Track the oldest XID that can be safely looked up in CLOG.

This provides infrastructure for looking up arbitrary, user-supplied
XIDs without a risk of scary-looking failures from within the clog
module.  Normally, the oldest XID that can be safely looked up in CLOG
is the same as the oldest XID that can reused without causing
wraparound, and the latter is already tracked.  However, while
truncation is in progress, the values are different, so we must
keep track of them separately.

Craig Ringer, reviewed by Simon Riggs and by me.

Discussion: http://postgr.es/m/CAMsr+YHQiWNEi0daCTboS40T+V5s_+dst3PYv_8v2wNVH+Xx4g@mail.gmail.com

doc/src/sgml/monitoring.sgml

@@ -1017,6 +1017,10 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
          <entry><literal>OldSnapshotTimeMapLock</></entry>
          <entry>Waiting to read or update old snapshot control information.</entry>
         </row>
+        <row>
+         <entry><literal>CLogTruncationLock</></entry>
+         <entry>Waiting to truncate the transaction log or waiting for transaction log truncation to finish.</entry>
+        </row>
         <row>
          <entry><literal>clog</></entry>
          <entry>Waiting for I/O on a clog (transaction status) buffer.</entry>

src/backend/access/rmgrdesc/clogdesc.c

@@ -23,12 +23,20 @@ clog_desc(StringInfo buf, XLogReaderState *record)
 	char	   *rec = XLogRecGetData(record);
 	uint8		info = XLogRecGetInfo(record) & ~XLR_INFO_MASK;
 
-	if (info == CLOG_ZEROPAGE || info == CLOG_TRUNCATE)
+	if (info == CLOG_ZEROPAGE)
 	{
 		int			pageno;
 
 		memcpy(&pageno, rec, sizeof(int));
-		appendStringInfo(buf, "%d", pageno);
+		appendStringInfo(buf, "page %d", pageno);
+	}
+	else if (info == CLOG_TRUNCATE)
+	{
+		xl_clog_truncate xlrec;
+
+		memcpy(&xlrec, rec, sizeof(xl_clog_truncate));
+		appendStringInfo(buf, "page %d; oldestXact %u",
+			xlrec.pageno, xlrec.oldestXact);
 	}
 }
 

src/backend/access/transam/clog.c

@@ -83,7 +83,8 @@ static SlruCtlData ClogCtlData;
 static int	ZeroCLOGPage(int pageno, bool writeXlog);
 static bool CLOGPagePrecedes(int page1, int page2);
 static void WriteZeroPageXlogRec(int pageno);
-static void WriteTruncateXlogRec(int pageno);
+static void WriteTruncateXlogRec(int pageno, TransactionId oldestXact,
+								 Oid oldestXidDb);
 static void TransactionIdSetPageStatus(TransactionId xid, int nsubxids,
 						   TransactionId *subxids, XidStatus status,
 						   XLogRecPtr lsn, int pageno);
@@ -640,7 +641,7 @@ ExtendCLOG(TransactionId newestXact)
  * the XLOG flush unless we have confirmed that there is a removable segment.
  */
 void
-TruncateCLOG(TransactionId oldestXact)
+TruncateCLOG(TransactionId oldestXact, Oid oldestxid_datoid)
 {
 	int			cutoffPage;
 
@@ -654,8 +655,26 @@ TruncateCLOG(TransactionId oldestXact)
 	if (!SlruScanDirectory(ClogCtl, SlruScanDirCbReportPresence, &cutoffPage))
 		return;					/* nothing to remove */
 
-	/* Write XLOG record and flush XLOG to disk */
-	WriteTruncateXlogRec(cutoffPage);
+	/*
+	 * Advance oldestClogXid before truncating clog, so concurrent xact status
+	 * lookups can ensure they don't attempt to access truncated-away clog.
+	 *
+	 * It's only necessary to do this if we will actually truncate away clog
+	 * pages.
+	 */
+	AdvanceOldestClogXid(oldestXact);
+
+	/* vac_truncate_clog already advanced oldestXid */
+	Assert(TransactionIdPrecedesOrEquals(oldestXact,
+		   ShmemVariableCache->oldestXid));
+
+	/*
+	 * Write XLOG record and flush XLOG to disk. We record the oldest xid we're
+	 * keeping information about here so we can ensure that it's always ahead
+	 * of clog truncation in case we crash, and so a standby finds out the new
+	 * valid xid before the next checkpoint.
+	 */
+	WriteTruncateXlogRec(cutoffPage, oldestXact, oldestxid_datoid);
 
 	/* Now we can remove the old CLOG segment(s) */
 	SimpleLruTruncate(ClogCtl, cutoffPage);
@@ -704,12 +723,17 @@ WriteZeroPageXlogRec(int pageno)
  * in TruncateCLOG().
  */
 static void
-WriteTruncateXlogRec(int pageno)
+WriteTruncateXlogRec(int pageno, TransactionId oldestXact, Oid oldestXactDb)
 {
 	XLogRecPtr	recptr;
+	xl_clog_truncate xlrec;
+
+	xlrec.pageno = pageno;
+	xlrec.oldestXact = oldestXact;
+	xlrec.oldestXactDb = oldestXactDb;
 
 	XLogBeginInsert();
-	XLogRegisterData((char *) (&pageno), sizeof(int));
+	XLogRegisterData((char *) (&xlrec), sizeof(xl_clog_truncate));
 	recptr = XLogInsert(RM_CLOG_ID, CLOG_TRUNCATE);
 	XLogFlush(recptr);
 }
@@ -742,17 +766,19 @@ clog_redo(XLogReaderState *record)
 	}
 	else if (info == CLOG_TRUNCATE)
 	{
-		int			pageno;
+		xl_clog_truncate xlrec;
 
-		memcpy(&pageno, XLogRecGetData(record), sizeof(int));
+		memcpy(&xlrec, XLogRecGetData(record), sizeof(xl_clog_truncate));
 
 		/*
 		 * During XLOG replay, latest_page_number isn't set up yet; insert a
 		 * suitable value to bypass the sanity test in SimpleLruTruncate.
 		 */
-		ClogCtl->shared->latest_page_number = pageno;
+		ClogCtl->shared->latest_page_number = xlrec.pageno;
+
+		AdvanceOldestClogXid(xlrec.oldestXact);
 
-		SimpleLruTruncate(ClogCtl, pageno);
+		SimpleLruTruncate(ClogCtl, xlrec.pageno);
 	}
 	else
 		elog(PANIC, "clog_redo: unknown op code %u", info);

src/backend/access/transam/transam.c

@@ -119,7 +119,7 @@ TransactionLogFetch(TransactionId transactionId)
  *		True iff transaction associated with the identifier did commit.
  *
  * Note:
- *		Assumes transaction identifier is valid.
+ *		Assumes transaction identifier is valid and exists in clog.
  */
 bool							/* true if given transaction committed */
 TransactionIdDidCommit(TransactionId transactionId)
@@ -175,7 +175,7 @@ TransactionIdDidCommit(TransactionId transactionId)
  *		True iff transaction associated with the identifier did abort.
  *
  * Note:
- *		Assumes transaction identifier is valid.
+ *		Assumes transaction identifier is valid and exists in clog.
  */
 bool							/* true if given transaction aborted */
 TransactionIdDidAbort(TransactionId transactionId)

src/backend/access/transam/varsup.c

@@ -259,7 +259,28 @@ ReadNewTransactionId(void)
 }
 
 /*
- * Determine the last safe XID to allocate given the currently oldest
+ * Advance the cluster-wide value for the oldest valid clog entry.
+ *
+ * We must acquire CLogTruncationLock to advance the oldestClogXid. It's not
+ * necessary to hold the lock during the actual clog truncation, only when we
+ * advance the limit, as code looking up arbitrary xids is required to hold
+ * CLogTruncationLock from when it tests oldestClogXid through to when it
+ * completes the clog lookup.
+ */
+void
+AdvanceOldestClogXid(TransactionId oldest_datfrozenxid)
+{
+	LWLockAcquire(CLogTruncationLock, LW_EXCLUSIVE);
+	if (TransactionIdPrecedes(ShmemVariableCache->oldestClogXid,
+		oldest_datfrozenxid))
+	{
+		ShmemVariableCache->oldestClogXid = oldest_datfrozenxid;
+	}
+	LWLockRelease(CLogTruncationLock);
+}
+
+/*
+ * Determine the last safe XID to allocate using the currently oldest
  * datfrozenxid (ie, the oldest XID that might exist in any database
  * of our cluster), and the OID of the (or a) database with that value.
  */

src/backend/access/transam/xlog.c

@@ -5016,6 +5016,7 @@ BootStrapXLOG(void)
 	ShmemVariableCache->nextOid = checkPoint.nextOid;
 	ShmemVariableCache->oidCount = 0;
 	MultiXactSetNextMXact(checkPoint.nextMulti, checkPoint.nextMultiOffset);
+	AdvanceOldestClogXid(checkPoint.oldestXid);
 	SetTransactionIdLimit(checkPoint.oldestXid, checkPoint.oldestXidDB);
 	SetMultiXactIdLimit(checkPoint.oldestMulti, checkPoint.oldestMultiDB, true);
 	SetCommitTsLimit(InvalidTransactionId, InvalidTransactionId);
@@ -6622,6 +6623,7 @@ StartupXLOG(void)
 	ShmemVariableCache->nextOid = checkPoint.nextOid;
 	ShmemVariableCache->oidCount = 0;
 	MultiXactSetNextMXact(checkPoint.nextMulti, checkPoint.nextMultiOffset);
+	AdvanceOldestClogXid(checkPoint.oldestXid);
 	SetTransactionIdLimit(checkPoint.oldestXid, checkPoint.oldestXidDB);
 	SetMultiXactIdLimit(checkPoint.oldestMulti, checkPoint.oldestMultiDB, true);
 	SetCommitTsLimit(checkPoint.oldestCommitTsXid,
@@ -8687,6 +8689,11 @@ CreateCheckPoint(int flags)
 
 	/*
 	 * Get the other info we need for the checkpoint record.
+	 *
+	 * We don't need to save oldestClogXid in the checkpoint, it only matters
+	 * for the short period in which clog is being truncated, and if we crash
+	 * during that we'll redo the clog truncation and fix up oldestClogXid
+	 * there.
 	 */
 	LWLockAcquire(XidGenLock, LW_SHARED);
 	checkPoint.nextXid = ShmemVariableCache->nextXid;
@@ -9616,6 +9623,10 @@ xlog_redo(XLogReaderState *record)
 
 		MultiXactAdvanceOldest(checkPoint.oldestMulti,
 							   checkPoint.oldestMultiDB);
+		/*
+		 * No need to set oldestClogXid here as well; it'll be set when we
+		 * redo an xl_clog_truncate if it changed since initialization.
+		 */
 		SetTransactionIdLimit(checkPoint.oldestXid, checkPoint.oldestXidDB);
 
 		/*

src/backend/commands/vacuum.c

@@ -1194,7 +1194,7 @@ vac_truncate_clog(TransactionId frozenXID,
 	/*
 	 * Truncate CLOG, multixact and CommitTs to the oldest computed value.
 	 */
-	TruncateCLOG(frozenXID);
+	TruncateCLOG(frozenXID, oldestxid_datoid);
 	TruncateCommitTs(frozenXID);
 	TruncateMultiXact(minMulti, minmulti_datoid);
 

src/backend/storage/lmgr/lwlocknames.txt

@@ -49,3 +49,4 @@ MultiXactTruncationLock				41
 OldSnapshotTimeMapLock				42
 BackendRandomLock					43
 LogicalRepWorkerLock				44
+CLogTruncationLock					45

src/include/access/clog.h

@@ -28,6 +28,12 @@ typedef int XidStatus;
 #define TRANSACTION_STATUS_ABORTED			0x02
 #define TRANSACTION_STATUS_SUB_COMMITTED	0x03
 
+typedef struct xl_clog_truncate
+{
+	int pageno;
+	TransactionId oldestXact;
+	Oid oldestXactDb;
+} xl_clog_truncate;
 
 extern void TransactionIdSetTreeStatus(TransactionId xid, int nsubxids,
 				   TransactionId *subxids, XidStatus status, XLogRecPtr lsn);
@@ -42,7 +48,7 @@ extern void TrimCLOG(void);
 extern void ShutdownCLOG(void);
 extern void CheckPointCLOG(void);
 extern void ExtendCLOG(TransactionId newestXact);
-extern void TruncateCLOG(TransactionId oldestXact);
+extern void TruncateCLOG(TransactionId oldestXact, Oid oldestxid_datoid);
 
 /* XLOG stuff */
 #define CLOG_ZEROPAGE		0x00

src/include/access/transam.h

@@ -134,6 +134,12 @@ typedef struct VariableCacheData
 	 */
 	TransactionId latestCompletedXid;	/* newest XID that has committed or
 										 * aborted */
+
+	/*
+	 * These fields are protected by CLogTruncationLock
+	 */
+	TransactionId oldestClogXid;	/* oldest it's safe to look up in clog */
+
 } VariableCacheData;
 
 typedef VariableCacheData *VariableCache;
@@ -173,6 +179,7 @@ extern TransactionId GetNewTransactionId(bool isSubXact);
 extern TransactionId ReadNewTransactionId(void);
 extern void SetTransactionIdLimit(TransactionId oldest_datfrozenxid,
 					  Oid oldest_datoid);
+extern void AdvanceOldestClogXid(TransactionId oldest_datfrozenxid);
 extern bool ForceTransactionIdLimitUpdate(void);
 extern Oid	GetNewObjectId(void);
 

src/include/access/xlog_internal.h

@@ -31,7 +31,7 @@
 /*
  * Each page of XLOG file has a header like this:
  */
-#define XLOG_PAGE_MAGIC 0xD096	/* can be used as WAL version indicator */
+#define XLOG_PAGE_MAGIC 0xD097	/* can be used as WAL version indicator */
 
 typedef struct XLogPageHeaderData
 {