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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.
This commit is contained in:
Antoine Pitrou 2013-09-07 23:38:37 +02:00
parent eda7c64151
commit 7b4769937f
7 changed files with 223 additions and 33 deletions
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26
Include/pystate.h
View File

@ -118,6 +118,32 @@ typedef struct _ts {
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;

4
Lib/_dummy_thread.py
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@ -81,6 +81,10 @@ def stack_size(size=None):
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.

70
Lib/test/test_threading.py
View File

@ -539,6 +539,40 @@ class ThreadTests(BaseTestCase):
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 @@ class ThreadJoinOnShutdown(BaseTestCase):
# 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 @@ class ThreadJoinOnShutdown(BaseTestCase):
# 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 @@ class SubinterpThreadingTests(BaseTestCase):
# 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

85
Lib/threading.py
View File

@ -33,6 +33,7 @@ __all__ = ['active_count', 'Condition', 'current_thread', 'enumerate', 'Event',
# 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 @@ class Thread:
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 @@ class Thread:
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 @@ class Thread:
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 @@ class Thread:
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 @@ class Thread:
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 @@ class _DummyThread(Thread):
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 @@ def _after_fork():
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()

4
Misc/NEWS
View File

@ -56,6 +56,10 @@ Core and Builtins
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.

62
Modules/_threadmodule.c
View File

@ -1172,6 +1172,66 @@ yet finished.\n\
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 @@ static PyMethodDef thread_methods[] = {
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 */
};

5
Python/pystate.c
View File

@ -208,6 +208,8 @@ new_threadstate(PyInterpreterState *interp, int init)
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 @@ tstate_delete_common(PyThreadState *tstate)
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);
}