text: /*-------------------------------------------------------------------------
   *
   * checkpointer.c
   *
   * The checkpointer is new as of Postgres 9.2.  It handles all checkpoints.
   * Checkpoints are automatically dispatched after a certain amount of time has
   * elapsed since the last one, and it can be signaled to perform requested
   * checkpoints as well.  (The GUC parameter that mandates a checkpoint every
   * so many WAL segments is implemented by having backends signal when they
   * fill WAL segments; the checkpointer itself doesn't watch for the
   * condition.)
   *
   * The checkpointer 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 SIGUSR2, which instructs the checkpointer to
   * execute a shutdown checkpoint and then exit(0).  (All backends must be
   * stopped before SIGUSR2 is issued!)  Emergency termination is by SIGQUIT;
   * like any backend, the checkpointer will simply abort and exit on SIGQUIT.
   *
   * If the checkpointer 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.  (Even if
   * shared memory isn't corrupted, we have lost information about which
   * files need to be fsync'd for the next checkpoint, and so a system
   * restart needs to be forced.)
   *
   *
   * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
   *
   *
   * IDENTIFICATION
   *	  src/backend/postmaster/checkpointer.c
   *
   *-------------------------------------------------------------------------
   */
  #include "postgres.h"
  
  #include <sys/time.h>
  #include <time.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 "replication/syncrep.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 "utils/guc.h"
  #include "utils/memutils.h"
  #include "utils/resowner.h"
  
  
  /*----------
   * Shared memory area for communication between checkpointer and backends
   *
   * The ckpt counters allow backends to watch for completion of a checkpoint
   * request they send.  Here's how it works:
   *	* At start of a checkpoint, checkpointer reads (and clears) the request
   *	  flags and increments ckpt_started, while holding ckpt_lck.
   *	* On completion of a checkpoint, checkpointer sets ckpt_done to
   *	  equal ckpt_started.
   *	* On failure of a checkpoint, checkpointer increments ckpt_failed
   *	  and sets ckpt_done to equal ckpt_started.
   *
   * The algorithm for backends is:
   *	1. Record current values of ckpt_failed and ckpt_started, and
   *	   set request flags, while holding ckpt_lck.
   *	2. Send signal to request checkpoint.
   *	3. Sleep until ckpt_started changes.  Now you know a checkpoint has
   *	   begun since you started this algorithm (although *not* that it was
   *	   specifically initiated by your signal), and that it is using your flags.
   *	4. Record new value of ckpt_started.
   *	5. Sleep until ckpt_done >= saved value of ckpt_started.  (Use modulo
   *	   arithmetic here in case counters wrap around.)  Now you know a
   *	   checkpoint has started and completed, but not whether it was
   *	   successful.
   *	6. If ckpt_failed is different from the originally saved value,
   *	   assume request failed; otherwise it was definitely successful.
   *
   * ckpt_flags holds the OR of the checkpoint request flags sent by all
   * requesting backends since the last checkpoint start.  The flags are
   * chosen so that OR'ing is the correct way to combine multiple requests.
   *
   * num_backend_writes is used to count the number of buffer writes performed
   * by user backend processes.  This counter should be wide enough that it
   * can't overflow during a single processing cycle.  num_backend_fsync
   * counts the subset of those writes that also had to do their own fsync,
   * because the checkpointer failed to absorb their request.
   *
   * The requests array holds fsync requests sent by backends and not yet
   * absorbed by the checkpointer.
   *
   * Unlike the checkpoint fields, num_backend_writes, num_backend_fsync, and
   * the requests fields are protected by CheckpointerCommLock.
   *----------
   */
  typedef struct
  {
  	SyncRequestType type;		/* request type */
  	FileTag		ftag;			/* file identifier */
  } CheckpointerRequest;
  
  typedef struct
  {
  	pid_t		checkpointer_pid;	/* PID (0 if not started) */
  
  	slock_t		ckpt_lck;		/* protects all the ckpt_* fields */
  
  	int			ckpt_started;	/* advances when checkpoint starts */
  	int			ckpt_done;		/* advances when checkpoint done */
  	int			ckpt_failed;	/* advances when checkpoint fails */
  
  	int			ckpt_flags;		/* checkpoint flags, as defined in xlog.h */
  
  	ConditionVariable start_cv; /* signaled when ckpt_started advances */
  	ConditionVariable done_cv;	/* signaled when ckpt_done advances */
  
  	uint32		num_backend_writes; /* counts user backend buffer writes */
  	uint32		num_backend_fsync;	/* counts user backend fsync calls */
  
  	int			num_requests;	/* current # of requests */
  	int			max_requests;	/* allocated array size */
  	CheckpointerRequest requests[FLEXIBLE_ARRAY_MEMBER];
  } CheckpointerShmemStruct;
  
  static CheckpointerShmemStruct *CheckpointerShmem;
  
  /* interval for calling AbsorbSyncRequests in CheckpointWriteDelay */
  #define WRITES_PER_ABSORB		1000
  
  /*
   * GUC parameters
   */
  int			CheckPointTimeout = 300;
  int			CheckPointWarning = 30;
  double		CheckPointCompletionTarget = 0.9;
  
  /*
   * Private state
   */
  static bool ckpt_active = false;
  
  /* these values are valid when ckpt_active is true: */
  static pg_time_t ckpt_start_time;
  static XLogRecPtr ckpt_start_recptr;
  static double ckpt_cached_elapsed;
  
  static pg_time_t last_checkpoint_time;
  static pg_time_t last_xlog_switch_time;
  
  /* Prototypes for private functions */
  
  static void HandleCheckpointerInterrupts(void);
  static void CheckArchiveTimeout(void);
  static bool IsCheckpointOnSchedule(double progress);
  static bool ImmediateCheckpointRequested(void);
  static bool CompactCheckpointerRequestQueue(void);
  static void UpdateSharedMemoryConfig(void);
  
  /* Signal handlers */
  static void ReqCheckpointHandler(SIGNAL_ARGS);
  
  
  /*
   * Main entry point for checkpointer process
   *
   * This is invoked from AuxiliaryProcessMain, which has already created the
   * basic execution environment, but not enabled signals yet.
   */
  void
  CheckpointerMain(void)
  {
  	sigjmp_buf	local_sigjmp_buf;
  	MemoryContext checkpointer_context;
  
  	CheckpointerShmem->checkpointer_pid = MyProcPid;
  
  	/*
  	 * Properly accept or ignore signals the postmaster might send us
  	 *
  	 * Note: we deliberately ignore SIGTERM, because during a standard Unix
  	 * system shutdown cycle, init will SIGTERM all processes at once.  We
  	 * want to wait for the backends to exit, whereupon the postmaster will
  	 * tell us it's okay to shut down (via SIGUSR2).
  	 */
  	pqsignal(SIGHUP, SignalHandlerForConfigReload);
  	pqsignal(SIGINT, ReqCheckpointHandler); /* request checkpoint */
  	pqsignal(SIGTERM, SIG_IGN); /* ignore SIGTERM */
  	/* SIGQUIT handler was already set up by InitPostmasterChild */
  	pqsignal(SIGALRM, SIG_IGN);
  	pqsignal(SIGPIPE, SIG_IGN);
  	pqsignal(SIGUSR1, procsignal_sigusr1_handler);
  	pqsignal(SIGUSR2, SignalHandlerForShutdownRequest);
  
  	/*
  	 * Reset some signals that are accepted by postmaster but not here
  	 */
  	pqsignal(SIGCHLD, SIG_DFL);
  
  	/*
  	 * Initialize so that first time-driven event happens at the correct time.
  	 */
  	last_checkpoint_time = last_xlog_switch_time = (pg_time_t) time(NULL);
  
  	/*
  	 * 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.
  	 */
  	checkpointer_context = AllocSetContextCreate(TopMemoryContext,
  												 "Checkpointer",
  												 ALLOCSET_DEFAULT_SIZES);
  	MemoryContextSwitchTo(checkpointer_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 checkpointer, but we do have LWLocks, buffers, and temp
  		 * files.
  		 */
  		LWLockReleaseAll();
  		ConditionVariableCancelSleep();
  		pgstat_report_wait_end();
  		AbortBufferIO();
  		UnlockBuffers();
  		ReleaseAuxProcessResources(false);
  		AtEOXact_Buffers(false);
  		AtEOXact_SMgr();
  		AtEOXact_Files(false);
  		AtEOXact_HashTables(false);
  
  		/* Warn any waiting backends that the checkpoint failed. */
  		if (ckpt_active)
  		{
  			SpinLockAcquire(&CheckpointerShmem->ckpt_lck);
  			CheckpointerShmem->ckpt_failed++;
  			CheckpointerShmem->ckpt_done = CheckpointerShmem->ckpt_started;
  			SpinLockRelease(&CheckpointerShmem->ckpt_lck);
  
  			ConditionVariableBroadcast(&CheckpointerShmem->done_cv);
  
  			ckpt_active = false;
  		}
  
  		/*
  		 * Now return to normal top-level context and clear ErrorContext for
  		 * next time.
  		 */
  		MemoryContextSwitchTo(checkpointer_context);
  		FlushErrorState();
  
  		/* Flush any leaked data in the top-level context */
  		MemoryContextResetAndDeleteChildren(checkpointer_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);
  
  	/*
  	 * Ensure all shared memory values are set correctly for the config. Doing
  	 * this here ensures no race conditions from other concurrent updaters.
  	 */
  	UpdateSharedMemoryConfig();
  
  	/*
  	 * Advertise our latch that backends can use to wake us up while we're
  	 * sleeping.
  	 */
  	ProcGlobal->checkpointerLatch = &MyProc->procLatch;
  
  	/*
  	 * Loop forever
  	 */
  	for (;;)
  	{
  		bool		do_checkpoint = false;
  		int			flags = 0;
  		pg_time_t	now;
  		int			elapsed_secs;
  		int			cur_timeout;
  
  		/* Clear any already-pending wakeups */
  		ResetLatch(MyLatch);
  
  		/*
  		 * Process any requests or signals received recently.
  		 */
  		AbsorbSyncRequests();
  		HandleCheckpointerInterrupts();
  
  		/*
  		 * Detect a pending checkpoint request by checking whether the flags
  		 * word in shared memory is nonzero.  We shouldn't need to acquire the
  		 * ckpt_lck for this.
  		 */
  		if (((volatile CheckpointerShmemStruct *) CheckpointerShmem)->ckpt_flags)
  		{
  			do_checkpoint = true;
  			BgWriterStats.m_requested_checkpoints++;
  		}
  
  		/*
  		 * Force a checkpoint if too much time has elapsed since the last one.
  		 * Note that we count a timed checkpoint in stats only when this
  		 * occurs without an external request, but we set the CAUSE_TIME flag
  		 * bit even if there is also an external request.
  		 */
  		now = (pg_time_t) time(NULL);
  		elapsed_secs = now - last_checkpoint_time;
  		if (elapsed_secs >= CheckPointTimeout)
  		{
  			if (!do_checkpoint)
  				BgWriterStats.m_timed_checkpoints++;
  			do_checkpoint = true;
  			flags |= CHECKPOINT_CAUSE_TIME;
  		}
  
  		/*
  		 * Do a checkpoint if requested.
  		 */
  		if (do_checkpoint)
  		{
  			bool		ckpt_performed = false;
  			bool		do_restartpoint;
  
  			/*
  			 * Check if we should perform a checkpoint or a restartpoint. As a
  			 * side-effect, RecoveryInProgress() initializes TimeLineID if
  			 * it's not set yet.
  			 */
  			do_restartpoint = RecoveryInProgress();
  
  			/*
  			 * Atomically fetch the request flags to figure out what kind of a
  			 * checkpoint we should perform, and increase the started-counter
  			 * to acknowledge that we've started a new checkpoint.
  			 */
  			SpinLockAcquire(&CheckpointerShmem->ckpt_lck);
  			flags |= CheckpointerShmem->ckpt_flags;
  			CheckpointerShmem->ckpt_flags = 0;
  			CheckpointerShmem->ckpt_started++;
  			SpinLockRelease(&CheckpointerShmem->ckpt_lck);
  
  			ConditionVariableBroadcast(&CheckpointerShmem->start_cv);
  
  			/*
  			 * The end-of-recovery checkpoint is a real checkpoint that's
  			 * performed while we're still in recovery.
  			 */
  			if (flags & CHECKPOINT_END_OF_RECOVERY)
  				do_restartpoint = false;
  
  			/*
  			 * We will warn if (a) too soon since last checkpoint (whatever
  			 * caused it) and (b) somebody set the CHECKPOINT_CAUSE_XLOG flag
  			 * since the last checkpoint start.  Note in particular that this
  			 * implementation will not generate warnings caused by
  			 * CheckPointTimeout < CheckPointWarning.
  			 */
  			if (!do_restartpoint &&
  				(flags & CHECKPOINT_CAUSE_XLOG) &&
  				elapsed_secs < CheckPointWarning)
  				ereport(LOG,
  						(errmsg_plural("checkpoints are occurring too frequently (%d second apart)",
  									   "checkpoints are occurring too frequently (%d seconds apart)",
  									   elapsed_secs,
  									   elapsed_secs),
  						 errhint("Consider increasing the configuration parameter \"max_wal_size\".")));
  
  			/*
  			 * Initialize checkpointer-private variables used during
  			 * checkpoint.
  			 */
  			ckpt_active = true;
  			if (do_restartpoint)
  				ckpt_start_recptr = GetXLogReplayRecPtr(NULL);
  			else
  				ckpt_start_recptr = GetInsertRecPtr();
  			ckpt_start_time = now;
  			ckpt_cached_elapsed = 0;
  
  			/*
  			 * Do the checkpoint.
  			 */
  			if (!do_restartpoint)
  			{
  				CreateCheckPoint(flags);
  				ckpt_performed = true;
  			}
  			else
  				ckpt_performed = CreateRestartPoint(flags);
  
  			/*
  			 * After any checkpoint, close all smgr files.  This is so we
  			 * won't hang onto smgr references to deleted files indefinitely.
  			 */
  			smgrcloseall();
  
  			/*
  			 * Indicate checkpoint completion to any waiting backends.
  			 */
  			SpinLockAcquire(&CheckpointerShmem->ckpt_lck);
  			CheckpointerShmem->ckpt_done = CheckpointerShmem->ckpt_started;
  			SpinLockRelease(&CheckpointerShmem->ckpt_lck);
  
  			ConditionVariableBroadcast(&CheckpointerShmem->done_cv);
  
  			if (ckpt_performed)
  			{
  				/*
  				 * Note we record the checkpoint start time not end time as
  				 * last_checkpoint_time.  This is so that time-driven
  				 * checkpoints happen at a predictable spacing.
  				 */
  				last_checkpoint_time = now;
  			}
  			else
  			{
  				/*
  				 * We were not able to perform the restartpoint (checkpoints
  				 * throw an ERROR in case of error).  Most likely because we
  				 * have not received any new checkpoint WAL records since the
  				 * last restartpoint. Try again in 15 s.
  				 */
  				last_checkpoint_time = now - CheckPointTimeout + 15;
  			}
  
  			ckpt_active = false;
  		}
  
  		/* Check for archive_timeout and switch xlog files if necessary. */
  		CheckArchiveTimeout();
  
  		/*
  		 * Send off activity statistics to the stats collector.  (The reason
  		 * why we re-use bgwriter-related code for this is that the bgwriter
  		 * and checkpointer used to be just one process.  It's probably not
  		 * worth the trouble to split the stats support into two independent
  		 * stats message types.)
  		 */
  		pgstat_send_bgwriter();
  
  		/* Send WAL statistics to the stats collector. */
  		pgstat_report_wal();
  
  		/*
  		 * If any checkpoint flags have been set, redo the loop to handle the
  		 * checkpoint without sleeping.
  		 */
  		if (((volatile CheckpointerShmemStruct *) CheckpointerShmem)->ckpt_flags)
  			continue;
  
  		/*
  		 * Sleep until we are signaled or it's time for another checkpoint or
  		 * xlog file switch.
  		 */
  		now = (pg_time_t) time(NULL);
  		elapsed_secs = now - last_checkpoint_time;
  		if (elapsed_secs >= CheckPointTimeout)
  			continue;			/* no sleep for us ... */
  		cur_timeout = CheckPointTimeout - elapsed_secs;
  		if (XLogArchiveTimeout > 0 && !RecoveryInProgress())
  		{
  			elapsed_secs = now - last_xlog_switch_time;
  			if (elapsed_secs >= XLogArchiveTimeout)
  				continue;		/* no sleep for us ... */
  			cur_timeout = Min(cur_timeout, XLogArchiveTimeout - elapsed_secs);
  		}
  
  		(void) WaitLatch(MyLatch,
  						 WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
  						 cur_timeout * 1000L /* convert to ms */ ,
  						 WAIT_EVENT_CHECKPOINTER_MAIN);
  	}
  }
  
  /*
   * Process any new interrupts.
   */
  static void
  HandleCheckpointerInterrupts(void)
  {
  	if (ProcSignalBarrierPending)
  		ProcessProcSignalBarrier();
  
  	if (ConfigReloadPending)
  	{
  		ConfigReloadPending = false;
  		ProcessConfigFile(PGC_SIGHUP);
  
  		/*
  		 * Checkpointer is the last process to shut down, so we ask it to hold
  		 * the keys for a range of other tasks required most of which have
  		 * nothing to do with checkpointing at all.
  		 *
  		 * For various reasons, some config values can change dynamically so
  		 * the primary copy of them is held in shared memory to make sure all
  		 * backends see the same value.  We make Checkpointer responsible for
  		 * updating the shared memory copy if the parameter setting changes
  		 * because of SIGHUP.
  		 */
  		UpdateSharedMemoryConfig();
  	}
  	if (ShutdownRequestPending)
  	{
  		/*
  		 * From here on, elog(ERROR) should end with exit(1), not send control
  		 * back to the sigsetjmp block above
  		 */
  		ExitOnAnyError = true;
  
  		/*
  		 * Close down the database.
  		 *
  		 * Since ShutdownXLOG() creates restartpoint or checkpoint, and
  		 * updates the statistics, increment the checkpoint request and send
  		 * the statistics to the stats collector.
  		 */
  		BgWriterStats.m_requested_checkpoints++;
  		ShutdownXLOG(0, 0);
  		pgstat_send_bgwriter();
  		pgstat_report_wal();
  
  		/* Normal exit from the checkpointer is here */
  		proc_exit(0);			/* done */
  	}
  }
  
  /*
   * CheckArchiveTimeout -- check for archive_timeout and switch xlog files
   *
   * This will switch to a new WAL file and force an archive file write if
   * meaningful activity is recorded in the current WAL file. This includes most
   * writes, including just a single checkpoint record, but excludes WAL records
   * that were inserted with the XLOG_MARK_UNIMPORTANT flag being set (like
   * snapshots of running transactions).  Such records, depending on
   * configuration, occur on regular intervals and don't contain important
   * information.  This avoids generating archives with a few unimportant
   * records.
   */
  static void
  CheckArchiveTimeout(void)
  {
  	pg_time_t	now;
  	pg_time_t	last_time;
  	XLogRecPtr	last_switch_lsn;
  
  	if (XLogArchiveTimeout <= 0 || RecoveryInProgress())
  		return;
  
  	now = (pg_time_t) time(NULL);
  
  	/* First we do a quick check using possibly-stale local state. */
  	if ((int) (now - last_xlog_switch_time) < XLogArchiveTimeout)
  		return;
  
  	/*
  	 * Update local state ... note that last_xlog_switch_time is the last time
  	 * a switch was performed *or requested*.
  	 */
  	last_time = GetLastSegSwitchData(&last_switch_lsn);
  
  	last_xlog_switch_time = Max(last_xlog_switch_time, last_time);
  
  	/* Now we can do the real checks */
  	if ((int) (now - last_xlog_switch_time) >= XLogArchiveTimeout)
  	{
  		/*
  		 * Switch segment only when "important" WAL has been logged since the
  		 * last segment switch (last_switch_lsn points to end of segment
  		 * switch occurred in).
  		 */
  		if (GetLastImportantRecPtr() > last_switch_lsn)
  		{
  			XLogRecPtr	switchpoint;
  
  			/* mark switch as unimportant, avoids triggering checkpoints */
  			switchpoint = RequestXLogSwitch(true);
  
  			/*
  			 * If the returned pointer points exactly to a segment boundary,
  			 * assume nothing happened.
  			 */
  			if (XLogSegmentOffset(switchpoint, wal_segment_size) != 0)
  				elog(DEBUG1, "write-ahead log switch forced (archive_timeout=%d)",
  					 XLogArchiveTimeout);
  		}
  
  		/*
  		 * Update state in any case, so we don't retry constantly when the
  		 * system is idle.
  		 */
  		last_xlog_switch_time = now;
  	}
  }
  
  /*
   * Returns true if an immediate checkpoint request is pending.  (Note that
   * this does not check the *current* checkpoint's IMMEDIATE flag, but whether
   * there is one pending behind it.)
   */
  static bool
  ImmediateCheckpointRequested(void)
  {
  	volatile CheckpointerShmemStruct *cps = CheckpointerShmem;
  
  	/*
  	 * We don't need to acquire the ckpt_lck in this case because we're only
  	 * looking at a single flag bit.
  	 */
  	if (cps->ckpt_flags & CHECKPOINT_IMMEDIATE)
  		return true;
  	return false;
  }
  
  /*
   * CheckpointWriteDelay -- control rate of checkpoint
   *
   * This function is called after each page write performed by BufferSync().
   * It is responsible for throttling BufferSync()'s write rate to hit
   * checkpoint_completion_target.
   *
   * The checkpoint request flags should be passed in; currently the only one
   * examined is CHECKPOINT_IMMEDIATE, which disables delays between writes.
   *
   * 'progress' is an estimate of how much of the work has been done, as a
   * fraction between 0.0 meaning none, and 1.0 meaning all done.
   */
  void
  CheckpointWriteDelay(int flags, double progress)
  {
  	static int	absorb_counter = WRITES_PER_ABSORB;
  
  	/* Do nothing if checkpoint is being executed by non-checkpointer process */
  	if (!AmCheckpointerProcess())
  		return;
  
  	/*
  	 * Perform the usual duties and take a nap, unless we're behind schedule,
  	 * in which case we just try to catch up as quickly as possible.
  	 */
  	if (!(flags & CHECKPOINT_IMMEDIATE) &&
  		!ShutdownRequestPending &&
  		!ImmediateCheckpointRequested() &&
  		IsCheckpointOnSchedule(progress))
  	{
  		if (ConfigReloadPending)
  		{
  			ConfigReloadPending = false;
  			ProcessConfigFile(PGC_SIGHUP);
  			/* update shmem copies of config variables */
  			UpdateSharedMemoryConfig();
  		}
  
  		AbsorbSyncRequests();
  		absorb_counter = WRITES_PER_ABSORB;
  
  		CheckArchiveTimeout();
  
  		/*
  		 * Report interim activity statistics to the stats collector.
  		 */
  		pgstat_send_bgwriter();
  
  		/*
  		 * This sleep used to be connected to bgwriter_delay, typically 200ms.
  		 * That resulted in more frequent wakeups if not much work to do.
  		 * Checkpointer and bgwriter are no longer related so take the Big
  		 * Sleep.
  		 */
  		pg_usleep(100000L);
  	}
  	else if (--absorb_counter <= 0)
  	{
  		/*
  		 * Absorb pending fsync requests after each WRITES_PER_ABSORB write
  		 * operations even when we don't sleep, to prevent overflow of the
  		 * fsync request queue.
  		 */
  		AbsorbSyncRequests();
  		absorb_counter = WRITES_PER_ABSORB;
  	}
  
  	/* Check for barrier events. */
  	if (ProcSignalBarrierPending)
  		ProcessProcSignalBarrier();
  }
  
  /*
   * IsCheckpointOnSchedule -- are we on schedule to finish this checkpoint
   *		 (or restartpoint) in time?
   *
   * Compares the current progress against the time/segments elapsed since last
   * checkpoint, and returns true if the progress we've made this far is greater
   * than the elapsed time/segments.
   */
  static bool
  IsCheckpointOnSchedule(double progress)
  {
  	XLogRecPtr	recptr;
  	struct timeval now;
  	double		elapsed_xlogs,
  				elapsed_time;
  
  	Assert(ckpt_active);
  
  	/* Scale progress according to checkpoint_completion_target. */
  	progress *= CheckPointCompletionTarget;
  
  	/*
  	 * Check against the cached value first. Only do the more expensive
  	 * calculations once we reach the target previously calculated. Since
  	 * neither time or WAL insert pointer moves backwards, a freshly
  	 * calculated value can only be greater than or equal to the cached value.
  	 */
  	if (progress < ckpt_cached_elapsed)
  		return false;
  
  	/*
  	 * Check progress against WAL segments written and CheckPointSegments.
  	 *
  	 * We compare the current WAL insert location against the location
  	 * computed before calling CreateCheckPoint. The code in XLogInsert that
  	 * actually triggers a checkpoint when CheckPointSegments is exceeded
  	 * compares against RedoRecPtr, so this is not completely accurate.
  	 * However, it's good enough for our purposes, we're only calculating an
  	 * estimate anyway.
  	 *
  	 * During recovery, we compare last replayed WAL record's location with
  	 * the location computed before calling CreateRestartPoint. That maintains
  	 * the same pacing as we have during checkpoints in normal operation, but
  	 * we might exceed max_wal_size by a fair amount. That's because there can
  	 * be a large gap between a checkpoint's redo-pointer and the checkpoint
  	 * record itself, and we only start the restartpoint after we've seen the
  	 * checkpoint record. (The gap is typically up to CheckPointSegments *
  	 * checkpoint_completion_target where checkpoint_completion_target is the
  	 * value that was in effect when the WAL was generated).
  	 */
  	if (RecoveryInProgress())
  		recptr = GetXLogReplayRecPtr(NULL);
  	else
  		recptr = GetInsertRecPtr();
  	elapsed_xlogs = (((double) (recptr - ckpt_start_recptr)) /
  					 wal_segment_size) / CheckPointSegments;
  
  	if (progress < elapsed_xlogs)
  	{
  		ckpt_cached_elapsed = elapsed_xlogs;
  		return false;
  	}
  
  	/*
  	 * Check progress against time elapsed and checkpoint_timeout.
  	 */
  	gettimeofday(&now, NULL);
  	elapsed_time = ((double) ((pg_time_t) now.tv_sec - ckpt_start_time) +
  					now.tv_usec / 1000000.0) / CheckPointTimeout;
  
  	if (progress < elapsed_time)
  	{
  		ckpt_cached_elapsed = elapsed_time;
  		return false;
  	}
  
  	/* It looks like we're on schedule. */
  	return true;
  }
  
  
  /* --------------------------------
   *		signal handler routines
   * --------------------------------
   */
  
  /* SIGINT: set flag to run a normal checkpoint right away */
  static void
  ReqCheckpointHandler(SIGNAL_ARGS)
  {
  	int			save_errno = errno;
  
  	/*
  	 * The signaling process should have set ckpt_flags nonzero, so all we
  	 * need do is ensure that our main loop gets kicked out of any wait.
  	 */
  	SetLatch(MyLatch);
  
  	errno = save_errno;
  }
  
  
  /* --------------------------------
   *		communication with backends
   * --------------------------------
   */
  
  /*
   * CheckpointerShmemSize
   *		Compute space needed for checkpointer-related shared memory
   */
  Size
  CheckpointerShmemSize(void)
  {
  	Size		size;
  
  	/*
  	 * Currently, the size of the requests[] array is arbitrarily set equal to
  	 * NBuffers.  This may prove too large or small ...
  	 */
  	size = offsetof(CheckpointerShmemStruct, requests);
  	size = add_size(size, mul_size(NBuffers, sizeof(CheckpointerRequest)));
  
  	return size;
  }
  
  /*
   * CheckpointerShmemInit
   *		Allocate and initialize checkpointer-related shared memory
   */
  void
  CheckpointerShmemInit(void)
  {
  	Size		size = CheckpointerShmemSize();
  	bool		found;
  
  	CheckpointerShmem = (CheckpointerShmemStruct *)
  		ShmemInitStruct("Checkpointer Data",
  						size,
  						&found);
  
  	if (!found)
  	{
  		/*
  		 * First time through, so initialize.  Note that we zero the whole
  		 * requests array; this is so that CompactCheckpointerRequestQueue can
  		 * assume that any pad bytes in the request structs are zeroes.
  		 */
  		MemSet(CheckpointerShmem, 0, size);
  		SpinLockInit(&CheckpointerShmem->ckpt_lck);
  		CheckpointerShmem->max_requests = NBuffers;
  		ConditionVariableInit(&CheckpointerShmem->start_cv);
  		ConditionVariableInit(&CheckpointerShmem->done_cv);
  	}
  }
  
  /*
   * RequestCheckpoint
   *		Called in backend processes to request a checkpoint
   *
   * flags is a bitwise OR of the following:
   *	CHECKPOINT_IS_SHUTDOWN: checkpoint is for database shutdown.
   *	CHECKPOINT_END_OF_RECOVERY: checkpoint is for end of WAL recovery.
   *	CHECKPOINT_IMMEDIATE: finish the checkpoint ASAP,
   *		ignoring checkpoint_completion_target parameter.
   *	CHECKPOINT_FORCE: force a checkpoint even if no XLOG activity has occurred
   *		since the last one (implied by CHECKPOINT_IS_SHUTDOWN or
   *		CHECKPOINT_END_OF_RECOVERY).
   *	CHECKPOINT_WAIT: wait for completion before returning (otherwise,
   *		just signal checkpointer to do it, and return).
   *	CHECKPOINT_CAUSE_XLOG: checkpoint is requested due to xlog filling.
   *		(This affects logging, and in particular enables CheckPointWarning.)
   */
  void
  RequestCheckpoint(int flags)
  {
  	int			ntries;
  	int			old_failed,
  				old_started;
  
  	/*
  	 * If in a standalone backend, just do it ourselves.
  	 */
  	if (!IsPostmasterEnvironment)
  	{
  		/*
  		 * There's no point in doing slow checkpoints in a standalone backend,
  		 * because there's no other backends the checkpoint could disrupt.
  		 */
  		CreateCheckPoint(flags | CHECKPOINT_IMMEDIATE);
  
  		/*
  		 * After any checkpoint, close all smgr files.  This is so we won't
  		 * hang onto smgr references to deleted files indefinitely.
  		 */
  		smgrcloseall();
  
  		return;
  	}
  
  	/*
  	 * Atomically set the request flags, and take a snapshot of the counters.
  	 * When we see ckpt_started > old_started, we know the flags we set here
  	 * have been seen by checkpointer.
  	 *
  	 * Note that we OR the flags with any existing flags, to avoid overriding
  	 * a "stronger" request by another backend.  The flag senses must be
  	 * chosen to make this work!
  	 */
  	SpinLockAcquire(&CheckpointerShmem->ckpt_lck);
  
  	old_failed = CheckpointerShmem->ckpt_failed;
  	old_started = CheckpointerShmem->ckpt_started;
  	CheckpointerShmem->ckpt_flags |= (flags | CHECKPOINT_REQUESTED);
  
  	SpinLockRelease(&CheckpointerShmem->ckpt_lck);
  
  	/*
  	 * Send signal to request checkpoint.  It's possible that the checkpointer
  	 * hasn't started yet, or is in process of restarting, so we will retry a
  	 * few times if needed.  (Actually, more than a few times, since on slow
  	 * or overloaded buildfarm machines, it's been observed that the
  	 * checkpointer can take several seconds to start.)  However, if not told
  	 * to wait for the checkpoint to occur, we consider failure to send the
  	 * signal to be nonfatal and merely LOG it.  The checkpointer should see
  	 * the request when it does start, with or without getting a signal.
  	 */
  #define MAX_SIGNAL_TRIES 600	/* max wait 60.0 sec */
  	for (ntries = 0;; ntries++)
  	{
  		if (CheckpointerShmem->checkpointer_pid == 0)
  		{
  			if (ntries >= MAX_SIGNAL_TRIES || !(flags & CHECKPOINT_WAIT))
  			{
  				elog((flags & CHECKPOINT_WAIT) ? ERROR : LOG,
  					 "could not signal for checkpoint: checkpointer is not running");
  				break;
  			}
  		}
  		else if (kill(CheckpointerShmem->checkpointer_pid, SIGINT) != 0)
  		{
  			if (ntries >= MAX_SIGNAL_TRIES || !(flags & CHECKPOINT_WAIT))
  			{
  				elog((flags & CHECKPOINT_WAIT) ? ERROR : LOG,
  					 "could not signal for checkpoint: %m");
  				break;
  			}
  		}
  		else
  			break;				/* signal sent successfully */
  
  		CHECK_FOR_INTERRUPTS();
  		pg_usleep(100000L);		/* wait 0.1 sec, then retry */
  	}
  
  	/*
  	 * If requested, wait for completion.  We detect completion according to
  	 * the algorithm given above.
  	 */
  	if (flags & CHECKPOINT_WAIT)
  	{
  		int			new_started,
  					new_failed;
  
  		/* Wait for a new checkpoint to start. */
  		ConditionVariablePrepareToSleep(&CheckpointerShmem->start_cv);
  		for (;;)
  		{
  			SpinLockAcquire(&CheckpointerShmem->ckpt_lck);
  			new_started = CheckpointerShmem->ckpt_started;
  			SpinLockRelease(&CheckpointerShmem->ckpt_lck);
  
  			if (new_started != old_started)
  				break;
  
  			ConditionVariableSleep(&CheckpointerShmem->start_cv,
  								   WAIT_EVENT_CHECKPOINT_START);
  		}
  		ConditionVariableCancelSleep();
  
  		/*
  		 * We are waiting for ckpt_done >= new_started, in a modulo sense.
  		 */
  		ConditionVariablePrepareToSleep(&CheckpointerShmem->done_cv);
  		for (;;)
  		{
  			int			new_done;
  
  			SpinLockAcquire(&CheckpointerShmem->ckpt_lck);
  			new_done = CheckpointerShmem->ckpt_done;
  			new_failed = CheckpointerShmem->ckpt_failed;
  			SpinLockRelease(&CheckpointerShmem->ckpt_lck);
  
  			if (new_done - new_started >= 0)
  				break;
  
  			ConditionVariableSleep(&CheckpointerShmem->done_cv,
  								   WAIT_EVENT_CHECKPOINT_DONE);
  		}
  		ConditionVariableCancelSleep();
  
  		if (new_failed != old_failed)
  			ereport(ERROR,
  					(errmsg("checkpoint request failed"),
  					 errhint("Consult recent messages in the server log for details.")));
  	}
  }
  
  /*
   * ForwardSyncRequest
   *		Forward a file-fsync request from a backend to the checkpointer
   *
   * Whenever a backend is compelled to write directly to a relation
   * (which should be seldom, if the background writer is getting its job done),
   * the backend calls this routine to pass over knowledge that the relation
   * is dirty and must be fsync'd before next checkpoint.  We also use this
   * opportunity to count such writes for statistical purposes.
   *
   * To avoid holding the lock for longer than necessary, we normally write
   * to the requests[] queue without checking for duplicates.  The checkpointer
   * will have to eliminate dups internally anyway.  However, if we discover
   * that the queue is full, we make a pass over the entire queue to compact
   * it.  This is somewhat expensive, but the alternative is for the backend
   * to perform its own fsync, which is far more expensive in practice.  It
   * is theoretically possible a backend fsync might still be necessary, if
   * the queue is full and contains no duplicate entries.  In that case, we
   * let the backend know by returning false.
   */
  bool
  ForwardSyncRequest(const FileTag *ftag, SyncRequestType type)
  {
  	CheckpointerRequest *request;
  	bool		too_full;
  
  	if (!IsUnderPostmaster)
  		return false;			/* probably shouldn't even get here */
  
  	if (AmCheckpointerProcess())
  		elog(ERROR, "ForwardSyncRequest must not be called in checkpointer");
  
  	LWLockAcquire(CheckpointerCommLock, LW_EXCLUSIVE);
  
  	/* Count all backend writes regardless of if they fit in the queue */
  	if (!AmBackgroundWriterProcess())
  		CheckpointerShmem->num_backend_writes++;
  
  	/*
  	 * If the checkpointer isn't running or the request queue is full, the
  	 * backend will have to perform its own fsync request.  But before forcing
  	 * that to happen, we can try to compact the request queue.
  	 */
  	if (CheckpointerShmem->checkpointer_pid == 0 ||
  		(CheckpointerShmem->num_requests >= CheckpointerShmem->max_requests &&
  		 !CompactCheckpointerRequestQueue()))
  	{
  		/*
  		 * Count the subset of writes where backends have to do their own
  		 * fsync
  		 */
  		if (!AmBackgroundWriterProcess())
  			CheckpointerShmem->num_backend_fsync++;
  		LWLockRelease(CheckpointerCommLock);
  		return false;
  	}
  
  	/* OK, insert request */
  	request = &CheckpointerShmem->requests[CheckpointerShmem->num_requests++];
  	request->ftag = *ftag;
  	request->type = type;
  
  	/* If queue is more than half full, nudge the checkpointer to empty it */
  	too_full = (CheckpointerShmem->num_requests >=
  				CheckpointerShmem->max_requests / 2);
  
  	LWLockRelease(CheckpointerCommLock);
  
  	/* ... but not till after we release the lock */
  	if (too_full && ProcGlobal->checkpointerLatch)
  		SetLatch(ProcGlobal->checkpointerLatch);
  
  	return true;
  }
  
  /*
   * CompactCheckpointerRequestQueue
   *		Remove duplicates from the request queue to avoid backend fsyncs.
   *		Returns "true" if any entries were removed.
   *
   * Although a full fsync request queue is not common, it can lead to severe
   * performance problems when it does happen.  So far, this situation has
   * only been observed to occur when the system is under heavy write load,
   * and especially during the "sync" phase of a checkpoint.  Without this
   * logic, each backend begins doing an fsync for every block written, which
   * gets very expensive and can slow down the whole system.
   *
   * Trying to do this every time the queue is full could lose if there
   * aren't any removable entries.  But that should be vanishingly rare in
   * practice: there's one queue entry per shared buffer.
   */
  static bool
  CompactCheckpointerRequestQueue(void)
  {
  	struct CheckpointerSlotMapping
  	{
  		CheckpointerRequest request;
  		int			slot;
  	};
  
  	int			n,
  				preserve_count;
  	int			num_skipped = 0;
  	HASHCTL		ctl;
  	HTAB	   *htab;
  	bool	   *skip_slot;
  
  	/* must hold CheckpointerCommLock in exclusive mode */
  	Assert(LWLockHeldByMe(CheckpointerCommLock));
  
  	/* Initialize skip_slot array */
  	skip_slot = palloc0(sizeof(bool) * CheckpointerShmem->num_requests);
  
  	/* Initialize temporary hash table */
  	ctl.keysize = sizeof(CheckpointerRequest);
  	ctl.entrysize = sizeof(struct CheckpointerSlotMapping);
  	ctl.hcxt = CurrentMemoryContext;
  
  	htab = hash_create("CompactCheckpointerRequestQueue",
  					   CheckpointerShmem->num_requests,
  					   &ctl,
  					   HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
  
  	/*
  	 * The basic idea here is that a request can be skipped if it's followed
  	 * by a later, identical request.  It might seem more sensible to work
  	 * backwards from the end of the queue and check whether a request is
  	 * *preceded* by an earlier, identical request, in the hopes of doing less
  	 * copying.  But that might change the semantics, if there's an
  	 * intervening SYNC_FORGET_REQUEST or SYNC_FILTER_REQUEST, so we do it
  	 * this way.  It would be possible to be even smarter if we made the code
  	 * below understand the specific semantics of such requests (it could blow
  	 * away preceding entries that would end up being canceled anyhow), but
  	 * it's not clear that the extra complexity would buy us anything.
  	 */
  	for (n = 0; n < CheckpointerShmem->num_requests; n++)
  	{
  		CheckpointerRequest *request;
  		struct CheckpointerSlotMapping *slotmap;
  		bool		found;
  
  		/*
  		 * We use the request struct directly as a hashtable key.  This
  		 * assumes that any padding bytes in the structs are consistently the
  		 * same, which should be okay because we zeroed them in
  		 * CheckpointerShmemInit.  Note also that RelFileNode had better
  		 * contain no pad bytes.
  		 */
  		request = &CheckpointerShmem->requests[n];
  		slotmap = hash_search(htab, request, HASH_ENTER, &found);
  		if (found)
  		{
  			/* Duplicate, so mark the previous occurrence as skippable */
  			skip_slot[slotmap->slot] = true;
  			num_skipped++;
  		}
  		/* Remember slot containing latest occurrence of this request value */
  		slotmap->slot = n;
  	}
  
  	/* Done with the hash table. */
  	hash_destroy(htab);
  
  	/* If no duplicates, we're out of luck. */
  	if (!num_skipped)
  	{
  		pfree(skip_slot);
  		return false;
  	}
  
  	/* We found some duplicates; remove them. */
  	preserve_count = 0;
  	for (n = 0; n < CheckpointerShmem->num_requests; n++)
  	{
  		if (skip_slot[n])
  			continue;
  		CheckpointerShmem->requests[preserve_count++] = CheckpointerShmem->requests[n];
  	}
  	ereport(DEBUG1,
  			(errmsg_internal("compacted fsync request queue from %d entries to %d entries",
  							 CheckpointerShmem->num_requests, preserve_count)));
  	CheckpointerShmem->num_requests = preserve_count;
  
  	/* Cleanup. */
  	pfree(skip_slot);
  	return true;
  }
  
  /*
   * AbsorbSyncRequests
   *		Retrieve queued sync requests and pass them to sync mechanism.
   *
   * This is exported because it must be called during CreateCheckPoint;
   * we have to be sure we have accepted all pending requests just before
   * we start fsync'ing.  Since CreateCheckPoint sometimes runs in
   * non-checkpointer processes, do nothing if not checkpointer.
   */
  void
  AbsorbSyncRequests(void)
  {
  	CheckpointerRequest *requests = NULL;
  	CheckpointerRequest *request;
  	int			n;
  
  	if (!AmCheckpointerProcess())
  		return;
  
  	LWLockAcquire(CheckpointerCommLock, LW_EXCLUSIVE);
  
  	/* Transfer stats counts into pending pgstats message */
  	BgWriterStats.m_buf_written_backend += CheckpointerShmem->num_backend_writes;
  	BgWriterStats.m_buf_fsync_backend += CheckpointerShmem->num_backend_fsync;
  
  	CheckpointerShmem->num_backend_writes = 0;
  	CheckpointerShmem->num_backend_fsync = 0;
  
  	/*
  	 * We try to avoid holding the lock for a long time by copying the request
  	 * array, and processing the requests after releasing the lock.
  	 *
  	 * Once we have cleared the requests from shared memory, we have to PANIC
  	 * if we then fail to absorb them (eg, because our hashtable runs out of
  	 * memory).  This is because the system cannot run safely if we are unable
  	 * to fsync what we have been told to fsync.  Fortunately, the hashtable
  	 * is so small that the problem is quite unlikely to arise in practice.
  	 */
  	n = CheckpointerShmem->num_requests;
  	if (n > 0)
  	{
  		requests = (CheckpointerRequest *) palloc(n * sizeof(CheckpointerRequest));
  		memcpy(requests, CheckpointerShmem->requests, n * sizeof(CheckpointerRequest));
  	}
  
  	START_CRIT_SECTION();
  
  	CheckpointerShmem->num_requests = 0;
  
  	LWLockRelease(CheckpointerCommLock);
  
  	for (request = requests; n > 0; request++, n--)
  		RememberSyncRequest(&request->ftag, request->type);
  
  	END_CRIT_SECTION();
  
  	if (requests)
  		pfree(requests);
  }
  
  /*
   * Update any shared memory configurations based on config parameters
   */
  static void
  UpdateSharedMemoryConfig(void)
  {
  	/* update global shmem state for sync rep */
  	SyncRepUpdateSyncStandbysDefined();
  
  	/*
  	 * If full_page_writes has been changed by SIGHUP, we update it in shared
  	 * memory and write an XLOG_FPW_CHANGE record.
  	 */
  	UpdateFullPageWrites();
  
  	elog(DEBUG2, "checkpointer updated shared memory configuration values");
  }
  
  /*
   * FirstCallSinceLastCheckpoint allows a process to take an action once
   * per checkpoint cycle by asynchronously checking for checkpoint completion.
   */
  bool
  FirstCallSinceLastCheckpoint(void)
  {
  	static int	ckpt_done = 0;
  	int			new_done;
  	bool		FirstCall = false;
  
  	SpinLockAcquire(&CheckpointerShmem->ckpt_lck);
  	new_done = CheckpointerShmem->ckpt_done;
  	SpinLockRelease(&CheckpointerShmem->ckpt_lck);
  
  	if (new_done != ckpt_done)
  		FirstCall = true;
  
  	ckpt_done = new_done;
  
  	return FirstCall;
  }
  