Adjust assertion in XLogDecodeNextRecord.

robertmhaas · robertmhaas · commit ec97db399f1e · 2022-08-18T12:22:20.000-04:00
As written, if you use XLogBeginRead() to position an xlogreader at the beginning of a WAL page and then try to read WAL, this assertion will fail. However, the header comment for XLogBeginRead() claims that positioning an xlogreader at the beginning of a page is valid, and the code here is perfectly able to cope with it. It's only the assertion that causes trouble. So relax it. This is formally a bug in all supported branches, but as it doesn't seem to have any consequences for current uses of the xlogreader facility, no back-patch, at least for now. Dilip Kumar and Robert Haas Discussion: http://postgr.es/m/CA+TgmoaJSs2_7WHW2GzFYe9+zfPtxBKvT3GW47+x=ptUE=cULw@mail.gmail.com
diff --git a/src/backend/access/transam/xlogreader.c b/src/backend/access/transam/xlogreader.c
@@ -576,10 +576,11 @@ XLogDecodeNextRecord(XLogReaderState *state, bool nonblocking)
 		/*
 		 * Caller supplied a position to start at.
 		 *
-		 * In this case, NextRecPtr should already be pointing to a valid
-		 * record starting position.
+		 * In this case, NextRecPtr should already be pointing either to a
+		 * valid record starting position or alternatively to the beginning of
+		 * a page. See the header comments for XLogBeginRead.
 		 */
-		Assert(XRecOffIsValid(RecPtr));
+		Assert(RecPtr % XLOG_BLCKSZ == 0 || XRecOffIsValid(RecPtr));
 		randAccess = true;
 	}
 

Original file line number	Diff line number	Diff line change
`@@ -576,10 +576,11 @@ XLogDecodeNextRecord(XLogReaderState *state, bool nonblocking)`
`576`	`576`	`/*`
`577`	`577`	`* Caller supplied a position to start at.`
`578`	`578`	`*`
`579`		`- * In this case, NextRecPtr should already be pointing to a valid`
`580`		`- * record starting position.`
	`579`	`+ * In this case, NextRecPtr should already be pointing either to a`
	`580`	`+ * valid record starting position or alternatively to the beginning of`
	`581`	`+ * a page. See the header comments for XLogBeginRead.`
`581`	`582`	`*/`
`582`		`- Assert(XRecOffIsValid(RecPtr));`
	`583`	`+ Assert(RecPtr % XLOG_BLCKSZ == 0 \|\| XRecOffIsValid(RecPtr));`
`583`	`584`	`randAccess = true;`
`584`	`585`	`}`
`585`	`586`