AssertionError from MultiError.__init__()

[belm0]

seen in Trio 0.12.1

Traceback (most recent call last):
  File "/usr/lib/python3.7/contextlib.py", line 635, in __aexit__
    cb_suppress = await cb(*exc_details)
  File "/usr/lib/python3.7/contextlib.py", line 539, in _exit_wrapper
    return await cm_exit(cm, exc_type, exc, tb)
  File "/.../python3.7/site-packages/anyio/_backends/_trio.py", line 123, in __aexit__
    raise TrioExceptionGroup(exc.exceptions) from None
  File "/.../python3.7/site-packages/trio/_core/_multierror.py", line 184, in __init__
    assert len(exceptions) == 1 and exceptions[0] is self
AssertionError

[belm0]

When __init__() is invoked, the exceptions class instance member is an empty tuple.

And here is the value of the exceptions arg, in all its glory (ConnectionClosed is from trio_websocket):

 [Cancelled(), <MultiError: Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, Cancelled(), ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, Cancelled(), Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, <MultiError: Cancelled(), Cancelled(), Cancelled(), <MultiError: Cancelled(), Cancelled(), Cancelled()>>, <MultiError: Cancelled(), Cancelled(), ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>>, <MultiError: Cancelled(), Cancelled(), ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>>>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), <MultiError: Cancelled(), Cancelled(), Cancelled()>>, <MultiError: Cancelled(), Cancelled(), <MultiError: Cancelled(), Cancelled(), Cancelled()>>, <MultiError: Cancelled(), Cancelled(), <MultiError: Cancelled(), Cancelled(), Cancelled()>>, <MultiError: Cancelled(), Cancelled(), <MultiError: Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled()>>, <MultiError: Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), <MultiError: Cancelled(), Cancelled(), Cancelled()>>, <MultiError: Cancelled(), Cancelled(), Cancelled(), <MultiError: Cancelled(), Cancelled(), Cancelled()>, Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), <MultiError: Cancelled(), Cancelled(), Cancelled(), Cancelled()>>, <MultiError: Cancelled(), Cancelled(), <MultiError: Cancelled(), Cancelled(), Cancelled()>>, <MultiError: Cancelled(), Cancelled(), <MultiError: Cancelled(), Cancelled(), Cancelled()>>, <MultiError: Cancelled(), Cancelled(), <MultiError: Cancelled(), Cancelled(), Cancelled()>>, <MultiError: Cancelled(), Cancelled(), <MultiError: Cancelled(), Cancelled(), Cancelled()>>, <MultiError: Cancelled(), Cancelled(), <MultiError: Cancelled(), Cancelled(), Cancelled()>>, <MultiError: Cancelled(), Cancelled(), <MultiError: Cancelled(), <MultiError: Cancelled(), Cancelled(), Cancelled()>, Cancelled()>>, <MultiError: Cancelled(), Cancelled(), <MultiError: Cancelled(), Cancelled(), <MultiError: Cancelled(), Cancelled(), Cancelled()>, Cancelled()>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, <MultiError: Cancelled(), Cancelled(), <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), <MultiError: Cancelled(), Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled()>>, Cancelled()>, <MultiError: Cancelled(), Cancelled(), Cancelled(), ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, <MultiError: Cancelled(), Cancelled(), Cancelled()>, Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), Cancelled(), <MultiError: Cancelled(), Cancelled(), Cancelled()>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, <MultiError: Cancelled(), Cancelled(), <MultiError: Cancelled(), Cancelled(), <MultiError: Cancelled(), Cancelled(), Cancelled()>>>>, <MultiError: Cancelled(), <MultiError: Cancelled(), Cancelled()>, <MultiError: Cancelled(), Cancelled(), <MultiError: Cancelled(), Cancelled()>>>, Cancelled(), ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>>, Cancelled()]

[belm0]

So it's related to anyio exception groups. The base class ExceptionGroup has an exceptions attribute, and TrioExceptionGroup inherits from ExceptionGroup and trio.MultiError.

Hmm, I see in anyio head some hack for Trio 0.12 to turn around and delete the attribute.

https://github.com/agronholm/anyio/blob/37c1e21c4edd2a5c35706144f6813139ec57af5f/anyio/exceptions.py#L10-L11

quite a mess all around...

[belm0]

I tried anyio HEAD. Same problem, although now the exceptions class instance member is populated rather than an empty tuple:

[ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, <MultiError: ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>>, <MultiError: ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, <MultiError: ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>>, <MultiError: ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>, ConnectionClosed<CloseReason<code=1006, name=ABNORMAL_CLOSURE, reason=None>>]

So it's to do with multiple inheritance and MultiError?

[njsmith]

I definitely haven't thought through how to support subclassing on MultiError... @agronholm, I guess you're the one doing the subclassing here; any thoughts? I think the proximal change that's triggering this is probably #1067 / #1066

[agronholm]

That change in trio was not particularly welcomed by yours truly, as it completely broke anyio compatibility and I had to do the hack mentioned by @belm0 to work around it. I've never seen guards like that anywhere else. How exactly did you trigger the problem @belm0 ?

[belm0]

Are we sure that MultiError worked correctly in the face of subclassing / multiple inheritance even prior to that change?

[agronholm]

Well, at least it was fine in anyio as far as I can tell.

[agronholm]

@belm0 tell me again, how are you triggering the problem on the latest master version of anyio? The test suite passes just fine.

[belm0]

It's a non-trivial app, I can't offer a repro at this point.

I'm sure that the behavior is different between anyio 1.0.0 and head, yet the latter is not sufficient.

From the debugging I did above, I think we can conclude that with subclassing and/or multiple inheritance there are other paths where __init__() may be called on an existing object, beyond what is assumed in the Trio code.

[agronholm]

Are you by any chance serializing the MultiErrors, possibly via multiprocessing?

[agronholm]

I guess not. But can you at least find out what exactly is being passed to TrioExceptionGroup's constructor?

[belm0]

Sorry, operator error here. Indeed, there is not an AssertionError from Trio with anyio HEAD.

So outstanding is to decide on policy about MultiError inheritance, and whether to make the internals less fragile.

[Zac-HD]

Closing this because we've dropped MultiError.