Avoid stack overflow in ShowTransactionStateRec()

akorotkov · akorotkov · commit 6f38c43eb135 · 2024-03-08T13:18:30.000+02:00
The function recurses, but didn't perform stack-depth checks. It's just a debugging aid, so instead of the usual check_stack_depth() call, stop the printing if we'd risk stack overflow. Here's an example of how to test this: (n=1000000; printf "BEGIN;"; for ((i=1;i<=$n;i++)); do printf "SAVEPOINT s$i;"; done; printf "SET log_min_messages = 'DEBUG5'; SAVEPOINT sp;") | psql >/dev/null In the passing, swap building the list of child XIDs and recursing to parent. That saves memory while recursing, reducing the risk of out of memory errors with lots of subtransactions. The saving is not very significant in practice, but this order seems more logical anyway. Report by Egor Chindyaskin and Alexander Lakhin. Discussion: https://www.postgresql.org/message-id/1672760457.940462079%40f306.i.mail.ru Author: Heikki Linnakangas Reviewed-by: Robert Haas, Andres Freund, Alexander Korotkov
diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
@@ -5583,8 +5583,22 @@ ShowTransactionStateRec(const char *str, TransactionState s)
 {
 	StringInfoData buf;
 
-	initStringInfo(&buf);
+	if (s->parent)
+	{
+		/*
+		 * Since this function recurses, it could be driven to stack overflow.
+		 * This is just a debugging aid, so we can leave out some details
+		 * instead of erroring out with check_stack_depth().
+		 */
+		if (stack_is_too_deep())
+			ereport(DEBUG5,
+					(errmsg_internal("%s(%d): parent omitted to avoid stack overflow",
+									 str, s->nestingLevel)));
+		else
+			ShowTransactionStateRec(str, s->parent);
+	}
 
+	initStringInfo(&buf);
 	if (s->nChildXids > 0)
 	{
 		int			i;
@@ -5593,10 +5607,6 @@ ShowTransactionStateRec(const char *str, TransactionState s)
 		for (i = 1; i < s->nChildXids; i++)
 			appendStringInfo(&buf, " %u", s->childXids[i]);
 	}
-
-	if (s->parent)
-		ShowTransactionStateRec(str, s->parent);
-
 	ereport(DEBUG5,
 			(errmsg_internal("%s(%d) name: %s; blockState: %s; state: %s, xid/subid/cid: %u/%u/%u%s%s",
 							 str, s->nestingLevel,
@@ -5608,7 +5618,6 @@ ShowTransactionStateRec(const char *str, TransactionState s)
 							 (unsigned int) currentCommandId,
 							 currentCommandIdUsed ? " (used)" : "",
 							 buf.data)));
-
 	pfree(buf.data);
 }
 
