Fix postgres_fdw to return the right ctid value in EvalPlanQual cases.

If a postgres_fdw foreign table is a non-locked source relation in an
UPDATE, DELETE, or SELECT FOR UPDATE/SHARE, and the query selects its
ctid column, the wrong value would be returned if an EvalPlanQual
recheck occurred.  This happened because the foreign table's result row
was copied via the ROW_MARK_COPY code path, and EvalPlanQualFetchRowMarks
just unconditionally set the reconstructed tuple's t_self to "invalid".

To fix that, we can have EvalPlanQualFetchRowMarks copy the composite
datum's t_ctid field, and be sure to initialize that along with t_self
when postgres_fdw constructs a tuple to return.

If we just did that much then EvalPlanQualFetchRowMarks would start
returning "(0,0)" as ctid for all other ROW_MARK_COPY cases, which perhaps
does not matter much, but then again maybe it might.  The cause of that is
that heap_form_tuple, which is the ultimate source of all composite datums,
simply leaves t_ctid as zeroes in newly constructed tuples.  That seems
like a bad idea on general principles: a field that's really not been
initialized shouldn't appear to have a valid value.  So let's eat the
trivial additional overhead of doing "ItemPointerSetInvalid(&(td->t_ctid))"
in heap_form_tuple.

This closes out our handling of Etsuro Fujita's report that tableoid and
ctid weren't correctly set in postgres_fdw EvalPlanQual cases.  Along the
way we did a great deal of work to improve FDWs' ability to control row
locking behavior; which was not wasted effort by any means, but it didn't
end up being a fix for this problem because that feature would be too
expensive for postgres_fdw to use all the time.

Although the fix for the tableoid misbehavior was back-patched, I'm
hesitant to do so here; it seems far less likely that people would care
about remote ctid than tableoid, and even such a minor behavioral change
as this in heap_form_tuple is perhaps best not back-patched.  So commit
to HEAD only, at least for the moment.

Etsuro Fujita, with some adjustments by me

contrib/postgres_fdw/postgres_fdw.c

@@ -2964,8 +2964,14 @@ make_tuple_from_result_row(PGresult *res,
 
 	tuple = heap_form_tuple(tupdesc, values, nulls);
 
+	/*
+	 * If we have a CTID to return, install it in both t_self and t_ctid.
+	 * t_self is the normal place, but if the tuple is converted to a
+	 * composite Datum, t_self will be lost; setting t_ctid allows CTID to be
+	 * preserved during EvalPlanQual re-evaluations (see ROW_MARK_COPY code).
+	 */
 	if (ctid)
-		tuple->t_self = *ctid;
+		tuple->t_self = tuple->t_data->t_ctid = *ctid;
 
 	/* Clean up */
 	MemoryContextReset(temp_context);

src/backend/access/common/heaptuple.c

@@ -727,6 +727,8 @@ heap_form_tuple(TupleDesc tupleDescriptor,
 	HeapTupleHeaderSetDatumLength(td, len);
 	HeapTupleHeaderSetTypeId(td, tupleDescriptor->tdtypeid);
 	HeapTupleHeaderSetTypMod(td, tupleDescriptor->tdtypmod);
+	/* We also make sure that t_ctid is invalid unless explicitly set */
+	ItemPointerSetInvalid(&(td->t_ctid));
 
 	HeapTupleHeaderSetNatts(td, numberOfAttributes);
 	td->t_hoff = hoff;

src/backend/executor/execMain.c

@@ -2613,10 +2613,11 @@ EvalPlanQualFetchRowMarks(EPQState *epqstate)
 
 			/* build a temporary HeapTuple control structure */
 			tuple.t_len = HeapTupleHeaderGetDatumLength(td);
-			ItemPointerSetInvalid(&(tuple.t_self));
+			tuple.t_data = td;
 			/* relation might be a foreign table, if so provide tableoid */
 			tuple.t_tableOid = erm->relid;
-			tuple.t_data = td;
+			/* also copy t_ctid in case there's valid data there */
+			tuple.t_self = td->t_ctid;
 
 			/* copy and store tuple */
 			EvalPlanQualSetTuple(epqstate, erm->rti,