Preserve CurrentMemoryContext across notify and sinval interrupts.

[pgsql.git] / contrib / pg_prewarm / autoprewarm.c

/*-------------------------------------------------------------------------
 *
 * 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) 2016-2024, 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 "pgstat.h"
#include "postmaster/bgworker.h"
#include "postmaster/interrupt.h"
#include "storage/buf_internals.h"
#include "storage/dsm.h"
#include "storage/dsm_registry.h"
#include "storage/fd.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/relfilenumbermap.h"
#include "utils/resowner.h"

#define AUTOPREWARM_FILE "autoprewarm.blocks"

/* Metadata for each block we dump. */
typedef struct BlockInfoRecord
{
        Oid                     database;
        Oid                     tablespace;
        RelFileNumber filenumber;
        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;

PGDLLEXPORT void autoprewarm_main(Datum main_arg);
PGDLLEXPORT 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 = 300; /* 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);

        MarkGUCPrefixReserved("pg_prewarm");

        /* 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.
         *
         * NB: Autoprewarm's state is stored in a DSM segment, and DSM segments
         * are detached before calling the on_shmem_exit callbacks, so we must put
         * apw_detach_shmem in the before_shmem_exit callback list.
         */
        before_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 %d",
                                                (int) 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 %d",
                                                (int) 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].filenumber,
                                   &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. */
        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->filenumber != blk->filenumber &&
                        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->filenumber != blk->filenumber)
                {
                        Oid                     reloid;

                        Assert(rel == NULL);
                        StartTransactionCommand();
                        reloid = RelidByRelfilenumber(blk->tablespace, blk->filenumber);
                        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->filenumber != blk->filenumber ||
                        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 %d",
                                                        (int) apw_state->pid_using_dumpfile)));

                ereport(LOG,
                                (errmsg("skipping block dump because it is already being performed by PID %d",
                                                (int) 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.dbOid;
                        block_info_array[num_blocks].tablespace = bufHdr->tag.spcOid;
                        block_info_array[num_blocks].filenumber =
                                BufTagGetRelNumber(&bufHdr->tag);
                        block_info_array[num_blocks].forknum =
                                BufTagGetForkNum(&bufHdr->tag);
                        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].filenumber,
                                          (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 %d",
                                                (int) 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);
}

static void
apw_init_state(void *ptr)
{
        AutoPrewarmSharedState *state = (AutoPrewarmSharedState *) ptr;

        LWLockInitialize(&state->lock, LWLockNewTrancheId());
        state->bgworker_pid = InvalidPid;
        state->pid_using_dumpfile = InvalidPid;
}

/*
 * 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;

        apw_state = GetNamedDSMSegment("autoprewarm",
                                                                   sizeof(AutoPrewarmSharedState),
                                                                   apw_init_state,
                                                                   &found);
        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 the configuration parameter \"%s\".", "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,
 * filenumber, 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(filenumber);
        cmp_member_elem(forknum);
        cmp_member_elem(blocknum);

        return 0;
}