Re: pgsql: Generic Messages for Logical Decoding

On Wed, Apr 6, 2016 at 6:08 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> Generic Messages for Logical Decoding
>
> API and mechanism to allow generic messages to be inserted into WAL that are
> intended to be read by logical decoding plugins. This commit adds an optional
> new callback to the logical decoding API.

When I specify very long text message, pg_xlogdump failed to dump
correctly the WAL file which should contain that inserted text message.
Isn't this a bug?

You can reproduce the problem by the following steps.

=# SELECT pg_xlogfile_name(pg_switch_xlog());
pg_xlogfile_name
--------------------------
000000010000000000000003
(1 row)

=# SELECT pg_xlogfile_name(pg_current_xlog_location());
pg_xlogfile_name
--------------------------
000000010000000000000003
(1 row)

=# SELECT pg_logical_emit_message(true, 'test',
repeat('0123456789ABCDEFG', 1024*1024));
pg_logical_emit_message
-------------------------
0/510CD40
(1 row)

=# SELECT pg_xlogfile_name(pg_current_xlog_location());
pg_xlogfile_name
--------------------------
000000010000000000000005
(1 row)

=# insert into t values(0,0);
INSERT 0 1
postgres=# SELECT pg_xlogfile_name(pg_current_xlog_location());
pg_xlogfile_name
--------------------------
000000010000000000000005
(1 row)

The WAL record of pg_logical_emit_message() should be stored in
000000010000000000000004 and 000000010000000000000005.
The WAL record of last insertion should be stored in
000000010000000000000005.
But the results of pg_xlogdump were wrong as follows.

$ pg_xlogdump data/pg_xlog/000000010000000000000004
rmgr: Standby len (rec/tot): 24/ 50, tx: 0, lsn:
0/04000028, prev 0/030146F8, desc: RUNNING_XACTS nextXid 863
latestCompletedXid 862 oldestRunningXid 863

$ pg_xlogdump data/pg_xlog/000000010000000000000005
pg_xlogdump: FATAL: could not find a valid record after 0/5000000

Regards,

--
Fujii Masao