Repair two problems with WAL logging of sequence nextvalI() ops, as

per recent pghackers discussion: force a new WAL record at first nextval
after a checkpoint, and ensure that xlog is flushed to disk if a nextval
record is the only thing emitted by a transaction.

src/backend/access/transam/xact.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/access/transam/xact.c,v 1.117 2002/03/06 06:09:21 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/access/transam/xact.c,v 1.118 2002/03/15 19:20:29 tgl Exp $
  *
  * NOTES
  *		Transaction aborts can now occur two ways:
@@ -546,32 +546,48 @@ RecordTransactionCommit(void)
 	xid = GetCurrentTransactionId();
 
 	/*
-	 * We needn't write anything in xlog or clog if the transaction was
+	 * We only need to log the commit in xlog and clog if the transaction made
-	 * read-only, which we check by testing if it made any xlog entries.
+	 * any transaction-controlled XLOG entries.  (Otherwise, its XID appears
+	 * nowhere in permanent storage, so no one will ever care if it
+	 * committed.)  However, we must flush XLOG to disk if we made any XLOG
+	 * entries, whether in or out of transaction control.  For example, if we
+	 * reported a nextval() result to the client, this ensures that any XLOG
+	 * record generated by nextval will hit the disk before we report the
+	 * transaction committed.
 	 */
-	if (MyLastRecPtr.xrecoff != 0)
+	if (MyXactMadeXLogEntry)
 	{
-		XLogRecData rdata;
-		xl_xact_commit xlrec;
 		XLogRecPtr	recptr;
 
 		BufmgrCommit();
 
+		START_CRIT_SECTION();
+
+		if (MyLastRecPtr.xrecoff != 0)
+		{
+			/* Need to emit a commit record */
+			XLogRecData rdata;
+			xl_xact_commit xlrec;
+
 			xlrec.xtime = time(NULL);
 			rdata.buffer = InvalidBuffer;
 			rdata.data = (char *) (&xlrec);
 			rdata.len = SizeOfXactCommit;
 			rdata.next = NULL;
 
-		START_CRIT_SECTION();
-
 			/*
-		 * SHOULD SAVE ARRAY OF RELFILENODE-s TO DROP
+			 * XXX SHOULD SAVE ARRAY OF RELFILENODE-s TO DROP
 			 */
 			recptr = XLogInsert(RM_XACT_ID, XLOG_XACT_COMMIT, &rdata);
+		}
+		else
+		{
+			/* Just flush through last record written by me */
+			recptr = ProcLastRecEnd;
+		}
 
 		/*
-		 * Sleep before commit! So we can flush more than one commit
+		 * Sleep before flush! So we can flush more than one commit
 		 * records per single fsync.  (The idea is some other backend may
 		 * do the XLogFlush while we're sleeping.  This needs work still,
 		 * because on most Unixen, the minimum select() delay is 10msec or
@@ -593,15 +609,17 @@ RecordTransactionCommit(void)
 
 		XLogFlush(recptr);
 
-		/* Break the chain of back-links in the XLOG records I output */
+		/* Mark the transaction committed in clog, if needed */
-		MyLastRecPtr.xrecoff = 0;
+		if (MyLastRecPtr.xrecoff != 0)
-
-		/* Mark the transaction committed in clog */
 			TransactionIdCommit(xid);
 
 		END_CRIT_SECTION();
 	}
 
+	/* Break the chain of back-links in the XLOG records I output */
+	MyLastRecPtr.xrecoff = 0;
+	MyXactMadeXLogEntry = false;
+
 	/* Show myself as out of the transaction in PROC array */
 	MyProc->logRec.xrecoff = 0;
 
@@ -689,8 +707,11 @@ RecordTransactionAbort(void)
 	TransactionId xid = GetCurrentTransactionId();
 
 	/*
-	 * We needn't write anything in xlog or clog if the transaction was
+	 * We only need to log the abort in xlog and clog if the transaction made
-	 * read-only, which we check by testing if it made any xlog entries.
+	 * any transaction-controlled XLOG entries.  (Otherwise, its XID appears
+	 * nowhere in permanent storage, so no one will ever care if it
+	 * committed.)  We do not flush XLOG to disk in any case, since the
+	 * default assumption after a crash would be that we aborted, anyway.
 	 *
 	 * Extra check here is to catch case that we aborted partway through
 	 * RecordTransactionCommit ...
@@ -714,11 +735,6 @@ RecordTransactionAbort(void)
 		 */
 		recptr = XLogInsert(RM_XACT_ID, XLOG_XACT_ABORT, &rdata);
 
-		/*
-		 * There's no need for XLogFlush here, since the default
-		 * assumption would be that we aborted, anyway.
-		 */
-
 		/* Mark the transaction aborted in clog */
 		TransactionIdAbort(xid);
 
@@ -727,6 +743,8 @@ RecordTransactionAbort(void)
 
 	/* Break the chain of back-links in the XLOG records I output */
 	MyLastRecPtr.xrecoff = 0;
+	MyXactMadeXLogEntry = false;
+
 	/* Show myself as out of the transaction in PROC array */
 	MyProc->logRec.xrecoff = 0;
 

src/backend/access/transam/xlog.c

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2001, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Header: /cvsroot/pgsql/src/backend/access/transam/xlog.c,v 1.89 2002/03/06 06:09:22 momjian Exp $
+ * $Header: /cvsroot/pgsql/src/backend/access/transam/xlog.c,v 1.90 2002/03/15 19:20:30 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -131,27 +131,36 @@ bool		InRecovery = false;
 
 /*
  * MyLastRecPtr points to the start of the last XLOG record inserted by the
- * current transaction.  If MyLastRecPtr.xrecoff == 0, then we are not in
+ * current transaction.  If MyLastRecPtr.xrecoff == 0, then the current
- * a transaction or the transaction has not yet made any loggable changes.
+ * xact hasn't yet inserted any transaction-controlled XLOG records.
  *
  * Note that XLOG records inserted outside transaction control are not
- * reflected into MyLastRecPtr.
+ * reflected into MyLastRecPtr.  They do, however, cause MyXactMadeXLogEntry
+ * to be set true.  The latter can be used to test whether the current xact
+ * made any loggable changes (including out-of-xact changes, such as
+ * sequence updates).
  */
 XLogRecPtr	MyLastRecPtr = {0, 0};
 
+bool		MyXactMadeXLogEntry = false;
+
 /*
  * ProcLastRecPtr points to the start of the last XLOG record inserted by the
  * current backend.  It is updated for all inserts, transaction-controlled
- * or not.
+ * or not.  ProcLastRecEnd is similar but points to end+1 of last record.
  */
 static XLogRecPtr ProcLastRecPtr = {0, 0};
 
+XLogRecPtr	ProcLastRecEnd = {0, 0};
+
 /*
  * RedoRecPtr is this backend's local copy of the REDO record pointer
  * (which is almost but not quite the same as a pointer to the most recent
  * CHECKPOINT record).	We update this from the shared-memory copy,
  * XLogCtl->Insert.RedoRecPtr, whenever we can safely do so (ie, when we
- * hold the Insert lock).  See XLogInsert for details.
+ * hold the Insert lock).  See XLogInsert for details.  We are also allowed
+ * to update from XLogCtl->Insert.RedoRecPtr if we hold the info_lck;
+ * see GetRedoRecPtr.
  */
 static XLogRecPtr RedoRecPtr;
 
@@ -272,7 +281,8 @@ typedef struct XLogCtlData
 	StartUpID	ThisStartUpID;
 
 	/* This value is not protected by *any* lock... */
-	XLogRecPtr	RedoRecPtr;		/* see SetRedoRecPtr/GetRedoRecPtr */
+	/* see SetSavedRedoRecPtr/GetSavedRedoRecPtr */
+	XLogRecPtr	SavedRedoRecPtr;
 
 	slock_t		info_lck;		/* locks shared LogwrtRqst/LogwrtResult */
 } XLogCtlData;
@@ -777,6 +787,7 @@ begin:;
 		MyLastRecPtr = RecPtr;
 	ProcLastRecPtr = RecPtr;
 	Insert->PrevRecord = RecPtr;
+	MyXactMadeXLogEntry = true;
 
 	Insert->currpos += SizeOfXLogRecord;
 	freespace -= SizeOfXLogRecord;
@@ -855,6 +866,8 @@ begin:;
 		SpinLockRelease_NoHoldoff(&xlogctl->info_lck);
 	}
 
+	ProcLastRecEnd = RecPtr;
+
 	END_CRIT_SECTION();
 
 	return (RecPtr);
@@ -2538,7 +2551,7 @@ StartupXLOG(void)
 
 	ThisStartUpID = checkPoint.ThisStartUpID;
 	RedoRecPtr = XLogCtl->Insert.RedoRecPtr =
-		XLogCtl->RedoRecPtr = checkPoint.redo;
+		XLogCtl->SavedRedoRecPtr = checkPoint.redo;
 
 	if (XLByteLT(RecPtr, checkPoint.redo))
 		elog(PANIC, "invalid redo in checkpoint record");
@@ -2824,32 +2837,47 @@ void
 SetThisStartUpID(void)
 {
 	ThisStartUpID = XLogCtl->ThisStartUpID;
-	RedoRecPtr = XLogCtl->RedoRecPtr;
+	RedoRecPtr = XLogCtl->SavedRedoRecPtr;
 }
 
 /*
  * CheckPoint process called by postmaster saves copy of new RedoRecPtr
- * in shmem (using SetRedoRecPtr).	When checkpointer completes, postmaster
+ * in shmem (using SetSavedRedoRecPtr).  When checkpointer completes,
- * calls GetRedoRecPtr to update its own copy of RedoRecPtr, so that
+ * postmaster calls GetSavedRedoRecPtr to update its own copy of RedoRecPtr,
- * subsequently-spawned backends will start out with a reasonably up-to-date
+ * so that subsequently-spawned backends will start out with a reasonably
- * local RedoRecPtr.  Since these operations are not protected by any lock
+ * up-to-date local RedoRecPtr.  Since these operations are not protected by
- * and copying an XLogRecPtr isn't atomic, it's unsafe to use either of these
+ * any lock and copying an XLogRecPtr isn't atomic, it's unsafe to use either
- * routines at other times!
+ * of these routines at other times!
- *
- * Note: once spawned, a backend must update its local RedoRecPtr from
- * XLogCtl->Insert.RedoRecPtr while holding the insert lock.  This is
- * done in XLogInsert().
  */
 void
-SetRedoRecPtr(void)
+SetSavedRedoRecPtr(void)
 {
-	XLogCtl->RedoRecPtr = RedoRecPtr;
+	XLogCtl->SavedRedoRecPtr = RedoRecPtr;
 }
 
 void
+GetSavedRedoRecPtr(void)
+{
+	RedoRecPtr = XLogCtl->SavedRedoRecPtr;
+}
+
+/*
+ * Once spawned, a backend may update its local RedoRecPtr from
+ * XLogCtl->Insert.RedoRecPtr; it must hold the insert lock or info_lck
+ * to do so.  This is done in XLogInsert() or GetRedoRecPtr().
+ */
+XLogRecPtr
 GetRedoRecPtr(void)
 {
-	RedoRecPtr = XLogCtl->RedoRecPtr;
+	/* use volatile pointer to prevent code rearrangement */
+	volatile XLogCtlData *xlogctl = XLogCtl;
+
+	SpinLockAcquire_NoHoldoff(&xlogctl->info_lck);
+	Assert(XLByteLE(RedoRecPtr, xlogctl->Insert.RedoRecPtr));
+	RedoRecPtr = xlogctl->Insert.RedoRecPtr;
+	SpinLockRelease_NoHoldoff(&xlogctl->info_lck);
+
+	return RedoRecPtr;
 }
 
 /*
@@ -2862,6 +2890,7 @@ ShutdownXLOG(void)
 
 	/* suppress in-transaction check in CreateCheckPoint */
 	MyLastRecPtr.xrecoff = 0;
+	MyXactMadeXLogEntry = false;
 
 	CritSectionCount++;
 	CreateDummyCaches();
@@ -2886,7 +2915,7 @@ CreateCheckPoint(bool shutdown)
 	uint32		_logId;
 	uint32		_logSeg;
 
-	if (MyLastRecPtr.xrecoff != 0)
+	if (MyXactMadeXLogEntry)
 		elog(ERROR, "CreateCheckPoint: cannot be called inside transaction block");
 
 	/*
@@ -2972,9 +3001,16 @@ CreateCheckPoint(bool shutdown)
 
 	/*
 	 * Here we update the shared RedoRecPtr for future XLogInsert calls;
-	 * this must be done while holding the insert lock.
+	 * this must be done while holding the insert lock AND the info_lck.
 	 */
-	RedoRecPtr = XLogCtl->Insert.RedoRecPtr = checkPoint.redo;
+	{
+		/* use volatile pointer to prevent code rearrangement */
+		volatile XLogCtlData *xlogctl = XLogCtl;
+
+		SpinLockAcquire_NoHoldoff(&xlogctl->info_lck);
+		RedoRecPtr = xlogctl->Insert.RedoRecPtr = checkPoint.redo;
+		SpinLockRelease_NoHoldoff(&xlogctl->info_lck);
+	}
 
 	/*
 	 * Get UNDO record ptr - this is oldest of PROC->logRec values. We do

src/backend/bootstrap/bootstrap.c

@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/bootstrap/bootstrap.c,v 1.123 2002/03/08 00:42:09 tgl Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/bootstrap/bootstrap.c,v 1.124 2002/03/15 19:20:34 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -386,7 +386,7 @@ BootstrapMain(int argc, char *argv[])
 				InitDummyProcess();		/* needed to get LWLocks */
 			CreateDummyCaches();
 			CreateCheckPoint(false);
-			SetRedoRecPtr();
+			SetSavedRedoRecPtr(); /* pass redo ptr back to postmaster */
 			proc_exit(0);		/* done */
 
 		case BS_XLOG_STARTUP:

src/backend/commands/sequence.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/commands/sequence.c,v 1.70 2002/03/06 06:09:35 momjian Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/commands/sequence.c,v 1.71 2002/03/15 19:20:35 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -286,6 +286,7 @@ nextval(PG_FUNCTION_ARGS)
 	char	   *seqname = get_seq_name(seqin);
 	SeqTable	elm;
 	Buffer		buf;
+	Page		page;
 	Form_pg_sequence seq;
 	int64		incby,
 				maxv,
@@ -316,6 +317,7 @@ nextval(PG_FUNCTION_ARGS)
 
 	seq = read_info("nextval", elm, &buf);		/* lock page' buffer and
 												 * read tuple */
+	page = BufferGetPage(buf);
 
 	last = next = result = seq->last_value;
 	incby = seq->increment_by;
@@ -331,11 +333,33 @@ nextval(PG_FUNCTION_ARGS)
 		log--;
 	}
 
+	/*
+	 * Decide whether we should emit a WAL log record.  If so, force up
+	 * the fetch count to grab SEQ_LOG_VALS more values than we actually
+	 * need to cache.  (These will then be usable without logging.)
+	 *
+	 * If this is the first nextval after a checkpoint, we must force
+	 * a new WAL record to be written anyway, else replay starting from the
+	 * checkpoint would fail to advance the sequence past the logged
+	 * values.  In this case we may as well fetch extra values.
+	 */
 	if (log < fetch)
 	{
-		fetch = log = fetch - log + SEQ_LOG_VALS;
+		/* forced log to satisfy local demand for values */
+		fetch = log = fetch + SEQ_LOG_VALS;
 		logit = true;
 	}
+	else
+	{
+		XLogRecPtr	redoptr = GetRedoRecPtr();
+
+		if (XLByteLE(PageGetLSN(page), redoptr))
+		{
+			/* last update of seq was before checkpoint */
+			fetch = log = fetch + SEQ_LOG_VALS;
+			logit = true;
+		}
+	}
 
 	while (fetch)				/* try to fetch cache [+ log ] numbers */
 	{
@@ -386,6 +410,9 @@ nextval(PG_FUNCTION_ARGS)
 		}
 	}
 
+	log -= fetch;				/* adjust for any unfetched numbers */
+	Assert(log >= 0);
+
 	/* save info in local cache */
 	elm->last = result;			/* last returned number */
 	elm->cached = last;			/* last fetched number */
@@ -396,7 +423,6 @@ nextval(PG_FUNCTION_ARGS)
 		xl_seq_rec	xlrec;
 		XLogRecPtr	recptr;
 		XLogRecData rdata[2];
-		Page		page = BufferGetPage(buf);
 
 		xlrec.node = elm->rel->rd_node;
 		rdata[0].buffer = InvalidBuffer;
@@ -417,15 +443,11 @@ nextval(PG_FUNCTION_ARGS)
 
 		PageSetLSN(page, recptr);
 		PageSetSUI(page, ThisStartUpID);
-
-		if (fetch)				/* not all numbers were fetched */
-			log -= fetch;
 	}
 
 	/* update on-disk data */
 	seq->last_value = last;		/* last fetched number */
 	seq->is_called = true;
-	Assert(log >= 0);
 	seq->log_cnt = log;			/* how much is logged */
 	END_CRIT_SECTION();
 

src/backend/postmaster/postmaster.c

@@ -37,7 +37,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/postmaster/postmaster.c,v 1.270 2002/03/04 01:46:03 tgl Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/postmaster/postmaster.c,v 1.271 2002/03/15 19:20:35 tgl Exp $
  *
  * NOTES
  *
@@ -1683,7 +1683,7 @@ CleanupProc(int pid,
 			{
 				checkpointed = time(NULL);
 				/* Update RedoRecPtr for future child backends */
-				GetRedoRecPtr();
+				GetSavedRedoRecPtr();
 			}
 		}
 		else

src/include/access/xlog.h

@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2001, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $Id: xlog.h,v 1.28 2001/11/05 17:46:31 momjian Exp $
+ * $Id: xlog.h,v 1.29 2002/03/15 19:20:36 tgl Exp $
  */
 #ifndef XLOG_H
 #define XLOG_H
@@ -178,6 +178,8 @@ typedef struct XLogRecData
 extern StartUpID ThisStartUpID; /* current SUI */
 extern bool InRecovery;
 extern XLogRecPtr MyLastRecPtr;
+extern bool MyXactMadeXLogEntry;
+extern XLogRecPtr ProcLastRecEnd;
 
 /* these variables are GUC parameters related to XLOG */
 extern int	CheckPointSegments;
@@ -205,8 +207,9 @@ extern void ShutdownXLOG(void);
 extern void CreateCheckPoint(bool shutdown);
 extern void SetThisStartUpID(void);
 extern void XLogPutNextOid(Oid nextOid);
-extern void SetRedoRecPtr(void);
+extern void SetSavedRedoRecPtr(void);
-extern void GetRedoRecPtr(void);
+extern void GetSavedRedoRecPtr(void);
+extern XLogRecPtr GetRedoRecPtr(void);
 
 /* in storage/ipc/sinval.c, but don't want to declare in sinval.h because
  * we'd have to include xlog.h into that ...