postgresql/src/backend/replication/syncrep.c

/*-------------------------------------------------------------------------
 *
 * syncrep.c
 *
 * Synchronous replication is new as of PostgreSQL 9.1.
 *
 * If requested, transaction commits wait until their commit LSN is
 * acknowledged by the sync standby.
 *
 * This module contains the code for waiting and release of backends.
 * All code in this module executes on the primary. The core streaming
 * replication transport remains within WALreceiver/WALsender modules.
 *
 * The essence of this design is that it isolates all logic about
 * waiting/releasing onto the primary. The primary defines which standbys
 * it wishes to wait for. The standby is completely unaware of the
 * durability requirements of transactions on the primary, reducing the
 * complexity of the code and streamlining both standby operations and
 * network bandwidth because there is no requirement to ship
 * per-transaction state information.
 *
 * Replication is either synchronous or not synchronous (async). If it is
 * async, we just fastpath out of here. If it is sync, then in 9.1 we wait
 * for the flush location on the standby before releasing the waiting backend.
 * Further complexity in that interaction is expected in later releases.
 *
 * The best performing way to manage the waiting backends is to have a
 * single ordered queue of waiting backends, so that we can avoid
 * searching the through all waiters each time we receive a reply.
 *
 * In 9.1 we support only a single synchronous standby, chosen from a
 * priority list of synchronous_standby_names. Before it can become the
 * synchronous standby it must have caught up with the primary; that may
 * take some time. Once caught up, the current highest priority standby
 * will release waiters from the queue.
 *
 * Portions Copyright (c) 2010-2012, PostgreSQL Global Development Group
 *
 * IDENTIFICATION
 *	  src/backend/replication/syncrep.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <unistd.h>

#include "access/xact.h"
#include "miscadmin.h"
#include "replication/syncrep.h"
#include "replication/walsender.h"
#include "replication/walsender_private.h"
#include "storage/pmsignal.h"
#include "storage/proc.h"
#include "tcop/tcopprot.h"
#include "utils/builtins.h"
#include "utils/ps_status.h"

/* User-settable parameters for sync rep */
char	   *SyncRepStandbyNames;

#define SyncStandbysDefined() \
	(SyncRepStandbyNames != NULL && SyncRepStandbyNames[0] != '\0')

#define SyncRepRequested() \
	(max_wal_senders > 0 && synchronous_commit > SYNCHRONOUS_COMMIT_LOCAL_FLUSH)

static bool announce_next_takeover = true;

static void SyncRepQueueInsert(void);
static void SyncRepCancelWait(void);

static int	SyncRepGetStandbyPriority(void);

#ifdef USE_ASSERT_CHECKING
static bool SyncRepQueueIsOrderedByLSN(void);
#endif

/*
 * ===========================================================
 * Synchronous Replication functions for normal user backends
 * ===========================================================
 */

/*
 * Wait for synchronous replication, if requested by user.
 *
 * Initially backends start in state SYNC_REP_NOT_WAITING and then
 * change that state to SYNC_REP_WAITING before adding ourselves
 * to the wait queue. During SyncRepWakeQueue() a WALSender changes
 * the state to SYNC_REP_WAIT_COMPLETE once replication is confirmed.
 * This backend then resets its state to SYNC_REP_NOT_WAITING.
 */
void
SyncRepWaitForLSN(XLogRecPtr XactCommitLSN)
{
	char	   *new_status = NULL;
	const char *old_status;

	/*
	 * Fast exit if user has not requested sync replication, or there are no
	 * sync replication standby names defined. Note that those standbys don't
	 * need to be connected.
	 */
	if (!SyncRepRequested() || !SyncStandbysDefined())
		return;

	Assert(SHMQueueIsDetached(&(MyProc->syncRepLinks)));
	Assert(WalSndCtl != NULL);

	LWLockAcquire(SyncRepLock, LW_EXCLUSIVE);
	Assert(MyProc->syncRepState == SYNC_REP_NOT_WAITING);

	/*
	 * We don't wait for sync rep if WalSndCtl->sync_standbys_defined is not
	 * set.  See SyncRepUpdateSyncStandbysDefined.
	 *
	 * Also check that the standby hasn't already replied. Unlikely race
	 * condition but we'll be fetching that cache line anyway so its likely to
	 * be a low cost check.
	 */
	if (!WalSndCtl->sync_standbys_defined ||
		XLByteLE(XactCommitLSN, WalSndCtl->lsn))
	{
		LWLockRelease(SyncRepLock);
		return;
	}

	/*
	 * Set our waitLSN so WALSender will know when to wake us, and add
	 * ourselves to the queue.
	 */
	MyProc->waitLSN = XactCommitLSN;
	MyProc->syncRepState = SYNC_REP_WAITING;
	SyncRepQueueInsert();
	Assert(SyncRepQueueIsOrderedByLSN());
	LWLockRelease(SyncRepLock);

	/* Alter ps display to show waiting for sync rep. */
	if (update_process_title)
	{
		int			len;

		old_status = get_ps_display(&len);
		new_status = (char *) palloc(len + 32 + 1);
		memcpy(new_status, old_status, len);
		sprintf(new_status + len, " waiting for %X/%X",
				XactCommitLSN.xlogid, XactCommitLSN.xrecoff);
		set_ps_display(new_status, false);
		new_status[len] = '\0'; /* truncate off " waiting ..." */
	}

	/*
	 * Wait for specified LSN to be confirmed.
	 *
	 * Each proc has its own wait latch, so we perform a normal latch
	 * check/wait loop here.
	 */
	for (;;)
	{
		int			syncRepState;

		/* Must reset the latch before testing state. */
		ResetLatch(&MyProc->procLatch);

		/*
		 * Try checking the state without the lock first.  There's no
		 * guarantee that we'll read the most up-to-date value, so if it looks
		 * like we're still waiting, recheck while holding the lock.  But if
		 * it looks like we're done, we must really be done, because once
		 * walsender changes the state to SYNC_REP_WAIT_COMPLETE, it will
		 * never update it again, so we can't be seeing a stale value in that
		 * case.
		 *
		 * Note: on machines with weak memory ordering, the acquisition of
		 * the lock is essential to avoid race conditions: we cannot be sure
		 * the sender's state update has reached main memory until we acquire
		 * the lock.  We could get rid of this dance if SetLatch/ResetLatch
		 * contained memory barriers.
		 */
		syncRepState = MyProc->syncRepState;
		if (syncRepState == SYNC_REP_WAITING)
		{
			LWLockAcquire(SyncRepLock, LW_SHARED);
			syncRepState = MyProc->syncRepState;
			LWLockRelease(SyncRepLock);
		}
		if (syncRepState == SYNC_REP_WAIT_COMPLETE)
			break;

		/*
		 * If a wait for synchronous replication is pending, we can neither
		 * acknowledge the commit nor raise ERROR or FATAL.  The latter would
		 * lead the client to believe that that the transaction aborted, which
		 * is not true: it's already committed locally. The former is no good
		 * either: the client has requested synchronous replication, and is
		 * entitled to assume that an acknowledged commit is also replicated,
		 * which might not be true. So in this case we issue a WARNING (which
		 * some clients may be able to interpret) and shut off further output.
		 * We do NOT reset ProcDiePending, so that the process will die after
		 * the commit is cleaned up.
		 */
		if (ProcDiePending)
		{
			ereport(WARNING,
					(errcode(ERRCODE_ADMIN_SHUTDOWN),
					 errmsg("canceling the wait for synchronous replication and terminating connection due to administrator command"),
					 errdetail("The transaction has already committed locally, but might not have been replicated to the standby.")));
			whereToSendOutput = DestNone;
			SyncRepCancelWait();
			break;
		}

		/*
		 * It's unclear what to do if a query cancel interrupt arrives.  We
		 * can't actually abort at this point, but ignoring the interrupt
		 * altogether is not helpful, so we just terminate the wait with a
		 * suitable warning.
		 */
		if (QueryCancelPending)
		{
			QueryCancelPending = false;
			ereport(WARNING,
					(errmsg("canceling wait for synchronous replication due to user request"),
					 errdetail("The transaction has already committed locally, but might not have been replicated to the standby.")));
			SyncRepCancelWait();
			break;
		}

		/*
		 * If the postmaster dies, we'll probably never get an
		 * acknowledgement, because all the wal sender processes will exit. So
		 * just bail out.
		 */
		if (!PostmasterIsAlive())
		{
			ProcDiePending = true;
			whereToSendOutput = DestNone;
			SyncRepCancelWait();
			break;
		}

		/*
		 * Wait on latch.  Any condition that should wake us up will set
		 * the latch, so no need for timeout.
		 */
		WaitLatch(&MyProc->procLatch, WL_LATCH_SET | WL_POSTMASTER_DEATH, -1);
	}

	/*
	 * WalSender has checked our LSN and has removed us from queue. Clean up
	 * state and leave.  It's OK to reset these shared memory fields without
	 * holding SyncRepLock, because any walsenders will ignore us anyway when
	 * we're not on the queue.
	 */
	Assert(SHMQueueIsDetached(&(MyProc->syncRepLinks)));
	MyProc->syncRepState = SYNC_REP_NOT_WAITING;
	MyProc->waitLSN.xlogid = 0;
	MyProc->waitLSN.xrecoff = 0;

	if (new_status)
	{
		/* Reset ps display */
		set_ps_display(new_status, false);
		pfree(new_status);
	}
}

/*
 * Insert MyProc into SyncRepQueue, maintaining sorted invariant.
 *
 * Usually we will go at tail of queue, though it's possible that we arrive
 * here out of order, so start at tail and work back to insertion point.
 */
static void
SyncRepQueueInsert(void)
{
	PGPROC	   *proc;

	proc = (PGPROC *) SHMQueuePrev(&(WalSndCtl->SyncRepQueue),
								   &(WalSndCtl->SyncRepQueue),
								   offsetof(PGPROC, syncRepLinks));

	while (proc)
	{
		/*
		 * Stop at the queue element that we should after to ensure the queue
		 * is ordered by LSN.
		 */
		if (XLByteLT(proc->waitLSN, MyProc->waitLSN))
			break;

		proc = (PGPROC *) SHMQueuePrev(&(WalSndCtl->SyncRepQueue),
									   &(proc->syncRepLinks),
									   offsetof(PGPROC, syncRepLinks));
	}

	if (proc)
		SHMQueueInsertAfter(&(proc->syncRepLinks), &(MyProc->syncRepLinks));
	else
		SHMQueueInsertAfter(&(WalSndCtl->SyncRepQueue), &(MyProc->syncRepLinks));
}

/*
 * Acquire SyncRepLock and cancel any wait currently in progress.
 */
static void
SyncRepCancelWait(void)
{
	LWLockAcquire(SyncRepLock, LW_EXCLUSIVE);
	if (!SHMQueueIsDetached(&(MyProc->syncRepLinks)))
		SHMQueueDelete(&(MyProc->syncRepLinks));
	MyProc->syncRepState = SYNC_REP_NOT_WAITING;
	LWLockRelease(SyncRepLock);
}

void
SyncRepCleanupAtProcExit(void)
{
	if (!SHMQueueIsDetached(&(MyProc->syncRepLinks)))
	{
		LWLockAcquire(SyncRepLock, LW_EXCLUSIVE);
		SHMQueueDelete(&(MyProc->syncRepLinks));
		LWLockRelease(SyncRepLock);
	}
}

/*
 * ===========================================================
 * Synchronous Replication functions for wal sender processes
 * ===========================================================
 */

/*
 * Take any action required to initialise sync rep state from config
 * data. Called at WALSender startup and after each SIGHUP.
 */
void
SyncRepInitConfig(void)
{
	int			priority;

	/*
	 * Determine if we are a potential sync standby and remember the result
	 * for handling replies from standby.
	 */
	priority = SyncRepGetStandbyPriority();
	if (MyWalSnd->sync_standby_priority != priority)
	{
		LWLockAcquire(SyncRepLock, LW_EXCLUSIVE);
		MyWalSnd->sync_standby_priority = priority;
		LWLockRelease(SyncRepLock);
		ereport(DEBUG1,
			(errmsg("standby \"%s\" now has synchronous standby priority %u",
					application_name, priority)));
	}
}

/*
 * Update the LSNs on each queue based upon our latest state. This
 * implements a simple policy of first-valid-standby-releases-waiter.
 *
 * Other policies are possible, which would change what we do here and what
 * perhaps also which information we store as well.
 */
void
SyncRepReleaseWaiters(void)
{
	volatile WalSndCtlData *walsndctl = WalSndCtl;
	volatile WalSnd *syncWalSnd = NULL;
	int			numprocs = 0;
	int			priority = 0;
	int			i;

	/*
	 * If this WALSender is serving a standby that is not on the list of
	 * potential standbys then we have nothing to do. If we are still starting
	 * up or still running base backup, then leave quickly also.
	 */
	if (MyWalSnd->sync_standby_priority == 0 ||
		MyWalSnd->state < WALSNDSTATE_STREAMING)
		return;

	/*
	 * We're a potential sync standby. Release waiters if we are the highest
	 * priority standby. If there are multiple standbys with same priorities
	 * then we use the first mentioned standby. If you change this, also
	 * change pg_stat_get_wal_senders().
	 */
	LWLockAcquire(SyncRepLock, LW_EXCLUSIVE);

	for (i = 0; i < max_wal_senders; i++)
	{
		/* use volatile pointer to prevent code rearrangement */
		volatile WalSnd *walsnd = &walsndctl->walsnds[i];

		if (walsnd->pid != 0 &&
			walsnd->sync_standby_priority > 0 &&
			(priority == 0 ||
			 priority > walsnd->sync_standby_priority))
		{
			priority = walsnd->sync_standby_priority;
			syncWalSnd = walsnd;
		}
	}

	/*
	 * We should have found ourselves at least.
	 */
	Assert(syncWalSnd);

	/*
	 * If we aren't managing the highest priority standby then just leave.
	 */
	if (syncWalSnd != MyWalSnd)
	{
		LWLockRelease(SyncRepLock);
		announce_next_takeover = true;
		return;
	}

	if (XLByteLT(walsndctl->lsn, MyWalSnd->flush))
	{
		/*
		 * Set the lsn first so that when we wake backends they will release
		 * up to this location.
		 */
		walsndctl->lsn = MyWalSnd->flush;
		numprocs = SyncRepWakeQueue(false);
	}

	LWLockRelease(SyncRepLock);

	elog(DEBUG3, "released %d procs up to %X/%X",
		 numprocs,
		 MyWalSnd->flush.xlogid,
		 MyWalSnd->flush.xrecoff);

	/*
	 * If we are managing the highest priority standby, though we weren't
	 * prior to this, then announce we are now the sync standby.
	 */
	if (announce_next_takeover)
	{
		announce_next_takeover = false;
		ereport(LOG,
				(errmsg("standby \"%s\" is now the synchronous standby with priority %u",
						application_name, MyWalSnd->sync_standby_priority)));
	}
}

/*
 * Check if we are in the list of sync standbys, and if so, determine
 * priority sequence. Return priority if set, or zero to indicate that
 * we are not a potential sync standby.
 *
 * Compare the parameter SyncRepStandbyNames against the application_name
 * for this WALSender, or allow any name if we find a wildcard "*".
 */
static int
SyncRepGetStandbyPriority(void)
{
	char	   *rawstring;
	List	   *elemlist;
	ListCell   *l;
	int			priority = 0;
	bool		found = false;

	/*
	 * Since synchronous cascade replication is not allowed, we always
	 * set the priority of cascading walsender to zero.
	 */
	if (am_cascading_walsender)
		return 0;

	/* Need a modifiable copy of string */
	rawstring = pstrdup(SyncRepStandbyNames);

	/* Parse string into list of identifiers */
	if (!SplitIdentifierString(rawstring, ',', &elemlist))
	{
		/* syntax error in list */
		pfree(rawstring);
		list_free(elemlist);
		/* GUC machinery will have already complained - no need to do again */
		return 0;
	}

	foreach(l, elemlist)
	{
		char	   *standby_name = (char *) lfirst(l);

		priority++;

		if (pg_strcasecmp(standby_name, application_name) == 0 ||
			pg_strcasecmp(standby_name, "*") == 0)
		{
			found = true;
			break;
		}
	}

	pfree(rawstring);
	list_free(elemlist);

	return (found ? priority : 0);
}

/*
 * Walk queue from head.  Set the state of any backends that need to be woken,
 * remove them from the queue, and then wake them.	Pass all = true to wake
 * whole queue; otherwise, just wake up to the walsender's LSN.
 *
 * Must hold SyncRepLock.
 */
int
SyncRepWakeQueue(bool all)
{
	volatile WalSndCtlData *walsndctl = WalSndCtl;
	PGPROC	   *proc = NULL;
	PGPROC	   *thisproc = NULL;
	int			numprocs = 0;

	Assert(SyncRepQueueIsOrderedByLSN());

	proc = (PGPROC *) SHMQueueNext(&(WalSndCtl->SyncRepQueue),
								   &(WalSndCtl->SyncRepQueue),
								   offsetof(PGPROC, syncRepLinks));

	while (proc)
	{
		/*
		 * Assume the queue is ordered by LSN
		 */
		if (!all && XLByteLT(walsndctl->lsn, proc->waitLSN))
			return numprocs;

		/*
		 * Move to next proc, so we can delete thisproc from the queue.
		 * thisproc is valid, proc may be NULL after this.
		 */
		thisproc = proc;
		proc = (PGPROC *) SHMQueueNext(&(WalSndCtl->SyncRepQueue),
									   &(proc->syncRepLinks),
									   offsetof(PGPROC, syncRepLinks));

		/*
		 * Set state to complete; see SyncRepWaitForLSN() for discussion of
		 * the various states.
		 */
		thisproc->syncRepState = SYNC_REP_WAIT_COMPLETE;

		/*
		 * Remove thisproc from queue.
		 */
		SHMQueueDelete(&(thisproc->syncRepLinks));

		/*
		 * Wake only when we have set state and removed from queue.
		 */
		SetLatch(&(thisproc->procLatch));

		numprocs++;
	}

	return numprocs;
}

/*
 * The background writer calls this as needed to update the shared
 * sync_standbys_defined flag, so that backends don't remain permanently wedged
 * if synchronous_standby_names is unset.  It's safe to check the current value
 * without the lock, because it's only ever updated by one process.  But we
 * must take the lock to change it.
 */
void
SyncRepUpdateSyncStandbysDefined(void)
{
	bool		sync_standbys_defined = SyncStandbysDefined();

	if (sync_standbys_defined != WalSndCtl->sync_standbys_defined)
	{
		LWLockAcquire(SyncRepLock, LW_EXCLUSIVE);

		/*
		 * If synchronous_standby_names has been reset to empty, it's futile
		 * for backends to continue to waiting.  Since the user no longer
		 * wants synchronous replication, we'd better wake them up.
		 */
		if (!sync_standbys_defined)
			SyncRepWakeQueue(true);

		/*
		 * Only allow people to join the queue when there are synchronous
		 * standbys defined.  Without this interlock, there's a race
		 * condition: we might wake up all the current waiters; then, some
		 * backend that hasn't yet reloaded its config might go to sleep on
		 * the queue (and never wake up).  This prevents that.
		 */
		WalSndCtl->sync_standbys_defined = sync_standbys_defined;

		LWLockRelease(SyncRepLock);
	}
}

#ifdef USE_ASSERT_CHECKING
static bool
SyncRepQueueIsOrderedByLSN(void)
{
	PGPROC	   *proc = NULL;
	XLogRecPtr	lastLSN;

	lastLSN.xlogid = 0;
	lastLSN.xrecoff = 0;

	proc = (PGPROC *) SHMQueueNext(&(WalSndCtl->SyncRepQueue),
								   &(WalSndCtl->SyncRepQueue),
								   offsetof(PGPROC, syncRepLinks));

	while (proc)
	{
		/*
		 * Check the queue is ordered by LSN and that multiple procs don't
		 * have matching LSNs
		 */
		if (XLByteLE(proc->waitLSN, lastLSN))
			return false;

		lastLSN = proc->waitLSN;

		proc = (PGPROC *) SHMQueueNext(&(WalSndCtl->SyncRepQueue),
									   &(proc->syncRepLinks),
									   offsetof(PGPROC, syncRepLinks));
	}

	return true;
}
#endif

/*
 * ===========================================================
 * Synchronous Replication functions executed by any process
 * ===========================================================
 */

bool
check_synchronous_standby_names(char **newval, void **extra, GucSource source)
{
	char	   *rawstring;
	List	   *elemlist;

	/* Need a modifiable copy of string */
	rawstring = pstrdup(*newval);

	/* Parse string into list of identifiers */
	if (!SplitIdentifierString(rawstring, ',', &elemlist))
	{
		/* syntax error in list */
		GUC_check_errdetail("List syntax is invalid.");
		pfree(rawstring);
		list_free(elemlist);
		return false;
	}

	/*
	 * Any additional validation of standby names should go here.
	 *
	 * Don't attempt to set WALSender priority because this is executed by
	 * postmaster at startup, not WALSender, so the application_name is not
	 * yet correctly set.
	 */

	pfree(rawstring);
	list_free(elemlist);

	return true;
}