Issue #14432: Generator now clears the borrowed reference to the thread state

Fix a crash when a generator is created in a C thread that is destroyed while
the generator is still used. The issue was that a generator contains a frame,
and the frame kept a reference to the Python state of the destroyed C thread.
The crash occurs when a trace function is setup.

Lib/test/test_threading.py

@@ -17,6 +17,10 @@ import weakref
 import os
 from test.script_helper import assert_python_ok, assert_python_failure
 import subprocess
+try:
+    import _testcapi
+except ImportError:
+    _testcapi = None
 
 from test import lock_tests
 
@@ -769,6 +773,45 @@ class ThreadJoinOnShutdown(BaseTestCase):
         for t in threads:
             t.join()
 
+    @unittest.skipIf(_testcapi is None, "need _testcapi module")
+    def test_frame_tstate_tracing(self):
+        # Issue #14432: Crash when a generator is created in a C thread that is
+        # destroyed while the generator is still used. The issue was that a
+        # generator contains a frame, and the frame kept a reference to the
+        # Python state of the destroyed C thread. The crash occurs when a trace
+        # function is setup.
+
+        def noop_trace(frame, event, arg):
+            # no operation
+            return noop_trace
+
+        def generator():
+            while 1:
+                yield "genereator"
+
+        def callback():
+            if callback.gen is None:
+                callback.gen = generator()
+            return next(callback.gen)
+        callback.gen = None
+
+        old_trace = sys.gettrace()
+        sys.settrace(noop_trace)
+        try:
+            # Install a trace function
+            threading.settrace(noop_trace)
+
+            # Create a generator in a C thread which exits after the call
+            _testcapi.call_in_temporary_c_thread(callback)
+
+            # Call the generator in a different Python thread, check that the
+            # generator didn't keep a reference to the destroyed thread state
+            for test in range(3):
+                # The trace function is still called here
+                callback()
+        finally:
+            sys.settrace(old_trace)
+
 
 class ThreadingExceptionTests(BaseTestCase):
     # A RuntimeError should be raised if Thread.start() is called

Misc/NEWS

@@ -10,6 +10,12 @@ What's New in Python 3.3.4 release candidate 1?
 Core and Builtins
 -----------------
 
+- Issue #14432: Generator now clears the borrowed reference to the thread
+  state. Fix a crash when a generator is created in a C thread that is
+  destroyed while the generator is still used. The issue was that a generator
+  contains a frame, and the frame kept a reference to the Python state of the
+  destroyed C thread. The crash occurs when a trace function is setup.
+
 - Issue #17576: Deprecation warning emitted now when __int__() or __index__()
   return not int instance.
 

Modules/_testcapimodule.c

@@ -2477,6 +2477,93 @@ test_pytime_object_to_timespec(PyObject *self, PyObject *args)
     return Py_BuildValue("Nl", _PyLong_FromTime_t(sec), nsec);
 }
 
+typedef struct {
+    PyThread_type_lock start_event;
+    PyThread_type_lock exit_event;
+    PyObject *callback;
+} test_c_thread_t;
+
+static void
+temporary_c_thread(void *data)
+{
+    test_c_thread_t *test_c_thread = data;
+    PyGILState_STATE state;
+    PyObject *res;
+
+    PyThread_release_lock(test_c_thread->start_event);
+
+    /* Allocate a Python thread state for this thread */
+    state = PyGILState_Ensure();
+
+    res = PyObject_CallFunction(test_c_thread->callback, "", NULL);
+    Py_CLEAR(test_c_thread->callback);
+
+    if (res == NULL) {
+        PyErr_Print();
+    }
+    else {
+        Py_DECREF(res);
+    }
+
+    /* Destroy the Python thread state for this thread */
+    PyGILState_Release(state);
+
+    PyThread_release_lock(test_c_thread->exit_event);
+
+    PyThread_exit_thread();
+}
+
+static PyObject *
+call_in_temporary_c_thread(PyObject *self, PyObject *callback)
+{
+    PyObject *res = NULL;
+    test_c_thread_t test_c_thread;
+    long thread;
+
+    PyEval_InitThreads();
+
+    test_c_thread.start_event = PyThread_allocate_lock();
+    test_c_thread.exit_event = PyThread_allocate_lock();
+    test_c_thread.callback = NULL;
+    if (!test_c_thread.start_event || !test_c_thread.exit_event) {
+        PyErr_SetString(PyExc_RuntimeError, "could not allocate lock");
+        goto exit;
+    }
+
+    Py_INCREF(callback);
+    test_c_thread.callback = callback;
+
+    PyThread_acquire_lock(test_c_thread.start_event, 1);
+    PyThread_acquire_lock(test_c_thread.exit_event, 1);
+
+    thread = PyThread_start_new_thread(temporary_c_thread, &test_c_thread);
+    if (thread == -1) {
+        PyErr_SetString(PyExc_RuntimeError, "unable to start the thread");
+        PyThread_release_lock(test_c_thread.start_event);
+        PyThread_release_lock(test_c_thread.exit_event);
+        goto exit;
+    }
+
+    PyThread_acquire_lock(test_c_thread.start_event, 1);
+    PyThread_release_lock(test_c_thread.start_event);
+
+    Py_BEGIN_ALLOW_THREADS
+        PyThread_acquire_lock(test_c_thread.exit_event, 1);
+        PyThread_release_lock(test_c_thread.exit_event);
+    Py_END_ALLOW_THREADS
+
+    Py_INCREF(Py_None);
+    res = Py_None;
+
+exit:
+    Py_CLEAR(test_c_thread.callback);
+    if (test_c_thread.start_event)
+        PyThread_free_lock(test_c_thread.start_event);
+    if (test_c_thread.exit_event)
+        PyThread_free_lock(test_c_thread.exit_event);
+    return res;
+}
+
 
 static PyMethodDef TestMethods[] = {
     {"raise_exception",         raise_exception,                 METH_VARARGS},
@@ -2574,6 +2661,8 @@ static PyMethodDef TestMethods[] = {
     {"pytime_object_to_time_t", test_pytime_object_to_time_t,  METH_VARARGS},
     {"pytime_object_to_timeval", test_pytime_object_to_timeval,  METH_VARARGS},
     {"pytime_object_to_timespec", test_pytime_object_to_timespec,  METH_VARARGS},
+    {"call_in_temporary_c_thread", call_in_temporary_c_thread, METH_O,
+     PyDoc_STR("set_error_class(error_class) -> None")},
     {NULL, NULL} /* sentinel */
 };
 

Objects/genobject.c

@@ -76,6 +76,7 @@ gen_send_ex(PyGenObject *gen, PyObject *arg, int exc)
 
     /* Generators always return to their most recent caller, not
      * necessarily their creator. */
+    f->f_tstate = tstate;
     Py_XINCREF(tstate->frame);
     assert(f->f_back == NULL);
     f->f_back = tstate->frame;
@@ -89,6 +90,8 @@ gen_send_ex(PyGenObject *gen, PyObject *arg, int exc)
      * cycle. */
     assert(f->f_back == tstate->frame);
     Py_CLEAR(f->f_back);
+    /* Clear the borrowed reference to the thread state */
+    f->f_tstate = NULL;
 
     /* If the generator just returned (as opposed to yielding), signal
      * that the generator is exhausted. */