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There is a race between when `Thread._tstate_lock` is released[^1] in `Thread._wait_for_tstate_lock()` and when `Thread._stop()` asserts[^2] that it is unlocked. Consider the following execution involving threads A, B, and C: 1. A starts. 2. B joins A, blocking on its `_tstate_lock`. 3. C joins A, blocking on its `_tstate_lock`. 4. A finishes and releases its `_tstate_lock`. 5. B acquires A's `_tstate_lock` in `_wait_for_tstate_lock()`, releases it, but is swapped out before calling `_stop()`. 6. C is scheduled, acquires A's `_tstate_lock` in `_wait_for_tstate_lock()` but is swapped out before releasing it. 7. B is scheduled, calls `_stop()`, which asserts that A's `_tstate_lock` is not held. However, C holds it, so the assertion fails. The race can be reproduced[^3] by inserting sleeps at the appropriate points in the threading code. To do so, run the `repro_join_race.py` from the linked repo. There are two main parts to this PR: 1. `_tstate_lock` is replaced with an event that is attached to `PyThreadState`. The event is set by the runtime prior to the thread being cleared (in the same place that `_tstate_lock` was released). `Thread.join()` blocks waiting for the event to be set. 2. `_PyInterpreterState_WaitForThreads()` provides the ability to wait for all non-daemon threads to exit. To do so, an `is_daemon` predicate was added to `PyThreadState`. This field is set each time a thread is created. `threading._shutdown()` now calls into `_PyInterpreterState_WaitForThreads()` instead of waiting on `_tstate_lock`s. [^1]: |
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| .. | ||
| cpython | ||
| internal | ||
| abstract.h | ||
| bltinmodule.h | ||
| boolobject.h | ||
| bytearrayobject.h | ||
| bytesobject.h | ||
| ceval.h | ||
| codecs.h | ||
| compile.h | ||
| complexobject.h | ||
| datetime.h | ||
| descrobject.h | ||
| dictobject.h | ||
| dynamic_annotations.h | ||
| enumobject.h | ||
| errcode.h | ||
| exports.h | ||
| fileobject.h | ||
| fileutils.h | ||
| floatobject.h | ||
| frameobject.h | ||
| genericaliasobject.h | ||
| import.h | ||
| interpreteridobject.h | ||
| intrcheck.h | ||
| iterobject.h | ||
| listobject.h | ||
| longobject.h | ||
| marshal.h | ||
| memoryobject.h | ||
| methodobject.h | ||
| modsupport.h | ||
| moduleobject.h | ||
| object.h | ||
| objimpl.h | ||
| opcode.h | ||
| opcode_ids.h | ||
| osdefs.h | ||
| osmodule.h | ||
| patchlevel.h | ||
| py_curses.h | ||
| pyatomic.h | ||
| pybuffer.h | ||
| pycapsule.h | ||
| pydtrace.d | ||
| pydtrace.h | ||
| pyerrors.h | ||
| pyexpat.h | ||
| pyframe.h | ||
| pyhash.h | ||
| pylifecycle.h | ||
| pymacconfig.h | ||
| pymacro.h | ||
| pymath.h | ||
| pymem.h | ||
| pyport.h | ||
| pystate.h | ||
| pystats.h | ||
| pystrcmp.h | ||
| pystrtod.h | ||
| Python.h | ||
| pythonrun.h | ||
| pythread.h | ||
| pytypedefs.h | ||
| rangeobject.h | ||
| README.rst | ||
| setobject.h | ||
| sliceobject.h | ||
| structmember.h | ||
| structseq.h | ||
| sysmodule.h | ||
| traceback.h | ||
| tupleobject.h | ||
| typeslots.h | ||
| unicodeobject.h | ||
| warnings.h | ||
| weakrefobject.h | ||
The Python C API ================ The C API is divided into these sections: 1. ``Include/``: Limited API 2. ``Include/cpython/``: CPython implementation details 3. ``Include/cpython/``, names with the ``PyUnstable_`` prefix: API that can change between minor releases 4. ``Include/internal/``, and any name with ``_`` prefix: The internal API Information on changing the C API is available `in the developer guide`_ .. _in the developer guide: https://devguide.python.org/c-api/