bug in update tuple routing with foreign partitions

Hi,

(added Fujita-san)

I noticed a bug with how UPDATE tuple routing initializes ResultRelInfos
to use for partition routing targets.  Specifically, the bug occurs when
UPDATE targets include a foreign partition that is locally modified (as
opposed to being modified directly on the remove server) and its
ResultRelInfo (called subplan result rel in the source code) is reused for
tuple routing:

-- setup
create extension postgres_fdw ;
create server loopback foreign data wrapper postgres_fdw;
create user mapping for current_user server loopback;
create table p (a int) partition by list (a);
create table p1 partition of p for values in (1);
create table p2base (a int check (a = 2));
create foreign table p2 partition of p for values in (2) server loopback
options (table_name 'p2base');
insert into p values (1), (2);

-- see in the plan that foreign partition p2 is locally modified
explain verbose update p set a = 2 from (values (1), (2)) s(x) where a =
s.x returning *;
                                   QUERY PLAN

─────────────────────────────────────────────────────────────────────────────────
 Update on public.p  (cost=0.05..236.97 rows=50 width=42)
   Output: p1.a, "*VALUES*".column1
   Update on public.p1
   Foreign Update on public.p2
     Remote SQL: UPDATE public.p2base SET a = $2 WHERE ctid = $1 RETURNING a
   ->  Hash Join  (cost=0.05..45.37 rows=26 width=42)
         Output: 2, p1.ctid, "*VALUES*".*, "*VALUES*".column1
         Hash Cond: (p1.a = "*VALUES*".column1)
         ->  Seq Scan on public.p1  (cost=0.00..35.50 rows=2550 width=10)
               Output: p1.ctid, p1.a
         ->  Hash  (cost=0.03..0.03 rows=2 width=32)
               Output: "*VALUES*".*, "*VALUES*".column1
               ->  Values Scan on "*VALUES*"  (cost=0.00..0.03 rows=2
width=32)
                     Output: "*VALUES*".*, "*VALUES*".column1
   ->  Hash Join  (cost=100.05..191.59 rows=24 width=42)
         Output: 2, p2.ctid, "*VALUES*".*, "*VALUES*".column1
         Hash Cond: (p2.a = "*VALUES*".column1)
         ->  Foreign Scan on public.p2  (cost=100.00..182.27 rows=2409
width=10)
               Output: p2.ctid, p2.a
               Remote SQL: SELECT a, ctid FROM public.p2base FOR UPDATE
         ->  Hash  (cost=0.03..0.03 rows=2 width=32)
               Output: "*VALUES*".*, "*VALUES*".column1
               ->  Values Scan on "*VALUES*"  (cost=0.00..0.03 rows=2
width=32)
                     Output: "*VALUES*".*, "*VALUES*".column1


-- as opposed to directly on remote side (because there's no local join)
explain verbose update p set a = 2 returning *;
                                 QUERY PLAN
─────────────────────────────────────────────────────────────────────────────
 Update on public.p  (cost=0.00..227.40 rows=5280 width=10)
   Output: p1.a
   Update on public.p1
   Foreign Update on public.p2
   ->  Seq Scan on public.p1  (cost=0.00..35.50 rows=2550 width=10)
         Output: 2, p1.ctid
   ->  Foreign Update on public.p2  (cost=100.00..191.90 rows=2730 width=10)
         Remote SQL: UPDATE public.p2base SET a = 2 RETURNING a
(8 rows)

Running the first update query crashes:

update p set a = 2 from (values (1), (2)) s(x) where a = s.x returning
tableoid::regclass, *;
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.

The problem seems to occur because ExecSetupPartitionTupleRouting thinks
it can reuse p2's ResultRelInfo for tuple routing.  In this case, it can't
be used, because its ri_FdwState contains information that will be needed
for postgresExecForeignUpdate to work, but it's still used today.  Because
it's assigned to be used for tuple routing, its ri_FdwState will be
overwritten by postgresBeginForeignInsert that's invoked by the tuple
routing code using the aforementioned ResultRelInfo.  Crash occurs when
postgresExecForeignUpdate () can't find the information it's expecting in
the ri_FdwState.

The solution is to teach ExecSetupPartitionTupleRouting to avoid using a
subplan result rel if its ri_FdwState is already set.  I was wondering if
it should also check ri_usesFdwDirectModify is true, but in that case,
ri_FdwState is unused, so it's perhaps safe for tuple routing code to
scribble on it.

I have attached 2 patches, one for PG 11 where the problem first appeared
and another for HEAD.  The patch for PG 11 is significantly bigger due to
having to handle the complexities of mapping of subplan result rel indexes
to leaf partition indexes.  Most of that complexity is gone in HEAD due to
3f2393edefa5, so the patch for HEAD is much simpler.  I've also added a
test in postgres_fdw.sql to exercise this test case.

Thanks,
Amit