Fix race condition in reading commit timestamps

alvherre · alvherre · commit 8eace46d34ab · 2017-01-19T18:24:17.000-03:00
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
diff --git a/src/backend/access/rmgrdesc/committsdesc.c b/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, "%d", pageno);
+		appendStringInfo(buf, "pageno %d, oldestXid %u",
+						 trunc->pageno, trunc->oldestXid);
 	}
 	else if (info == COMMIT_TS_SETTS)
 	{
diff --git a/src/backend/access/transam/commit_ts.c b/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)
 	{
diff --git a/src/backend/commands/vacuum.c b/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);
 }
 
 
diff --git a/src/include/access/commit_ts.h b/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);
diff --git a/src/include/access/xlog_internal.h b/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
 {

Original file line number	Diff line number	Diff line change
`@@ -33,10 +33,10 @@ commit_ts_desc(StringInfo buf, XLogReaderState *record)`
`33`	`33`	`}`
`34`	`34`	`else if (info == COMMIT_TS_TRUNCATE)`
`35`	`35`	`{`
`36`		`- int pageno;`
	`36`	`+ xl_commit_ts_truncate trunc = (xl_commit_ts_truncate ) rec;`
`37`	`37`
`38`		`- memcpy(&pageno, rec, sizeof(int));`
`39`		`- appendStringInfo(buf, "%d", pageno);`
	`38`	`+ appendStringInfo(buf, "pageno %d, oldestXid %u",`
	`39`	`+ trunc->pageno, trunc->oldestXid);`
`40`	`40`	`}`
`41`	`41`	`else if (info == COMMIT_TS_SETTS)`
`42`	`42`	`{`
Original file line number	Diff line number	Diff line change
`@@ -31,7 +31,7 @@`
`31`	`31`	`/*`
`32`	`32`	`* Each page of XLOG file has a header like this:`
`33`	`33`	`*/`
`34`		`-#define XLOG_PAGE_MAGIC 0xD093 /* can be used as WAL version indicator */`
	`34`	`+#define XLOG_PAGE_MAGIC 0xD094 /* can be used as WAL version indicator */`
`35`	`35`
`36`	`36`	`typedef struct XLogPageHeaderData`
`37`	`37`	`{`