bogus assert in logicalmsg_desc

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