#include "postgres.h"
#include <signal.h>
#include <unistd.h>
#include "access/xlog.h"
#include "libpq/pqsignal.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/auxprocess.h"
#include "postmaster/interrupt.h"
#include "postmaster/walwriter.h"
#include "storage/aio_subsys.h"
#include "storage/bufmgr.h"
#include "storage/condition_variable.h"
#include "storage/fd.h"
#include "storage/lwlock.h"
#include "storage/proc.h"
#include "storage/procsignal.h"
#include "storage/smgr.h"
#include "utils/hsearch.h"
#include "utils/memutils.h"
#include "utils/resowner.h"

Include dependency graph for walwriter.c:

Macros
#define	LOOPS_UNTIL_HIBERNATE 50

#define	HIBERNATE_FACTOR 25

Functions
void	WalWriterMain (const void *startup_data, size_t startup_data_len)

Variables
int	WalWriterDelay = 200

int	WalWriterFlushAfter = DEFAULT_WAL_WRITER_FLUSH_AFTER

Macro Definition Documentation

◆ HIBERNATE_FACTOR

#define HIBERNATE_FACTOR 25

Definition at line 79 of file walwriter.c.

◆ LOOPS_UNTIL_HIBERNATE

#define LOOPS_UNTIL_HIBERNATE 50

Definition at line 78 of file walwriter.c.

Function Documentation

◆ WalWriterMain()

void WalWriterMain	(	const void *	startup_data,
		size_t	startup_data_len
	)

Definition at line 88 of file walwriter.c.

{
    sigjmp_buf  local_sigjmp_buf;
    MemoryContext walwriter_context;
    int         left_till_hibernate;
    bool        hibernating;
 
    Assert(startup_data_len == 0);
 
    MyBackendType = B_WAL_WRITER;
    AuxiliaryProcessMainCommon();
 
    /*
     * Properly accept or ignore signals the postmaster might send us
     *
     * We have no particular use for SIGINT at the moment, but seems
     * reasonable to treat like SIGTERM.
     */
    pqsignal(SIGHUP, SignalHandlerForConfigReload);
    pqsignal(SIGINT, SignalHandlerForShutdownRequest);
    pqsignal(SIGTERM, SignalHandlerForShutdownRequest);
    /* SIGQUIT handler was already set up by InitPostmasterChild */
    pqsignal(SIGALRM, SIG_IGN);
    pqsignal(SIGPIPE, SIG_IGN);
    pqsignal(SIGUSR1, procsignal_sigusr1_handler);
    pqsignal(SIGUSR2, SIG_IGN); /* not used */
 
    /*
     * Reset some signals that are accepted by postmaster but not here
     */
    pqsignal(SIGCHLD, SIG_DFL);
 
    /*
     * Create a memory context that we will do all our work in.  We do this so
     * that we can reset the context during error recovery and thereby avoid
     * possible memory leaks.  Formerly this code just ran in
     * TopMemoryContext, but resetting that would be a really bad idea.
     */
    walwriter_context = AllocSetContextCreate(TopMemoryContext,
                                              "Wal Writer",
                                              ALLOCSET_DEFAULT_SIZES);
    MemoryContextSwitchTo(walwriter_context);
 
    /*
     * If an exception is encountered, processing resumes here.
     *
     * You might wonder why this isn't coded as an infinite loop around a
     * PG_TRY construct.  The reason is that this is the bottom of the
     * exception stack, and so with PG_TRY there would be no exception handler
     * in force at all during the CATCH part.  By leaving the outermost setjmp
     * always active, we have at least some chance of recovering from an error
     * during error recovery.  (If we get into an infinite loop thereby, it
     * will soon be stopped by overflow of elog.c's internal state stack.)
     *
     * Note that we use sigsetjmp(..., 1), so that the prevailing signal mask
     * (to wit, BlockSig) will be restored when longjmp'ing to here.  Thus,
     * signals other than SIGQUIT will be blocked until we complete error
     * recovery.  It might seem that this policy makes the HOLD_INTERRUPTS()
     * call redundant, but it is not since InterruptPending might be set
     * already.
     */
    if (sigsetjmp(local_sigjmp_buf, 1) != 0)
    {
        /* Since not using PG_TRY, must reset error stack by hand */
        error_context_stack = NULL;
 
        /* Prevent interrupts while cleaning up */
        HOLD_INTERRUPTS();
 
        /* Report the error to the server log */
        EmitErrorReport();
 
        /*
         * These operations are really just a minimal subset of
         * AbortTransaction().  We don't have very many resources to worry
         * about in walwriter, but we do have LWLocks, and perhaps buffers?
         */
        LWLockReleaseAll();
        ConditionVariableCancelSleep();
        pgstat_report_wait_end();
        pgaio_error_cleanup();
        UnlockBuffers();
        ReleaseAuxProcessResources(false);
        AtEOXact_Buffers(false);
        AtEOXact_SMgr();
        AtEOXact_Files(false);
        AtEOXact_HashTables(false);
 
        /*
         * Now return to normal top-level context and clear ErrorContext for
         * next time.
         */
        MemoryContextSwitchTo(walwriter_context);
        FlushErrorState();
 
        /* Flush any leaked data in the top-level context */
        MemoryContextReset(walwriter_context);
 
        /* Now we can allow interrupts again */
        RESUME_INTERRUPTS();
 
        /*
         * Sleep at least 1 second after any error.  A write error is likely
         * to be repeated, and we don't want to be filling the error logs as
         * fast as we can.
         */
        pg_usleep(1000000L);
    }
 
    /* We can now handle ereport(ERROR) */
    PG_exception_stack = &local_sigjmp_buf;
 
    /*
     * Unblock signals (they were blocked when the postmaster forked us)
     */
    sigprocmask(SIG_SETMASK, &UnBlockSig, NULL);
 
    /*
     * Reset hibernation state after any error.
     */
    left_till_hibernate = LOOPS_UNTIL_HIBERNATE;
    hibernating = false;
    SetWalWriterSleeping(false);
 
    /*
     * Advertise our proc number that backends can use to wake us up while
     * we're sleeping.
     */
    ProcGlobal->walwriterProc = MyProcNumber;
 
    /*
     * Loop forever
     */
    for (;;)
    {
        long        cur_timeout;
 
        /*
         * Advertise whether we might hibernate in this cycle.  We do this
         * before resetting the latch to ensure that any async commits will
         * see the flag set if they might possibly need to wake us up, and
         * that we won't miss any signal they send us.  (If we discover work
         * to do in the last cycle before we would hibernate, the global flag
         * will be set unnecessarily, but little harm is done.)  But avoid
         * touching the global flag if it doesn't need to change.
         */
        if (hibernating != (left_till_hibernate <= 1))
        {
            hibernating = (left_till_hibernate <= 1);
            SetWalWriterSleeping(hibernating);
        }
 
        /* Clear any already-pending wakeups */
        ResetLatch(MyLatch);
 
        /* Process any signals received recently */
        ProcessMainLoopInterrupts();
 
        /*
         * Do what we're here for; then, if XLogBackgroundFlush() found useful
         * work to do, reset hibernation counter.
         */
        if (XLogBackgroundFlush())
            left_till_hibernate = LOOPS_UNTIL_HIBERNATE;
        else if (left_till_hibernate > 0)
            left_till_hibernate--;
 
        /* report pending statistics to the cumulative stats system */
        pgstat_report_wal(false);
 
        /*
         * Sleep until we are signaled or WalWriterDelay has elapsed.  If we
         * haven't done anything useful for quite some time, lengthen the
         * sleep time so as to reduce the server's idle power consumption.
         */
        if (left_till_hibernate > 0)
            cur_timeout = WalWriterDelay;   /* in ms */
        else
            cur_timeout = WalWriterDelay * HIBERNATE_FACTOR;
 
        (void) WaitLatch(MyLatch,
                         WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
                         cur_timeout,
                         WAIT_EVENT_WAL_WRITER_MAIN);
    }
}