--- /dev/null
+/*-------------------------------------------------------------------------
+ *
+ * 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) 2010-2011, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ * $PostgreSQL$
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include <unistd.h>
+
+#include "access/xact.h"
+#include "access/xlog_internal.h"
+#include "miscadmin.h"
+#include "postmaster/autovacuum.h"
+#include "replication/syncrep.h"
+#include "replication/walsender.h"
+#include "storage/latch.h"
+#include "storage/ipc.h"
+#include "storage/pmsignal.h"
+#include "storage/proc.h"
+#include "utils/builtins.h"
+#include "utils/guc.h"
+#include "utils/guc_tables.h"
+#include "utils/memutils.h"
+#include "utils/ps_status.h"
+
+/* User-settable parameters for sync rep */
+bool sync_rep_mode = false; /* Only set in user backends */
+char *SyncRepStandbyNames;
+
+static bool sync_standbys_defined = false; /* Is there at least one name? */
+static bool announce_next_takeover = true;
+
+static void SyncRepQueueInsert(void);
+
+static int SyncRepGetStandbyPriority(void);
+static bool SyncRepQueueIsOrderedByLSN(void);
+
+/*
+ * ===========================================================
+ * Synchronous Replication functions for normal user backends
+ * ===========================================================
+ */
+
+/*
+ * Wait for synchronous replication, if requested by user.
+ */
+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() || !sync_standbys_defined)
+ return;
+
+ Assert(SHMQueueIsDetached(&(MyProc->syncRepLinks)));
+
+ /*
+ * 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 (;;)
+ {
+ ResetLatch(&MyProc->waitLatch);
+
+ /*
+ * Synchronous Replication state machine within user backend
+ *
+ * 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 when
+ * we exit normally, or SYNC_REP_MUST_DISCONNECT in abnormal cases.
+ *
+ * We read MyProc->syncRepState without SyncRepLock, which
+ * assumes that read access is atomic.
+ */
+ switch (MyProc->syncRepState)
+ {
+ case SYNC_REP_NOT_WAITING:
+ /*
+ * Set our waitLSN so WALSender will know when to wake us.
+ * We set this before we add ourselves to queue, so that
+ * any proc on the queue can be examined freely without
+ * taking a lock on each process in the queue, as long as
+ * they hold SyncRepLock.
+ */
+ MyProc->waitLSN = XactCommitLSN;
+ MyProc->syncRepState = SYNC_REP_WAITING;
+
+ /*
+ * Add to queue while holding lock.
+ */
+ LWLockAcquire(SyncRepLock, LW_EXCLUSIVE);
+ 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 ..." */
+ }
+
+ break;
+
+ case SYNC_REP_WAITING:
+ /*
+ * Check for conditions that would cause us to leave the
+ * wait state before the LSN has been reached.
+ */
+ if (!PostmasterIsAlive(true))
+ {
+ LWLockAcquire(SyncRepLock, LW_EXCLUSIVE);
+ SHMQueueDelete(&(MyProc->syncRepLinks));
+ LWLockRelease(SyncRepLock);
+
+ MyProc->syncRepState = SYNC_REP_MUST_DISCONNECT;
+ return;
+ }
+
+ /*
+ * We don't receive SIGHUPs at this point, so resetting
+ * synchronous_standby_names has no effect on waiters.
+ */
+
+ /* Continue waiting */
+
+ break;
+
+ case SYNC_REP_WAIT_COMPLETE:
+ /*
+ * WalSender has checked our LSN and has removed us from
+ * queue. Cleanup local state and leave.
+ */
+ 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);
+ }
+
+ return;
+
+ case SYNC_REP_MUST_DISCONNECT:
+ return;
+
+ default:
+ elog(FATAL, "invalid syncRepState");
+ }
+
+ /*
+ * Wait on latch for up to 60 seconds. This allows us to
+ * check for postmaster death regularly while waiting.
+ * Note that timeout here does not necessarily release from loop.
+ */
+ WaitLatch(&MyProc->waitLatch, 60000000L);
+ }
+}
+
+/*
+ * Insert MyProc into SyncRepQueue, maintaining sorted invariant.
+ *
+ * Usually we will go at tail of queue, though its 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));
+}
+
+void
+SyncRepCleanupAtProcExit(int code, Datum arg)
+{
+ if (!SHMQueueIsDetached(&(MyProc->syncRepLinks)))
+ {
+ LWLockAcquire(SyncRepLock, LW_EXCLUSIVE);
+ SHMQueueDelete(&(MyProc->syncRepLinks));
+ LWLockRelease(SyncRepLock);
+ }
+
+ DisownLatch(&MyProc->waitLatch);
+}
+
+/*
+ * ===========================================================
+ * 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;
+
+ /* 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);
+ ereport(FATAL,
+ (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ errmsg("invalid list syntax for parameter \"synchronous_standby_names\"")));
+ 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 setting setting the state of any backends that
+ * need to be woken, remove them from the queue and then wake them.
+ * Set all = true to wake whole queue, or just up to 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.
+ */
+ Assert(SHMQueueIsDetached(&(thisproc->syncRepLinks)));
+ Assert(thisproc->syncRepState == SYNC_REP_WAIT_COMPLETE);
+ SetLatch(&(thisproc->waitLatch));
+
+ numprocs++;
+ }
+
+ return numprocs;
+}
+
+#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
+ * ===========================================================
+ */
+
+const char *
+assign_synchronous_standby_names(const char *newval, bool doit, 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 */
+ pfree(rawstring);
+ list_free(elemlist);
+ ereport(FATAL,
+ (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ errmsg("invalid list syntax for parameter \"synchronous_standby_names\"")));
+ return NULL;
+ }
+
+ /*
+ * Is there at least one sync standby? If so cache this knowledge to
+ * improve performance of SyncRepWaitForLSN() for all-async configs.
+ */
+ if (doit && list_length(elemlist) > 0)
+ sync_standbys_defined = true;
+
+ /*
+ * 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 newval;
+}
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