Description

gpshead

opened on Jan 10, 2026

Issue body actions

When a module import fails, other threads can receive a stale module reference that is no longer in sys.modules.

Race condition

1. Thread 1 starts importing module X, adds it to sys.modules
2. Thread 2 checks sys.modules, finds module X, takes the "skip lock" fast path
3. Thread 1's import fails, removes module X from sys.modules
4. Thread 2 returns the stale module reference (not in sys.modules)

Affected code paths

• Lib/importlib/_bootstrap.py:_find_and_load() lines 1268-1282
• Python/import.c:PyImport_GetModule()
• Python/import.c:PyImport_ImportModuleLevelObject()

All three have an optimization that skips locking when a module appears already loaded, but none verify the module is still valid after checking _initializing.

Reproducer

import sys
import threading
import tempfile
import os

with tempfile.TemporaryDirectory() as tmpdir:
    sys.path.insert(0, tmpdir)
    
    # Module that partially initializes then fails
    with open(os.path.join(tmpdir, "failing_mod.py"), "w") as f:
        f.write("import time; time.sleep(0.05); raise RuntimeError('fail')")
    
    results = []
    def do_import(delay):
        import time; time.sleep(delay)
        try:
            mod = __import__("failing_mod")
            if "failing_mod" not in sys.modules:
                results.append("RACE: got module not in sys.modules")
        except RuntimeError:
            results.append("ok")
    
    t1 = threading.Thread(target=do_import, args=(0,))
    t2 = threading.Thread(target=do_import, args=(0.01,))
    t1.start(); t2.start(); t1.join(); t2.join()
    print(results)  # May show "RACE" on affected builds

Linked PRs

• gh-143650: Fix importlib race condition on import failure #143651
• [3.14] gh-143650: Fix importlib race condition on import failure (GH-143651) #144662
• [3.13] gh-143650: Fix importlib race condition on import failure (GH-143651) #144697

Activity

1. gpshead

   self-assigned this

   on Jan 10, 2026
2. StanFromIreland

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   topic-importlib

   on Jan 10, 2026
3. gpshead

   marked No protection: import numpy in two different threads can lead to race-condition #91238 as a duplicate of this issue on Jan 11, 2026
4. encukou

   added a commit that references this issue on Feb 10, 2026

   gh-143650: Fix importlib race condition on import failure (GH-143651)

   ac8b5b6
5. miss-islington

   added a commit that references this issue on Feb 10, 2026

   pythongh-143650: Fix importlib race condition on import failure (pyth…

   6cc1f0c
6. gpshead

   added 3 commits that reference this issue on Feb 11, 2026

   [3.14] gh-143650: Fix importlib race condition on import failure (GH-…

   7d07470

   pythongh-143650: Fix importlib race condition on import failure (pyth…

   d6e953f

   [3.13] gh-143650: Fix importlib race condition on import failure (GH-…

   c117ef1
7. encukou

   closed this as completedon Feb 11, 2026
8. thunder-coding

   added a commit that references this issue on Feb 15, 2026

   pythongh-143650: Fix importlib race condition on import failure (pyth…

   30ed3bd
9. brijkapadia

   added a commit that references this issue on Feb 28, 2026

   pythongh-143650: Fix importlib race condition on import failure (pyth…

   6aa0be4
10. ljfp

    added a commit that references this issue on Apr 25, 2026

    pythongh-143650: Fix importlib race condition on import failure (pyth…

    40af15d
11. Pyosch

    added a commit that references this issue on Jun 17, 2026

    fix(startup): remove background preloader daemon to stop import race

    4cc0a51

Issue actions