Install an error check into cancel_before_shmem_exit().

tglsfdc · tglsfdc · commit c9ae5cbb8834 · 2020-09-08T15:54:25.000-04:00
Historically, cancel_before_shmem_exit() just silently did nothing if the specified callback wasn't the top-of-stack. The folly of ignoring this case was exposed by the bugs fixed in 3036401 and bab1500, so let's make it throw elog(ERROR) instead. There is a decent argument to be made that PG_ENSURE_ERROR_CLEANUP should use some separate infrastructure, so it wouldn't break if something inside the guarded code decides to register a new before_shmem_exit callback. However, a survey of the surviving uses of before_shmem_exit() and PG_ENSURE_ERROR_CLEANUP doesn't show any plausible conflicts of that sort today, so for now we'll forgo the extra complexity. (It will almost certainly become necessary if anyone ever wants to wrap PG_ENSURE_ERROR_CLEANUP around arbitrary user-defined actions, though.) No backpatch, since this is developer support not a production issue. Bharath Rupireddy, per advice from Andres Freund, Robert Haas, and myself Discussion: https://postgr.es/m/CALj2ACWk7j4F2v2fxxYfrroOF=AdFNPr1WsV+AGtHAFQOqm_pw@mail.gmail.com
diff --git a/src/backend/storage/ipc/ipc.c b/src/backend/storage/ipc/ipc.c
@@ -381,9 +381,9 @@ on_shmem_exit(pg_on_exit_callback function, Datum arg)
  *		cancel_before_shmem_exit
  *
  *		this function removes a previously-registered before_shmem_exit
- *		callback.  For simplicity, only the latest entry can be
- *		removed.  (We could work harder but there is no need for
- *		current uses.)
+ *		callback.  We only look at the latest entry for removal, as we
+ * 		expect callers to add and remove temporary before_shmem_exit
+ * 		callbacks in strict LIFO order.
  * ----------------------------------------------------------------
  */
 void
@@ -394,6 +394,9 @@ cancel_before_shmem_exit(pg_on_exit_callback function, Datum arg)
 		== function &&
 		before_shmem_exit_list[before_shmem_exit_index - 1].arg == arg)
 		--before_shmem_exit_index;
+	else
+		elog(ERROR, "before_shmem_exit callback (%p,0x%llx) is not the latest entry",
+			 function, (long long) arg);
 }
 
 /* ----------------------------------------------------------------
