postgres_fdw: incomplete subabort cleanup of connections used in async execution

Hi,

While working on [1], I noticed $SUBJECT: postgres_fdw resets the
per-connection states of connections, which store async requests sent
to remote servers in async_capable mode, during post-abort
(pgfdw_xact_callback()), but it fails to do so during post-subabort
(pgfdw_subxact_callback()). This causes a crash when re-executing a
query that was aborted in a subtransaction:

postgres=# create table t (a text, b text);
postgres=# create or replace function slow_data (name text, duration
float) returns setof t as $$ begin perform pg_sleep(duration); return
query select name, generate_series(1, 1000)::text; end; $$ language
plpgsql;
postgres=# create view v1 as select * from slow_data('foo', 2.5);
postgres=# create view v2 as select * from slow_data('bar', 5.0);
postgres=# create extension postgres_fdw;
postgres=# create server loopback1 foreign data wrapper postgres_fdw
options (dbname 'postgres', async_capable 'true');
postgres=# create server loopback2 foreign data wrapper postgres_fdw
options (dbname 'postgres', async_capable 'true');
postgres=# create user mapping for current_user server loopback1;
postgres=# create user mapping for current_user server loopback2;
postgres=# create foreign table list_p1 (a text, b text) server
loopback1 options (table_name 'v1');
postgres=# create foreign table list_p2 (a text, b text) server
loopback2 options (table_name 'v2');
postgres=# create table list_pt (a text, b text) partition by list (a);
postgres=# alter table list_pt attach partition list_p1 for values in ('foo');
postgres=# alter table list_pt attach partition list_p2 for values in ('bar');

postgres=# begin;
BEGIN
postgres=*# savepoint s1;
SAVEPOINT
postgres=*# select count(*) from list_pt;
^CCancel request sent
ERROR: canceling statement due to user request
postgres=!# rollback to savepoint s1;
ROLLBACK
postgres=*# select count(*) from list_pt;
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.

When canceling the SELECT, an async request sent to a remote server
using a connection is canceled, but it’s stored in the per-connection
state of the connection even after the failed subtransaction for the
reason above, so when re-executing the SELECT, postgres_fdw processes
the invalid async request to re-use the connection in GetConnection(),
causing a segmentation fault. This would be my oversight in commit
27e1f1456. :-(

To fix, I modified pgfdw_abort_cleanup() to reset the per-connection
state in the post-subabort case as well. Also, I modified the
initialization so that it’s done only if necessary, to save cycles,
and improved a comment on the initialization a bit. Attached is a
patch for that.

Best regards,
Etsuro Fujita