Add test to WAL replay to verify that xl_prev points back to the previous

WAL record; this is necessary to be sure we recognize stale WAL records
when a WAL page was only partially written during a system crash.

1 files changed, 34 insertions, 3 deletions

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 682a6323e0e..8b086b78f3c 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.180.4.2 2005/04/15 22:49:45 momjian Exp $
+ * $PostgreSQL: pgsql/src/backend/access/transam/xlog.c,v 1.180.4.3 2005/05/31 19:10:39 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -423,8 +423,8 @@ static char *readRecordBuf = NULL;
 static uint32 readRecordBufSize = 0;
 
 /* State information for XLOG reading */
-static XLogRecPtr ReadRecPtr;
-static XLogRecPtr EndRecPtr;
+static XLogRecPtr ReadRecPtr;				/* start of last record read */
+static XLogRecPtr EndRecPtr;				/* end+1 of last record read */
 static XLogRecord *nextRecord = NULL;
 static TimeLineID lastPageTLI = 0;
 
@@ -2503,6 +2503,37 @@ got_record:;
 					 record->xl_rmid, RecPtr->xlogid, RecPtr->xrecoff)));
 		goto next_record_is_invalid;
 	}
+	if (randAccess)
+	{
+		/*
+		 * We can't exactly verify the prev-link, but surely it should be
+		 * less than the record's own address.
+		 */
+		if (!XLByteLT(record->xl_prev, *RecPtr))
+		{
+			ereport(emode,
+					(errmsg("record with incorrect prev-link %X/%X at %X/%X",
+							record->xl_prev.xlogid, record->xl_prev.xrecoff,
+							RecPtr->xlogid, RecPtr->xrecoff)));
+			goto next_record_is_invalid;
+		}
+	}
+	else
+	{
+		/*
+		 * Record's prev-link should exactly match our previous location.
+		 * This check guards against torn WAL pages where a stale but
+		 * valid-looking WAL record starts on a sector boundary.
+		 */
+		if (!XLByteEQ(record->xl_prev, ReadRecPtr))
+		{
+			ereport(emode,
+					(errmsg("record with incorrect prev-link %X/%X at %X/%X",
+							record->xl_prev.xlogid, record->xl_prev.xrecoff,
+							RecPtr->xlogid, RecPtr->xrecoff)));
+			goto next_record_is_invalid;
+		}
+	}
 
 	/*
 	 * Compute total length of record including any appended backup