Fix race condition in reading commit timestamps

If a user requests the commit timestamp for a transaction old enough
that its data is concurrently being truncated away by vacuum at just the
right time, they would receive an ugly internal file-not-found error
message from slru.c rather than the expected NULL return value.

In a primary server, the window for the race is very small: the lookup
has to occur exactly between the two calls by vacuum, and there's not a
lot that happens between them (mostly just a multixact truncate).  In a
standby server, however, the window is larger because the truncation is
executed as soon as the WAL record for it is replayed, but the advance
of the oldest-Xid is not executed until the next checkpoint record.

To fix in the primary, simply reverse the order of operations in
vac_truncate_clog.  To fix in the standby, augment the WAL truncation
record so that the standby is aware of the new oldest-XID value and can
apply the update immediately.  WAL version bumped because of this.

No backpatch, because of the low importance of the bug and its rarity.

Author: Craig Ringer
Reviewed-By: Petr Jelínek, Peter Eisentraut
Discussion: https://postgr.es/m/CAMsr+YFhVtRQT1VAwC+WGbbxZZRzNou=N9Ed-FrCqkwQ8H8oJQ@mail.gmail.com

src/backend/access/rmgrdesc/committsdesc.c

@@ -33,10 +33,10 @@ commit_ts_desc(StringInfo buf, XLogReaderState *record)
 	}
 	else if (info == COMMIT_TS_TRUNCATE)
 	{
-		int			pageno;
+		xl_commit_ts_truncate *trunc = (xl_commit_ts_truncate *) rec;
 
-		memcpy(&pageno, rec, sizeof(int));
+		appendStringInfo(buf, "pageno %d, oldestXid %u",
-		appendStringInfo(buf, "%d", pageno);
+						 trunc->pageno, trunc->oldestXid);
 	}
 	else if (info == COMMIT_TS_SETTS)
 	{

src/backend/access/transam/commit_ts.c

@@ -113,7 +113,7 @@ static bool CommitTsPagePrecedes(int page1, int page2);
 static void ActivateCommitTs(void);
 static void DeactivateCommitTs(void);
 static void WriteZeroPageXlogRec(int pageno);
-static void WriteTruncateXlogRec(int pageno);
+static void WriteTruncateXlogRec(int pageno, TransactionId oldestXid);
 static void WriteSetTimestampXlogRec(TransactionId mainxid, int nsubxids,
 						 TransactionId *subxids, TimestampTz timestamp,
 						 RepOriginId nodeid);
@@ -824,7 +824,7 @@ TruncateCommitTs(TransactionId oldestXact)
 		return;					/* nothing to remove */
 
 	/* Write XLOG record */
-	WriteTruncateXlogRec(cutoffPage);
+	WriteTruncateXlogRec(cutoffPage, oldestXact);
 
 	/* Now we can remove the old CommitTs segment(s) */
 	SimpleLruTruncate(CommitTsCtl, cutoffPage);
@@ -910,10 +910,15 @@ WriteZeroPageXlogRec(int pageno)
  * Write a TRUNCATE xlog record
  */
 static void
-WriteTruncateXlogRec(int pageno)
+WriteTruncateXlogRec(int pageno, TransactionId oldestXid)
 {
+	xl_commit_ts_truncate xlrec;
+
+	xlrec.pageno = pageno;
+	xlrec.oldestXid = oldestXid;
+
 	XLogBeginInsert();
-	XLogRegisterData((char *) (&pageno), sizeof(int));
+	XLogRegisterData((char *) (&xlrec), SizeOfCommitTsTruncate);
 	(void) XLogInsert(RM_COMMIT_TS_ID, COMMIT_TS_TRUNCATE);
 }
 
@@ -967,17 +972,17 @@ commit_ts_redo(XLogReaderState *record)
 	}
 	else if (info == COMMIT_TS_TRUNCATE)
 	{
-		int			pageno;
+		xl_commit_ts_truncate *trunc = (xl_commit_ts_truncate *) XLogRecGetData(record);
 
-		memcpy(&pageno, XLogRecGetData(record), sizeof(int));
+		AdvanceOldestCommitTsXid(trunc->oldestXid);
 
 		/*
 		 * During XLOG replay, latest_page_number isn't set up yet; insert a
 		 * suitable value to bypass the sanity test in SimpleLruTruncate.
 		 */
-		CommitTsCtl->shared->latest_page_number = pageno;
+		CommitTsCtl->shared->latest_page_number = trunc->pageno;
 
-		SimpleLruTruncate(CommitTsCtl, pageno);
+		SimpleLruTruncate(CommitTsCtl, trunc->pageno);
 	}
 	else if (info == COMMIT_TS_SETTS)
 	{

src/backend/commands/vacuum.c

@@ -1152,6 +1152,15 @@ vac_truncate_clog(TransactionId frozenXID,
 	if (bogus)
 		return;
 
+	/*
+	 * Advance the oldest value for commit timestamps before truncating, so
+	 * that if a user requests a timestamp for a transaction we're truncating
+	 * away right after this point, they get NULL instead of an ugly "file not
+	 * found" error from slru.c.  This doesn't matter for xact/multixact
+	 * because they are not subject to arbitrary lookups from users.
+	 */
+	AdvanceOldestCommitTsXid(frozenXID);
+
 	/*
 	 * Truncate CLOG, multixact and CommitTs to the oldest computed value.
 	 */
@@ -1167,7 +1176,6 @@ vac_truncate_clog(TransactionId frozenXID,
 	 */
 	SetTransactionIdLimit(frozenXID, oldestxid_datoid);
 	SetMultiXactIdLimit(minMulti, minmulti_datoid);
-	AdvanceOldestCommitTsXid(frozenXID);
 }
 
 

src/include/access/commit_ts.h

@@ -61,6 +61,14 @@ typedef struct xl_commit_ts_set
 #define SizeOfCommitTsSet	(offsetof(xl_commit_ts_set, mainxid) + \
 							 sizeof(TransactionId))
 
+typedef struct xl_commit_ts_truncate
+{
+	int			pageno;
+	TransactionId oldestXid;
+} xl_commit_ts_truncate;
+
+#define SizeOfCommitTsTruncate	(offsetof(xl_commit_ts_truncate, oldestXid) + \
+								 sizeof(TransactionId))
 
 extern void commit_ts_redo(XLogReaderState *record);
 extern void commit_ts_desc(StringInfo buf, XLogReaderState *record);

src/include/access/xlog_internal.h

@@ -31,7 +31,7 @@
 /*
  * Each page of XLOG file has a header like this:
  */
-#define XLOG_PAGE_MAGIC 0xD093	/* can be used as WAL version indicator */
+#define XLOG_PAGE_MAGIC 0xD094	/* can be used as WAL version indicator */
 
 typedef struct XLogPageHeaderData
 {