Fix unsafe memory management in CloneRowTriggersToPartition().

It's not really supported to call systable_getnext() in a different
memory context than systable_beginscan() was called in, and it's
*definitely* not safe to do so and then reset that context between
calls.  I'm not very clear on how this code survived
CLOBBER_CACHE_ALWAYS testing ... but Alexander Lakhin found a case
that would crash it pretty reliably.

Per bug #15828.  Fix, and backpatch to v11 where this code came in.

Discussion: https://postgr.es/m/15828-f6ddd7df4852f473@postgresql.org

src/backend/commands/tablecmds.c

@@ -15772,8 +15772,7 @@ CloneRowTriggersToPartition(Relation parent, Relation partition)
 	ScanKeyData key;
 	SysScanDesc scan;
 	HeapTuple	tuple;
-	MemoryContext oldcxt,
-				perTupCxt;
+	MemoryContext perTupCxt;
 
 	ScanKeyInit(&key, Anum_pg_trigger_tgrelid, BTEqualStrategyNumber,
 				F_OIDEQ, ObjectIdGetDatum(RelationGetRelid(parent)));
@@ -15783,18 +15782,16 @@ CloneRowTriggersToPartition(Relation parent, Relation partition)
 
 	perTupCxt = AllocSetContextCreate(CurrentMemoryContext,
 									  "clone trig", ALLOCSET_SMALL_SIZES);
-	oldcxt = MemoryContextSwitchTo(perTupCxt);
 
 	while (HeapTupleIsValid(tuple = systable_getnext(scan)))
 	{
-		Form_pg_trigger trigForm;
+		Form_pg_trigger trigForm = (Form_pg_trigger) GETSTRUCT(tuple);
 		CreateTrigStmt *trigStmt;
 		Node	   *qual = NULL;
 		Datum		value;
 		bool		isnull;
 		List	   *cols = NIL;
-
-		trigForm = (Form_pg_trigger) GETSTRUCT(tuple);
+		MemoryContext oldcxt;
 
 		/*
 		 * Ignore statement-level triggers; those are not cloned.
@@ -15813,6 +15810,9 @@ CloneRowTriggersToPartition(Relation parent, Relation partition)
 			elog(ERROR, "unexpected trigger \"%s\" found",
 				 NameStr(trigForm->tgname));
 
+		/* Use short-lived context for CREATE TRIGGER */
+		oldcxt = MemoryContextSwitchTo(perTupCxt);
+
 		/*
 		 * If there is a WHEN clause, generate a 'cooked' version of it that's
 		 * appropriate for the partition.
@@ -15876,10 +15876,10 @@ CloneRowTriggersToPartition(Relation parent, Relation partition)
 					  trigForm->tgfoid, trigForm->oid, qual,
 					  false, true);
 
+		MemoryContextSwitchTo(oldcxt);
 		MemoryContextReset(perTupCxt);
 	}
 
-	MemoryContextSwitchTo(oldcxt);
 	MemoryContextDelete(perTupCxt);
 
 	systable_endscan(scan);