unlogged);
static void apw_start_leader_worker(void);
static void apw_start_database_worker(void);
static bool apw_init_shmem(void);
static void apw_detach_shmem(int code, Datum arg);
static int  apw_compare_blockinfo(const void *p, const void *q);

/* Pointer to shared-memory state. */
static AutoPrewarmSharedState *apw_state = NULL;

/* GUC variables. */
static bool autoprewarm = true; /* start worker? */
static int  autoprewarm_interval;   /* dump interval */

/*
 * Module load callback.
 */
void
_PG_init(void)
{
    DefineCustomIntVariable("pg_prewarm.autoprewarm_interval",
                            "Sets the interval between dumps of shared buffers",
                            "If set to zero, time-based dumping is disabled.",
                            &autoprewarm_interval,
                            300,
                            0, INT_MAX / 1000,
                            PGC_SIGHUP,
                            GUC_UNIT_S,
                            NULL,
                            NULL,
                            NULL);

    if (!process_shared_preload_libraries_in_progress)
        return;

    /* can't define PGC_POSTMASTER variable after startup */
    DefineCustomBoolVariable("pg_prewarm.autoprewarm",
                             "Starts the autoprewarm worker.",
                             NULL,
                             &autoprewarm,
                             true,
                             PGC_POSTMASTER,
                             0,
                             NULL,
                             NULL,
                             NULL);

    EmitWarningsOnPlaceholders("pg_prewarm");

    RequestAddinShmemSpace(MAXALIGN(sizeof(AutoPrewarmSharedState)));

    /* Register autoprewarm worker, if enabled. */
    if (autoprewarm)
        apw_start_leader_worker();
}

/*
 * Main entry point for the leader autoprewarm process.  Per-database workers
 * have a separate entry point.
 */
void
autoprewarm_main(Datum main_arg)
{
    bool        first_time = true;
    bool        final_dump_allowed = true;
    TimestampTz last_dump_time = 0;

    /* Establish signal handlers; once that's done, unblock signals. */
    pqsignal(SIGTERM, SignalHandlerForShutdownRequest);
    pqsignal(SIGHUP, SignalHandlerForConfigReload);
    pqsignal(SIGUSR1, procsignal_sigusr1_handler);
    BackgroundWorkerUnblockSignals();

    /* Create (if necessary) and attach to our shared memory area. */
    if (apw_init_shmem())
        first_time = false;

    /* Set on-detach hook so that our PID will be cleared on exit. */
    on_shmem_exit(apw_detach_shmem, 0);

    /*
     * Store our PID in the shared memory area --- unless there's already
     * another worker running, in which case just exit.
     */
    LWLockAcquire(&apw_state->lock, LW_EXCLUSIVE);
    if (apw_state->bgworker_pid != InvalidPid)
    {
        LWLockRelease(&apw_state->lock);
        ereport(LOG,
                (errmsg("autoprewarm worker is already running under PID %lu",
                        (unsigned long) apw_state->bgworker_pid)));
        return;
    }
    apw_state->bgworker_pid = MyProcPid;
    LWLockRelease(&apw_state->lock);

    /*
     * Preload buffers from the dump file only if we just created the shared
     * memory region.  Otherwise, it's either already been done or shouldn't
     * be done - e.g. because the old dump file has been overwritten since the
     * server was started.
     *
     * There's not much point in performing a dump immediately after we finish
     * preloading; so, if we do end up preloading, consider the last dump time
     * to be equal to the current time.
     *
     * If apw_load_buffers() is terminated early by a shutdown request,
     * prevent dumping out our state below the loop, because we'd effectively
     * just truncate the saved state to however much we'd managed to preload.
     */
    if (first_time)
    {
        apw_load_buffers();
        final_dump_allowed = !ShutdownRequestPending;
        last_dump_time = GetCurrentTimestamp();
    }

    /* Periodically dump buffers until terminated. */
    while (!ShutdownRequestPending)
    {
        /* In case of a SIGHUP, just reload the configuration. */
        if (ConfigReloadPending)
        {
            ConfigReloadPending = false;
            ProcessConfigFile(PGC_SIGHUP);
        }

        if (autoprewarm_interval <= 0)
        {
            /* We're only dumping at shutdown, so just wait forever. */
            (void) WaitLatch(MyLatch,
                             WL_LATCH_SET | WL_EXIT_ON_PM_DEATH,
                             -1L,
                             PG_WAIT_EXTENSION);
        }
        else
        {
            TimestampTz next_dump_time;
            long        delay_in_ms;

            /* Compute the next dump time. */
            next_dump_time =
                TimestampTzPlusMilliseconds(last_dump_time,
                                            autoprewarm_interval * 1000);
            delay_in_ms =
                TimestampDifferenceMilliseconds(GetCurrentTimestamp(),
                                                next_dump_time);

            /* Perform a dump if it's time. */
            if (delay_in_ms <= 0)
            {
                last_dump_time = GetCurrentTimestamp();
                apw_dump_now(true, false);
                continue;
            }

            /* Sleep until the next dump time. */
            (void) WaitLatch(MyLatch,
                             WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
                             delay_in_ms,
                             PG_WAIT_EXTENSION);
        }

        /* Reset the latch, loop. */
        ResetLatch(MyLatch);
    }

    /*
     * Dump one last time.  We assume this is probably the result of a system
     * shutdown, although it's possible that we've merely been terminated.
     */
    if (final_dump_allowed)
        apw_dump_now(true, true);
}

/*
 * Read the dump file and launch per-database workers one at a time to
 * prewarm the buffers found there.
 */
static void
apw_load_buffers(void)
{
    FILE       *file = NULL;
    int         num_elements,
                i;
    BlockInfoRecord *blkinfo;
    dsm_segment *seg;

    /*
     * Skip the prewarm if the dump file is in use; otherwise, prevent any
     * other process from writing it while we're using it.
     */
    LWLockAcquire(&apw_state->lock, LW_EXCLUSIVE);
    if (apw_state->pid_using_dumpfile == InvalidPid)
        apw_state->pid_using_dumpfile = MyProcPid;
    else
    {
        LWLockRelease(&apw_state->lock);
        ereport(LOG,
                (errmsg("skipping prewarm because block dump file is being written by PID %lu",
                        (unsigned long) apw_state->pid_using_dumpfile)));
        return;
    }
    LWLockRelease(&apw_state->lock);

    /*
     * Open the block dump file.  Exit quietly if it doesn't exist, but report
     * any other error.
     */
    file = AllocateFile(AUTOPREWARM_FILE, "r");
    if (!file)
    {
        if (errno == ENOENT)
        {
            LWLockAcquire(&apw_state->lock, LW_EXCLUSIVE);
            apw_state->pid_using_dumpfile = InvalidPid;
            LWLockRelease(&apw_state->lock);
            return;             /* No file to load. */
        }
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not read file \"%s\": %m",
                        AUTOPREWARM_FILE)));
    }

    /* First line of the file is a record count. */
    if (fscanf(file, "<<%d>>\n", &num_elements) != 1)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not read from file \"%s\": %m",
                        AUTOPREWARM_FILE)));

    /* Allocate a dynamic shared memory segment to store the record data. */
    seg = dsm_create(sizeof(BlockInfoRecord) * num_elements, 0);
    blkinfo = (BlockInfoRecord *) dsm_segment_address(seg);

    /* Read records, one per line. */
    for (i = 0; i < num_elements; i++)
    {
        unsigned    forknum;

        if (fscanf(file, "%u,%u,%u,%u,%u\n", &blkinfo[i].database,
                   &blkinfo[i].tablespace, &blkinfo[i].filenode,
                   &forknum, &blkinfo[i].blocknum) != 5)
            ereport(ERROR,
                    (errmsg("autoprewarm block dump file is corrupted at line %d",
                            i + 1)));
        blkinfo[i].forknum = forknum;
    }

    FreeFile(file);

    /* Sort the blocks to be loaded. */
    pg_qsort(blkinfo, num_elements, sizeof(BlockInfoRecord),
             apw_compare_blockinfo);

    /* Populate shared memory state. */
    apw_state->block_info_handle = dsm_segment_handle(seg);
    apw_state->prewarm_start_idx = apw_state->prewarm_stop_idx = 0;
    apw_state->prewarmed_blocks = 0;

    /* Get the info position of the first block of the next database. */
    while (apw_state->prewarm_start_idx < num_elements)
    {
        int         j = apw_state->prewarm_start_idx;
        Oid         current_db = blkinfo[j].database;

        /*
         * Advance the prewarm_stop_idx to the first BlockInfoRecord that does
         * not belong to this database.
         */
        j++;
        while (j < num_elements)
        {
            if (current_db != blkinfo[j].database)
            {
                /*
                 * Combine BlockInfoRecords for global objects with those of
                 * the database.
                 */
                if (current_db != InvalidOid)
                    break;
                current_db = blkinfo[j].database;
            }

            j++;
        }

        /*
         * If we reach this point with current_db == InvalidOid, then only
         * BlockInfoRecords belonging to global objects exist.  We can't
         * prewarm without a database connection, so just bail out.
         */
        if (current_db == InvalidOid)
            break;

        /* Configure stop point and database for next per-database worker. */
        apw_state->prewarm_stop_idx = j;
        apw_state->database = current_db;
        Assert(apw_state->prewarm_start_idx < apw_state->prewarm_stop_idx);

        /* If we've run out of free buffers, don't launch another worker. */
        if (!have_free_buffer())
            break;

        /*
         * Likewise, don't launch if we've already been told to shut down.
         * (The launch would fail anyway, but we might as well skip it.)
         */
        if (ShutdownRequestPending)
            break;

        /*
         * Start a per-database worker to load blocks for this database; this
         * function will return once the per-database worker exits.
         */
        apw_start_database_worker();

        /* Prepare for next database. */
        apw_state->prewarm_start_idx = apw_state->prewarm_stop_idx;
    }

    /* Clean up. */
    dsm_detach(seg);
    LWLockAcquire(&apw_state->lock, LW_EXCLUSIVE);
    apw_state->block_info_handle = DSM_HANDLE_INVALID;
    apw_state->pid_using_dumpfile = InvalidPid;
    LWLockRelease(&apw_state->lock);

    /* Report our success, if we were able to finish. */
    if (!ShutdownRequestPending)
        ereport(LOG,
                (errmsg("autoprewarm successfully prewarmed %d of %d previously-loaded blocks",
                        apw_state->prewarmed_blocks, num_elements)));
}

/*
 * Prewarm all blocks for one database (and possibly also global objects, if
 * those got grouped with this database).
 */
void
autoprewarm_database_main(Datum main_arg)
{
    int         pos;
    BlockInfoRecord *block_info;
    Relation    rel = NULL;
    BlockNumber nblocks = 0;
    BlockInfoRecord *old_blk = NULL;
    dsm_segment *seg;

    /* Establish signal handlers; once that's done, unblock signals. */
    pqsignal(SIGTERM, die);
    BackgroundWorkerUnblockSignals();

    /* Connect to correct database and get block information. */
    apw_init_shmem();
    seg = dsm_attach(apw_state->block_info_handle);
    if (seg == NULL)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("could not map dynamic shared memory segment")));
    BackgroundWorkerInitializeConnectionByOid(apw_state->database, InvalidOid, 0);
    block_info = (BlockInfoRecord *) dsm_segment_address(seg);
    pos = apw_state->prewarm_start_idx;

    /*
     * Loop until we run out of blocks to prewarm or until we run out of free
     * buffers.
     */
    while (pos < apw_state->prewarm_stop_idx && have_free_buffer())
    {
        BlockInfoRecord *blk = &block_info[pos++];
        Buffer      buf;

        CHECK_FOR_INTERRUPTS();

        /*
         * Quit if we've reached records for another database. If previous
         * blocks are of some global objects, then continue pre-warming.
         */
        if (old_blk != NULL && old_blk->database != blk->database &&
            old_blk->database != 0)
            break;

        /*
         * As soon as we encounter a block of a new relation, close the old
         * relation. Note that rel will be NULL if try_relation_open failed
         * previously; in that case, there is nothing to close.
         */
        if (old_blk != NULL && old_blk->filenode != blk->filenode &&
            rel != NULL)
        {
            relation_close(rel, AccessShareLock);
            rel = NULL;
            CommitTransactionCommand();
        }

        /*
         * Try to open each new relation, but only once, when we first
         * encounter it. If it's been dropped, skip the associated blocks.
         */
        if (old_blk == NULL || old_blk->filenode != blk->filenode)
        {
            Oid         reloid;

            Assert(rel == NULL);
            StartTransactionCommand();
            reloid = RelidByRelfilenode(blk->tablespace, blk->filenode);
            if (OidIsValid(reloid))
                rel = try_relation_open(reloid, AccessShareLock);

            if (!rel)
                CommitTransactionCommand();
        }
        if (!rel)
        {
            old_blk = blk;
            continue;
        }

        /* Once per fork, check for fork existence and size. */
        if (old_blk == NULL ||
            old_blk->filenode != blk->filenode ||
            old_blk->forknum != blk->forknum)
        {
            /*
             * smgrexists is not safe for illegal forknum, hence check whether
             * the passed forknum is valid before using it in smgrexists.
             */
            if (blk->forknum > InvalidForkNumber &&
                blk->forknum <= MAX_FORKNUM &&
                smgrexists(RelationGetSmgr(rel), blk->forknum))
                nblocks = RelationGetNumberOfBlocksInFork(rel, blk->forknum);
            else
                nblocks = 0;
        }

        /* Check whether blocknum is valid and within fork file size. */
        if (blk->blocknum >= nblocks)
        {
            /* Move to next forknum. */
            old_blk = blk;
            continue;
        }

        /* Prewarm buffer. */
        buf = ReadBufferExtended(rel, blk->forknum, blk->blocknum, RBM_NORMAL,
                                 NULL);
        if (BufferIsValid(buf))
        {
            apw_state->prewarmed_blocks++;
            ReleaseBuffer(buf);
        }

        old_blk = blk;
    }

    dsm_detach(seg);

    /* Release lock on previous relation. */
    if (rel)
    {
        relation_close(rel, AccessShareLock);
        CommitTransactionCommand();
    }
}

/*
 * Dump information on blocks in shared buffers.  We use a text format here
 * so that it's easy to understand and even change the file contents if
 * necessary.
 * Returns the number of blocks dumped.
 */
static int
apw_dump_now(bool is_bgworker, bool dump_unlogged)
{
    int         num_blocks;
    int         i;
    int         ret;
    BlockInfoRecord *block_info_array;
    BufferDesc *bufHdr;
    FILE       *file;
    char        transient_dump_file_path[MAXPGPATH];
    pid_t       pid;

    LWLockAcquire(&apw_state->lock, LW_EXCLUSIVE);
    pid = apw_state->pid_using_dumpfile;
    if (apw_state->pid_using_dumpfile == InvalidPid)
        apw_state->pid_using_dumpfile = MyProcPid;
    LWLockRelease(&apw_state->lock);

    if (pid != InvalidPid)
    {
        if (!is_bgworker)
            ereport(ERROR,
                    (errmsg("could not perform block dump because dump file is being used by PID %lu",
                            (unsigned long) apw_state->pid_using_dumpfile)));

        ereport(LOG,
                (errmsg("skipping block dump because it is already being performed by PID %lu",
                        (unsigned long) apw_state->pid_using_dumpfile)));
        return 0;
    }

    block_info_array =
        (BlockInfoRecord *) palloc(sizeof(BlockInfoRecord) * NBuffers);

    for (num_blocks = 0, i = 0; i < NBuffers; i++)
    {
        uint32      buf_state;

        CHECK_FOR_INTERRUPTS();

        bufHdr = GetBufferDescriptor(i);

        /* Lock each buffer header before inspecting. */
        buf_state = LockBufHdr(bufHdr);

        /*
         * Unlogged tables will be automatically truncated after a crash or
         * unclean shutdown. In such cases we need not prewarm them. Dump them
         * only if requested by caller.
         */
        if (buf_state & BM_TAG_VALID &&
            ((buf_state & BM_PERMANENT) || dump_unlogged))
        {
            block_info_array[num_blocks].database = bufHdr->tag.rnode.dbNode;
            block_info_array[num_blocks].tablespace = bufHdr->tag.rnode.spcNode;
            block_info_array[num_blocks].filenode = bufHdr->tag.rnode.relNode;
            block_info_array[num_blocks].forknum = bufHdr->tag.forkNum;
            block_info_array[num_blocks].blocknum = bufHdr->tag.blockNum;
            ++num_blocks;
        }

        UnlockBufHdr(bufHdr, buf_state);
    }

    snprintf(transient_dump_file_path, MAXPGPATH, "%s.tmp", AUTOPREWARM_FILE);
    file = AllocateFile(transient_dump_file_path, "w");
    if (!file)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not open file \"%s\": %m",
                        transient_dump_file_path)));

    ret = fprintf(file, "<<%d>>\n", num_blocks);
    if (ret < 0)
    {
        int         save_errno = errno;

        FreeFile(file);
        unlink(transient_dump_file_path);
        errno = save_errno;
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not write to file \"%s\": %m",
                        transient_dump_file_path)));
    }

    for (i = 0; i < num_blocks; i++)
    {
        CHECK_FOR_INTERRUPTS();

        ret = fprintf(file, "%u,%u,%u,%u,%u\n",
                      block_info_array[i].database,
                      block_info_array[i].tablespace,
                      block_info_array[i].filenode,
                      (uint32) block_info_array[i].forknum,
                      block_info_array[i].blocknum);
        if (ret < 0)
        {
            int         save_errno = errno;

            FreeFile(file);
            unlink(transient_dump_file_path);
            errno = save_errno;
            ereport(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not write to file \"%s\": %m",
                            transient_dump_file_path)));
        }
    }

    pfree(block_info_array);

    /*
     * Rename transient_dump_file_path to AUTOPREWARM_FILE to make things
     * permanent.
     */
    ret = FreeFile(file);
    if (ret != 0)
    {
        int         save_errno = errno;

        unlink(transient_dump_file_path);
        errno = save_errno;
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not close file \"%s\": %m",
                        transient_dump_file_path)));
    }

    (void) durable_rename(transient_dump_file_path, AUTOPREWARM_FILE, ERROR);
    apw_state->pid_using_dumpfile = InvalidPid;

    ereport(DEBUG1,
            (errmsg_internal("wrote block details for %d blocks", num_blocks)));
    return num_blocks;
}

/*
 * SQL-callable function to launch autoprewarm.
 */
Datum
autoprewarm_start_worker(PG_FUNCTION_ARGS)
{
    pid_t       pid;

    if (!autoprewarm)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("autoprewarm is disabled")));

    apw_init_shmem();
    LWLockAcquire(&apw_state->lock, LW_EXCLUSIVE);
    pid = apw_state->bgworker_pid;
    LWLockRelease(&apw_state->lock);

    if (pid != InvalidPid)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("autoprewarm worker is already running under PID %lu",
                        (unsigned long) pid)));

    apw_start_leader_worker();

    PG_RETURN_VOID();
}

/*
 * SQL-callable function to perform an immediate block dump.
 *
 * Note: this is declared to return int8, as insurance against some
 * very distant day when we might make NBuffers wider than int.
 */
Datum
autoprewarm_dump_now(PG_FUNCTION_ARGS)
{
    int         num_blocks;

    apw_init_shmem();

    PG_ENSURE_ERROR_CLEANUP(apw_detach_shmem, 0);
    {
        num_blocks = apw_dump_now(false, true);
    }
    PG_END_ENSURE_ERROR_CLEANUP(apw_detach_shmem, 0);

    PG_RETURN_INT64((int64) num_blocks);
}

/*
 * Allocate and initialize autoprewarm related shared memory, if not already
 * done, and set up backend-local pointer to that state.  Returns true if an
 * existing shared memory segment was found.
 */
static bool
apw_init_shmem(void)
{
    bool        found;

    LWLockAcquire(AddinShmemInitLock, LW_EXCLUSIVE);
    apw_state = ShmemInitStruct("autoprewarm",
                                sizeof(AutoPrewarmSharedState),
                                &found);
    if (!found)
    {
        /* First time through ... */
        LWLockInitialize(&apw_state->lock, LWLockNewTrancheId());
        apw_state->bgworker_pid = InvalidPid;
        apw_state->pid_using_dumpfile = InvalidPid;
    }
    LWLockRelease(AddinShmemInitLock);

    LWLockRegisterTranche(apw_state->lock.tranche, "autoprewarm");

    return found;
}

/*
 * Clear our PID from autoprewarm shared state.
 */
static void
apw_detach_shmem(int code, Datum arg)
{
    LWLockAcquire(&apw_state->lock, LW_EXCLUSIVE);
    if (apw_state->pid_using_dumpfile == MyProcPid)
        apw_state->pid_using_dumpfile = InvalidPid;
    if (apw_state->bgworker_pid == MyProcPid)
        apw_state->bgworker_pid = InvalidPid;
    LWLockRelease(&apw_state->lock);
}

/*
 * Start autoprewarm leader worker process.
 */
static void
apw_start_leader_worker(void)
{
    BackgroundWorker worker;
    BackgroundWorkerHandle *handle;
    BgwHandleStatus status;
    pid_t       pid;

    MemSet(&worker, 0, sizeof(BackgroundWorker));
    worker.bgw_flags = BGWORKER_SHMEM_ACCESS;
    worker.bgw_start_time = BgWorkerStart_ConsistentState;
    strcpy(worker.bgw_library_name, "pg_prewarm");
    strcpy(worker.bgw_function_name, "autoprewarm_main");
    strcpy(worker.bgw_name, "autoprewarm leader");
    strcpy(worker.bgw_type, "autoprewarm leader");

    if (process_shared_preload_libraries_in_progress)
    {
        RegisterBackgroundWorker(&worker);
        return;
    }

    /* must set notify PID to wait for startup */
    worker.bgw_notify_pid = MyProcPid;

    if (!RegisterDynamicBackgroundWorker(&worker, &handle))
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_RESOURCES),
                 errmsg("could not register background process"),
                 errhint("You may need to increase max_worker_processes.")));

    status = WaitForBackgroundWorkerStartup(handle, &pid);
    if (status != BGWH_STARTED)
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_RESOURCES),
                 errmsg("could not start background process"),
                 errhint("More details may be available in the server log.")));
}

/*
 * Start autoprewarm per-database worker process.
 */
static void
apw_start_database_worker(void)
{
    BackgroundWorker worker;
    BackgroundWorkerHandle *handle;

    MemSet(&worker, 0, sizeof(BackgroundWorker));
    worker.bgw_flags =
        BGWORKER_SHMEM_ACCESS | BGWORKER_BACKEND_DATABASE_CONNECTION;
    worker.bgw_start_time = BgWorkerStart_ConsistentState;
    worker.bgw_restart_time = BGW_NEVER_RESTART;
    strcpy(worker.bgw_library_name, "pg_prewarm");
    strcpy(worker.bgw_function_name, "autoprewarm_database_main");
    strcpy(worker.bgw_name, "autoprewarm worker");
    strcpy(worker.bgw_type, "autoprewarm worker");

    /* must set notify PID to wait for shutdown */
    worker.bgw_notify_pid = MyProcPid;

    if (!RegisterDynamicBackgroundWorker(&worker, &handle))
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_RESOURCES),
                 errmsg("registering dynamic bgworker autoprewarm failed"),
                 errhint("Consider increasing configuration parameter \"max_worker_processes\".")));

    /*
     * Ignore return value; if it fails, postmaster has died, but we have
     * checks for that elsewhere.
     */
    WaitForBackgroundWorkerShutdown(handle);
}

/* Compare member elements to check whether they are not equal. */
#define cmp_member_elem(fld)    \
do { \
    if (a->fld < b->fld)        \
        return -1;              \
    else if (a->fld > b->fld)   \
        return 1;               \
} while(0)

/*
 * apw_compare_blockinfo
 *
 * We depend on all records for a particular database being consecutive
 * in the dump file; each per-database worker will preload blocks until
 * it sees a block for some other database.  Sorting by tablespace,
 * filenode, forknum, and blocknum isn't critical for correctness, but
 * helps us get a sequential I/O pattern.
 */
static int
apw_compare_blockinfo(const void *p, const void *q)
{
    const BlockInfoRecord *a = (const BlockInfoRecord *) p;
    const BlockInfoRecord *b = (const BlockInfoRecord *) q;

    cmp_member_elem(database);
    cmp_member_elem(tablespace);
    cmp_member_elem(filenode);
    cmp_member_elem(forknum);
    cmp_member_elem(blocknum);

    return 0;
}