Keep CommitTs module in sync in standby and master

We allow this module to be turned off on restarts, so a restart time
check is enough to activate or deactivate the module; however, if there
is a standby replaying WAL emitted from a master which is restarted, but
the standby isn't, the state in the standby becomes inconsistent and can
easily be crashed.

Fix by activating and deactivating the module during WAL replay on
parameter change as well as on system start.

Problem reported by Fujii Masao in
http://www.postgresql.org/message-id/CAHGQGwFhJ3CnHo1CELEfay18yg_RA-XZT-7D8NuWUoYSZ90r4Q@mail.gmail.com

Author: Petr Jelínek

Author: alvherre

src/backend/access/transam/commit_ts.c

@@ -557,6 +557,12 @@ StartupCommitTs(void)
 	TransactionId xid = ShmemVariableCache->nextXid;
 	int			pageno = TransactionIdToCTsPage(xid);
 
+	if (track_commit_timestamp)
+	{
+		ActivateCommitTs();
+		return;
+	}
+
 	LWLockAcquire(CommitTsControlLock, LW_EXCLUSIVE);
 
 	/*
@@ -569,16 +575,33 @@ StartupCommitTs(void)
 
 /*
  * This must be called ONCE during postmaster or standalone-backend startup,
- * when commit timestamp is enabled.  Must be called after recovery has
- * finished.
+ * when commit timestamp is enabled, after recovery has finished.
+ */
+void
+CompleteCommitTsInitialization(void)
+{
+	if (!track_commit_timestamp)
+		DeactivateCommitTs(true);
+}
+
+/*
+ * Activate this module whenever necessary.
+ * 		This must happen during postmaster or standalong-backend startup,
+ * 		or during WAL replay anytime the track_commit_timestamp setting is
+ * 		changed in the master.
+ *
+ * The reason why this SLRU needs separate activation/deactivation functions is
+ * that it can be enabled/disabled during start and the activation/deactivation
+ * on master is propagated to slave via replay. Other SLRUs don't have this
+ * property and they can be just initialized during normal startup.
  *
  * This is in charge of creating the currently active segment, if it's not
  * already there.  The reason for this is that the server might have been
  * running with this module disabled for a while and thus might have skipped
  * the normal creation point.
  */
 void
-CompleteCommitTsInitialization(void)
+ActivateCommitTs(void)
 {
 	TransactionId xid = ShmemVariableCache->nextXid;
 	int			pageno = TransactionIdToCTsPage(xid);
@@ -590,22 +613,6 @@ CompleteCommitTsInitialization(void)
 	CommitTsCtl->shared->latest_page_number = pageno;
 	LWLockRelease(CommitTsControlLock);
 
-	/*
-	 * If this module is not currently enabled, make sure we don't hand back
-	 * possibly-invalid data; also remove segments of old data.
-	 */
-	if (!track_commit_timestamp)
-	{
-		LWLockAcquire(CommitTsLock, LW_EXCLUSIVE);
-		ShmemVariableCache->oldestCommitTs = InvalidTransactionId;
-		ShmemVariableCache->newestCommitTs = InvalidTransactionId;
-		LWLockRelease(CommitTsLock);
-
-		TruncateCommitTs(ReadNewTransactionId());
-
-		return;
-	}
-
 	/*
 	 * If CommitTs is enabled, but it wasn't in the previous server run, we
 	 * need to set the oldest and newest values to the next Xid; that way, we
@@ -640,6 +647,37 @@ CompleteCommitTsInitialization(void)
 	}
 }
 
+/*
+ * Deactivate this module.
+ *
+ * This must be called when the track_commit_timestamp parameter is turned off.
+ * This happens during postmaster or standalone-backend startup, or during WAL
+ * replay.
+ *
+ * Resets CommitTs into invalid state to make sure we don't hand back
+ * possibly-invalid data; also removes segments of old data.
+ */
+void
+DeactivateCommitTs(bool do_wal)
+{
+	TransactionId xid = ShmemVariableCache->nextXid;
+	int			pageno = TransactionIdToCTsPage(xid);
+
+	/*
+	 * Re-Initialize our idea of the latest page number.
+	 */
+	LWLockAcquire(CommitTsControlLock, LW_EXCLUSIVE);
+	CommitTsCtl->shared->latest_page_number = pageno;
+	LWLockRelease(CommitTsControlLock);
+
+	LWLockAcquire(CommitTsLock, LW_EXCLUSIVE);
+	ShmemVariableCache->oldestCommitTs = InvalidTransactionId;
+	ShmemVariableCache->newestCommitTs = InvalidTransactionId;
+	LWLockRelease(CommitTsLock);
+
+	TruncateCommitTs(ReadNewTransactionId(), do_wal);
+}
+
 /*
  * This must be called ONCE during postmaster or standalone-backend shutdown
  */
@@ -705,7 +743,7 @@ ExtendCommitTs(TransactionId newestXact)
  * Note that we don't need to flush XLOG here.
  */
 void
-TruncateCommitTs(TransactionId oldestXact)
+TruncateCommitTs(TransactionId oldestXact, bool do_wal)
 {
 	int			cutoffPage;
 
@@ -721,7 +759,8 @@ TruncateCommitTs(TransactionId oldestXact)
 		return;					/* nothing to remove */
 
 	/* Write XLOG record */
-	WriteTruncateXlogRec(cutoffPage);
+	if (do_wal)
+		WriteTruncateXlogRec(cutoffPage);
 
 	/* Now we can remove the old CommitTs segment(s) */
 	SimpleLruTruncate(CommitTsCtl, cutoffPage);

src/backend/access/transam/xlog.c

@@ -5688,6 +5688,19 @@ do { \
 						minValue))); \
 } while(0)
 
+#define RecoveryRequiresBoolParameter(param_name, currValue, masterValue) \
+do { \
+	bool _currValue = (currValue); \
+	bool _masterValue = (masterValue); \
+	if (_currValue != _masterValue) \
+		ereport(ERROR, \
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE), \
+				 errmsg("hot standby is not possible because it requires \"%s\" to be same on master and standby (master has \"%s\", standby has \"%s\")", \
+						param_name, \
+						_masterValue ? "true" : "false", \
+						_currValue ? "true" : "false"))); \
+} while(0)
+
 /*
  * Check to see if required parameters are set high enough on this server
  * for various aspects of recovery operation.
@@ -5730,6 +5743,9 @@ CheckRequiredParameterValues(void)
 		RecoveryRequiresIntParameter("max_locks_per_transaction",
 									 max_locks_per_xact,
 									 ControlFile->max_locks_per_xact);
+		RecoveryRequiresBoolParameter("track_commit_timestamp",
+									  track_commit_timestamp,
+									  ControlFile->track_commit_timestamp);
 	}
 }
 
@@ -9118,7 +9134,6 @@ xlog_redo(XLogReaderState *record)
 		ControlFile->max_locks_per_xact = xlrec.max_locks_per_xact;
 		ControlFile->wal_level = xlrec.wal_level;
 		ControlFile->wal_log_hints = xlrec.wal_log_hints;
-		ControlFile->track_commit_timestamp = xlrec.track_commit_timestamp;
 
 		/*
 		 * Update minRecoveryPoint to ensure that if recovery is aborted, we
@@ -9136,6 +9151,25 @@ xlog_redo(XLogReaderState *record)
 			ControlFile->minRecoveryPointTLI = ThisTimeLineID;
 		}
 
+		/*
+		 * Update the commit timestamp tracking. If there was a change
+		 * it needs to be activated or deactivated accordingly.
+		 */
+		if (track_commit_timestamp != xlrec.track_commit_timestamp)
+		{
+			track_commit_timestamp = xlrec.track_commit_timestamp;
+			ControlFile->track_commit_timestamp = track_commit_timestamp;
+			if (track_commit_timestamp)
+				ActivateCommitTs();
+			else
+				/*
+				 * We can't create a new WAL record here, but that's OK as
+				 * master did the WAL logging already and we will replay the
+				 * record from master in case we crash.
+				 */
+				DeactivateCommitTs(false);
+		}
+
 		UpdateControlFile();
 		LWLockRelease(ControlFileLock);
 

src/backend/commands/vacuum.c

@@ -1087,7 +1087,7 @@ vac_truncate_clog(TransactionId frozenXID,
 	 * checkpoint.
 	 */
 	TruncateCLOG(frozenXID);
-	TruncateCommitTs(frozenXID);
+	TruncateCommitTs(frozenXID, true);
 
 	/*
 	 * Update the wrap limit for GetNewTransactionId and creation of new

src/include/access/commit_ts.h

@@ -39,11 +39,13 @@ extern Size CommitTsShmemSize(void);
 extern void CommitTsShmemInit(void);
 extern void BootStrapCommitTs(void);
 extern void StartupCommitTs(void);
+extern void ActivateCommitTs(void);
+extern void DeactivateCommitTs(bool do_wal);
 extern void CompleteCommitTsInitialization(void);
 extern void ShutdownCommitTs(void);
 extern void CheckPointCommitTs(void);
 extern void ExtendCommitTs(TransactionId newestXact);
-extern void TruncateCommitTs(TransactionId oldestXact);
+extern void TruncateCommitTs(TransactionId oldestXact, bool do_wal);
 extern void SetCommitTsLimit(TransactionId oldestXact,
 				 TransactionId newestXact);
 extern void AdvanceOldestCommitTs(TransactionId oldestXact);