From aa3256ada3eb8b292e9d6ed03e34ac884176b04d Mon Sep 17 00:00:00 2001
From: Kadir Can Ozden <101993364+bysiber@users.noreply.github.com>
Date: Fri, 20 Feb 2026 09:25:39 +0300
Subject: [PATCH] Fix _Once.ensure() to propagate handshake failure to
 concurrent waiters
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When two tasks use an SSLStream concurrently (one sending, one
receiving), both call _Once.ensure() to trigger the lazy handshake.
If the first task starts the handshake and it fails (e.g. certificate
error, connection reset), the exception propagates to that task but
_done is never set.  The second task, already waiting on _done.wait(),
blocks indefinitely — the Event is never signalled and started is
permanently True, so there is no recovery path.

Store the failure exception and set _done even on error, so that
all waiters wake up and receive a BrokenResourceError chained from
the original handshake exception.
---
 src/trio/_ssl.py | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/src/trio/_ssl.py b/src/trio/_ssl.py
index 52c5137ea1..a5ade68b46 100644
--- a/src/trio/_ssl.py
+++ b/src/trio/_ssl.py
@@ -223,7 +223,7 @@ class NeedHandshakeError(Exception):
 
 
 class _Once:
-    __slots__ = ("_afn", "_args", "_done", "started")
+    __slots__ = ("_afn", "_args", "_done", "_failure", "started")
 
     def __init__(
         self,
@@ -234,16 +234,26 @@ def __init__(
         self._args = args
         self.started = False
         self._done = _sync.Event()
+        self._failure: BaseException | None = None
 
     async def ensure(self, *, checkpoint: bool) -> None:
         if not self.started:
             self.started = True
-            await self._afn(*self._args)
+            try:
+                await self._afn(*self._args)
+            except BaseException as exc:
+                self._failure = exc
+                self._done.set()
+                raise
             self._done.set()
         elif not checkpoint and self._done.is_set():
+            if self._failure is not None:
+                raise trio.BrokenResourceError from self._failure
             return
         else:
             await self._done.wait()
+            if self._failure is not None:
+                raise trio.BrokenResourceError from self._failure
 
     @property
     def done(self) -> bool: