Don't throw an error if a queued AFTER trigger no longer exists.

afterTriggerInvokeEvents and AfterTriggerExecute have always
treated it as an error if the trigger OID mentioned in a queued
after-trigger event can't be found.  However, that fails to
account for the edge case where the trigger's been dropped in
the current transaction since queueing the event.  There seems
no very good reason to disallow that case, so instead silently
do nothing if the trigger OID can't be found.

This does give up a little bit of bug-detection ability, but I don't
recall that these error messages have ever actually revealed a bug,
so it seems mostly theoretical.  Alternatives such as marking
pending events DONE at the time of dropping a trigger would be
complicated and perhaps introduce bugs of their own.

Per bug #18517 from Alexander Lakhin.  Back-patch to all
supported branches.

Discussion: https://postgr.es/m/18517-af2d19882240902c@postgresql.org

src/backend/commands/trigger.c

@@ -3997,8 +3997,12 @@ AfterTriggerExecute(EState *estate,
 	bool		should_free_new = false;
 
 	/*
-	 * Locate trigger in trigdesc.
+	 * Locate trigger in trigdesc.  It might not be present, and in fact the
+	 * trigdesc could be NULL, if the trigger was dropped since the event was
+	 * queued.  In that case, silently do nothing.
 	 */
+	if (trigdesc == NULL)
+		return;
 	for (tgindx = 0; tgindx < trigdesc->numtriggers; tgindx++)
 	{
 		if (trigdesc->triggers[tgindx].tgoid == tgoid)
@@ -4008,7 +4012,7 @@ AfterTriggerExecute(EState *estate,
 		}
 	}
 	if (LocTriggerData.tg_trigger == NULL)
-		elog(ERROR, "could not find trigger %u", tgoid);
+		return;
 
 	/*
 	 * If doing EXPLAIN ANALYZE, start charging time to this trigger. We want
@@ -4332,6 +4336,7 @@ afterTriggerInvokeEvents(AfterTriggerEventList *events,
 					/* Catch calls with insufficient relcache refcounting */
 					Assert(!RelationHasReferenceCountZero(rel));
 					trigdesc = rInfo->ri_TrigDesc;
+					/* caution: trigdesc could be NULL here */
 					finfo = rInfo->ri_TrigFunctions;
 					instr = rInfo->ri_TrigInstrument;
 					if (slot1 != NULL)
@@ -4347,9 +4352,6 @@ afterTriggerInvokeEvents(AfterTriggerEventList *events,
 						slot2 = MakeSingleTupleTableSlot(rel->rd_att,
 														 &TTSOpsMinimalTuple);
 					}
-					if (trigdesc == NULL)	/* should not happen */
-						elog(ERROR, "relation %u has no triggers",
-							 evtshared->ats_relid);
 				}
 
 				/*

src/test/regress/expected/triggers.out

@@ -3286,6 +3286,17 @@ select * from trig_table;
 
 drop table refd_table, trig_table;
 --
+-- Test that we can drop a not-yet-fired deferred trigger
+--
+create table refd_table (id int primary key);
+create table trig_table (fk int references refd_table initially deferred);
+begin;
+insert into trig_table values (1);
+drop table refd_table cascade;
+NOTICE:  drop cascades to constraint trig_table_fk_fkey on table trig_table
+commit;
+drop table trig_table;
+--
 -- self-referential FKs are even more fun
 --
 create table self_ref (a int primary key,

src/test/regress/sql/triggers.sql

@@ -2406,6 +2406,20 @@ select * from trig_table;
 
 drop table refd_table, trig_table;
 
+--
+-- Test that we can drop a not-yet-fired deferred trigger
+--
+
+create table refd_table (id int primary key);
+create table trig_table (fk int references refd_table initially deferred);
+
+begin;
+insert into trig_table values (1);
+drop table refd_table cascade;
+commit;
+
+drop table trig_table;
+
 --
 -- self-referential FKs are even more fun
 --