Fix use-after-free bug with event triggers and ALTER TABLE.

tglsfdc · tglsfdc · commit cc623ed2f4ac · 2020-10-27T15:37:13.000-04:00
EventTriggerAlterTableEnd neglected to make sure that it built its output list in the right context. In simple cases this was masked because the function is called in PortalContext which will be sufficiently long-lived anyway; but that doesn't make it not a bug. Commit ced138e fixed this in HEAD and v13, but mistakenly chose not to back-patch further. Back-patch the same code change all the way (I didn't bother with the test case though, as it would prove nothing in pre-v13 branches). Per report from Arseny Sher. Original fix by Jehan-Guillaume de Rorthais. Discussion: https://postgr.es/m/877drcyprb.fsf@ars-thinkpad Discussion: https://postgr.es/m/20200902193715.6e0269d4@firost
diff --git a/src/backend/commands/event_trigger.c b/src/backend/commands/event_trigger.c
@@ -1802,9 +1802,15 @@ EventTriggerAlterTableEnd(void)
 	/* If no subcommands, don't collect */
 	if (list_length(currentEventTriggerState->currentCommand->d.alterTable.subcmds) != 0)
 	{
+		MemoryContext oldcxt;
+
+		oldcxt = MemoryContextSwitchTo(currentEventTriggerState->cxt);
+
 		currentEventTriggerState->commandList =
 			lappend(currentEventTriggerState->commandList,
 					currentEventTriggerState->currentCommand);
+
+		MemoryContextSwitchTo(oldcxt);
 	}
 	else
 		pfree(currentEventTriggerState->currentCommand);
