bogus assert in logicalmsg_desc

[Tomas Vondra]

Hi,

while experimenting with logical messages, I ran into this assert in
logicalmsg_desc:

    Assert(prefix[xlrec->prefix_size] != '\0');

This seems to be incorrect, because LogLogicalMessage does this:

    xlrec.prefix_size = strlen(prefix) + 1;

So prefix_size includes the null byte, so the assert points out at the
first payload byte. And of course, the check should be "==" because we
expect the byte to be \0, not the other way around.

It's pretty simple to make this crash by writing a logical message where
the first payload byte is \0, e.g. like this:

    select pg_logical_emit_message(true, 'a'::text, '\x00'::bytea);

and then running pg_waldump on the WAL segment.

Attached is a patch addressing this. This was added in 14, so we should
backpatch to that version.


regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
diff --git a/src/backend/access/rmgrdesc/logicalmsgdesc.c b/src/backend/access/rmgrdesc/logicalmsgdesc.c
index 099e11a84e7..08e03aa30d1 100644
--- a/src/backend/access/rmgrdesc/logicalmsgdesc.c
+++ b/src/backend/access/rmgrdesc/logicalmsgdesc.c
@@ -28,7 +28,7 @@ logicalmsg_desc(StringInfo buf, XLogReaderState *record)
 		char	   *message = xlrec->message + xlrec->prefix_size;
 		char	   *sep = "";
 
-		Assert(prefix[xlrec->prefix_size] != '\0');
+		Assert(prefix[xlrec->prefix_size - 1] == '\0');
 
 		appendStringInfo(buf, "%s, prefix \"%s\"; payload (%zu bytes): ",
 						 xlrec->transactional ? "transactional" : "non-transactional",