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

1 files changed, 36 insertions, 10 deletions

diff --git a/src/backend/access/transam/clog.c b/src/backend/access/transam/clog.c
index 5b1d13dac1b..2d335109303 100644
--- a/src/backend/access/transam/clog.c
+++ b/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);