Apparent segfault in immutables.Map.__init__

[njsmith]

We're getting an intermittent segfault in the contextvars library, on our MacOS CI: https://ci.cryptography.io/blue/rest/organizations/jenkins/pipelines/python-trio/pipelines/trio/branches/PR-575/runs/2/nodes/6/steps/33/log/?start=0

The crash-handler traceback shows it as happening on line 27 of contextvars/__init__.py, which seems to be:

        self._data = immutables.Map()

So our current hypothesis is that immutables.Map() sometimes segfaults? Does that ring any bells?

We don't know how to reproduce it locally, but I believe that's the python.org build of 3.6.1.

Previous discussion: python-trio/trio#200 (comment)

[njsmith]

More details: this seems to happen consistently with CPython 3.6.1 or 3.6.2 on MacOS, but not earlier or later 3.6.x versions, and not on Linux at all: https://travis-ci.org/python-trio/trio/builds/409675722

Of course, it looks like some weird memory corruption thing, so it's possible the bug is latent in other configurations and those are just the builds where it happens to trigger a segfault. But useful for reproducing, maybe.

I also managed to get a traceback out:

(lldb) bt all
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0xd)
  * frame #0: 0x0000000100148dcc Python`visit_decref + 12
    frame #1: 0x0000000102c81435 _map.cpython-36m-darwin.so`map_tp_traverse + 21
    frame #2: 0x0000000100149db7 Python`collect + 439
    frame #3: 0x000000010014aa65 Python`_PyObject_GC_Alloc + 357
    frame #4: 0x000000010014ab13 Python`_PyObject_GC_NewVar + 51
    frame #5: 0x0000000102c8182c _map.cpython-36m-darwin.so`map_tp_new + 236
    frame #6: 0x0000000100080096 Python`type_call + 38
    frame #7: 0x000000010000e8d4 Python`_PyObject_FastCallDict + 180
    frame #8: 0x00000001000f3225 Python`call_function + 165

[1st1]

I think the bug is already fixed in immutables, i just need to bump the dep. Can you try to install the latest immutables and see if the bug is still there?

[njsmith]

Forcing a pip install immutables==0.6 does seem to have fix the issue: https://travis-ci.org/python-trio/trio/builds/409727578

[1st1]

I've released contextvars 2.3 that use the fixed immutables package. Closing this now.

[njsmith]

Thanks!