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.
   *
   * The bgwriter is started by the postmaster as soon as the startup subprocess
   * finishes, or as soon as recovery begins if we are doing archive recovery.
   * It remains alive until the postmaster commands it to terminate.
   * 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) 1996-2014, PostgreSQL Global Development Group
   *
   *
   * IDENTIFICATION
   *	  src/backend/postmaster/bgwriter.c
   *
   *-------------------------------------------------------------------------
   */
  #include "postgres.h"
  
  #include <signal.h>
  #include <sys/time.h>
  #include <time.h>
  #include <unistd.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 "storage/bufmgr.h"
  #include "storage/buf_internals.h"
  #include "storage/fd.h"
  #include "storage/ipc.h"
  #include "storage/lwlock.h"
  #include "storage/proc.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;
  
  /*
   * 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 bg_quickdie(SIGNAL_ARGS);
  static void BgSigHupHandler(SIGNAL_ARGS);
  static void ReqShutdownHandler(SIGNAL_ARGS);
  static void bgwriter_sigusr1_handler(SIGNAL_ARGS);
  
  
  /*
   * 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;
  
  	/*
  	 * If possible, make this process a group leader, so that the postmaster
  	 * can signal any child processes too.  (bgwriter probably never has any
  	 * child processes, but for consistency we make all postmaster child
  	 * processes do this.)
  	 */
  #ifdef HAVE_SETSID
  	if (setsid() < 0)
  		elog(FATAL, "setsid() failed: %m");
  #endif
  
  	/*
  	 * Properly accept or ignore signals the postmaster might send us.
  	 *
  	 * bgwriter doesn't participate in ProcSignal signalling, but a SIGUSR1
  	 * handler is still needed for latch wakeups.
  	 */
  	pqsignal(SIGHUP, BgSigHupHandler);	/* set flag to read config file */
  	pqsignal(SIGINT, SIG_IGN);
  	pqsignal(SIGTERM, ReqShutdownHandler);		/* shutdown */
  	pqsignal(SIGQUIT, bg_quickdie);		/* hard crash time */
  	pqsignal(SIGALRM, SIG_IGN);
  	pqsignal(SIGPIPE, SIG_IGN);
  	pqsignal(SIGUSR1, bgwriter_sigusr1_handler);
  	pqsignal(SIGUSR2, SIG_IGN);
  
  	/*
  	 * Reset some signals that are accepted by postmaster but not here
  	 */
  	pqsignal(SIGCHLD, SIG_DFL);
  	pqsignal(SIGTTIN, SIG_DFL);
  	pqsignal(SIGTTOU, SIG_DFL);
  	pqsignal(SIGCONT, SIG_DFL);
  	pqsignal(SIGWINCH, SIG_DFL);
  
  	/* We allow SIGQUIT (quickdie) at all times */
  	sigdelset(&BlockSig, SIGQUIT);
  
  	/*
  	 * Create a resource owner to keep track of our resources (currently only
  	 * buffer pins).
  	 */
  	CurrentResourceOwner = ResourceOwnerCreate(NULL, "Background Writer");
  
  	/*
  	 * 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_MINSIZE,
  											 ALLOCSET_DEFAULT_INITSIZE,
  											 ALLOCSET_DEFAULT_MAXSIZE);
  	MemoryContextSwitchTo(bgwriter_context);
  
  	/*
  	 * If an exception is encountered, processing resumes here.
  	 *
  	 * See notes in postgres.c about the design of this coding.
  	 */
  	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();
  		AbortBufferIO();
  		UnlockBuffers();
  		/* buffer pins are released here: */
  		ResourceOwnerRelease(CurrentResourceOwner,
  							 RESOURCE_RELEASE_BEFORE_LOCKS,
  							 false, true);
  		/* we needn't bother with the other ResourceOwnerRelease phases */
  		AtEOXact_Buffers(false);
  		AtEOXact_SMgr();
  		AtEOXact_Files();
  		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);
  
  		/* 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.
  	 */
  	prev_hibernate = false;
  
  	/*
  	 * Loop forever
  	 */
  	for (;;)
  	{
  		bool		can_hibernate;
  		int			rc;
  
  		/* Clear any already-pending wakeups */
  		ResetLatch(&MyProc->procLatch);
  
  		if (got_SIGHUP)
  		{
  			got_SIGHUP = false;
  			ProcessConfigFile(PGC_SIGHUP);
  		}
  		if (shutdown_requested)
  		{
  			/*
  			 * From here on, elog(ERROR) should end with exit(1), not send
  			 * control back to the sigsetjmp block above
  			 */
  			ExitOnAnyError = true;
  			/* Normal exit from the bgwriter is here */
  			proc_exit(0);		/* done */
  		}
  
  		/*
  		 * Do one cycle of dirty-buffer writing.
  		 */
  		can_hibernate = BgBufferSync();
  
  		/*
  		 * 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_snap_interval_ms is met strictly. To make sure
  		 * we're not waking the disk up unneccesarily 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 its the only process thats
  		 * 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 some xlog record has
  			 * been inserted.
  			 */
  			if (now >= timeout &&
  				last_snapshot_lsn != GetXLogInsertRecPtr())
  			{
  				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(&MyProc->procLatch,
  					   WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH,
  					   BgWriterDelay /* ms */ );
  
  		/*
  		 * 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->procLatch);
  			/* Sleep ... */
  			rc = WaitLatch(&MyProc->procLatch,
  						   WL_LATCH_SET | WL_TIMEOUT | WL_POSTMASTER_DEATH,
  						   BgWriterDelay * HIBERNATE_FACTOR);
  			/* Reset the notification request in case we timed out */
  			StrategyNotifyBgWriter(NULL);
  		}
  
  		/*
  		 * 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);
  
  		prev_hibernate = can_hibernate;
  	}
  }
  
  
  /* --------------------------------
   *		signal handler routines
   * --------------------------------
   */
  
  /*
   * bg_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
  bg_quickdie(SIGNAL_ARGS)
  {
  	PG_SETMASK(&BlockSig);
  
  	/*
  	 * We DO NOT want to run proc_exit() 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.  Now that
  	 * there's an atexit callback to prevent third-party code from breaking
  	 * things by calling exit() directly, we have to reset the callbacks
  	 * explicitly to make this work as intended.
  	 */
  	on_exit_reset();
  
  	/*
  	 * Note we do exit(2) not exit(0).  This is to force the postmaster into a
  	 * system reset cycle if some idiot DBA 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
  BgSigHupHandler(SIGNAL_ARGS)
  {
  	int			save_errno = errno;
  
  	got_SIGHUP = true;
  	if (MyProc)
  		SetLatch(&MyProc->procLatch);
  
  	errno = save_errno;
  }
  
  /* SIGTERM: set flag to shutdown and exit */
  static void
  ReqShutdownHandler(SIGNAL_ARGS)
  {
  	int			save_errno = errno;
  
  	shutdown_requested = true;
  	if (MyProc)
  		SetLatch(&MyProc->procLatch);
  
  	errno = save_errno;
  }
  
  /* SIGUSR1: used for latch wakeups */
  static void
  bgwriter_sigusr1_handler(SIGNAL_ARGS)
  {
  	int			save_errno = errno;
  
  	latch_sigusr1_handler();
  
  	errno = save_errno;
  }
  