Fix postgres_fdw failure with whole-row Vars of type RECORD.

Commit 86dc90056 expects that FDWs can cope with whole-row Vars for
their tables, even if the Vars are marked with vartype RECORDOID.
Previously, whole-row Vars generated by the planner had vartype equal
to the relevant table's rowtype OID.  (The point behind this change is
to enable sharing of resjunk columns across inheritance child tables.)

It turns out that postgres_fdw fails to cope with this, though through
bad fortune none of its test cases exposed that.  Things mostly work,
but when we try to read back a value of such a Var, the expected
rowtype is not available to record_in().  Fortunately, it's not
difficult to hack up the tupdesc that controls this process to
substitute the foreign table's rowtype for RECORDOID.  Thus we can
solve the runtime problem while still sharing the resjunk column
with other tables.

Per report from Alexander Pyhalov.

Discussion: https://postgr.es/m/7817fb9ebd6661cdf9b67dec6e129a78@postgrespro.ru

1 files changed, 53 insertions, 2 deletions

diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.c
index c48a421e88b..0843ed9dba2 100644
--- a/contrib/postgres_fdw/postgres_fdw.c
+++ b/contrib/postgres_fdw/postgres_fdw.c
@@ -1440,6 +1440,57 @@ postgresGetForeignPlan(PlannerInfo *root,
 }
 
 /*
+ * Construct a tuple descriptor for the scan tuples handled by a foreign join.
+ */
+static TupleDesc
+get_tupdesc_for_join_scan_tuples(ForeignScanState *node)
+{
+	ForeignScan *fsplan = (ForeignScan *) node->ss.ps.plan;
+	EState	   *estate = node->ss.ps.state;
+	TupleDesc	tupdesc;
+
+	/*
+	 * The core code has already set up a scan tuple slot based on
+	 * fsplan->fdw_scan_tlist, and this slot's tupdesc is mostly good enough,
+	 * but there's one case where it isn't.  If we have any whole-row row
+	 * identifier Vars, they may have vartype RECORD, and we need to replace
+	 * that with the associated table's actual composite type.  This ensures
+	 * that when we read those ROW() expression values from the remote server,
+	 * we can convert them to a composite type the local server knows.
+	 */
+	tupdesc = CreateTupleDescCopy(node->ss.ss_ScanTupleSlot->tts_tupleDescriptor);
+	for (int i = 0; i < tupdesc->natts; i++)
+	{
+		Form_pg_attribute att = TupleDescAttr(tupdesc, i);
+		Var		   *var;
+		RangeTblEntry *rte;
+		Oid			reltype;
+
+		/* Nothing to do if it's not a generic RECORD attribute */
+		if (att->atttypid != RECORDOID || att->atttypmod >= 0)
+			continue;
+
+		/*
+		 * If we can't identify the referenced table, do nothing.  This'll
+		 * likely lead to failure later, but perhaps we can muddle through.
+		 */
+		var = (Var *) list_nth_node(TargetEntry, fsplan->fdw_scan_tlist,
+									i)->expr;
+		if (!IsA(var, Var) || var->varattno != 0)
+			continue;
+		rte = list_nth(estate->es_range_table, var->varno - 1);
+		if (rte->rtekind != RTE_RELATION)
+			continue;
+		reltype = get_rel_type_id(rte->relid);
+		if (!OidIsValid(reltype))
+			continue;
+		att->atttypid = reltype;
+		/* shouldn't need to change anything else */
+	}
+	return tupdesc;
+}
+
+/*
  * postgresBeginForeignScan
  *		Initiate an executor scan of a foreign PostgreSQL table.
  */
@@ -1523,7 +1574,7 @@ postgresBeginForeignScan(ForeignScanState *node, int eflags)
 	else
 	{
 		fsstate->rel = NULL;
-		fsstate->tupdesc = node->ss.ss_ScanTupleSlot->tts_tupleDescriptor;
+		fsstate->tupdesc = get_tupdesc_for_join_scan_tuples(node);
 	}
 
 	fsstate->attinmeta = TupleDescGetAttInMetadata(fsstate->tupdesc);
@@ -2631,7 +2682,7 @@ postgresBeginDirectModify(ForeignScanState *node, int eflags)
 		TupleDesc	tupdesc;
 
 		if (fsplan->scan.scanrelid == 0)
-			tupdesc = node->ss.ss_ScanTupleSlot->tts_tupleDescriptor;
+			tupdesc = get_tupdesc_for_join_scan_tuples(node);
 		else
 			tupdesc = RelationGetDescr(dmstate->rel);