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#
# On Unix we run a server process which keeps track of unlinked
# semaphores. The server ignores SIGINT and SIGTERM and reads from a
# pipe. Every other process of the program has a copy of the writable
# end of the pipe, so we get EOF when all other processes have exited.
# Then the server process unlinks any remaining semaphore names.
#
# This is important because the system only supports a limited number
# of named semaphores, and they will not be automatically removed till
# the next reboot. Without this semaphore tracker process, "killall
# python" would probably leave unlinked semaphores.
#
import os
import signal
import sys
import threading
import warnings
import _multiprocessing
from . import spawn
from . import util
__all__ = ['ensure_running', 'register', 'unregister']
_HAVE_SIGMASK = hasattr(signal, 'pthread_sigmask')
_IGNORED_SIGNALS = (signal.SIGINT, signal.SIGTERM)
class SemaphoreTracker(object):
def __init__(self):
self._lock = threading.Lock()
self._fd = None
self._pid = None
def getfd(self):
self.ensure_running()
return self._fd
def ensure_running(self):
'''Make sure that semaphore tracker process is running.
This can be run from any process. Usually a child process will use
the semaphore created by its parent.'''
with self._lock:
if self._fd is not None:
# semaphore tracker was launched before, is it still running?
if self._check_alive():
# => still alive
return
# => dead, launch it again
os.close(self._fd)
# Clean-up to avoid dangling processes.
try:
# _pid can be None if this process is a child from another
# python process, which has started the semaphore_tracker.
if self._pid is not None:
os.waitpid(self._pid, 0)
except ChildProcessError:
# The semaphore_tracker has already been terminated.
pass
self._fd = None
self._pid = None
warnings.warn('semaphore_tracker: process died unexpectedly, '
'relaunching. Some semaphores might leak.')
fds_to_pass = []
try:
fds_to_pass.append(sys.stderr.fileno())
except Exception:
pass
cmd = 'from multiprocessing.semaphore_tracker import main;main(%d)'
r, w = os.pipe()
try:
fds_to_pass.append(r)
# process will out live us, so no need to wait on pid
exe = spawn.get_executable()
args = [exe] + util._args_from_interpreter_flags()
args += ['-c', cmd % r]
# 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
else:
self._fd = w
self._pid = pid
finally:
os.close(r)
def _check_alive(self):
'''Check that the pipe has not been closed by sending a probe.'''
try:
# We cannot use send here as it calls ensure_running, creating
# a cycle.
os.write(self._fd, b'PROBE:0\n')
except OSError:
return False
else:
return True
def register(self, name):
'''Register name of semaphore with semaphore tracker.'''
self._send('REGISTER', name)
def unregister(self, name):
'''Unregister name of semaphore with semaphore tracker.'''
self._send('UNREGISTER', name)
def _send(self, cmd, name):
self.ensure_running()
msg = '{0}:{1}\n'.format(cmd, name).encode('ascii')
if len(name) > 512:
# posix guarantees that writes to a pipe of less than PIPE_BUF
# bytes are atomic, and that PIPE_BUF >= 512
raise ValueError('name too long')
nbytes = os.write(self._fd, msg)
assert nbytes == len(msg), "nbytes {0:n} but len(msg) {1:n}".format(
nbytes, len(msg))
_semaphore_tracker = SemaphoreTracker()
ensure_running = _semaphore_tracker.ensure_running
register = _semaphore_tracker.register
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:
f.close()
except Exception:
pass
cache = set()
try:
# keep track of registered/unregistered semaphores
with open(fd, 'rb') as f:
for line in f:
try:
cmd, name = line.strip().split(b':')
if cmd == b'REGISTER':
cache.add(name)
elif cmd == b'UNREGISTER':
cache.remove(name)
elif cmd == b'PROBE':
pass
else:
raise RuntimeError('unrecognized command %r' % cmd)
except Exception:
try:
sys.excepthook(*sys.exc_info())
except:
pass
finally:
# all processes have terminated; cleanup any remaining semaphores
if cache:
try:
warnings.warn('semaphore_tracker: There appear to be %d '
'leaked semaphores to clean up at shutdown' %
len(cache))
except Exception:
pass
for name in cache:
# For some reason the process which created and registered this
# semaphore has failed to unregister it. Presumably it has died.
# We therefore unlink it.
try:
name = name.decode('ascii')
try:
_multiprocessing.sem_unlink(name)
except Exception as e:
warnings.warn('semaphore_tracker: %r: %s' % (name, e))
finally:
pass
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