postgres_fdw: Oddity in pushing down inherited UPDATE/DELETE joinsto remote servers

While doing a bit more review of the partitionwise-join-fix patch, I
noticed $SUBJECT.  Here is an example that causes an assertion failure
"TRAP: FailedAssertion("!(foreignrel)", File: "postgres_fdw.c", Line:
2213)":

postgres=# create table t1 (a int, b text);
CREATE TABLE
postgres=# create table t2 (a int, b text);
CREATE TABLE
postgres=# create foreign table ft1 (a int, b text) server loopback
options (table_name 't1');
CREATE FOREIGN TABLE
postgres=# create foreign table ft2 (a int, b text) server loopback
options (table_name 't2');
CREATE FOREIGN TABLE
postgres=# insert into ft1 values (1, 'foo');
INSERT 0 1
postgres=# insert into ft1 values (2, 'bar');
INSERT 0 1
postgres=# insert into ft2 values (1, 'test1');
INSERT 0 1
postgres=# insert into ft2 values (2, 'test2');
INSERT 0 1
postgres=# analyze ft1;
ANALYZE
postgres=# analyze ft2;
ANALYZE
postgres=# create table parent (a int, b text);
CREATE TABLE
postgres=# alter foreign table ft1 inherit parent;
ALTER FOREIGN TABLE
postgres=# update parent set b = ft2.b from ft2 where parent.a = ft2.a;
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.
!>

I think the reason for that is: in that case we try to find the target
foreign-join RelOptInfo using find_join_rel in postgresPlanDirectModify,
but can't find it, because the given root is the *parent* root and
doesn't have join RelOptInfos with it.  To fix this, I'd like to propose
to modify make_modifytable so that in case of an inherited
UPDATE/DELETE, it passes to PlanDirectModify the per-child modified
subroot, not the parent root, for the FDW to get the foreign-join
RelOptInfo from the given root in PlanDirectModify.  I think that that
would be more consistent with the non-inherited UPDATE/DELETE case in
that the FDW can look at any join RelOptInfos in the given root in
PlanDirectModify, which I think would be a good thing because some FDWs
might need to do that for some reason.  For the same reason, I'd also
like to propose to pass to PlanForeignModify the per-child modified
subroot, not the parent root, as for PlanDirectModify.  Attached is a
proposed patch for that.  I'll add this to the open items list for PG11.

Best regards,
Etsuro Fujita