TSAN reports multiple data races, SEGV errors, and thread leaks when running the full test suite

Bug report

Bug description:

Description:

When running the full test suite (./python -m test) on a debug build of the main branch configured with ThreadSanitizer (TSAN) and the experimental JIT, the test run terminates with a FAILURE result.

The attached logs show thousands of warnings from TSAN, including:

1. Massive numbers of data race warnings, primarily during the parallel initialization of sub-interpreters.
2. Several DEADLYSIGNAL (SEGV) reports, causing processes to abort.
3. Multiple thread leak warnings related to multiprocessing tests.
4. Failures in several core tests, including test_datetime, test_faulthandler, test_pyrepl, and test.test_multiprocessing_forkserver.test_manager.

Build & Environment

• CPython Version: 3.15.0a1+ (heads/main:7ac94fcb1d0, Oct 16 2025)
• Platform: Linux-6.12.48+deb13-cloud-amd64-x86_64-with-glibc2.41
• Compiler: Clang 19.1.7
• Configuration command:

  CC=clang CXX=clang++ ./configure --with-thread-sanitizer --with-pydebug --enable-experimental-jit=yes
  

• Build command:

  make -j$(nproc)
  

• Test command:

  ./python -m test -j$(nproc)
  

Summary of Failures

The test suite concludes with the following result:

== Tests result: FAILURE ==

4 tests failed:
    test.test_multiprocessing_forkserver.test_manager test_datetime
    test_faulthandler test_pyrepl

Key Issues Reported by ThreadSanitizer

The full logs will be attached to this report, but here are the primary categories of errors observed.

1. Data Races during Sub-interpreter Initialization

Thousands of data race warnings are reported when multiple threads attempt to initialize global state concurrently. This is especially prominent in test_datetime, which fails with an exit code from TSAN.

• Example Data Race Warning:

  ==================
  WARNING: ThreadSanitizer: data race (pid=1111680)
    Write of size 8 at 0x56174f42e490 by thread T2:
      #0 _PyExc_InitTypes /root/cpython/main/Objects/exceptions.c:4428:32
      #1 pycore_init_types /root/cpython/main/Python/pylifecycle.c:732:9
      ...
  
    Previous write of size 8 at 0x56174f42e490 by thread T1:
      #0 _PyExc_InitTypes /root/cpython/main/Objects/exceptions.c:4428:32
      #1 pycore_init_types /root/cpython/main/Python/pylifecycle.c:732:9
      ...
  
    Location is global '_PyExc_ReferenceError' of size 416 at 0x56174f42e300 (python+0xa57490)
  ...
  SUMMARY: ThreadSanitizer: data race /root/cpython/main/Objects/exceptions.c:4428:32 in _PyExc_InitTypes
  ==================
  

• Resulting test_datetime Failure:

  test test_datetime failed -- Traceback (most recent call last):
  ...
  AssertionError: Process return code is 66
  command line: ['/root/cpython/main/python', '-X', 'faulthandler', '-I', '-c', "if True:\n from concurrent.futures import InterpreterPoolExecutor\n\n def func():\n import _datetime\n ..."]
  

2. SEGV Errors and Process Aborts

Multiple tests trigger fatal segmentation faults, which are caught and reported by TSAN.

• Example SEGV Report:

  ThreadSanitizer:DEADLYSIGNAL
  ==1101487==ERROR: ThreadSanitizer: SEGV on unknown address 0x00000010ceaf (pc 0x7f05f669195c bp 0x000000000000 sp 0x7ffde6ad06a0 T1101487)
  ==1101487==The signal is caused by a READ memory access.
      #0 <null> <null> (libc.so.6+0x9495c)
      #1 raise <null> (libc.so.6+0x3fcc1)
      #2 raise <null> (python+0x103503)
      #3 faulthandler_raise_sigsegv /root/cpython/main/./Modules/faulthandler.c:1158:5
      ...
  SUMMARY: ThreadSanitizer: SEGV (/lib/x86_64-linux-gnu/libc.so.6+0x9495c) 
  ==1101487==ABORTING
  

3. Thread Leaks

TSAN reports that threads created during multiprocessing tests are not properly joined, leading to resource leaks.

• Example Thread Leak Warning:

  ==================
  WARNING: ThreadSanitizer: thread leak (pid=1106511)
    Thread T2 'Thread-2 (handl' (tid=1106562, finished) created by thread T1 at:
      #0 pthread_create <null> (python+0xfaa35)
      #1 do_start_joinable_thread /root/cpython/main/Python/thread_pthread.h:281:14
      ...
  SUMMARY: ThreadSanitizer: thread leak (/root/cpython/main/python+0xfaa35) in pthread_create
  ==================
  

CPython versions tested on:

3.15, CPython main branch

Operating systems tested on:

Linux

Start by reproducing the report with the listed configure, build, and ./python -m test commands. Read the TSAN traces in Objects/exceptions.c, Python/pylifecycle.c, Modules/faulthandler.c, and Python/thread_pthread.h, then isolate the failures in test_datetime, test_faulthandler, test_pyrepl, and test.test_multiprocessing_forkserver.test_manager. Done requires a scoped cause and a verified resolution for the reported races, crashes, and leaks.