Fix handling of multiple AFTER ROW triggers on a foreign table.

AfterTriggerExecute() retrieves a fresh tuple or pair of tuples from a
tuplestore and then stores the tuple(s) in the passed-in slot(s) if
AFTER_TRIGGER_FDW_FETCH, while it uses the most-recently-retrieved
tuple(s) stored in the slot(s) if AFTER_TRIGGER_FDW_REUSE.  This was
done correctly before 12, but commit ff11e7f4 broke it by mistakenly
clearing the tuple(s) stored in the slot(s) in that function, leading to
an assertion failure as reported in bug #16139 from Alexander Lakhin.

Also, fix some other issues with the aforementioned commit in passing:

* For tg_newslot, which is a slot added to the TriggerData struct by the
  commit to store new updated tuples, it didn't ensure the slot was NULL
  if there was no such tuple.
* The commit failed to update the documentation about the trigger
  interface.

Author: Etsuro Fujita
Backpatch-through: 12
Discussion: https://postgr.es/m/16139-94f9ccf0db6119ec%40postgresql.org

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -6459,6 +6459,36 @@ DROP TRIGGER trig_row_after ON rem1;
 DROP TRIGGER trig_stmt_before ON rem1;
 DROP TRIGGER trig_stmt_after ON rem1;
 DELETE from rem1;
+-- Test multiple AFTER ROW triggers on a foreign table
+CREATE TRIGGER trig_row_after1
+AFTER INSERT OR UPDATE OR DELETE ON rem1
+FOR EACH ROW EXECUTE PROCEDURE trigger_data(23,'skidoo');
+CREATE TRIGGER trig_row_after2
+AFTER INSERT OR UPDATE OR DELETE ON rem1
+FOR EACH ROW EXECUTE PROCEDURE trigger_data(23,'skidoo');
+insert into rem1 values(1,'insert');
+NOTICE:  trig_row_after1(23, skidoo) AFTER ROW INSERT ON rem1
+NOTICE:  NEW: (1,insert)
+NOTICE:  trig_row_after2(23, skidoo) AFTER ROW INSERT ON rem1
+NOTICE:  NEW: (1,insert)
+update rem1 set f2  = 'update' where f1 = 1;
+NOTICE:  trig_row_after1(23, skidoo) AFTER ROW UPDATE ON rem1
+NOTICE:  OLD: (1,insert),NEW: (1,update)
+NOTICE:  trig_row_after2(23, skidoo) AFTER ROW UPDATE ON rem1
+NOTICE:  OLD: (1,insert),NEW: (1,update)
+update rem1 set f2 = f2 || f2;
+NOTICE:  trig_row_after1(23, skidoo) AFTER ROW UPDATE ON rem1
+NOTICE:  OLD: (1,update),NEW: (1,updateupdate)
+NOTICE:  trig_row_after2(23, skidoo) AFTER ROW UPDATE ON rem1
+NOTICE:  OLD: (1,update),NEW: (1,updateupdate)
+delete from rem1;
+NOTICE:  trig_row_after1(23, skidoo) AFTER ROW DELETE ON rem1
+NOTICE:  OLD: (1,updateupdate)
+NOTICE:  trig_row_after2(23, skidoo) AFTER ROW DELETE ON rem1
+NOTICE:  OLD: (1,updateupdate)
+-- cleanup
+DROP TRIGGER trig_row_after1 ON rem1;
+DROP TRIGGER trig_row_after2 ON rem1;
 -- Test WHEN conditions
 CREATE TRIGGER trig_row_before_insupd
 BEFORE INSERT OR UPDATE ON rem1
@@ -6689,7 +6719,7 @@ NOTICE:  trig_row_after(23, skidoo) AFTER ROW INSERT ON rem1
 NOTICE:  NEW: (13,"test triggered !")
   ctid  
 --------
- (0,29)
+ (0,32)
 (1 row)
 
 -- cleanup

contrib/postgres_fdw/sql/postgres_fdw.sql

@@ -1485,6 +1485,23 @@ DROP TRIGGER trig_stmt_after ON rem1;
 
 DELETE from rem1;
 
+-- Test multiple AFTER ROW triggers on a foreign table
+CREATE TRIGGER trig_row_after1
+AFTER INSERT OR UPDATE OR DELETE ON rem1
+FOR EACH ROW EXECUTE PROCEDURE trigger_data(23,'skidoo');
+
+CREATE TRIGGER trig_row_after2
+AFTER INSERT OR UPDATE OR DELETE ON rem1
+FOR EACH ROW EXECUTE PROCEDURE trigger_data(23,'skidoo');
+
+insert into rem1 values(1,'insert');
+update rem1 set f2  = 'update' where f1 = 1;
+update rem1 set f2 = f2 || f2;
+delete from rem1;
+
+-- cleanup
+DROP TRIGGER trig_row_after1 ON rem1;
+DROP TRIGGER trig_row_after2 ON rem1;
 
 -- Test WHEN conditions
 

doc/src/sgml/trigger.sgml

@@ -511,8 +511,8 @@ typedef struct TriggerData
     HeapTuple        tg_trigtuple;
     HeapTuple        tg_newtuple;
     Trigger         *tg_trigger;
-    Buffer           tg_trigtuplebuf;
-    Buffer           tg_newtuplebuf;
+    TupleTableSlot  *tg_trigslot;
+    TupleTableSlot  *tg_newslot;
     Tuplestorestate *tg_oldtable;
     Tuplestorestate *tg_newtable;
 } TriggerData;
@@ -714,21 +714,21 @@ typedef struct Trigger
      </varlistentry>
 
      <varlistentry>
-      <term><structfield>tg_trigtuplebuf</structfield></term>
+      <term><structfield>tg_trigslot</structfield></term>
       <listitem>
        <para>
-        The buffer containing <structfield>tg_trigtuple</structfield>, or <symbol>InvalidBuffer</symbol> if there
-        is no such tuple or it is not stored in a disk buffer.
+        The slot containing <structfield>tg_trigtuple</structfield>,
+        or a <symbol>NULL</symbol> pointer if there is no such tuple.
        </para>
       </listitem>
      </varlistentry>
 
      <varlistentry>
-      <term><structfield>tg_newtuplebuf</structfield></term>
+      <term><structfield>tg_newslot</structfield></term>
       <listitem>
        <para>
-        The buffer containing <structfield>tg_newtuple</structfield>, or <symbol>InvalidBuffer</symbol> if there
-        is no such tuple or it is not stored in a disk buffer.
+        The slot containing <structfield>tg_newtuple</structfield>,
+        or a <symbol>NULL</symbol> pointer if there is no such tuple.
        </para>
       </listitem>
      </varlistentry>

src/backend/commands/trigger.c

@@ -4255,12 +4255,17 @@ AfterTriggerExecute(EState *estate,
 			LocTriggerData.tg_trigtuple =
 				ExecFetchSlotHeapTuple(trig_tuple_slot1, true, &should_free_trig);
 
-			LocTriggerData.tg_newslot = trig_tuple_slot2;
-			LocTriggerData.tg_newtuple =
-				((evtshared->ats_event & TRIGGER_EVENT_OPMASK) ==
-				 TRIGGER_EVENT_UPDATE) ?
-				ExecFetchSlotHeapTuple(trig_tuple_slot2, true, &should_free_new) : NULL;
-
+			if ((evtshared->ats_event & TRIGGER_EVENT_OPMASK) ==
+				TRIGGER_EVENT_UPDATE)
+			{
+				LocTriggerData.tg_newslot = trig_tuple_slot2;
+				LocTriggerData.tg_newtuple =
+					ExecFetchSlotHeapTuple(trig_tuple_slot2, true, &should_free_new);
+			}
+			else
+			{
+				LocTriggerData.tg_newtuple = NULL;
+			}
 			break;
 
 		default:
@@ -4355,10 +4360,14 @@ AfterTriggerExecute(EState *estate,
 	if (should_free_new)
 		heap_freetuple(LocTriggerData.tg_newtuple);
 
-	if (LocTriggerData.tg_trigslot)
-		ExecClearTuple(LocTriggerData.tg_trigslot);
-	if (LocTriggerData.tg_newslot)
-		ExecClearTuple(LocTriggerData.tg_newslot);
+	/* don't clear slots' contents if foreign table */
+	if (trig_tuple_slot1 == NULL)
+	{
+		if (LocTriggerData.tg_trigslot)
+			ExecClearTuple(LocTriggerData.tg_trigslot);
+		if (LocTriggerData.tg_newslot)
+			ExecClearTuple(LocTriggerData.tg_newslot);
+	}
 
 	/*
 	 * If doing EXPLAIN ANALYZE, stop charging time to this trigger, and count
@@ -4512,13 +4521,14 @@ afterTriggerInvokeEvents(AfterTriggerEventList *events,
 					trigdesc = rInfo->ri_TrigDesc;
 					finfo = rInfo->ri_TrigFunctions;
 					instr = rInfo->ri_TrigInstrument;
+					if (slot1 != NULL)
+					{
+						ExecDropSingleTupleTableSlot(slot1);
+						ExecDropSingleTupleTableSlot(slot2);
+						slot1 = slot2 = NULL;
+					}
 					if (rel->rd_rel->relkind == RELKIND_FOREIGN_TABLE)
 					{
-						if (slot1 != NULL)
-						{
-							ExecDropSingleTupleTableSlot(slot1);
-							ExecDropSingleTupleTableSlot(slot2);
-						}
 						slot1 = MakeSingleTupleTableSlot(rel->rd_att,
 														 &TTSOpsMinimalTuple);
 						slot2 = MakeSingleTupleTableSlot(rel->rd_att,