Issue #18808: Thread.join() now waits for the underlying thread state to be destroyed before returning.
This prevents unpredictable aborts in Py_EndInterpreter() when some non-daemon threads are still running.
diff --git a/Include/pystate.h b/Include/pystate.h
index e41fe4c..ddc6892 100644
--- a/Include/pystate.h
+++ b/Include/pystate.h
@@ -118,6 +118,32 @@
int trash_delete_nesting;
PyObject *trash_delete_later;
+ /* Called when a thread state is deleted normally, but not when it
+ * is destroyed after fork().
+ * Pain: to prevent rare but fatal shutdown errors (issue 18808),
+ * Thread.join() must wait for the join'ed thread's tstate to be unlinked
+ * from the tstate chain. That happens at the end of a thread's life,
+ * in pystate.c.
+ * The obvious way doesn't quite work: create a lock which the tstate
+ * unlinking code releases, and have Thread.join() wait to acquire that
+ * lock. The problem is that we _are_ at the end of the thread's life:
+ * if the thread holds the last reference to the lock, decref'ing the
+ * lock will delete the lock, and that may trigger arbitrary Python code
+ * if there's a weakref, with a callback, to the lock. But by this time
+ * _PyThreadState_Current is already NULL, so only the simplest of C code
+ * can be allowed to run (in particular it must not be possible to
+ * release the GIL).
+ * So instead of holding the lock directly, the tstate holds a weakref to
+ * the lock: that's the value of on_delete_data below. Decref'ing a
+ * weakref is harmless.
+ * on_delete points to _threadmodule.c's static release_sentinel() function.
+ * After the tstate is unlinked, release_sentinel is called with the
+ * weakref-to-lock (on_delete_data) argument, and release_sentinel releases
+ * the indirectly held lock.
+ */
+ void (*on_delete)(void *);
+ void *on_delete_data;
+
/* XXX signal handlers should also be here */
} PyThreadState;
diff --git a/Lib/_dummy_thread.py b/Lib/_dummy_thread.py
index 13b1f26..b67cfb9 100644
--- a/Lib/_dummy_thread.py
+++ b/Lib/_dummy_thread.py
@@ -81,6 +81,10 @@
raise error("setting thread stack size not supported")
return 0
+def _set_sentinel():
+ """Dummy implementation of _thread._set_sentinel()."""
+ return LockType()
+
class LockType(object):
"""Class implementing dummy implementation of _thread.LockType.
diff --git a/Lib/test/test_threading.py b/Lib/test/test_threading.py
index 971a635..79b10ed 100644
--- a/Lib/test/test_threading.py
+++ b/Lib/test/test_threading.py
@@ -539,6 +539,40 @@
self.assertEqual(err, b"")
self.assertEqual(data, "Thread-1\nTrue\nTrue\n")
+ def test_tstate_lock(self):
+ # Test an implementation detail of Thread objects.
+ started = _thread.allocate_lock()
+ finish = _thread.allocate_lock()
+ started.acquire()
+ finish.acquire()
+ def f():
+ started.release()
+ finish.acquire()
+ time.sleep(0.01)
+ # The tstate lock is None until the thread is started
+ t = threading.Thread(target=f)
+ self.assertIs(t._tstate_lock, None)
+ t.start()
+ started.acquire()
+ self.assertTrue(t.is_alive())
+ # The tstate lock can't be acquired when the thread is running
+ # (or suspended).
+ tstate_lock = t._tstate_lock
+ self.assertFalse(tstate_lock.acquire(timeout=0), False)
+ finish.release()
+ # When the thread ends, the state_lock can be successfully
+ # acquired.
+ self.assertTrue(tstate_lock.acquire(timeout=5), False)
+ # But is_alive() is still True: we hold _tstate_lock now, which
+ # prevents is_alive() from knowing the thread's end-of-life C code
+ # is done.
+ self.assertTrue(t.is_alive())
+ # Let is_alive() find out the C code is done.
+ tstate_lock.release()
+ self.assertFalse(t.is_alive())
+ # And verify the thread disposed of _tstate_lock.
+ self.assertTrue(t._tstate_lock is None)
+
class ThreadJoinOnShutdown(BaseTestCase):
@@ -669,7 +703,7 @@
# someone else tries to fix this test case by acquiring this lock
# before forking instead of resetting it, the test case will
# deadlock when it shouldn't.
- condition = w._block
+ condition = w._stopped._cond
orig_acquire = condition.acquire
call_count_lock = threading.Lock()
call_count = 0
@@ -733,7 +767,7 @@
# causes the worker to fork. At this point, the problematic waiter
# lock has been acquired once by the waiter and has been put onto
# the waiters list.
- condition = w._block
+ condition = w._stopped._cond
orig_release_save = condition._release_save
def my_release_save():
global start_fork
@@ -867,6 +901,38 @@
# The thread was joined properly.
self.assertEqual(os.read(r, 1), b"x")
+ def test_threads_join_2(self):
+ # Same as above, but a delay gets introduced after the thread's
+ # Python code returned but before the thread state is deleted.
+ # To achieve this, we register a thread-local object which sleeps
+ # a bit when deallocated.
+ r, w = os.pipe()
+ self.addCleanup(os.close, r)
+ self.addCleanup(os.close, w)
+ code = r"""if 1:
+ import os
+ import threading
+ import time
+
+ class Sleeper:
+ def __del__(self):
+ time.sleep(0.05)
+
+ tls = threading.local()
+
+ def f():
+ # Sleep a bit so that the thread is still running when
+ # Py_EndInterpreter is called.
+ time.sleep(0.05)
+ tls.x = Sleeper()
+ os.write(%d, b"x")
+ threading.Thread(target=f).start()
+ """ % (w,)
+ ret = _testcapi.run_in_subinterp(code)
+ self.assertEqual(ret, 0)
+ # The thread was joined properly.
+ self.assertEqual(os.read(r, 1), b"x")
+
def test_daemon_threads_fatal_error(self):
subinterp_code = r"""if 1:
import os
diff --git a/Lib/threading.py b/Lib/threading.py
index b6d19d5..d49be0b 100644
--- a/Lib/threading.py
+++ b/Lib/threading.py
@@ -33,6 +33,7 @@
# Rename some stuff so "from threading import *" is safe
_start_new_thread = _thread.start_new_thread
_allocate_lock = _thread.allocate_lock
+_set_sentinel = _thread._set_sentinel
get_ident = _thread.get_ident
ThreadError = _thread.error
try:
@@ -548,28 +549,33 @@
else:
self._daemonic = current_thread().daemon
self._ident = None
+ self._tstate_lock = None
self._started = Event()
- self._stopped = False
- self._block = Condition(Lock())
+ self._stopped = Event()
self._initialized = True
# sys.stderr is not stored in the class like
# sys.exc_info since it can be changed between instances
self._stderr = _sys.stderr
_dangling.add(self)
- def _reset_internal_locks(self):
+ def _reset_internal_locks(self, is_alive):
# private! Called by _after_fork() to reset our internal locks as
# they may be in an invalid state leading to a deadlock or crash.
- if hasattr(self, '_block'): # DummyThread deletes _block
- self._block.__init__()
self._started._reset_internal_locks()
+ self._stopped._reset_internal_locks()
+ if is_alive:
+ self._set_tstate_lock()
+ else:
+ # The thread isn't alive after fork: it doesn't have a tstate
+ # anymore.
+ self._tstate_lock = None
def __repr__(self):
assert self._initialized, "Thread.__init__() was not called"
status = "initial"
if self._started.is_set():
status = "started"
- if self._stopped:
+ if self._stopped.is_set():
status = "stopped"
if self._daemonic:
status += " daemon"
@@ -625,9 +631,18 @@
def _set_ident(self):
self._ident = get_ident()
+ def _set_tstate_lock(self):
+ """
+ Set a lock object which will be released by the interpreter when
+ the underlying thread state (see pystate.h) gets deleted.
+ """
+ self._tstate_lock = _set_sentinel()
+ self._tstate_lock.acquire()
+
def _bootstrap_inner(self):
try:
self._set_ident()
+ self._set_tstate_lock()
self._started.set()
with _active_limbo_lock:
_active[self._ident] = self
@@ -691,10 +706,7 @@
pass
def _stop(self):
- self._block.acquire()
- self._stopped = True
- self._block.notify_all()
- self._block.release()
+ self._stopped.set()
def _delete(self):
"Remove current thread from the dict of currently running threads."
@@ -738,21 +750,29 @@
raise RuntimeError("cannot join thread before it is started")
if self is current_thread():
raise RuntimeError("cannot join current thread")
+ if not self.is_alive():
+ return
+ self._stopped.wait(timeout)
+ if self._stopped.is_set():
+ self._wait_for_tstate_lock(timeout is None)
- self._block.acquire()
- try:
- if timeout is None:
- while not self._stopped:
- self._block.wait()
- else:
- deadline = _time() + timeout
- while not self._stopped:
- delay = deadline - _time()
- if delay <= 0:
- break
- self._block.wait(delay)
- finally:
- self._block.release()
+ def _wait_for_tstate_lock(self, block):
+ # Issue #18808: wait for the thread state to be gone.
+ # When self._stopped is set, the Python part of the thread is done,
+ # but the thread's tstate has not yet been destroyed. The C code
+ # releases self._tstate_lock when the C part of the thread is done
+ # (the code at the end of the thread's life to remove all knowledge
+ # of the thread from the C data structures).
+ # This method waits to acquire _tstate_lock if `block` is True, or
+ # sees whether it can be acquired immediately if `block` is False.
+ # If it does acquire the lock, the C code is done, and _tstate_lock
+ # is set to None.
+ lock = self._tstate_lock
+ if lock is None:
+ return # already determined that the C code is done
+ if lock.acquire(block):
+ lock.release()
+ self._tstate_lock = None
@property
def name(self):
@@ -771,7 +791,14 @@
def is_alive(self):
assert self._initialized, "Thread.__init__() not called"
- return self._started.is_set() and not self._stopped
+ if not self._started.is_set():
+ return False
+ if not self._stopped.is_set():
+ return True
+ # The Python part of the thread is done, but the C part may still be
+ # waiting to run.
+ self._wait_for_tstate_lock(False)
+ return self._tstate_lock is not None
isAlive = is_alive
@@ -854,11 +881,6 @@
def __init__(self):
Thread.__init__(self, name=_newname("Dummy-%d"), daemon=True)
- # Thread._block consumes an OS-level locking primitive, which
- # can never be used by a _DummyThread. Since a _DummyThread
- # instance is immortal, that's bad, so release this resource.
- del self._block
-
self._started.set()
self._set_ident()
with _active_limbo_lock:
@@ -952,15 +974,16 @@
for thread in _enumerate():
# Any lock/condition variable may be currently locked or in an
# invalid state, so we reinitialize them.
- thread._reset_internal_locks()
if thread is current:
# There is only one active thread. We reset the ident to
# its new value since it can have changed.
+ thread._reset_internal_locks(True)
ident = get_ident()
thread._ident = ident
new_active[ident] = thread
else:
# All the others are already stopped.
+ thread._reset_internal_locks(False)
thread._stop()
_limbo.clear()
diff --git a/Misc/NEWS b/Misc/NEWS
index 757ac89..d375a3a 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -56,6 +56,10 @@
Library
-------
+- Issue #18808: Thread.join() now waits for the underlying thread state to
+ be destroyed before returning. This prevents unpredictable aborts in
+ Py_EndInterpreter() when some non-daemon threads are still running.
+
- Issue #18458: Prevent crashes with newer versions of libedit. Its readline
emulation has changed from 0-based indexing to 1-based like gnu readline.
diff --git a/Modules/_threadmodule.c b/Modules/_threadmodule.c
index cbb2901..d83d117 100644
--- a/Modules/_threadmodule.c
+++ b/Modules/_threadmodule.c
@@ -1172,6 +1172,66 @@
This function is meant for internal and specialized purposes only.\n\
In most applications `threading.enumerate()` should be used instead.");
+static void
+release_sentinel(void *wr)
+{
+ /* Tricky: this function is called when the current thread state
+ is being deleted. Therefore, only simple C code can safely
+ execute here. */
+ PyObject *obj = PyWeakref_GET_OBJECT(wr);
+ lockobject *lock;
+ if (obj != Py_None) {
+ assert(Py_TYPE(obj) == &Locktype);
+ lock = (lockobject *) obj;
+ if (lock->locked) {
+ PyThread_release_lock(lock->lock_lock);
+ lock->locked = 0;
+ }
+ }
+ /* Deallocating a weakref with a NULL callback only calls
+ PyObject_GC_Del(), which can't call any Python code. */
+ Py_DECREF(wr);
+}
+
+static PyObject *
+thread__set_sentinel(PyObject *self)
+{
+ PyObject *wr;
+ PyThreadState *tstate = PyThreadState_Get();
+ lockobject *lock;
+
+ if (tstate->on_delete_data != NULL) {
+ /* We must support the re-creation of the lock from a
+ fork()ed child. */
+ assert(tstate->on_delete == &release_sentinel);
+ wr = (PyObject *) tstate->on_delete_data;
+ tstate->on_delete = NULL;
+ tstate->on_delete_data = NULL;
+ Py_DECREF(wr);
+ }
+ lock = newlockobject();
+ if (lock == NULL)
+ return NULL;
+ /* The lock is owned by whoever called _set_sentinel(), but the weakref
+ hangs to the thread state. */
+ wr = PyWeakref_NewRef((PyObject *) lock, NULL);
+ if (wr == NULL) {
+ Py_DECREF(lock);
+ return NULL;
+ }
+ tstate->on_delete_data = (void *) wr;
+ tstate->on_delete = &release_sentinel;
+ return (PyObject *) lock;
+}
+
+PyDoc_STRVAR(_set_sentinel_doc,
+"_set_sentinel() -> lock\n\
+\n\
+Set a sentinel lock that will be released when the current thread\n\
+state is finalized (after it is untied from the interpreter).\n\
+\n\
+This is a private API for the threading module.");
+
static PyObject *
thread_stack_size(PyObject *self, PyObject *args)
{
@@ -1247,6 +1307,8 @@
METH_NOARGS, _count_doc},
{"stack_size", (PyCFunction)thread_stack_size,
METH_VARARGS, stack_size_doc},
+ {"_set_sentinel", (PyCFunction)thread__set_sentinel,
+ METH_NOARGS, _set_sentinel_doc},
{NULL, NULL} /* sentinel */
};
diff --git a/Python/pystate.c b/Python/pystate.c
index 924b6a2..ecd00ce 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -208,6 +208,8 @@
tstate->trash_delete_nesting = 0;
tstate->trash_delete_later = NULL;
+ tstate->on_delete = NULL;
+ tstate->on_delete_data = NULL;
if (init)
_PyThreadState_Init(tstate);
@@ -390,6 +392,9 @@
if (tstate->next)
tstate->next->prev = tstate->prev;
HEAD_UNLOCK();
+ if (tstate->on_delete != NULL) {
+ tstate->on_delete(tstate->on_delete_data);
+ }
PyMem_RawFree(tstate);
}