[Alexander Pyhalov <a.pyhalov@postgrespro.ru>]

Hi.

Recent commit 555276f8594087ba15e0d58e38cd2186b9f39f6d introduced final 
cleanup of node->as_eventset in ExecAppendAsyncEventWait().
Unfortunately, now this function can return in the middle of TRY/FINALLY 
block, without restoring PG_exception_stack.

We found this while working on our FDW. Unfortunately, I couldn't 
reproduce the issue with postgres_fdw, but it seems it is also affected.

The following patch heals the issue.

-- l
Best regards,
Alexander Pyhalov,
Postgres Professional

Attachments:

  [text/x-diff] 0001-Avoid-corrupting-PG_exception_stack-in-ExecAppendAsy.patch (1.9K, ../425fa29a429b21b0332737c42a4fdc70@postgrespro.ru/2-0001-Avoid-corrupting-PG_exception_stack-in-ExecAppendAsy.patch)
  download | inline diff:
From 025f40894d6d8f499144f0f7c45c0a124a46c408 Mon Sep 17 00:00:00 2001
From: Alexander Pyhalov <a.pyhalov@postgrespro.ru>
Date: Fri, 23 Feb 2024 13:06:40 +0300
Subject: [PATCH] Avoid corrupting PG_exception_stack in
 ExecAppendAsyncEventWait()

After commit 555276f8594087ba15e0d58e38cd2186b9f39f6d
ExecAppendAsyncEventWait() could corrupt PG_exception_stack
after returning in the the middle of PG_TRY()/PG_END_TRY() block.
It should exit only after PG_END_TRY().
---
 src/backend/executor/nodeAppend.c | 27 ++++++++++++++-------------
 1 file changed, 14 insertions(+), 13 deletions(-)

diff --git a/src/backend/executor/nodeAppend.c b/src/backend/executor/nodeAppend.c
index baba3ceea23..42962e80177 100644
--- a/src/backend/executor/nodeAppend.c
+++ b/src/backend/executor/nodeAppend.c
@@ -1052,21 +1052,22 @@ ExecAppendAsyncEventWait(AppendState *node)
 		 */
 		if (GetNumRegisteredWaitEvents(node->as_eventset) == 1)
 		{
-			FreeWaitEventSet(node->as_eventset);
-			node->as_eventset = NULL;
-			return;
+			/* Return after PG_TRY()/PG_END_TRY() block */
+			noccurred = 0;
 		}
+		else
+		{
+			/* Return at most EVENT_BUFFER_SIZE events in one call. */
+			if (nevents > EVENT_BUFFER_SIZE)
+				nevents = EVENT_BUFFER_SIZE;
 
-		/* Return at most EVENT_BUFFER_SIZE events in one call. */
-		if (nevents > EVENT_BUFFER_SIZE)
-			nevents = EVENT_BUFFER_SIZE;
-
-		/*
-		 * If the timeout is -1, wait until at least one event occurs.  If the
-		 * timeout is 0, poll for events, but do not wait at all.
-		 */
-		noccurred = WaitEventSetWait(node->as_eventset, timeout, occurred_event,
-									 nevents, WAIT_EVENT_APPEND_READY);
+			/*
+			 * If the timeout is -1, wait until at least one event occurs.  If
+			 * the timeout is 0, poll for events, but do not wait at all.
+			 */
+			noccurred = WaitEventSetWait(node->as_eventset, timeout, occurred_event,
+										 nevents, WAIT_EVENT_APPEND_READY);
+		}
 	}
 	PG_FINALLY();
 	{
-- 
2.34.1