auxprocess.c

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/*-------------------------------------------------------------------------
 * auxprocess.c
 *    functions related to auxiliary processes.
 *
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/postmaster/auxprocess.c
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
 
#include <unistd.h>
#include <signal.h>
 
#include "access/xlog.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/auxprocess.h"
#include "storage/condition_variable.h"
#include "storage/ipc.h"
#include "storage/proc.h"
#include "storage/procsignal.h"
#include "utils/memutils.h"
#include "utils/ps_status.h"
#include "utils/wait_event.h"
 
 
static void ShutdownAuxiliaryProcess(int code, Datum arg);
 
 
/*
 *   AuxiliaryProcessMainCommon
 *
 *   Common initialization code for auxiliary processes, such as the bgwriter,
 *   walwriter, walreceiver, and the startup process.
 */
void
AuxiliaryProcessMainCommon(void)
{
    Assert(IsUnderPostmaster);
 
    /* Release postmaster's working memory context */
    if (PostmasterContext)
    {
        MemoryContextDelete(PostmasterContext);
        PostmasterContext = NULL;
    }
 
    init_ps_display(NULL);
 
    Assert(GetProcessingMode() == InitProcessing);
 
    IgnoreSystemIndexes = true;
 
    /*
     * As an auxiliary process, we aren't going to do the full InitPostgres
     * pushups, but there are a couple of things that need to get lit up even
     * in an auxiliary process.
     */
 
    /*
     * Create a PGPROC so we can use LWLocks and access shared memory.
     */
    InitAuxiliaryProcess();
 
    BaseInit();
 
    /*
     * Prevent consuming interrupts between setting ProcSignalInit and setting
     * the initial local data checksum value.  If a barrier is emitted, and
     * absorbed, before local cached state is initialized the state transition
     * can be invalid.
     */
    HOLD_INTERRUPTS();
 
    ProcSignalInit(NULL, 0);
 
    /*
     * Initialize a local cache of the data_checksum_version, to be updated by
     * the procsignal-based barriers.
     *
     * This intentionally happens after initializing the procsignal, otherwise
     * we might miss a state change. This means we can get a barrier for the
     * state we've just initialized - but it can happen only once.
     *
     * The postmaster (which is what gets forked into the new child process)
     * does not handle barriers, therefore it may not have the current value
     * of LocalDataChecksumVersion value (it'll have the value read from the
     * control file, which may be arbitrarily old).
     *
     * NB: Even if the postmaster handled barriers, the value might still be
     * stale, as it might have changed after this process forked.
     */
    InitLocalDataChecksumState();
 
    RESUME_INTERRUPTS();
 
    /*
     * Auxiliary processes don't run transactions, but they may need a
     * resource owner anyway to manage buffer pins acquired outside
     * transactions (and, perhaps, other things in future).
     */
    CreateAuxProcessResourceOwner();
 
 
    /* Initialize backend status information */
    pgstat_beinit();
    pgstat_bestart_initial();
    pgstat_bestart_final();
 
    /* register a before-shutdown callback for LWLock cleanup */
    before_shmem_exit(ShutdownAuxiliaryProcess, 0);
 
    SetProcessingMode(NormalProcessing);
}
 
/*
 * Begin shutdown of an auxiliary process.  This is approximately the equivalent
 * of ShutdownPostgres() in postinit.c.  We can't run transactions in an
 * auxiliary process, so most of the work of AbortTransaction() is not needed,
 * but we do need to make sure we've released any LWLocks we are holding.
 * (This is only critical during an error exit.)
 */
static void
ShutdownAuxiliaryProcess(int code, Datum arg)
{
    LWLockReleaseAll();
    ConditionVariableCancelSleep();
    pgstat_report_wait_end();
}