pg_stash_advice.c

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/*-------------------------------------------------------------------------
 *
 * pg_stash_advice.c
 *    core infrastructure for pg_stash_advice contrib module
 *
 * Copyright (c) 2016-2026, PostgreSQL Global Development Group
 *
 *    contrib/pg_stash_advice/pg_stash_advice.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include "common/hashfn.h"
#include "common/string.h"
#include "miscadmin.h"
#include "nodes/queryjumble.h"
#include "pg_plan_advice.h"
#include "pg_stash_advice.h"
#include "postmaster/bgworker.h"
#include "storage/dsm_registry.h"
#include "utils/guc.h"
#include "utils/memutils.h"
 
PG_MODULE_MAGIC;
 
/* Shared memory hash table parameters */
static dshash_parameters pgsa_stash_dshash_parameters = {
    NAMEDATALEN,
    sizeof(pgsa_stash),
    dshash_strcmp,
    dshash_strhash,
    dshash_strcpy,
    LWTRANCHE_INVALID           /* gets set at runtime */
};
 
static dshash_parameters pgsa_entry_dshash_parameters = {
    sizeof(pgsa_entry_key),
    sizeof(pgsa_entry),
    dshash_memcmp,
    dshash_memhash,
    dshash_memcpy,
    LWTRANCHE_INVALID           /* gets set at runtime */
};
 
/* GUC variables */
static char *pg_stash_advice_stash_name = "";
bool        pg_stash_advice_persist = true;
int         pg_stash_advice_persist_interval = 30;
 
/* Shared memory pointers */
pgsa_shared_state *pgsa_state;
dsa_area   *pgsa_dsa_area;
dshash_table *pgsa_stash_dshash;
dshash_table *pgsa_entry_dshash;
 
/* Other global variables */
static MemoryContext pg_stash_advice_mcxt;
 
/* Function prototypes */
static char *pgsa_advisor(PlannerGlobal *glob,
                          Query *parse,
                          const char *query_string,
                          int cursorOptions,
                          ExplainState *es);
static bool pgsa_check_stash_name_guc(char **newval, void **extra,
                                      GucSource source);
static void pgsa_init_shared_state(void *ptr, void *arg);
static bool pgsa_is_identifier(char *str);
 
/* Stash name -> stash ID hash table */
#define SH_PREFIX pgsa_stash_name_table
#define SH_ELEMENT_TYPE pgsa_stash_name
#define SH_KEY_TYPE uint64
#define SH_KEY pgsa_stash_id
#define SH_HASH_KEY(tb, key) hash_bytes((const unsigned char *) &(key), sizeof(uint64))
#define SH_EQUAL(tb, a, b) (a == b)
#define SH_SCOPE extern
#define SH_DEFINE
#include "lib/simplehash.h"
 
/*
 * Initialize this module.
 */
void
_PG_init(void)
{
    void        (*add_advisor_fn) (pg_plan_advice_advisor_hook hook);
 
    /* If compute_query_id = 'auto', we would like query IDs. */
    EnableQueryId();
 
    /* Define our GUCs. */
    if (process_shared_preload_libraries_in_progress)
        DefineCustomBoolVariable("pg_stash_advice.persist",
                                 "Save and restore advice stash contents across restarts.",
                                 NULL,
                                 &pg_stash_advice_persist,
                                 true,
                                 PGC_POSTMASTER,
                                 0,
                                 NULL,
                                 NULL,
                                 NULL);
    else
        pg_stash_advice_persist = false;
 
    DefineCustomIntVariable("pg_stash_advice.persist_interval",
                            "Interval between advice stash saves, in seconds.",
                            NULL,
                            &pg_stash_advice_persist_interval,
                            30,
                            0,
                            3600,
                            PGC_SIGHUP,
                            GUC_UNIT_S,
                            NULL,
                            NULL,
                            NULL);
 
    DefineCustomStringVariable("pg_stash_advice.stash_name",
                               "Name of the advice stash to be used in this session.",
                               NULL,
                               &pg_stash_advice_stash_name,
                               "",
                               PGC_USERSET,
                               0,
                               pgsa_check_stash_name_guc,
                               NULL,
                               NULL);
 
    MarkGUCPrefixReserved("pg_stash_advice");
 
    /* Start the background worker for persistence, if enabled. */
    if (pg_stash_advice_persist)
        pgsa_start_worker();
 
    /* Tell pg_plan_advice that we want to provide advice strings. */
    add_advisor_fn =
        load_external_function("pg_plan_advice", "pg_plan_advice_add_advisor",
                               true, NULL);
    (*add_advisor_fn) (pgsa_advisor);
}
 
/*
 * Get the advice string that has been configured for this query, if any,
 * and return it. Otherwise, return NULL.
 */
static char *
pgsa_advisor(PlannerGlobal *glob, Query *parse,
             const char *query_string, int cursorOptions,
             ExplainState *es)
{
    pgsa_entry_key key;
    pgsa_entry *entry;
    char       *advice_string;
    uint64      stash_id;
 
    /*
     * Exit quickly if the stash name is empty or there's no query ID.
     */
    if (pg_stash_advice_stash_name[0] == '\0' || parse->queryId == 0)
        return NULL;
 
    /* Attach to dynamic shared memory if not already done. */
    if (unlikely(pgsa_entry_dshash == NULL))
        pgsa_attach();
 
    /* If stash data is still being restored from disk, ignore. */
    if (pg_atomic_unlocked_test_flag(&pgsa_state->stashes_ready))
        return NULL;
 
    /*
     * Translate pg_stash_advice.stash_name to an integer ID.
     *
     * pgsa_check_stash_name_guc() has already validated the advice stash
     * name, so we don't need to call pgsa_check_stash_name() here.
     */
    stash_id = pgsa_lookup_stash_id(pg_stash_advice_stash_name);
    if (stash_id == 0)
        return NULL;
 
    /*
     * Look up the advice string for the given stash ID + query ID.
     *
     * If we find an advice string, we copy it into the current memory
     * context, presumably short-lived, so that we can release the lock on the
     * dshash entry. pg_plan_advice only needs the value to remain allocated
     * long enough for it to be parsed, so this should be good enough.
     */
    memset(&key, 0, sizeof(pgsa_entry_key));
    key.pgsa_stash_id = stash_id;
    key.queryId = parse->queryId;
    entry = dshash_find(pgsa_entry_dshash, &key, false);
    if (entry == NULL)
        return NULL;
    if (entry->advice_string == InvalidDsaPointer)
        advice_string = NULL;
    else
        advice_string = pstrdup(dsa_get_address(pgsa_dsa_area,
                                                entry->advice_string));
    dshash_release_lock(pgsa_entry_dshash, entry);
 
    /* If we found an advice string, emit a debug message. */
    if (advice_string != NULL)
        elog(DEBUG2, "supplying automatic advice for stash \"%s\", query ID %" PRId64 ": %s",
             pg_stash_advice_stash_name, key.queryId, advice_string);
 
    return advice_string;
}
 
/*
 * Attach to various structures in dynamic shared memory.
 *
 * This function is designed to be resilient against errors. That is, if it
 * fails partway through, it should be possible to call it again, repeat no
 * work already completed, and potentially succeed or at least get further if
 * whatever caused the previous failure has been corrected.
 */
void
pgsa_attach(void)
{
    bool        found;
    MemoryContext oldcontext;
 
    /*
     * Create a memory context to make sure that any control structures
     * allocated in local memory are sufficiently persistent.
     */
    if (pg_stash_advice_mcxt == NULL)
        pg_stash_advice_mcxt = AllocSetContextCreate(TopMemoryContext,
                                                     "pg_stash_advice",
                                                     ALLOCSET_DEFAULT_SIZES);
    oldcontext = MemoryContextSwitchTo(pg_stash_advice_mcxt);
 
    /* Attach to the fixed-size state object if not already done. */
    if (pgsa_state == NULL)
        pgsa_state = GetNamedDSMSegment("pg_stash_advice",
                                        sizeof(pgsa_shared_state),
                                        pgsa_init_shared_state,
                                        &found, NULL);
 
    /* Attach to the DSA area if not already done. */
    if (pgsa_dsa_area == NULL)
    {
        dsa_handle  area_handle;
 
        LWLockAcquire(&pgsa_state->lock, LW_EXCLUSIVE);
        area_handle = pgsa_state->area;
        if (area_handle == DSA_HANDLE_INVALID)
        {
            pgsa_dsa_area = dsa_create(pgsa_state->dsa_tranche);
            dsa_pin(pgsa_dsa_area);
            pgsa_state->area = dsa_get_handle(pgsa_dsa_area);
            LWLockRelease(&pgsa_state->lock);
        }
        else
        {
            LWLockRelease(&pgsa_state->lock);
            pgsa_dsa_area = dsa_attach(area_handle);
        }
        dsa_pin_mapping(pgsa_dsa_area);
    }
 
    /* Attach to the stash_name->stash_id hash table if not already done. */
    if (pgsa_stash_dshash == NULL)
    {
        dshash_table_handle stash_handle;
 
        LWLockAcquire(&pgsa_state->lock, LW_EXCLUSIVE);
        pgsa_stash_dshash_parameters.tranche_id = pgsa_state->stash_tranche;
        stash_handle = pgsa_state->stash_hash;
        if (stash_handle == DSHASH_HANDLE_INVALID)
        {
            pgsa_stash_dshash = dshash_create(pgsa_dsa_area,
                                              &pgsa_stash_dshash_parameters,
                                              NULL);
            pgsa_state->stash_hash =
                dshash_get_hash_table_handle(pgsa_stash_dshash);
            LWLockRelease(&pgsa_state->lock);
        }
        else
        {
            LWLockRelease(&pgsa_state->lock);
            pgsa_stash_dshash = dshash_attach(pgsa_dsa_area,
                                              &pgsa_stash_dshash_parameters,
                                              stash_handle, NULL);
        }
    }
 
    /* Attach to the entry hash table if not already done. */
    if (pgsa_entry_dshash == NULL)
    {
        dshash_table_handle entry_handle;
 
        LWLockAcquire(&pgsa_state->lock, LW_EXCLUSIVE);
        pgsa_entry_dshash_parameters.tranche_id = pgsa_state->entry_tranche;
        entry_handle = pgsa_state->entry_hash;
        if (entry_handle == DSHASH_HANDLE_INVALID)
        {
            pgsa_entry_dshash = dshash_create(pgsa_dsa_area,
                                              &pgsa_entry_dshash_parameters,
                                              NULL);
            pgsa_state->entry_hash =
                dshash_get_hash_table_handle(pgsa_entry_dshash);
            LWLockRelease(&pgsa_state->lock);
        }
        else
        {
            LWLockRelease(&pgsa_state->lock);
            pgsa_entry_dshash = dshash_attach(pgsa_dsa_area,
                                              &pgsa_entry_dshash_parameters,
                                              entry_handle, NULL);
        }
    }
 
    /* Restore previous memory context. */
    MemoryContextSwitchTo(oldcontext);
}
 
/*
 * Error out if the stashes have not been loaded from disk yet.
 */
void
pgsa_check_lockout(void)
{
    if (pg_atomic_unlocked_test_flag(&pgsa_state->stashes_ready))
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("stash modifications are not allowed because \"%s\" has not been loaded yet",
                        PGSA_DUMP_FILE)));
}
 
/*
 * Check whether an advice stash name is legal, and signal an error if not.
 *
 * Keep this in sync with pgsa_check_stash_name_guc, below.
 */
void
pgsa_check_stash_name(char *stash_name)
{
    /* Reject empty advice stash name. */
    if (stash_name[0] == '\0')
        ereport(ERROR,
                errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                errmsg("advice stash name may not be zero length"));
 
    /* Reject overlong advice stash names. */
    if (strlen(stash_name) + 1 > NAMEDATALEN)
        ereport(ERROR,
                errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                errmsg("advice stash names may not be longer than %d bytes",
                       NAMEDATALEN - 1));
 
    /*
     * Reject non-ASCII advice stash names, since advice stashes are visible
     * across all databases and the encodings of those databases might differ.
     */
    if (!pg_is_ascii(stash_name))
        ereport(ERROR,
                errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                errmsg("advice stash name must not contain non-ASCII characters"));
 
    /*
     * Reject things that do not look like identifiers, since the ability to
     * create an advice stash with non-printable characters or weird symbols
     * in the name is not likely to be useful to anyone.
     */
    if (!pgsa_is_identifier(stash_name))
        ereport(ERROR,
                errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                errmsg("advice stash name must begin with a letter or underscore and contain only letters, digits, and underscores"));
}
 
/*
 * As above, but for the GUC check_hook. We allow the empty string here,
 * though, as equivalent to disabling the feature.
 */
static bool
pgsa_check_stash_name_guc(char **newval, void **extra, GucSource source)
{
    char       *stash_name = *newval;
 
    /* Reject overlong advice stash names. */
    if (strlen(stash_name) + 1 > NAMEDATALEN)
    {
        GUC_check_errcode(ERRCODE_INVALID_PARAMETER_VALUE);
        GUC_check_errdetail("advice stash names may not be longer than %d bytes",
                            NAMEDATALEN - 1);
        return false;
    }
 
    /*
     * Reject non-ASCII advice stash names, since advice stashes are visible
     * across all databases and the encodings of those databases might differ.
     */
    if (!pg_is_ascii(stash_name))
    {
        GUC_check_errcode(ERRCODE_INVALID_PARAMETER_VALUE);
        GUC_check_errdetail("advice stash name must not contain non-ASCII characters");
        return false;
    }
 
    /*
     * Reject things that do not look like identifiers, since the ability to
     * create an advice stash with non-printable characters or weird symbols
     * in the name is not likely to be useful to anyone.
     */
    if (!pgsa_is_identifier(stash_name))
    {
        GUC_check_errcode(ERRCODE_INVALID_PARAMETER_VALUE);
        GUC_check_errdetail("advice stash name must begin with a letter or underscore and contain only letters, digits, and underscores");
        return false;
    }
 
    return true;
}
 
/*
 * Create an advice stash.
 */
void
pgsa_create_stash(char *stash_name)
{
    pgsa_stash *stash;
    bool        found;
 
    Assert(LWLockHeldByMeInMode(&pgsa_state->lock, LW_EXCLUSIVE));
 
    /* Create a stash with this name, unless one already exists. */
    stash = dshash_find_or_insert(pgsa_stash_dshash, stash_name, &found);
    if (found)
        ereport(ERROR,
                errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                errmsg("advice stash \"%s\" already exists", stash_name));
    stash->pgsa_stash_id = pgsa_state->next_stash_id++;
    dshash_release_lock(pgsa_stash_dshash, stash);
 
    /* Bump change count. */
    pg_atomic_add_fetch_u64(&pgsa_state->change_count, 1);
}
 
/*
 * Remove any stored advice string for the given advice stash and query ID.
 */
void
pgsa_clear_advice_string(char *stash_name, int64 queryId)
{
    pgsa_entry *entry;
    pgsa_entry_key key;
    uint64      stash_id;
    dsa_pointer old_dp;
 
    Assert(LWLockHeldByMe(&pgsa_state->lock));
 
    /* Translate the stash name to an integer ID. */
    if ((stash_id = pgsa_lookup_stash_id(stash_name)) == 0)
        ereport(ERROR,
                errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                errmsg("advice stash \"%s\" does not exist", stash_name));
 
    /*
     * Look for an existing entry, and free it. But, be sure to save the
     * pointer to the associated advice string, if any.
     */
    memset(&key, 0, sizeof(pgsa_entry_key));
    key.pgsa_stash_id = stash_id;
    key.queryId = queryId;
    entry = dshash_find(pgsa_entry_dshash, &key, true);
    if (entry == NULL)
        old_dp = InvalidDsaPointer;
    else
    {
        old_dp = entry->advice_string;
        dshash_delete_entry(pgsa_entry_dshash, entry);
    }
 
    /* Now we free the advice string as well, if there was one. */
    if (old_dp != InvalidDsaPointer)
        dsa_free(pgsa_dsa_area, old_dp);
 
    /* Bump change count. */
    pg_atomic_add_fetch_u64(&pgsa_state->change_count, 1);
}
 
/*
 * Drop an advice stash.
 */
void
pgsa_drop_stash(char *stash_name)
{
    pgsa_entry *entry;
    pgsa_stash *stash;
    dshash_seq_status iterator;
    uint64      stash_id;
 
    Assert(LWLockHeldByMeInMode(&pgsa_state->lock, LW_EXCLUSIVE));
 
    /* Remove the entry for this advice stash. */
    stash = dshash_find(pgsa_stash_dshash, stash_name, true);
    if (stash == NULL)
        ereport(ERROR,
                errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                errmsg("advice stash \"%s\" does not exist", stash_name));
    stash_id = stash->pgsa_stash_id;
    dshash_delete_entry(pgsa_stash_dshash, stash);
 
    /*
     * Now remove all the entries. Since pgsa_state->lock must be held at
     * least in shared mode to insert entries into pgsa_entry_dshash, it
     * doesn't matter whether we do this before or after deleting the entry
     * from pgsa_stash_dshash.
     */
    dshash_seq_init(&iterator, pgsa_entry_dshash, true);
    while ((entry = dshash_seq_next(&iterator)) != NULL)
    {
        if (stash_id == entry->key.pgsa_stash_id)
        {
            if (entry->advice_string != InvalidDsaPointer)
                dsa_free(pgsa_dsa_area, entry->advice_string);
            dshash_delete_current(&iterator);
        }
    }
    dshash_seq_term(&iterator);
 
    /* Bump change count. */
    pg_atomic_add_fetch_u64(&pgsa_state->change_count, 1);
}
 
/*
 * Remove all stashes and entries from shared memory.
 *
 * This is intended to be called before reloading from a dump file, so that
 * a failed previous attempt doesn't leave stale data behind.
 */
void
pgsa_reset_all_stashes(void)
{
    dshash_seq_status iter;
    pgsa_entry *entry;
 
    Assert(LWLockHeldByMeInMode(&pgsa_state->lock, LW_EXCLUSIVE));
 
    /* Remove all stashes. */
    dshash_seq_init(&iter, pgsa_stash_dshash, true);
    while (dshash_seq_next(&iter) != NULL)
        dshash_delete_current(&iter);
    dshash_seq_term(&iter);
 
    /* Remove all entries. */
    dshash_seq_init(&iter, pgsa_entry_dshash, true);
    while ((entry = dshash_seq_next(&iter)) != NULL)
    {
        if (entry->advice_string != InvalidDsaPointer)
            dsa_free(pgsa_dsa_area, entry->advice_string);
        dshash_delete_current(&iter);
    }
    dshash_seq_term(&iter);
 
    /* Reset the stash ID counter. */
    pgsa_state->next_stash_id = UINT64CONST(1);
}
 
/*
 * Initialize shared state when first created.
 */
static void
pgsa_init_shared_state(void *ptr, void *arg)
{
    pgsa_shared_state *state = (pgsa_shared_state *) ptr;
 
    LWLockInitialize(&state->lock,
                     LWLockNewTrancheId("pg_stash_advice_lock"));
    state->dsa_tranche = LWLockNewTrancheId("pg_stash_advice_dsa");
    state->stash_tranche = LWLockNewTrancheId("pg_stash_advice_stash");
    state->entry_tranche = LWLockNewTrancheId("pg_stash_advice_entry");
    state->next_stash_id = UINT64CONST(1);
    state->area = DSA_HANDLE_INVALID;
    state->stash_hash = DSHASH_HANDLE_INVALID;
    state->entry_hash = DSHASH_HANDLE_INVALID;
    state->bgworker_pid = InvalidPid;
    pg_atomic_init_flag(&state->stashes_ready);
    pg_atomic_init_u64(&state->change_count, 0);
 
    /*
     * If this module was loaded via shared_preload_libraries, then
     * pg_stash_advice_persist is a GUC variable. If it's true, that means
     * that we should lock out manual stash modifications until the dump file
     * has been successfully loaded. If it's false, there's nothing to load,
     * so we set stashes_ready immediately.
     *
     * If this module was not loaded via shared_preload_libraries, then
     * pg_stash_advice_persist is not a GUC variable, but it will be false,
     * which leads to the correct behavior.
     */
    if (!pg_stash_advice_persist)
        pg_atomic_test_set_flag(&state->stashes_ready);
}
 
/*
 * Check whether a string looks like a valid identifier. It must contain only
 * ASCII identifier characters, and must not begin with a digit.
 */
static bool
pgsa_is_identifier(char *str)
{
    if (*str >= '0' && *str <= '9')
        return false;
 
    while (*str != '\0')
    {
        char        c = *str++;
 
        if ((c < '0' || c > '9') && (c < 'a' || c > 'z') &&
            (c < 'A' || c > 'Z') && c != '_')
            return false;
    }
 
    return true;
}
 
/*
 * Look up the integer ID that corresponds to the given stash name.
 *
 * Returns 0 if no such stash exists.
 */
uint64
pgsa_lookup_stash_id(char *stash_name)
{
    pgsa_stash *stash;
    uint64      stash_id;
 
    /* Search the shared hash table. */
    stash = dshash_find(pgsa_stash_dshash, stash_name, false);
    if (stash == NULL)
        return 0;
    stash_id = stash->pgsa_stash_id;
    dshash_release_lock(pgsa_stash_dshash, stash);
 
    return stash_id;
}
 
/*
 * Store a new or updated advice string for the given advice stash and query ID.
 */
void
pgsa_set_advice_string(char *stash_name, int64 queryId, char *advice_string)
{
    pgsa_entry *entry;
    bool        found;
    pgsa_entry_key key;
    uint64      stash_id;
    dsa_pointer new_dp;
    dsa_pointer old_dp;
 
    /*
     * The caller must hold our lock, at least in shared mode.  This is
     * important for two reasons.
     *
     * First, it holds off interrupts, so that we can't bail out of this code
     * after allocating DSA memory for the advice string and before storing
     * the resulting pointer somewhere that others can find it.
     *
     * Second, we need to avoid a race against pgsa_drop_stash(). That
     * function removes a stash_name->stash_id mapping and all the entries for
     * that stash_id. Without the lock, there's a race condition no matter
     * which of those things it does first, because as soon as we've looked up
     * the stash ID, that whole function can execute before we do the rest of
     * our work, which would result in us adding an entry for a stash that no
     * longer exists.
     */
    Assert(LWLockHeldByMe(&pgsa_state->lock));
 
    /* Look up the stash ID. */
    if ((stash_id = pgsa_lookup_stash_id(stash_name)) == 0)
        ereport(ERROR,
                errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                errmsg("advice stash \"%s\" does not exist", stash_name));
 
    /* Allocate space for the advice string. */
    new_dp = dsa_allocate(pgsa_dsa_area, strlen(advice_string) + 1);
    strcpy(dsa_get_address(pgsa_dsa_area, new_dp), advice_string);
 
    /* Attempt to insert an entry into the hash table. */
    memset(&key, 0, sizeof(pgsa_entry_key));
    key.pgsa_stash_id = stash_id;
    key.queryId = queryId;
    entry = dshash_find_or_insert_extended(pgsa_entry_dshash, &key, &found,
                                           DSHASH_INSERT_NO_OOM);
 
    /*
     * If it didn't work, bail out, being careful to free the shared memory
     * we've already allocated before, since error cleanup will not do so.
     */
    if (entry == NULL)
    {
        dsa_free(pgsa_dsa_area, new_dp);
        ereport(ERROR,
                errcode(ERRCODE_OUT_OF_MEMORY),
                errmsg("out of memory"),
                errdetail("could not insert advice string into shared hash table"));
    }
 
    /* Update the entry and release the lock. */
    old_dp = found ? entry->advice_string : InvalidDsaPointer;
    entry->advice_string = new_dp;
    dshash_release_lock(pgsa_entry_dshash, entry);
 
    /*
     * We're not safe from leaks yet!
     *
     * There's now a pointer to new_dp in the entry that we just updated, but
     * that means that there's no longer anything pointing to old_dp.
     */
    if (DsaPointerIsValid(old_dp))
        dsa_free(pgsa_dsa_area, old_dp);
 
    /* Bump change count. */
    pg_atomic_add_fetch_u64(&pgsa_state->change_count, 1);
}
 
/*
 * Start our worker process.
 */
void
pgsa_start_worker(void)
{
    BackgroundWorker worker = {0};
    BackgroundWorkerHandle *handle;
    BgwHandleStatus status;
    pid_t       pid;
 
    worker.bgw_flags = BGWORKER_SHMEM_ACCESS;
    worker.bgw_start_time = BgWorkerStart_ConsistentState;
    worker.bgw_restart_time = BGW_DEFAULT_RESTART_INTERVAL;
    strcpy(worker.bgw_library_name, "pg_stash_advice");
    strcpy(worker.bgw_function_name, "pg_stash_advice_worker_main");
    strcpy(worker.bgw_name, "pg_stash_advice worker");
    strcpy(worker.bgw_type, "pg_stash_advice worker");
 
    /*
     * If process_shared_preload_libraries_in_progress = true, we may be in
     * the postmaster, in which case this will really register the worker, or
     * we may be in a child process in an EXEC_BACKEND build, in which case it
     * will silently do nothing (which is the correct behavior).
     */
    if (process_shared_preload_libraries_in_progress)
    {
        RegisterBackgroundWorker(&worker);
        return;
    }
 
    /*
     * If process_shared_preload_libraries_in_progress = false, we're being
     * asked to start the worker after system startup time. In other words,
     * unless this is single-user mode, we're not in the postmaster, so we
     * should use RegisterDynamicBackgroundWorker and then wait for startup to
     * complete. (If we do happen to be in single-user mode, this will error
     * out, which is fine.)
     */
    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.")));
}