Fix system column accesses in ON CONFLICT ... RETURNING.

After 277cb789 ON CONFLICT ... SET ... RETURNING failed with
ERROR:  virtual tuple table slot does not have system attributes
when taking the update path, as the slot used to insert into the
table (and then process RETURNING) was defined to be a virtual slot in
that commit. Virtual slots don't support system columns except for
tableoid and ctid, as the other system columns are AM dependent.

Fix that by using a slot of the table's type. Add tests for system
column accesses in ON CONFLICT ...  RETURNING.

Reported-By: Roby, bisected to the relevant commit by Jeff Janes
Author: Andres Freund
Discussion: https://postgr.es/m/73436355-6432-49B1-92ED-1FE4F7E7E100@finefun.com.au
Backpatch: 12-, where the bug was introduced in 277cb789

src/backend/executor/nodeModifyTable.c

@@ -2542,11 +2542,16 @@ ExecInitModifyTable(ModifyTable *node, EState *estate, int eflags)
 			table_slot_create(resultRelInfo->ri_RelationDesc,
 							  &mtstate->ps.state->es_tupleTable);
 
-		/* create the tuple slot for the UPDATE SET projection */
+		/*
+		 * Create the tuple slot for the UPDATE SET projection. We want a slot
+		 * of the table's type here, because the slot will be used to insert
+		 * into the table, and for RETURNING processing - which may access
+		 * system attributes.
+		 */
 		tupDesc = ExecTypeFromTL((List *) node->onConflictSet);
 		resultRelInfo->ri_onConflict->oc_ProjSlot =
 			ExecInitExtraTupleSlot(mtstate->ps.state, tupDesc,
-								   &TTSOpsVirtual);
+								   table_slot_callbacks(resultRelInfo->ri_RelationDesc));
 
 		/* build UPDATE SET projection state */
 		resultRelInfo->ri_onConflict->oc_ProjInfo =

src/test/regress/expected/update.out

@@ -227,6 +227,29 @@ INSERT INTO upsert_test VALUES (1, 'Bat') ON CONFLICT(a)
  1 | Foo, Correlated, Excluded
 (1 row)
 
+-- ON CONFLICT using system attributes in RETURNING, testing both the
+-- inserting and updating paths. See bug report at:
+-- https://www.postgresql.org/message-id/73436355-6432-49B1-92ED-1FE4F7E7E100%40finefun.com.au
+CREATE FUNCTION xid_current() RETURNS xid LANGUAGE SQL AS $$SELECT (txid_current() % ((1::int8<<32)))::text::xid;$$;
+INSERT INTO upsert_test VALUES (2, 'Beeble') ON CONFLICT(a)
+  DO UPDATE SET (b, a) = (SELECT b || ', Excluded', a from upsert_test i WHERE i.a = excluded.a)
+  RETURNING tableoid::regclass, xmin = xid_current() AS xmin_correct, xmax = 0 AS xmax_correct;
+  tableoid   | xmin_correct | xmax_correct 
+-------------+--------------+--------------
+ upsert_test | t            | t
+(1 row)
+
+-- currently xmax is set after a conflict - that's probably not good,
+-- but it seems worthwhile to have to be explicit if that changes.
+INSERT INTO upsert_test VALUES (2, 'Brox') ON CONFLICT(a)
+  DO UPDATE SET (b, a) = (SELECT b || ', Excluded', a from upsert_test i WHERE i.a = excluded.a)
+  RETURNING tableoid::regclass, xmin = xid_current() AS xmin_correct, xmax = xid_current() AS xmax_correct;
+  tableoid   | xmin_correct | xmax_correct 
+-------------+--------------+--------------
+ upsert_test | t            | t
+(1 row)
+
+DROP FUNCTION xid_current();
 DROP TABLE update_test;
 DROP TABLE upsert_test;
 ---------------------------

src/test/regress/sql/update.sql

@@ -114,6 +114,21 @@ INSERT INTO upsert_test VALUES (1, 'Bat') ON CONFLICT(a)
   DO UPDATE SET (b, a) = (SELECT b || ', Excluded', a from upsert_test i WHERE i.a = excluded.a)
   RETURNING *;
 
+-- ON CONFLICT using system attributes in RETURNING, testing both the
+-- inserting and updating paths. See bug report at:
+-- https://www.postgresql.org/message-id/73436355-6432-49B1-92ED-1FE4F7E7E100%40finefun.com.au
+CREATE FUNCTION xid_current() RETURNS xid LANGUAGE SQL AS $$SELECT (txid_current() % ((1::int8<<32)))::text::xid;$$;
+INSERT INTO upsert_test VALUES (2, 'Beeble') ON CONFLICT(a)
+  DO UPDATE SET (b, a) = (SELECT b || ', Excluded', a from upsert_test i WHERE i.a = excluded.a)
+  RETURNING tableoid::regclass, xmin = xid_current() AS xmin_correct, xmax = 0 AS xmax_correct;
+-- currently xmax is set after a conflict - that's probably not good,
+-- but it seems worthwhile to have to be explicit if that changes.
+INSERT INTO upsert_test VALUES (2, 'Brox') ON CONFLICT(a)
+  DO UPDATE SET (b, a) = (SELECT b || ', Excluded', a from upsert_test i WHERE i.a = excluded.a)
+  RETURNING tableoid::regclass, xmin = xid_current() AS xmin_correct, xmax = xid_current() AS xmax_correct;
+
+
+DROP FUNCTION xid_current();
 DROP TABLE update_test;
 DROP TABLE upsert_test;