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, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *      contrib/worker_spi/worker_spi.c
 *
 * -------------------------------------------------------------------------
 */
#include "postgres.h"

/* These are always necessary for a bgworker */
#include "miscadmin.h"
#include "postmaster/bgworker.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;

void    _PG_init(void);

/* flags set by signal handlers */
static volatile sig_atomic_t got_sighup = false;
static volatile sig_atomic_t got_sigterm = false;

/* GUC variables */
static int  worker_spi_naptime = 10;
static int  worker_spi_total_workers = 2;


typedef struct worktable
{
    const char     *schema;
    const char     *name;
} worktable;

/*
 * Signal handler for SIGTERM
 *      Set a flag to let the main loop to terminate, and set our latch to wake
 *      it up.
 */
static void
worker_spi_sigterm(SIGNAL_ARGS)
{
    int         save_errno = errno;

    got_sigterm = true;
    if (MyProc)
        SetLatch(&MyProc->procLatch);

    errno = save_errno;
}

/*
 * Signal handler for SIGHUP
 *      Set a flag to let the main loop to reread the config file, and set
 *      our latch to wake it up.
 */
static void
worker_spi_sighup(SIGNAL_ARGS)
{
    got_sighup = true;
    if (MyProc)
        SetLatch(&MyProc->procLatch);
}

/*
 * 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 spi_worker schema");

    /* XXX could we use CREATE SCHEMA IF NOT EXISTS? */
    initStringInfo(&buf);
    appendStringInfo(&buf, "select count(*) from pg_namespace where nspname = '%s'",
                     table->schema);

    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 = DatumGetInt32(SPI_getbinval(SPI_tuptable->vals[0],
                                       SPI_tuptable->tupdesc,
                                       1, &isnull));
    if (isnull)
        elog(FATAL, "null result");

    if (ntup == 0)
    {
        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();

        ret = SPI_execute(buf.data, false, 0);

        if (ret != SPI_OK_UTILITY)
            elog(FATAL, "failed to create my schema");
    }

    SPI_finish();
    PopActiveSnapshot();
    CommitTransactionCommand();
    pgstat_report_activity(STATE_IDLE, NULL);
}

static void
worker_spi_main(void *main_arg)
{
    worktable      *table = (worktable *) main_arg;
    StringInfoData  buf;

    /* We're now ready to receive signals */
    BackgroundWorkerUnblockSignals();

    /* Connect to our database */
    BackgroundWorkerInitializeConnection("postgres", NULL);

    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 the SIGTERM handler tells us to terminate
     */
    while (!got_sigterm)
    {
        int     ret;
        int     rc;

        /*
         * 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.
         */
        rc = WaitLatch(&MyProc->procLatch,
                       WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH,
                       worker_spi_naptime * 1000L);
        ResetLatch(&MyProc->procLatch);

        /* emergency bailout if postmaster has died */
        if (rc & WL_POSTMASTER_DEATH)
            proc_exit(1);

        /*
         * In case of a SIGHUP, just reload the configuration.
         */
        if (got_sighup)
        {
            got_sighup = 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());
        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();
        pgstat_report_activity(STATE_IDLE, NULL);
    }

    proc_exit(0);
}

/*
 * 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;
    worktable          *table;
    unsigned int        i;
    char                name[20];

    /* get the configuration */
    DefineCustomIntVariable("worker_spi.naptime",
                "Duration between each check (in seconds).",
                NULL,
                &worker_spi_naptime,
                10,
                1,
                INT_MAX,
                PGC_SIGHUP,
                0,
                NULL,
                NULL,
                NULL);
    DefineCustomIntVariable("worker_spi.total_workers",
                "Number of workers.",
                NULL,
                &worker_spi_total_workers,
                2,
                1,
                100,
                PGC_POSTMASTER,
                0,
                NULL,
                NULL,
                NULL);

    /* set up common data for all our workers */
    worker.bgw_flags = BGWORKER_SHMEM_ACCESS |
        BGWORKER_BACKEND_DATABASE_CONNECTION;
    worker.bgw_start_time = BgWorkerStart_RecoveryFinished;
    worker.bgw_restart_time = BGW_NEVER_RESTART;
    worker.bgw_main = worker_spi_main;
    worker.bgw_sighup = worker_spi_sighup;
    worker.bgw_sigterm = worker_spi_sigterm;

    /*
     * Now fill in worker-specific data, and do the actual registrations.
     */
    for (i = 1; i <= worker_spi_total_workers; i++)
    {
        sprintf(name, "worker %d", i);
        worker.bgw_name = pstrdup(name);

        table = palloc(sizeof(worktable));
        sprintf(name, "schema%d", i);
        table->schema = pstrdup(name);
        table->name = pstrdup("counted");
        worker.bgw_main_arg = (void *) table;

        RegisterBackgroundWorker(&worker);
    }
}