gh-87135: Hang non-main threads that attempt to acquire the GIL during finalization (GH-105805)

Instead of surprise crashes and memory corruption, we now hang threads that attempt to re-enter the Python interpreter after Python runtime finalization has started. These are typically daemon threads (our long standing mis-feature) but could also be threads spawned by extension modules that then try to call into Python. This marks the `PyThread_exit_thread` public C API as deprecated as there is no plausible safe way to accomplish that on any supported platform in the face of things like C++ code with finalizers anywhere on a thread's stack. Doing this was the least bad option.

Co-authored-by: Gregory P. Smith <greg@krypto.org>

Doc/c-api/init.rst

@@ -430,7 +430,11 @@ Initializing and finalizing the interpreter
    Some memory allocated by extension modules may not be freed.  Some extensions may not
    work properly if their initialization routine is called more than once; this can
    happen if an application calls :c:func:`Py_Initialize` and :c:func:`Py_FinalizeEx`
-   more than once.
+   more than once.  :c:func:`Py_FinalizeEx` must not be called recursively from
+   within itself.  Therefore, it must not be called by any code that may be run
+   as part of the interpreter shutdown process, such as :py:mod:`atexit`
+   handlers, object finalizers, or any code that may be run while flushing the
+   stdout and stderr files.
 
    .. audit-event:: cpython._PySys_ClearAuditHooks "" c.Py_FinalizeEx
 
@@ -960,6 +964,37 @@ thread, where the CPython global runtime was originally initialized.
 The only exception is if :c:func:`exec` will be called immediately
 after.
 
+.. _cautions-regarding-runtime-finalization:
+
+Cautions regarding runtime finalization
+---------------------------------------
+
+In the late stage of :term:`interpreter shutdown`, after attempting to wait for
+non-daemon threads to exit (though this can be interrupted by
+:class:`KeyboardInterrupt`) and running the :mod:`atexit` functions, the runtime
+is marked as *finalizing*: :c:func:`_Py_IsFinalizing` and
+:func:`sys.is_finalizing` return true.  At this point, only the *finalization
+thread* that initiated finalization (typically the main thread) is allowed to
+acquire the :term:`GIL`.
+
+If any thread, other than the finalization thread, attempts to acquire the GIL
+during finalization, either explicitly via :c:func:`PyGILState_Ensure`,
+:c:macro:`Py_END_ALLOW_THREADS`, :c:func:`PyEval_AcquireThread`, or
+:c:func:`PyEval_AcquireLock`, or implicitly when the interpreter attempts to
+reacquire it after having yielded it, the thread enters **a permanently blocked
+state** where it remains until the program exits.  In most cases this is
+harmless, but this can result in deadlock if a later stage of finalization
+attempts to acquire a lock owned by the blocked thread, or otherwise waits on
+the blocked thread.
+
+Gross? Yes. This prevents random crashes and/or unexpectedly skipped C++
+finalizations further up the call stack when such threads were forcibly exited
+here in CPython 3.13 and earlier. The CPython runtime GIL acquiring C APIs
+have never had any error reporting or handling expectations at GIL acquisition
+time that would've allowed for graceful exit from this situation. Changing that
+would require new stable C APIs and rewriting the majority of C code in the
+CPython ecosystem to use those with error handling.
+
 
 High-level API
 --------------
@@ -1033,11 +1068,14 @@ code, or when embedding the Python interpreter:
    ensues.
 
    .. note::
-      Calling this function from a thread when the runtime is finalizing
-      will terminate the thread, even if the thread was not created by Python.
-      You can use :c:func:`Py_IsFinalizing` or :func:`sys.is_finalizing` to
-      check if the interpreter is in process of being finalized before calling
-      this function to avoid unwanted termination.
+      Calling this function from a thread when the runtime is finalizing will
+      hang the thread until the program exits, even if the thread was not
+      created by Python.  Refer to
+      :ref:`cautions-regarding-runtime-finalization` for more details.
+
+   .. versionchanged:: next
+      Hangs the current thread, rather than terminating it, if called while the
+      interpreter is finalizing.
 
 .. c:function:: PyThreadState* PyThreadState_Get()
 
@@ -1092,11 +1130,14 @@ with sub-interpreters:
    to call arbitrary Python code.  Failure is a fatal error.
 
    .. note::
-      Calling this function from a thread when the runtime is finalizing
-      will terminate the thread, even if the thread was not created by Python.
-      You can use :c:func:`Py_IsFinalizing` or :func:`sys.is_finalizing` to
-      check if the interpreter is in process of being finalized before calling
-      this function to avoid unwanted termination.
+      Calling this function from a thread when the runtime is finalizing will
+      hang the thread until the program exits, even if the thread was not
+      created by Python.  Refer to
+      :ref:`cautions-regarding-runtime-finalization` for more details.
+
+   .. versionchanged:: next
+      Hangs the current thread, rather than terminating it, if called while the
+      interpreter is finalizing.
 
 .. c:function:: void PyGILState_Release(PyGILState_STATE)
 
@@ -1374,17 +1415,20 @@ All of the following functions must be called after :c:func:`Py_Initialize`.
    If this thread already has the lock, deadlock ensues.
 
    .. note::
-      Calling this function from a thread when the runtime is finalizing
-      will terminate the thread, even if the thread was not created by Python.
-      You can use :c:func:`Py_IsFinalizing` or :func:`sys.is_finalizing` to
-      check if the interpreter is in process of being finalized before calling
-      this function to avoid unwanted termination.
+      Calling this function from a thread when the runtime is finalizing will
+      hang the thread until the program exits, even if the thread was not
+      created by Python.  Refer to
+      :ref:`cautions-regarding-runtime-finalization` for more details.
 
    .. versionchanged:: 3.8
       Updated to be consistent with :c:func:`PyEval_RestoreThread`,
       :c:func:`Py_END_ALLOW_THREADS`, and :c:func:`PyGILState_Ensure`,
       and terminate the current thread if called while the interpreter is finalizing.
 
+   .. versionchanged:: next
+      Hangs the current thread, rather than terminating it, if called while the
+      interpreter is finalizing.
+
    :c:func:`PyEval_RestoreThread` is a higher-level function which is always
    available (even when threads have not been initialized).