bpo-33613, test_semaphore_tracker_sigint: fix race condition (#7850)
Fail `test_semaphore_tracker_sigint` if no warnings are expected and one is received.
Fix race condition when the child receives SIGINT before it can register signal handlers for it.
The race condition occurs when the parent calls
`_semaphore_tracker.ensure_running()` (which in turn spawns the
semaphore_tracker using `_posixsubprocess.fork_exec`), the child
registers the signal handlers and the parent tries to kill the child.
What seem to happen is that in some slow systems, the parent sends the
signal to kill the child before the child protects against the signal.
diff --git a/Lib/multiprocessing/semaphore_tracker.py b/Lib/multiprocessing/semaphore_tracker.py
index 3e31bf8..82833bc 100644
--- a/Lib/multiprocessing/semaphore_tracker.py
+++ b/Lib/multiprocessing/semaphore_tracker.py
@@ -23,6 +23,9 @@
__all__ = ['ensure_running', 'register', 'unregister']
+_HAVE_SIGMASK = hasattr(signal, 'pthread_sigmask')
+_IGNORED_SIGNALS = (signal.SIGINT, signal.SIGTERM)
+
class SemaphoreTracker(object):
@@ -43,10 +46,16 @@
with self._lock:
if self._pid is not None:
# semaphore tracker was launched before, is it still running?
- pid, status = os.waitpid(self._pid, os.WNOHANG)
- if not pid:
- # => still alive
- return
+ try:
+ pid, _ = os.waitpid(self._pid, os.WNOHANG)
+ except ChildProcessError:
+ # The process terminated
+ pass
+ else:
+ if not pid:
+ # => still alive
+ return
+
# => dead, launch it again
os.close(self._fd)
self._fd = None
@@ -68,7 +77,19 @@
exe = spawn.get_executable()
args = [exe] + util._args_from_interpreter_flags()
args += ['-c', cmd % r]
- pid = util.spawnv_passfds(exe, args, fds_to_pass)
+ # bpo-33613: Register a signal mask that will block the signals.
+ # This signal mask will be inherited by the child that is going
+ # to be spawned and will protect the child from a race condition
+ # that can make the child die before it registers signal handlers
+ # for SIGINT and SIGTERM. The mask is unregistered after spawning
+ # the child.
+ try:
+ if _HAVE_SIGMASK:
+ signal.pthread_sigmask(signal.SIG_BLOCK, _IGNORED_SIGNALS)
+ pid = util.spawnv_passfds(exe, args, fds_to_pass)
+ finally:
+ if _HAVE_SIGMASK:
+ signal.pthread_sigmask(signal.SIG_UNBLOCK, _IGNORED_SIGNALS)
except:
os.close(w)
raise
@@ -104,12 +125,13 @@
unregister = _semaphore_tracker.unregister
getfd = _semaphore_tracker.getfd
-
def main(fd):
'''Run semaphore tracker.'''
# protect the process from ^C and "killall python" etc
signal.signal(signal.SIGINT, signal.SIG_IGN)
signal.signal(signal.SIGTERM, signal.SIG_IGN)
+ if _HAVE_SIGMASK:
+ signal.pthread_sigmask(signal.SIG_UNBLOCK, _IGNORED_SIGNALS)
for f in (sys.stdin, sys.stdout):
try:
diff --git a/Lib/test/_test_multiprocessing.py b/Lib/test/_test_multiprocessing.py
index 749cf8c..a5509ce 100644
--- a/Lib/test/_test_multiprocessing.py
+++ b/Lib/test/_test_multiprocessing.py
@@ -20,6 +20,7 @@
import struct
import operator
import weakref
+import warnings
import test.support
import test.support.script_helper
from test import support
@@ -4517,17 +4518,19 @@
# bpo-31310: if the semaphore tracker process has died, it should
# be restarted implicitly.
from multiprocessing.semaphore_tracker import _semaphore_tracker
- _semaphore_tracker.ensure_running()
pid = _semaphore_tracker._pid
+ if pid is not None:
+ os.kill(pid, signal.SIGKILL)
+ os.waitpid(pid, 0)
+ with warnings.catch_warnings(record=True) as all_warn:
+ _semaphore_tracker.ensure_running()
+ pid = _semaphore_tracker._pid
+
os.kill(pid, signum)
time.sleep(1.0) # give it time to die
ctx = multiprocessing.get_context("spawn")
- with contextlib.ExitStack() as stack:
- if should_die:
- stack.enter_context(self.assertWarnsRegex(
- UserWarning,
- "semaphore_tracker: process died"))
+ with warnings.catch_warnings(record=True) as all_warn:
sem = ctx.Semaphore()
sem.acquire()
sem.release()
@@ -4537,11 +4540,23 @@
del sem
gc.collect()
self.assertIsNone(wr())
+ if should_die:
+ self.assertEqual(len(all_warn), 1)
+ the_warn = all_warn[0]
+ issubclass(the_warn.category, UserWarning)
+ self.assertTrue("semaphore_tracker: process died"
+ in str(the_warn.message))
+ else:
+ self.assertEqual(len(all_warn), 0)
def test_semaphore_tracker_sigint(self):
# Catchable signal (ignored by semaphore tracker)
self.check_semaphore_tracker_death(signal.SIGINT, False)
+ def test_semaphore_tracker_sigterm(self):
+ # Catchable signal (ignored by semaphore tracker)
+ self.check_semaphore_tracker_death(signal.SIGTERM, False)
+
def test_semaphore_tracker_sigkill(self):
# Uncatchable signal.
self.check_semaphore_tracker_death(signal.SIGKILL, True)
diff --git a/Misc/NEWS.d/next/Library/2018-07-31-23-33-06.bpo-33613.Cdnt0i.rst b/Misc/NEWS.d/next/Library/2018-07-31-23-33-06.bpo-33613.Cdnt0i.rst
new file mode 100644
index 0000000..9574e43
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2018-07-31-23-33-06.bpo-33613.Cdnt0i.rst
@@ -0,0 +1,3 @@
+Fix a race condition in ``multiprocessing.semaphore_tracker`` when the
+tracker receives SIGINT before it can register signal handlers for ignoring
+it.