worker_spi.c

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/* -------------------------------------------------------------------------
 *
 * worker_spi.c
 *      Sample background worker code that demonstrates various coding
 *      patterns: establishing a database connection; starting and committing
 *      transactions; using GUC variables, and heeding SIGHUP to reread
 *      the configuration file; reporting to pg_stat_activity; using the
 *      process latch to sleep and exit in case of postmaster death.
 *
 * This code connects to a database, creates a schema and table, and summarizes
 * the numbers contained therein.  To see it working, insert an initial value
 * with "total" type and some initial value; then insert some other rows with
 * "delta" type.  Delta rows will be deleted by this worker and their values
 * aggregated into the total.
 *
 * Copyright (c) 2013-2023, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *      src/test/modules/worker_spi/worker_spi.c
 *
 * -------------------------------------------------------------------------
 */
#include "postgres.h"
 
/* These are always necessary for a bgworker */
#include "miscadmin.h"
#include "postmaster/bgworker.h"
#include "postmaster/interrupt.h"
#include "storage/ipc.h"
#include "storage/latch.h"
#include "storage/lwlock.h"
#include "storage/proc.h"
#include "storage/shmem.h"
 
/* these headers are used by this particular worker's code */
#include "access/xact.h"
#include "executor/spi.h"
#include "fmgr.h"
#include "lib/stringinfo.h"
#include "pgstat.h"
#include "utils/builtins.h"
#include "utils/snapmgr.h"
#include "tcop/utility.h"
 
PG_MODULE_MAGIC;
 
PG_FUNCTION_INFO_V1(worker_spi_launch);
 
PGDLLEXPORT void worker_spi_main(Datum main_arg) pg_attribute_noreturn();
 
/* GUC variables */
static int  worker_spi_naptime = 10;
static int  worker_spi_total_workers = 2;
static char *worker_spi_database = NULL;
 
/* value cached, fetched from shared memory */
static uint32 worker_spi_wait_event_main = 0;
 
typedef struct worktable
{
    const char *schema;
    const char *name;
} worktable;
 
/*
 * Initialize workspace for a worker process: create the schema if it doesn't
 * already exist.
 */
static void
initialize_worker_spi(worktable *table)
{
    int         ret;
    int         ntup;
    bool        isnull;
    StringInfoData buf;
 
    SetCurrentStatementStartTimestamp();
    StartTransactionCommand();
    SPI_connect();
    PushActiveSnapshot(GetTransactionSnapshot());
    pgstat_report_activity(STATE_RUNNING, "initializing worker_spi schema");
 
    /* XXX could we use CREATE SCHEMA IF NOT EXISTS? */
    initStringInfo(&buf);
    appendStringInfo(&buf, "select count(*) from pg_namespace where nspname = '%s'",
                     table->schema);
 
    debug_query_string = buf.data;
    ret = SPI_execute(buf.data, true, 0);
    if (ret != SPI_OK_SELECT)
        elog(FATAL, "SPI_execute failed: error code %d", ret);
 
    if (SPI_processed != 1)
        elog(FATAL, "not a singleton result");
 
    ntup = DatumGetInt64(SPI_getbinval(SPI_tuptable->vals[0],
                                       SPI_tuptable->tupdesc,
                                       1, &isnull));
    if (isnull)
        elog(FATAL, "null result");
 
    if (ntup == 0)
    {
        debug_query_string = NULL;
        resetStringInfo(&buf);
        appendStringInfo(&buf,
                         "CREATE SCHEMA \"%s\" "
                         "CREATE TABLE \"%s\" ("
                         "      type text CHECK (type IN ('total', 'delta')), "
                         "      value   integer)"
                         "CREATE UNIQUE INDEX \"%s_unique_total\" ON \"%s\" (type) "
                         "WHERE type = 'total'",
                         table->schema, table->name, table->name, table->name);
 
        /* set statement start time */
        SetCurrentStatementStartTimestamp();
 
        debug_query_string = buf.data;
        ret = SPI_execute(buf.data, false, 0);
 
        if (ret != SPI_OK_UTILITY)
            elog(FATAL, "failed to create my schema");
 
        debug_query_string = NULL;  /* rest is not statement-specific */
    }
 
    SPI_finish();
    PopActiveSnapshot();
    CommitTransactionCommand();
    debug_query_string = NULL;
    pgstat_report_activity(STATE_IDLE, NULL);
}
 
void
worker_spi_main(Datum main_arg)
{
    int         index = DatumGetInt32(main_arg);
    worktable  *table;
    StringInfoData buf;
    char        name[20];
 
    table = palloc(sizeof(worktable));
    sprintf(name, "schema%d", index);
    table->schema = pstrdup(name);
    table->name = pstrdup("counted");
 
    /* Establish signal handlers before unblocking signals. */
    pqsignal(SIGHUP, SignalHandlerForConfigReload);
    pqsignal(SIGTERM, die);
 
    /* We're now ready to receive signals */
    BackgroundWorkerUnblockSignals();
 
    /* Connect to our database */
    BackgroundWorkerInitializeConnection(worker_spi_database, NULL, 0);
 
    elog(LOG, "%s initialized with %s.%s",
         MyBgworkerEntry->bgw_name, table->schema, table->name);
    initialize_worker_spi(table);
 
    /*
     * Quote identifiers passed to us.  Note that this must be done after
     * initialize_worker_spi, because that routine assumes the names are not
     * quoted.
     *
     * Note some memory might be leaked here.
     */
    table->schema = quote_identifier(table->schema);
    table->name = quote_identifier(table->name);
 
    initStringInfo(&buf);
    appendStringInfo(&buf,
                     "WITH deleted AS (DELETE "
                     "FROM %s.%s "
                     "WHERE type = 'delta' RETURNING value), "
                     "total AS (SELECT coalesce(sum(value), 0) as sum "
                     "FROM deleted) "
                     "UPDATE %s.%s "
                     "SET value = %s.value + total.sum "
                     "FROM total WHERE type = 'total' "
                     "RETURNING %s.value",
                     table->schema, table->name,
                     table->schema, table->name,
                     table->name,
                     table->name);
 
    /*
     * Main loop: do this until SIGTERM is received and processed by
     * ProcessInterrupts.
     */
    for (;;)
    {
        int         ret;
 
        /* First time, allocate or get the custom wait event */
        if (worker_spi_wait_event_main == 0)
            worker_spi_wait_event_main = WaitEventExtensionNew("worker_spi_main");
 
        /*
         * Background workers mustn't call usleep() or any direct equivalent:
         * instead, they may wait on their process latch, which sleeps as
         * necessary, but is awakened if postmaster dies.  That way the
         * background process goes away immediately in an emergency.
         */
        (void) WaitLatch(MyLatch,
                         WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
                         worker_spi_naptime * 1000L,
                         worker_spi_wait_event_main);
        ResetLatch(MyLatch);
 
        CHECK_FOR_INTERRUPTS();
 
        /*
         * In case of a SIGHUP, just reload the configuration.
         */
        if (ConfigReloadPending)
        {
            ConfigReloadPending = false;
            ProcessConfigFile(PGC_SIGHUP);
        }
 
        /*
         * Start a transaction on which we can run queries.  Note that each
         * StartTransactionCommand() call should be preceded by a
         * SetCurrentStatementStartTimestamp() call, which sets both the time
         * for the statement we're about the run, and also the transaction
         * start time.  Also, each other query sent to SPI should probably be
         * preceded by SetCurrentStatementStartTimestamp(), so that statement
         * start time is always up to date.
         *
         * The SPI_connect() call lets us run queries through the SPI manager,
         * and the PushActiveSnapshot() call creates an "active" snapshot
         * which is necessary for queries to have MVCC data to work on.
         *
         * The pgstat_report_activity() call makes our activity visible
         * through the pgstat views.
         */
        SetCurrentStatementStartTimestamp();
        StartTransactionCommand();
        SPI_connect();
        PushActiveSnapshot(GetTransactionSnapshot());
        debug_query_string = buf.data;
        pgstat_report_activity(STATE_RUNNING, buf.data);
 
        /* We can now execute queries via SPI */
        ret = SPI_execute(buf.data, false, 0);
 
        if (ret != SPI_OK_UPDATE_RETURNING)
            elog(FATAL, "cannot select from table %s.%s: error code %d",
                 table->schema, table->name, ret);
 
        if (SPI_processed > 0)
        {
            bool        isnull;
            int32       val;
 
            val = DatumGetInt32(SPI_getbinval(SPI_tuptable->vals[0],
                                              SPI_tuptable->tupdesc,
                                              1, &isnull));
            if (!isnull)
                elog(LOG, "%s: count in %s.%s is now %d",
                     MyBgworkerEntry->bgw_name,
                     table->schema, table->name, val);
        }
 
        /*
         * And finish our transaction.
         */
        SPI_finish();
        PopActiveSnapshot();
        CommitTransactionCommand();
        debug_query_string = NULL;
        pgstat_report_stat(true);
        pgstat_report_activity(STATE_IDLE, NULL);
    }
 
    /* Not reachable */
}
 
/*
 * Entrypoint of this module.
 *
 * We register more than one worker process here, to demonstrate how that can
 * be done.
 */
void
_PG_init(void)
{
    BackgroundWorker worker;
 
    /* get the configuration */
 
    /*
     * These GUCs are defined even if this library is not loaded with
     * shared_preload_libraries, for worker_spi_launch().
     */
    DefineCustomIntVariable("worker_spi.naptime",
                            "Duration between each check (in seconds).",
                            NULL,
                            &worker_spi_naptime,
                            10,
                            1,
                            INT_MAX,
                            PGC_SIGHUP,
                            0,
                            NULL,
                            NULL,
                            NULL);
 
    DefineCustomStringVariable("worker_spi.database",
                               "Database to connect to.",
                               NULL,
                               &worker_spi_database,
                               "postgres",
                               PGC_SIGHUP,
                               0,
                               NULL, NULL, NULL);
 
    if (!process_shared_preload_libraries_in_progress)
        return;
 
    DefineCustomIntVariable("worker_spi.total_workers",
                            "Number of workers.",
                            NULL,
                            &worker_spi_total_workers,
                            2,
                            1,
                            100,
                            PGC_POSTMASTER,
                            0,
                            NULL,
                            NULL,
                            NULL);
 
    MarkGUCPrefixReserved("worker_spi");
 
    /* set up common data for all our workers */
    memset(&worker, 0, sizeof(worker));
    worker.bgw_flags = BGWORKER_SHMEM_ACCESS |
        BGWORKER_BACKEND_DATABASE_CONNECTION;
    worker.bgw_start_time = BgWorkerStart_RecoveryFinished;
    worker.bgw_restart_time = BGW_NEVER_RESTART;
    sprintf(worker.bgw_library_name, "worker_spi");
    sprintf(worker.bgw_function_name, "worker_spi_main");
    worker.bgw_notify_pid = 0;
 
    /*
     * Now fill in worker-specific data, and do the actual registrations.
     */
    for (int i = 1; i <= worker_spi_total_workers; i++)
    {
        snprintf(worker.bgw_name, BGW_MAXLEN, "worker_spi worker %d", i);
        snprintf(worker.bgw_type, BGW_MAXLEN, "worker_spi");
        worker.bgw_main_arg = Int32GetDatum(i);
 
        RegisterBackgroundWorker(&worker);
    }
}
 
/*
 * Dynamically launch an SPI worker.
 */
Datum
worker_spi_launch(PG_FUNCTION_ARGS)
{
    int32       i = PG_GETARG_INT32(0);
    BackgroundWorker worker;
    BackgroundWorkerHandle *handle;
    BgwHandleStatus status;
    pid_t       pid;
 
    memset(&worker, 0, sizeof(worker));
    worker.bgw_flags = BGWORKER_SHMEM_ACCESS |
        BGWORKER_BACKEND_DATABASE_CONNECTION;
    worker.bgw_start_time = BgWorkerStart_RecoveryFinished;
    worker.bgw_restart_time = BGW_NEVER_RESTART;
    sprintf(worker.bgw_library_name, "worker_spi");
    sprintf(worker.bgw_function_name, "worker_spi_main");
    snprintf(worker.bgw_name, BGW_MAXLEN, "worker_spi dynamic worker %d", i);
    snprintf(worker.bgw_type, BGW_MAXLEN, "worker_spi dynamic");
    worker.bgw_main_arg = Int32GetDatum(i);
    /* set bgw_notify_pid so that we can use WaitForBackgroundWorkerStartup */
    worker.bgw_notify_pid = MyProcPid;
 
    if (!RegisterDynamicBackgroundWorker(&worker, &handle))
        PG_RETURN_NULL();
 
    status = WaitForBackgroundWorkerStartup(handle, &pid);
 
    if (status == BGWH_STOPPED)
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_RESOURCES),
                 errmsg("could not start background process"),
                 errhint("More details may be available in the server log.")));
    if (status == BGWH_POSTMASTER_DIED)
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_RESOURCES),
                 errmsg("cannot start background processes without postmaster"),
                 errhint("Kill all remaining database processes and restart the database.")));
    Assert(status == BGWH_STARTED);
 
    PG_RETURN_INT32(pid);
}