text: /*-------------------------------------------------------------------------
   *
   * bgwriter.c
   *
   * The background writer (bgwriter) is new as of Postgres 8.0.  It attempts
   * to keep regular backends from having to write out dirty shared buffers
   * (which they would only do when needing to free a shared buffer to read in
   * another page).  In the best scenario all writes from shared buffers will
   * be issued by the background writer process.  However, regular backends are
   * still empowered to issue writes if the bgwriter fails to maintain enough
   * clean shared buffers.
   *
   * As of Postgres 9.2 the bgwriter no longer handles checkpoints.
   *
   * Normal termination is by SIGTERM, which instructs the bgwriter to exit(0).
   * Emergency termination is by SIGQUIT; like any backend, the bgwriter will
   * simply abort and exit on SIGQUIT.
   *
   * If the bgwriter 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) 1996-2022, PostgreSQL Global Development Group
   *
   *
   * IDENTIFICATION
   *	  src/backend/postmaster/bgwriter.c
   *
   *-------------------------------------------------------------------------
   */
  #include "postgres.h"
  
  #include "access/xlog.h"
  #include "access/xlog_internal.h"
  #include "libpq/pqsignal.h"
  #include "miscadmin.h"
  #include "pgstat.h"
  #include "postmaster/bgwriter.h"
  #include "postmaster/interrupt.h"
  #include "storage/buf_internals.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/procsignal.h"
  #include "storage/shmem.h"
  #include "storage/smgr.h"
  #include "storage/spin.h"
  #include "storage/standby.h"
  #include "utils/guc.h"
  #include "utils/memutils.h"
  #include "utils/resowner.h"
  #include "utils/timestamp.h"
  
  /*
   * GUC parameters
   */
  int			BgWriterDelay = 200;
  
  /*
   * Multiplier to apply to BgWriterDelay when we decide to hibernate.
   * (Perhaps this needs to be configurable?)
   */
  #define HIBERNATE_FACTOR			50
  
  /*
   * Interval in which standby snapshots are logged into the WAL stream, in
   * milliseconds.
   */
  #define LOG_SNAPSHOT_INTERVAL_MS 15000
  
  /*
   * LSN and timestamp at which we last issued a LogStandbySnapshot(), to avoid
   * doing so too often or repeatedly if there has been no other write activity
   * in the system.
   */
  static TimestampTz last_snapshot_ts;
  static XLogRecPtr last_snapshot_lsn = InvalidXLogRecPtr;
  
  
  /*
   * Main entry point for bgwriter process
   *
   * This is invoked from AuxiliaryProcessMain, which has already created the
   * basic execution environment, but not enabled signals yet.
   */
  void
  BackgroundWriterMain(void)
  {
  	sigjmp_buf	local_sigjmp_buf;
  	MemoryContext bgwriter_context;
  	bool		prev_hibernate;
  	WritebackContext wb_context;
  
  	/*
  	 * Properly accept or ignore signals that might be sent to us.
  	 */
  	pqsignal(SIGHUP, SignalHandlerForConfigReload);
  	pqsignal(SIGINT, SIG_IGN);
  	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);
  
  	/*
  	 * Reset some signals that are accepted by postmaster but not here
  	 */
  	pqsignal(SIGCHLD, SIG_DFL);
  
  	/*
  	 * We just started, assume there has been either a shutdown or
  	 * end-of-recovery snapshot.
  	 */
  	last_snapshot_ts = GetCurrentTimestamp();
  
  	/*
  	 * 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.
  	 */
  	bgwriter_context = AllocSetContextCreate(TopMemoryContext,
  											 "Background Writer",
  											 ALLOCSET_DEFAULT_SIZES);
  	MemoryContextSwitchTo(bgwriter_context);
  
  	WritebackContextInit(&wb_context, &bgwriter_flush_after);
  
  	/*
  	 * 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 bgwriter, but we do have LWLocks, buffers, and temp files.
  		 */
  		LWLockReleaseAll();
  		ConditionVariableCancelSleep();
  		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(bgwriter_context);
  		FlushErrorState();
  
  		/* Flush any leaked data in the top-level context */
  		MemoryContextResetAndDeleteChildren(bgwriter_context);
  
  		/* re-initialize to avoid repeated errors causing problems */
  		WritebackContextInit(&wb_context, &bgwriter_flush_after);
  
  		/* 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();
  
  		/* Report wait end here, when there is no further possibility of wait */
  		pgstat_report_wait_end();
  	}
  
  	/* 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.
  	 */
  	prev_hibernate = false;
  
  	/*
  	 * Loop forever
  	 */
  	for (;;)
  	{
  		bool		can_hibernate;
  		int			rc;
  
  		/* Clear any already-pending wakeups */
  		ResetLatch(MyLatch);
  
  		HandleMainLoopInterrupts();
  
  		/*
  		 * Do one cycle of dirty-buffer writing.
  		 */
  		can_hibernate = BgBufferSync(&wb_context);
  
  		/*
  		 * Send off activity statistics to the stats collector
  		 */
  		pgstat_send_bgwriter();
  
  		if (FirstCallSinceLastCheckpoint())
  		{
  			/*
  			 * After any checkpoint, close all smgr files.  This is so we
  			 * won't hang onto smgr references to deleted files indefinitely.
  			 */
  			smgrcloseall();
  		}
  
  		/*
  		 * Log a new xl_running_xacts every now and then so replication can
  		 * get into a consistent state faster (think of suboverflowed
  		 * snapshots) and clean up resources (locks, KnownXids*) more
  		 * frequently. The costs of this are relatively low, so doing it 4
  		 * times (LOG_SNAPSHOT_INTERVAL_MS) a minute seems fine.
  		 *
  		 * We assume the interval for writing xl_running_xacts is
  		 * significantly bigger than BgWriterDelay, so we don't complicate the
  		 * overall timeout handling but just assume we're going to get called
  		 * often enough even if hibernation mode is active. It's not that
  		 * important that LOG_SNAPSHOT_INTERVAL_MS is met strictly. To make
  		 * sure we're not waking the disk up unnecessarily on an idle system
  		 * we check whether there has been any WAL inserted since the last
  		 * time we've logged a running xacts.
  		 *
  		 * We do this logging in the bgwriter as it is the only process that
  		 * is run regularly and returns to its mainloop all the time. E.g.
  		 * Checkpointer, when active, is barely ever in its mainloop and thus
  		 * makes it hard to log regularly.
  		 */
  		if (XLogStandbyInfoActive() && !RecoveryInProgress())
  		{
  			TimestampTz timeout = 0;
  			TimestampTz now = GetCurrentTimestamp();
  
  			timeout = TimestampTzPlusMilliseconds(last_snapshot_ts,
  												  LOG_SNAPSHOT_INTERVAL_MS);
  
  			/*
  			 * Only log if enough time has passed and interesting records have
  			 * been inserted since the last snapshot.  Have to compare with <=
  			 * instead of < because GetLastImportantRecPtr() points at the
  			 * start of a record, whereas last_snapshot_lsn points just past
  			 * the end of the record.
  			 */
  			if (now >= timeout &&
  				last_snapshot_lsn <= GetLastImportantRecPtr())
  			{
  				last_snapshot_lsn = LogStandbySnapshot();
  				last_snapshot_ts = now;
  			}
  		}
  
  		/*
  		 * Sleep until we are signaled or BgWriterDelay has elapsed.
  		 *
  		 * Note: the feedback control loop in BgBufferSync() expects that we
  		 * will call it every BgWriterDelay msec.  While it's not critical for
  		 * correctness that that be exact, the feedback loop might misbehave
  		 * if we stray too far from that.  Hence, avoid loading this process
  		 * down with latch events that are likely to happen frequently during
  		 * normal operation.
  		 */
  		rc = WaitLatch(MyLatch,
  					   WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
  					   BgWriterDelay /* ms */ , WAIT_EVENT_BGWRITER_MAIN);
  
  		/*
  		 * If no latch event and BgBufferSync says nothing's happening, extend
  		 * the sleep in "hibernation" mode, where we sleep for much longer
  		 * than bgwriter_delay says.  Fewer wakeups save electricity.  When a
  		 * backend starts using buffers again, it will wake us up by setting
  		 * our latch.  Because the extra sleep will persist only as long as no
  		 * buffer allocations happen, this should not distort the behavior of
  		 * BgBufferSync's control loop too badly; essentially, it will think
  		 * that the system-wide idle interval didn't exist.
  		 *
  		 * There is a race condition here, in that a backend might allocate a
  		 * buffer between the time BgBufferSync saw the alloc count as zero
  		 * and the time we call StrategyNotifyBgWriter.  While it's not
  		 * critical that we not hibernate anyway, we try to reduce the odds of
  		 * that by only hibernating when BgBufferSync says nothing's happening
  		 * for two consecutive cycles.  Also, we mitigate any possible
  		 * consequences of a missed wakeup by not hibernating forever.
  		 */
  		if (rc == WL_TIMEOUT && can_hibernate && prev_hibernate)
  		{
  			/* Ask for notification at next buffer allocation */
  			StrategyNotifyBgWriter(MyProc->pgprocno);
  			/* Sleep ... */
  			(void) WaitLatch(MyLatch,
  							 WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
  							 BgWriterDelay * HIBERNATE_FACTOR,
  							 WAIT_EVENT_BGWRITER_HIBERNATE);
  			/* Reset the notification request in case we timed out */
  			StrategyNotifyBgWriter(-1);
  		}
  
  		prev_hibernate = can_hibernate;
  	}
  }
  