From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Fri, 6 Jan 2006 00:16:09 +0000 (+0000)
Subject: Convert Assert checking for empty page into a regular test and elog.
X-Git-Url: http://git.postgresql.org/gitweb/static/gitweb.js?a=commitdiff_plain;h=c80137b34c74361e0554add92350c7f7df148403;p=users%2Fbernd%2Fpostgres.git

Convert Assert checking for empty page into a regular test and elog.
The consequences of overwriting a non-empty page are bad enough that
we should not omit this test in production builds.
---

diff --git a/src/backend/access/heap/hio.c b/src/backend/access/heap/hio.c
index 5a950ddf37..2546e72c62 100644
--- a/src/backend/access/heap/hio.c
+++ b/src/backend/access/heap/hio.c
@@ -246,13 +246,6 @@ RelationGetBufferForTuple(Relation relation, Size len,
 	 */
 	buffer = ReadBuffer(relation, P_NEW);
 
-	/*
-	 * Release the file-extension lock; it's now OK for someone else to
-	 * extend the relation some more.
-	 */
-	if (needLock)
-		UnlockPage(relation, 0, ExclusiveLock);
-
 	/*
 	 * We can be certain that locking the otherBuffer first is OK, since
 	 * it must have a lower page number.
@@ -261,11 +254,31 @@ RelationGetBufferForTuple(Relation relation, Size len,
 		LockBuffer(otherBuffer, BUFFER_LOCK_EXCLUSIVE);
 
 	/*
-	 * We need to initialize the empty new page.
+	 * Now acquire lock on the new page.
 	 */
 	LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
+
+	/*
+	 * Release the file-extension lock; it's now OK for someone else to
+	 * extend the relation some more.  Note that we cannot release this
+	 * lock before we have buffer lock on the new page, or we risk a
+	 * race condition against vacuumlazy.c --- see comments therein.
+	 */
+	if (needLock)
+		UnlockPage(relation, 0, ExclusiveLock);
+
+	/*
+	 * We need to initialize the empty new page.  Double-check that it really
+	 * is empty (this should never happen, but if it does we don't want to
+	 * risk wiping out valid data).
+	 */
 	pageHeader = (Page) BufferGetPage(buffer);
-	Assert(PageIsNew((PageHeader) pageHeader));
+
+	if (!PageIsNew((PageHeader) pageHeader))
+		elog(ERROR, "page %u of relation \"%s\" should be empty but is not",
+			 BufferGetBlockNumber(buffer),
+			 RelationGetRelationName(relation));
+
 	PageInit(pageHeader, BufferGetPageSize(buffer), 0);
 
 	if (len > PageGetFreeSpace(pageHeader))