Directly constructed `uvloop.loop.Server(None)` segfaults in `close()`

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
68/100
Issue type
Bug
Clarity
Mostly clear
Activity status
Active
Tech stack
python
Domain
backend, networking

Research direction

Start with the Server.close and Server._unref paths identified in uvloop/loop.c, then reproduce the failure with the Python snippet using Server(None). Trace the close path under ASan and verify that the reproducer raises a Python exception or otherwise completes without terminating the interpreter.

Written by the indexing model from the issue text.

Description

Summary

uvloop.loop.Server(None) constructs an object, but calling close() on it terminates the interpreter.
I found this while fuzzing Python C extension modules.
I realize this is not a realistic input or usage pattern, but I would expect a Python exception rather than a process crash.

Versions

uvloop 0.22.1, CPython 3.12.3, Debian 12 x86_64, glibc 2.36

Reproducer
from uvloop.loop import Server

server = Server(None)
server.close()

Segmentation fault (core dumped)
ASan/UBSan result

I built uvloop 0.22.1 from source with Clang 18 using ASan and UBSan instrumentation.
ASan reports a read from address 0x158 on the Server.close() path:

AddressSanitizer:DEADLYSIGNAL
ERROR: AddressSanitizer: SEGV on unknown address 0x000000000158
The signal is caused by a READ memory access.
Hint: address points to the zero page.

    #0 PyType_IsSubtype
    #1 CPython set operation
    #2 __Pyx_PySet_Discard  uvloop/loop.c:194862:17
    #3 Server._unref        uvloop/loop.c:160673:15
    #4 Server.close         uvloop/loop.c:161676:19

SUMMARY: AddressSanitizer: SEGV in PyType_IsSubtype

The sanitizer process exits with code 134 after ASan aborts.
UBSan did not emit a separate diagnostic before the ASan failure.

Dominant language
Cython
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