PostgreSQL/src/backend/postmaster/walwriter.c

/*-------------------------------------------------------------------------
 *
 * walwriter.c
 *
 * The WAL writer background process is new as of Postgres 8.3.  It attempts
 * to keep regular backends from having to write out (and fsync) WAL pages.
 * Also, it guarantees that transaction commit records that weren't synced
 * to disk immediately upon commit (ie, were "asynchronously committed")
 * will reach disk within a knowable time --- which, as it happens, is at
 * most three times the wal_writer_delay cycle time.
 *
 * Note that as with the bgwriter for shared buffers, regular backends are
 * still empowered to issue WAL writes and fsyncs when the walwriter doesn't
 * keep up. This means that the WALWriter is not an essential process and
 * can shutdown quickly when requested.
 *
 * Because the walwriter's cycle is directly linked to the maximum delay
 * before async-commit transactions are guaranteed committed, it's probably
 * unwise to load additional functionality onto it.  For instance, if you've
 * got a yen to create xlog segments further in advance, that'd be better done
 * in bgwriter than in walwriter.
 *
 * The walwriter is started by the postmaster as soon as the startup subprocess
 * finishes.  It remains alive until the postmaster commands it to terminate.
 * Normal termination is by SIGTERM, which instructs the walwriter to exit(0).
 * Emergency termination is by SIGQUIT; like any backend, the walwriter will
 * simply abort and exit on SIGQUIT.
 *
 * If the walwriter exits unexpectedly, the postmaster treats that the same
 * as a backend crash: shared memory may be corrupted, so remaining backends
 * should be killed by SIGQUIT and then a recovery cycle started.
 *
 *
 * Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
 *
 *
 * IDENTIFICATION
 *	  src/backend/postmaster/walwriter.c
 *
 *-------------------------------------------------------------------------
 */
#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/walwriter.h"
#include "storage/bufmgr.h"
#include "storage/condition_variable.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/lwlock.h"
#include "storage/proc.h"
#include "storage/smgr.h"
#include "utils/guc.h"
#include "utils/hsearch.h"
#include "utils/memutils.h"
#include "utils/resowner.h"


/*
 * GUC parameters
 */
int			WalWriterDelay = 200;
int			WalWriterFlushAfter = 128;

/*
 * Number of do-nothing loops before lengthening the delay time, and the
 * multiplier to apply to WalWriterDelay when we do decide to hibernate.
 * (Perhaps these need to be configurable?)
 */
#define LOOPS_UNTIL_HIBERNATE		50
#define HIBERNATE_FACTOR			25

/*
 * Flags set by interrupt handlers for later service in the main loop.
 */
static volatile sig_atomic_t got_SIGHUP = false;
static volatile sig_atomic_t shutdown_requested = false;

/* Signal handlers */
static void wal_quickdie(SIGNAL_ARGS);
static void WalSigHupHandler(SIGNAL_ARGS);
static void WalShutdownHandler(SIGNAL_ARGS);
static void walwriter_sigusr1_handler(SIGNAL_ARGS);

/*
 * Main entry point for walwriter process
 *
 * This is invoked from AuxiliaryProcessMain, which has already created the
 * basic execution environment, but not enabled signals yet.
 */
void
WalWriterMain(void)
{
	sigjmp_buf	local_sigjmp_buf;
	MemoryContext walwriter_context;
	int			left_till_hibernate;
	bool		hibernating;

	/*
	 * 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, WalSigHupHandler); /* set flag to read config file */
	pqsignal(SIGINT, WalShutdownHandler);	/* request shutdown */
	pqsignal(SIGTERM, WalShutdownHandler);	/* request shutdown */
	pqsignal(SIGQUIT, wal_quickdie);	/* hard crash time */
	pqsignal(SIGALRM, SIG_IGN);
	pqsignal(SIGPIPE, SIG_IGN);
	pqsignal(SIGUSR1, walwriter_sigusr1_handler);
	pqsignal(SIGUSR2, SIG_IGN); /* not used */

	/*
	 * Reset some signals that are accepted by postmaster but not here
	 */
	pqsignal(SIGCHLD, SIG_DFL);

	/* We allow SIGQUIT (quickdie) at all times */
	sigdelset(&BlockSig, SIGQUIT);

	/*
	 * 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.
	 *
	 * This code is heavily based on bgwriter.c, q.v.
	 */
	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();
		AbortBufferIO();
		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 */
		MemoryContextResetAndDeleteChildren(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);

		/*
		 * Close all open files after any error.  This is helpful on Windows,
		 * where holding deleted files open causes various strange errors.
		 * It's not clear we need it elsewhere, but shouldn't hurt.
		 */
		smgrcloseall();
	}

	/* We can now handle ereport(ERROR) */
	PG_exception_stack = &local_sigjmp_buf;

	/*
	 * Unblock signals (they were blocked when the postmaster forked us)
	 */
	PG_SETMASK(&UnBlockSig);

	/*
	 * Reset hibernation state after any error.
	 */
	left_till_hibernate = LOOPS_UNTIL_HIBERNATE;
	hibernating = false;
	SetWalWriterSleeping(false);

	/*
	 * Advertise our latch that backends can use to wake us up while we're
	 * sleeping.
	 */
	ProcGlobal->walwriterLatch = &MyProc->procLatch;

	/*
	 * Loop forever
	 */
	for (;;)
	{
		long		cur_timeout;
		int			rc;

		/*
		 * 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 requests or signals received recently.
		 */
		if (got_SIGHUP)
		{
			got_SIGHUP = false;
			ProcessConfigFile(PGC_SIGHUP);
		}
		if (shutdown_requested)
		{
			/* Normal exit from the walwriter is here */
			proc_exit(0);		/* done */
		}

		/*
		 * 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--;

		/*
		 * 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;

		rc = WaitLatch(MyLatch,
					   WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH,
					   cur_timeout,
					   WAIT_EVENT_WAL_WRITER_MAIN);

		/*
		 * Emergency bailout if postmaster has died.  This is to avoid the
		 * necessity for manual cleanup of all postmaster children.
		 */
		if (rc & WL_POSTMASTER_DEATH)
			exit(1);
	}
}


/* --------------------------------
 *		signal handler routines
 * --------------------------------
 */

/*
 * wal_quickdie() occurs when signalled SIGQUIT by the postmaster.
 *
 * Some backend has bought the farm,
 * so we need to stop what we're doing and exit.
 */
static void
wal_quickdie(SIGNAL_ARGS)
{
	/*
	 * We DO NOT want to run proc_exit() or atexit() callbacks -- we're here
	 * because shared memory may be corrupted, so we don't want to try to
	 * clean up our transaction.  Just nail the windows shut and get out of
	 * town.  The callbacks wouldn't be safe to run from a signal handler,
	 * anyway.
	 *
	 * Note we do _exit(2) not _exit(0).  This is to force the postmaster into
	 * a system reset cycle if someone sends a manual SIGQUIT to a random
	 * backend.  This is necessary precisely because we don't clean up our
	 * shared memory state.  (The "dead man switch" mechanism in pmsignal.c
	 * should ensure the postmaster sees this as a crash, too, but no harm in
	 * being doubly sure.)
	 */
	_exit(2);
}

/* SIGHUP: set flag to re-read config file at next convenient time */
static void
WalSigHupHandler(SIGNAL_ARGS)
{
	int			save_errno = errno;

	got_SIGHUP = true;
	SetLatch(MyLatch);

	errno = save_errno;
}

/* SIGTERM: set flag to exit normally */
static void
WalShutdownHandler(SIGNAL_ARGS)
{
	int			save_errno = errno;

	shutdown_requested = true;
	SetLatch(MyLatch);

	errno = save_errno;
}

/* SIGUSR1: used for latch wakeups */
static void
walwriter_sigusr1_handler(SIGNAL_ARGS)
{
	int			save_errno = errno;

	latch_sigusr1_handler();

	errno = save_errno;
}