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Fix _Once.ensure() to propagate handshake failure to concurrent waiters #3397

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Fix _Once.ensure() to propagate handshake failure to concurrent waiters #3397

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14 changes: 12 additions & 2 deletions src/trio/_ssl.py
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Original file line number Diff line number Diff line change
Expand Up @@ -223,7 +223,7 @@ class NeedHandshakeError(Exception):


class _Once:
__slots__ = ("_afn", "_args", "_done", "started")
__slots__ = ("_afn", "_args", "_done", "_failure", "started")

def __init__(
self,
Expand All @@ -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
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@A5rocks A5rocks Feb 20, 2026

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This is a bad idea, because a) the stack frames for exc will be mutated, so you will have weird stack traces for raise ... from self._failure and b) this will lead to a refcycle I believe.

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:
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