BUG #13844: Logical decoding bug with subxact + row locking

The following bug has been logged on the website:

Bug reference: 13844
Logged by: Jarred Ward
Email address: jarred(at)webriots(dot)com
PostgreSQL version: 9.4.5
Operating system: Linux
Description:

This is a pretty serious logical decoding bug that bit us pretty hard
in production.

Run the following code:

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CREATE TABLE foo(a SERIAL PRIMARY KEY, b TEXT);

SELECT pg_create_logical_replication_slot('regression_slot',
'test_decoding');

INSERT INTO foo (b) VALUES ('john');

BEGIN;
SELECT * FROM foo WHERE b = 'john' FOR UPDATE;
SAVEPOINT sp1;
INSERT INTO foo (b) VALUES ('jane');
ROLLBACK TO SAVEPOINT sp1;
INSERT INTO foo (b) VALUES ('doe');
END;

SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL,
NULL);

----------------------------------------------------------------------------

The last select above should return the decoded WAL, but instead it
returns:

----------------------------------------------------------------------------
ERROR: subxact logged without previous toplevel record

----------------------------------------------------------------------------

And the process is disconnected with no way to recover the replication
slot without dropping and recreating.

If SELECT statement in the transaction is changed the following:

----------------------------------------------------------------------------
SELECT * FROM foo WHERE b = 'john' FOR UPDATE;

----------------------------------------------------------------------------

to remove the row level locking the correct decoded change set is
returned.