[3.13] gh-87135: Hang non-main threads that attempt to acquire the GIL during finalization (GH-105805) (GH-137827)

* [3.13] 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.

(cherry picked from commit 8cc5aa47ee)

Co-authored-by: Jeremy Maitin-Shepard <jeremy@jeremyms.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>

* state "3.13.7 and earlier"
* backport: do not add the deprecated marker
* fix Py_IsFinalizing doc ref

---------

Co-authored-by: Jeremy Maitin-Shepard <jeremy@jeremyms.com>

Modules/_testcapimodule.c

@@ -3322,6 +3322,34 @@ test_atexit(PyObject *self, PyObject *Py_UNUSED(args))
     Py_RETURN_NONE;
 }
 
+// Used by `finalize_thread_hang`.
+#ifdef _POSIX_THREADS
+static void finalize_thread_hang_cleanup_callback(void *Py_UNUSED(arg)) {
+    // Should not reach here.
+    Py_FatalError("pthread thread termination was triggered unexpectedly");
+}
+#endif
+
+// Tests that finalization does not trigger pthread cleanup.
+//
+// Must be called with a single nullary callable function that should block
+// (with GIL released) until finalization is in progress.
+static PyObject *
+finalize_thread_hang(PyObject *self, PyObject *callback)
+{
+    // WASI builds some pthread stuff but doesn't have these APIs today?
+#if defined(_POSIX_THREADS) && !defined(__wasi__)
+    pthread_cleanup_push(finalize_thread_hang_cleanup_callback, NULL);
+#endif
+    PyObject_CallNoArgs(callback);
+    // Should not reach here.
+    Py_FatalError("thread unexpectedly did not hang");
+#if defined(_POSIX_THREADS) && !defined(__wasi__)
+    pthread_cleanup_pop(0);
+#endif
+    Py_RETURN_NONE;
+}
+
 
 static void
 tracemalloc_track_race_thread(void *data)
@@ -3590,6 +3618,7 @@ static PyMethodDef TestMethods[] = {
     {"test_atexit", test_atexit, METH_NOARGS},
     {"tracemalloc_track_race", tracemalloc_track_race, METH_NOARGS},
     {"toggle_reftrace_printer", toggle_reftrace_printer, METH_O},
+    {"finalize_thread_hang", finalize_thread_hang, METH_O, NULL},
     {NULL, NULL} /* sentinel */
 };