Fix trigger drop procedure

After commit 123cc697a8, we remove redundant FK action triggers during
partition ATTACH by merely deleting the catalog tuple, but that's wrong:
it should use performDeletion() instead.  Repair, and make the comments
more explicit.

Per code review from Tom Lane.

Discussion: https://postgr.es/m/18885.1549642539@sss.pgh.pa.us

src/backend/commands/tablecmds.c

@@ -7919,10 +7919,12 @@ CloneFkReferencing(Relation pg_constraint, Relation parentRel,
 			ReleaseSysCache(partcontup);
 
 			/*
-			 * Looks good!  Attach this constraint.  Note that the action
-			 * triggers are no longer needed, so remove them.  We identify
-			 * them because they have our constraint OID, as well as being
-			 * on the referenced rel.
+			 * Looks good!  Attach this constraint.  The action triggers in
+			 * the new partition become redundant -- the parent table already
+			 * has equivalent ones, and those will be able to reach the
+			 * partition.  Remove the ones in the partition.  We identify them
+			 * because they have our constraint OID, as well as being on the
+			 * referenced rel.
 			 */
 			trigrel = heap_open(TriggerRelationId, RowExclusiveLock);
 			ScanKeyInit(&key,
@@ -7935,17 +7937,30 @@ CloneFkReferencing(Relation pg_constraint, Relation parentRel,
 			while ((trigtup = systable_getnext(scan)) != NULL)
 			{
 				Form_pg_trigger	trgform = (Form_pg_trigger) GETSTRUCT(trigtup);
+				ObjectAddress	trigger;
 
 				if (trgform->tgconstrrelid != fk->conrelid)
 					continue;
 				if (trgform->tgrelid != fk->confrelid)
 					continue;
 
-				deleteDependencyRecordsForClass(TriggerRelationId,
+				/*
+				 * The constraint is originally set up to contain this trigger
+				 * as an implementation object, so there's a dependency record
+				 * that links the two; however, since the trigger is no longer
+				 * needed, we remove the dependency link in order to be able
+				 * to drop the trigger while keeping the constraint intact.
+				 */
+				deleteDependencyRecordsFor(TriggerRelationId,
 										   trgform->oid,
-												ConstraintRelationId,
-												DEPENDENCY_INTERNAL);
-				CatalogTupleDelete(trigrel, &trigtup->t_self);
+										   false);
+				/* make dependency deletion visible to performDeletion */
+				CommandCounterIncrement();
+				ObjectAddressSet(trigger, TriggerRelationId,
+								 trgform->oid);
+				performDeletion(&trigger, DROP_RESTRICT, 0);
+				/* make trigger drop visible, in case the loop iterates */
+				CommandCounterIncrement();
 			}
 
 			systable_endscan(scan);