postgres_fdw: Fix incorrect handling of row movement for remote partitions.

Commit 3d956d9562 added support for update row movement in postgres_fdw.
This patch fixes the following issues introduced by that commit:

* When a remote partition chosen to insert routed rows into was also an
  UPDATE subplan target rel that would be updated later, the UPDATE that
  used a direct modification plan modified those routed rows incorrectly
  because those routed rows were visible to the later UPDATE command.
  The right fix for this would be to have some way in postgres_fdw in
  which the later UPDATE command ignores those routed rows, but it seems
  hard to do so with the current infrastructure.  For now throw an error
  in that case.

* When a remote partition chosen to insert routed rows into was also an
  UPDATE subplan target rel, fmstate created for the UPDATE that used a
  non-direct modification plan was mistakenly overridden by another
  fmstate created for inserting those routed rows into the partition.
  This caused 1) server crash when the partition would be updated later,
  and 2) resource leak when the partition had been already updated.  To
  avoid that, adjust the treatment of the fmstate for the inserting.  As
  for #1, since we would also have the incorrectness issue as mentioned
  above, error out in that case as well.

Update the docs to mention that postgres_fdw currently does not handle
the case where a remote partition chosen to insert a routed row into is
also an UPDATE subplan target rel that will be updated later.

Author: Amit Langote and Etsuro Fujita
Reviewed-by: Amit Langote
Backpatch-through: 11 where row movement in postgres_fdw was added
Discussion: https://postgr.es/m/21e7eaa4-0d4d-20c2-a1f7-c7e96f4ce440@lab.ntt.co.jp

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -1943,6 +1943,51 @@ update utrtest set a = 1 where a = 2 returning *;
 
 drop trigger loct_br_insert_trigger on loct;
 
+-- We can move rows to a foreign partition that has been updated already,
+-- but can't move rows to a foreign partition that hasn't been updated yet
+
+delete from utrtest;
+insert into utrtest values (1, 'foo');
+insert into utrtest values (2, 'qux');
+
+-- Test the former case:
+-- with a direct modification plan
+explain (verbose, costs off)
+update utrtest set a = 1 returning *;
+update utrtest set a = 1 returning *;
+
+delete from utrtest;
+insert into utrtest values (1, 'foo');
+insert into utrtest values (2, 'qux');
+
+-- with a non-direct modification plan
+explain (verbose, costs off)
+update utrtest set a = 1 from (values (1), (2)) s(x) where a = s.x returning *;
+update utrtest set a = 1 from (values (1), (2)) s(x) where a = s.x returning *;
+
+-- Change the definition of utrtest so that the foreign partition get updated
+-- after the local partition
+delete from utrtest;
+alter table utrtest detach partition remp;
+drop foreign table remp;
+alter table loct drop constraint loct_a_check;
+alter table loct add check (a in (3));
+create foreign table remp (a int check (a in (3)), b text) server loopback options (table_name 'loct');
+alter table utrtest attach partition remp for values in (3);
+insert into utrtest values (2, 'qux');
+insert into utrtest values (3, 'xyzzy');
+
+-- Test the latter case:
+-- with a direct modification plan
+explain (verbose, costs off)
+update utrtest set a = 3 returning *;
+update utrtest set a = 3 returning *; -- ERROR
+
+-- with a non-direct modification plan
+explain (verbose, costs off)
+update utrtest set a = 3 from (values (2), (3)) s(x) where a = s.x returning *;
+update utrtest set a = 3 from (values (2), (3)) s(x) where a = s.x returning *; -- ERROR
+
 drop table utrtest;
 drop table loct;