test_interpreters Failing Intermittently Under Py_DISABLE_GIL

@colesbury

Bug report

Bug description:

On one of my recent PRs, gh-118157, I was getting consistent failures on CI in test_interpreters. Different tests would fail, but the error was always the same:

(expand)

0:08:24 load avg: 4.18 Re-running 1 failed tests in verbose mode in subprocesses
0:08:24 load avg: 4.18 Run 1 test in parallel using 1 worker process (timeout: 10 min, worker timeout: 15 min)
0:08:26 load avg: 3.93 [1/1/1] test_interpreters failed (6 failures)
Re-running test_interpreters in verbose mode (matching: test_display_preserved_exception, test_gh_109793, test_sys_path_0, test_sys_path_0, test_sys_path_0, test_sys_path_0)
test_display_preserved_exception (test.test_interpreters.test_api.TestInterpreterExec.test_display_preserved_exception) ... FAIL
test_gh_109793 (test.test_interpreters.test_lifecycle.FinalizationTests.test_gh_109793) ... FAIL
test_sys_path_0 (test.test_interpreters.test_lifecycle.StartupTests.test_sys_path_0) ... 
### start subtest debug ###
# python3 script.py failed:
--- stdout ---
{
    "main": "/tmp/test_python_tk7ha1xh/test_interpreters_76l2y2f4",
    "sub": "/tmp/test_python_tk7ha1xh/test_interpreters_76l2y2f4"
}
------
--- stderr ---
Exception ignored in: <function Interpreter.__del__ at 0x2000119aed0>
Traceback (most recent call last):
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/support/interpreters/__init__.py", line 157, in __del__
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/support/interpreters/__init__.py", line 173, in _decref
TypeError: catching classes that do not inherit from BaseException is not allowed
Fatal Python error: PyInterpreterState_Delete: remaining subinterpreters
Python runtime state: finalizing (tstate=0x00005639aed8ea10)

------
### end subtest debug ###
  test_sys_path_0 (test.test_interpreters.test_lifecycle.StartupTests.test_sys_path_0) [python3 script.py] ... FAIL
### start subtest debug ###
# python3 -m script failed:
--- stdout ---
{
    "main": "/tmp/test_python_tk7ha1xh/test_interpreters_76l2y2f4",
    "sub": "/tmp/test_python_tk7ha1xh/test_interpreters_76l2y2f4"
}
------
--- stderr ---
Exception ignored in: <function Interpreter.__del__ at 0x2000117c0d0>
Traceback (most recent call last):
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/support/interpreters/__init__.py", line 157, in __del__
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/support/interpreters/__init__.py", line 173, in _decref
TypeError: catching classes that do not inherit from BaseException is not allowed
Fatal Python error: PyInterpreterState_Delete: remaining subinterpreters
Python runtime state: finalizing (tstate=0x0000564591b58a10)

------
### end subtest debug ###
  test_sys_path_0 (test.test_interpreters.test_lifecycle.StartupTests.test_sys_path_0) [python3 -m script] ... FAIL
### start subtest debug ###
# python3 -m pkg failed:
--- stdout ---
{
    "main": "/tmp/test_python_tk7ha1xh/test_interpreters_76l2y2f4",
    "sub": "/tmp/test_python_tk7ha1xh/test_interpreters_76l2y2f4"
}
------
--- stderr ---
Exception ignored in: <function Interpreter.__del__ at 0x2000118c0d0>
Traceback (most recent call last):
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/support/interpreters/__init__.py", line 157, in __del__
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/support/interpreters/__init__.py", line 173, in _decref
TypeError: catching classes that do not inherit from BaseException is not allowed
Fatal Python error: PyInterpreterState_Delete: remaining subinterpreters
Python runtime state: finalizing (tstate=0x000055aaa11eaa10)

------
### end subtest debug ###
  test_sys_path_0 (test.test_interpreters.test_lifecycle.StartupTests.test_sys_path_0) [python3 -m pkg] ... FAIL
### start subtest debug ###
# python3 -m pkg.script failed:
--- stdout ---
{
    "main": "/tmp/test_python_tk7ha1xh/test_interpreters_76l2y2f4",
    "sub": "/tmp/test_python_tk7ha1xh/test_interpreters_76l2y2f4"
}
------
--- stderr ---
Exception ignored in: <function Interpreter.__del__ at 0x2000118c0d0>
Traceback (most recent call last):
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/support/interpreters/__init__.py", line 157, in __del__
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/support/interpreters/__init__.py", line 173, in _decref
TypeError: catching classes that do not inherit from BaseException is not allowed
Fatal Python error: PyInterpreterState_Delete: remaining subinterpreters
Python runtime state: finalizing (tstate=0x00005637ca7cfa10)

------
### end subtest debug ###
  test_sys_path_0 (test.test_interpreters.test_lifecycle.StartupTests.test_sys_path_0) [python3 -m pkg.script] ... FAIL

======================================================================
FAIL: test_display_preserved_exception (test.test_interpreters.test_api.TestInterpreterExec.test_display_preserved_exception)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/support/__init__.py", line 2603, in wrapper
    return func(*args, **kwargs)
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_interpreters/test_api.py", line 764, in test_display_preserved_exception
    self.assertEqual(stderr, dedent(f"""\
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
        Traceback (most recent call last):
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    ...<18 lines>...
        RuntimeError: uh-oh!
        ^^^^^^^^^^^^^^^^^^^^
        """))
        ^^^^^
AssertionError: 'Trac[778 chars]oh!\nException ignored in: <function Interpret[519 chars]\n\n' != 'Trac[778 chars]oh!\n'
  Traceback (most recent call last):
    File "/tmp/test_python_tk7ha1xh/tmpkqxiakqz/script.py", line 9, in <module>
      interp.exec(script)
      ~~~~~~~~~~~^^^^^^^^
    File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/support/interpreters/__init__.py", line 227, in exec
      raise ExecutionFailed(excinfo)
  test.support.interpreters.ExecutionFailed: RuntimeError: uh-oh!
  
  Uncaught in the interpreter:
  
  Traceback (most recent call last):
    File "/tmp/test_python_tk7ha1xh/tmpkqxiakqz/script.py", line 6, in script
      spam.eggs()
      ~~~~~~~~~^^
    File "/tmp/test_python_tk7ha1xh/tmpkqxiakqz/spam.py", line 6, in eggs
      ham()
      ~~~^^
    File "/tmp/test_python_tk7ha1xh/tmpkqxiakqz/spam.py", line 3, in ham
      raise RuntimeError('uh-oh!')
  RuntimeError: uh-oh!
- Exception ignored in: <function Interpreter.__del__ at 0x2000119aed0>
- Traceback (most recent call last):
-   File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/support/interpreters/__init__.py", line 157, in __del__
-   File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/support/interpreters/__init__.py", line 173, in _decref
- TypeError: catching classes that do not inherit from BaseException is not allowed
- Fatal Python error: PyInterpreterState_Delete: remaining subinterpreters
- Python runtime state: finalizing (tstate=0x000055b06f0a1a10)
- 


======================================================================
FAIL: test_gh_10[979](https://github.com/python/cpython/actions/runs/8978599353/job/24660038142#step:19:980)3 (test.test_interpreters.test_lifecycle.FinalizationTests.test_gh_109793)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_interpreters/test_lifecycle.py", line 185, in test_gh_109793
    self.assertEqual(proc.returncode, 1)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^
AssertionError: -6 != 1

======================================================================
FAIL: test_sys_path_0 (test.test_interpreters.test_lifecycle.StartupTests.test_sys_path_0) [python3 script.py]
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_interpreters/test_lifecycle.py", line 29, in subTest
    yield ctx
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_interpreters/test_lifecycle.py", line 157, in test_sys_path_0
    out = self.run_python(argv, cwd=cwd)
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_interpreters/test_lifecycle.py", line 114, in run_python
    self.assertEqual(proc.returncode, 0)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^
AssertionError: -6 != 0

======================================================================
FAIL: test_sys_path_0 (test.test_interpreters.test_lifecycle.StartupTests.test_sys_path_0) [python3 -m script]
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_interpreters/test_lifecycle.py", line 29, in subTest
    yield ctx
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_interpreters/test_lifecycle.py", line 157, in test_sys_path_0
    out = self.run_python(argv, cwd=cwd)
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_interpreters/test_lifecycle.py", line 114, in run_python
    self.assertEqual(proc.returncode, 0)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^
AssertionError: -6 != 0

======================================================================
FAIL: test_sys_path_0 (test.test_interpreters.test_lifecycle.StartupTests.test_sys_path_0) [python3 -m pkg]
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_interpreters/test_lifecycle.py", line 29, in subTest
    yield ctx
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_interpreters/test_lifecycle.py", line 157, in test_sys_path_0
    out = self.run_python(argv, cwd=cwd)
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_interpreters/test_lifecycle.py", line 114, in run_python
    self.assertEqual(proc.returncode, 0)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^
AssertionError: -6 != 0

======================================================================
FAIL: test_sys_path_0 (test.test_interpreters.test_lifecycle.StartupTests.test_sys_path_0) [python3 -m pkg.script]
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_interpreters/test_lifecycle.py", line 29, in subTest
    yield ctx
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_interpreters/test_lifecycle.py", line 157, in test_sys_path_0
    out = self.run_python(argv, cwd=cwd)
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/test_interpreters/test_lifecycle.py", line 114, in run_python
    self.assertEqual(proc.returncode, 0)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^
AssertionError: -6 != 0

----------------------------------------------------------------------
Ran 3 tests in 1.720s

FAILED (failures=6)
test test_interpreters failed
1 test failed again:
    test_interpreters

The error looks like this:

Exception ignored in: <function Interpreter.__del__ at 0x2000117c0d0>
Traceback (most recent call last):
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/support/interpreters/__init__.py", line 157, in __del__
  File "/home/runner/work/cpython/cpython-ro-srcdir/Lib/test/support/interpreters/__init__.py", line 173, in _decref
TypeError: catching classes that do not inherit from BaseException is not allowed
Fatal Python error: PyInterpreterState_Delete: remaining subinterpreters
Python runtime state: finalizing (tstate=0x0000564591b58a10)

I'm fairly sure that means one of the 3 calls in _interpreters._decref() is raising a bogus exception.

It isn't clear to me how gh-118157 would have caused this to happen. As a very temporary solution, I've skipped test_interpreters on free-threading builds.

CC @colesbury

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

I think what's happening is:

The free-threaded build is immortalizing things like modules and their module.__dict__. This is not behavior that we want long term, but it's an important performance work around for 3.13. (See Temporarily immortalize objects that use deferred reference counting #117783)
This leads to slightly different behavior during interpreter shutdown. A lot of finalizers normally get called during finalize_remove_modules, but here that's not happening because the module's dictionary was immortalized.
Instead, the finalizer is called slightly later in finalize_modules_clear_weaklist when we start clearing the module's __dict__ in _PyModule_Clear.

By the time Interpreter.__del__ is called, a lot of global variables including _interpreters have already been deleted from the module's dictionary so you get a bunch of weird errors, and the interpreter is not deleted.

That makes sense. You have any thoughts already on how to resolve this?

I'm not sure. We could call the finalizers on all GC-enabled objects during shutdown in the free-threaded build (before we start clearing module's dictionaries). I think that makes logical sense: everything is going away, why not call their finalizers? That seems to fix test_interpreters, but a few other test cases would need tweaking.

On current main, test_interpreters passes under free-threading as deferred reference counting is implemented now, the tests can be enabled back now.

== CPython 3.15.0a0 free-threading build (heads/main-dirty:75e2c5dd346, Jul 14 2025, 17:20:43) [Clang 17.0.0 (clang-1700.0.13.5)]
== macOS-15.5-arm64-arm-64bit-Mach-O little-endian
== Python build: free_threading debug
== cwd: /Users/kumaraditya/work/cpython/build/test_python_worker_53278æ
== CPU count: 10
== encodings: locale=UTF-8 FS=utf-8
== resources: all test resources are disabled, use -u option to unskip tests

Using random seed: 2517149026
0:00:00 load avg: 8.09 Run 1 test sequentially in a single process
0:00:00 load avg: 8.09 [1/1] test_interpreters
0:00:07 load avg: 7.60 [1/1] test_interpreters passed

== Tests result: SUCCESS ==

1 test OK.

Total duration: 7.1 sec
Total tests: run=165 skipped=8
Total test files: run=1/1
Result: SUCCESS

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test_interpreters Failing Intermittently Under Py_DISABLE_GIL #118731

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test_interpreters Failing Intermittently Under Py_DISABLE_GIL #118731

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Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

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colesbury commented on May 7, 2024

ericsnowcurrently commented on May 7, 2024

colesbury commented on May 7, 2024

kumaraditya303 commented on Jul 14, 2025

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