bpo-35621: Support running subprocesses in asyncio when loop is executed in non-main thread (GH-14344)
(cherry picked from commit 0d671c04c39b52e44597491b893eb0b6c86b3d45)
Co-authored-by: Andrew Svetlov <andrew.svetlov@gmail.com>
diff --git a/Lib/asyncio/unix_events.py b/Lib/asyncio/unix_events.py
index 28128d2..d7a4af8 100644
--- a/Lib/asyncio/unix_events.py
+++ b/Lib/asyncio/unix_events.py
@@ -2,6 +2,7 @@
import errno
import io
+import itertools
import os
import selectors
import signal
@@ -12,7 +13,6 @@
import threading
import warnings
-
from . import base_events
from . import base_subprocess
from . import constants
@@ -29,7 +29,9 @@
__all__ = (
'SelectorEventLoop',
'AbstractChildWatcher', 'SafeChildWatcher',
- 'FastChildWatcher', 'DefaultEventLoopPolicy',
+ 'FastChildWatcher',
+ 'MultiLoopChildWatcher', 'ThreadedChildWatcher',
+ 'DefaultEventLoopPolicy',
)
@@ -184,6 +186,13 @@
stdin, stdout, stderr, bufsize,
extra=None, **kwargs):
with events.get_child_watcher() as watcher:
+ if not watcher.is_active():
+ # Check early.
+ # Raising exception before process creation
+ # prevents subprocess execution if the watcher
+ # is not ready to handle it.
+ raise RuntimeError("asyncio.get_child_watcher() is not activated, "
+ "subprocess support is not installed.")
waiter = self.create_future()
transp = _UnixSubprocessTransport(self, protocol, args, shell,
stdin, stdout, stderr, bufsize,
@@ -838,6 +847,15 @@
"""
raise NotImplementedError()
+ def is_active(self):
+ """Return ``True`` if the watcher is active and is used by the event loop.
+
+ Return True if the watcher is installed and ready to handle process exit
+ notifications.
+
+ """
+ raise NotImplementedError()
+
def __enter__(self):
"""Enter the watcher's context and allow starting new processes
@@ -849,6 +867,20 @@
raise NotImplementedError()
+def _compute_returncode(status):
+ if os.WIFSIGNALED(status):
+ # The child process died because of a signal.
+ return -os.WTERMSIG(status)
+ elif os.WIFEXITED(status):
+ # The child process exited (e.g sys.exit()).
+ return os.WEXITSTATUS(status)
+ else:
+ # The child exited, but we don't understand its status.
+ # This shouldn't happen, but if it does, let's just
+ # return that status; perhaps that helps debug it.
+ return status
+
+
class BaseChildWatcher(AbstractChildWatcher):
def __init__(self):
@@ -858,6 +890,9 @@
def close(self):
self.attach_loop(None)
+ def is_active(self):
+ return self._loop is not None and self._loop.is_running()
+
def _do_waitpid(self, expected_pid):
raise NotImplementedError()
@@ -898,19 +933,6 @@
'exception': exc,
})
- def _compute_returncode(self, status):
- if os.WIFSIGNALED(status):
- # The child process died because of a signal.
- return -os.WTERMSIG(status)
- elif os.WIFEXITED(status):
- # The child process exited (e.g sys.exit()).
- return os.WEXITSTATUS(status)
- else:
- # The child exited, but we don't understand its status.
- # This shouldn't happen, but if it does, let's just
- # return that status; perhaps that helps debug it.
- return status
-
class SafeChildWatcher(BaseChildWatcher):
"""'Safe' child watcher implementation.
@@ -934,11 +956,6 @@
pass
def add_child_handler(self, pid, callback, *args):
- if self._loop is None:
- raise RuntimeError(
- "Cannot add child handler, "
- "the child watcher does not have a loop attached")
-
self._callbacks[pid] = (callback, args)
# Prevent a race condition in case the child is already terminated.
@@ -974,7 +991,7 @@
# The child process is still alive.
return
- returncode = self._compute_returncode(status)
+ returncode = _compute_returncode(status)
if self._loop.get_debug():
logger.debug('process %s exited with returncode %s',
expected_pid, returncode)
@@ -1035,11 +1052,6 @@
def add_child_handler(self, pid, callback, *args):
assert self._forks, "Must use the context manager"
- if self._loop is None:
- raise RuntimeError(
- "Cannot add child handler, "
- "the child watcher does not have a loop attached")
-
with self._lock:
try:
returncode = self._zombies.pop(pid)
@@ -1072,7 +1084,7 @@
# A child process is still alive.
return
- returncode = self._compute_returncode(status)
+ returncode = _compute_returncode(status)
with self._lock:
try:
@@ -1101,6 +1113,209 @@
callback(pid, returncode, *args)
+class MultiLoopChildWatcher(AbstractChildWatcher):
+ """A watcher that doesn't require running loop in the main thread.
+
+ This implementation registers a SIGCHLD signal handler on
+ instantiation (which may conflict with other code that
+ install own handler for this signal).
+
+ The solution is safe but it has a significant overhead when
+ handling a big number of processes (*O(n)* each time a
+ SIGCHLD is received).
+ """
+
+ # Implementation note:
+ # The class keeps compatibility with AbstractChildWatcher ABC
+ # To achieve this it has empty attach_loop() method
+ # and doesn't accept explicit loop argument
+ # for add_child_handler()/remove_child_handler()
+ # but retrieves the current loop by get_running_loop()
+
+ def __init__(self):
+ self._callbacks = {}
+ self._saved_sighandler = None
+
+ def is_active(self):
+ return self._saved_sighandler is not None
+
+ def close(self):
+ self._callbacks.clear()
+ if self._saved_sighandler is not None:
+ handler = signal.getsignal(signal.SIGCHLD)
+ if handler != self._sig_chld:
+ logger.warning("SIGCHLD handler was changed by outside code")
+ else:
+ signal.signal(signal.SIGCHLD, self._saved_sighandler)
+ self._saved_sighandler = None
+
+ def __enter__(self):
+ return self
+
+ def __exit__(self, exc_type, exc_val, exc_tb):
+ pass
+
+ def add_child_handler(self, pid, callback, *args):
+ loop = events.get_running_loop()
+ self._callbacks[pid] = (loop, callback, args)
+
+ # Prevent a race condition in case the child is already terminated.
+ self._do_waitpid(pid)
+
+ def remove_child_handler(self, pid):
+ try:
+ del self._callbacks[pid]
+ return True
+ except KeyError:
+ return False
+
+ def attach_loop(self, loop):
+ # Don't save the loop but initialize itself if called first time
+ # The reason to do it here is that attach_loop() is called from
+ # unix policy only for the main thread.
+ # Main thread is required for subscription on SIGCHLD signal
+ if self._saved_sighandler is None:
+ self._saved_sighandler = signal.signal(signal.SIGCHLD, self._sig_chld)
+ if self._saved_sighandler is None:
+ logger.warning("Previous SIGCHLD handler was set by non-Python code, "
+ "restore to default handler on watcher close.")
+ self._saved_sighandler = signal.SIG_DFL
+
+ # Set SA_RESTART to limit EINTR occurrences.
+ signal.siginterrupt(signal.SIGCHLD, False)
+
+ def _do_waitpid_all(self):
+ for pid in list(self._callbacks):
+ self._do_waitpid(pid)
+
+ def _do_waitpid(self, expected_pid):
+ assert expected_pid > 0
+
+ try:
+ pid, status = os.waitpid(expected_pid, os.WNOHANG)
+ except ChildProcessError:
+ # The child process is already reaped
+ # (may happen if waitpid() is called elsewhere).
+ pid = expected_pid
+ returncode = 255
+ logger.warning(
+ "Unknown child process pid %d, will report returncode 255",
+ pid)
+ debug_log = False
+ else:
+ if pid == 0:
+ # The child process is still alive.
+ return
+
+ returncode = _compute_returncode(status)
+ debug_log = True
+ try:
+ loop, callback, args = self._callbacks.pop(pid)
+ except KeyError: # pragma: no cover
+ # May happen if .remove_child_handler() is called
+ # after os.waitpid() returns.
+ logger.warning("Child watcher got an unexpected pid: %r",
+ pid, exc_info=True)
+ else:
+ if loop.is_closed():
+ logger.warning("Loop %r that handles pid %r is closed", loop, pid)
+ else:
+ if debug_log and loop.get_debug():
+ logger.debug('process %s exited with returncode %s',
+ expected_pid, returncode)
+ loop.call_soon_threadsafe(callback, pid, returncode, *args)
+
+ def _sig_chld(self, signum, frame):
+ try:
+ self._do_waitpid_all()
+ except (SystemExit, KeyboardInterrupt):
+ raise
+ except BaseException:
+ logger.warning('Unknown exception in SIGCHLD handler', exc_info=True)
+
+
+class ThreadedChildWatcher(AbstractChildWatcher):
+ """Threaded child watcher implementation.
+
+ The watcher uses a thread per process
+ for waiting for the process finish.
+
+ It doesn't require subscription on POSIX signal
+ but a thread creation is not free.
+
+ The watcher has O(1) complexity, its perfomance doesn't depend
+ on amount of spawn processes.
+ """
+
+ def __init__(self):
+ self._pid_counter = itertools.count(0)
+ self._threads = {}
+
+ def is_active(self):
+ return True
+
+ def close(self):
+ pass
+
+ def __enter__(self):
+ return self
+
+ def __exit__(self, exc_type, exc_val, exc_tb):
+ pass
+
+ def __del__(self, _warn=warnings.warn):
+ threads = [thread for thread in list(self._threads.values())
+ if thread.is_alive()]
+ if threads:
+ _warn(f"{self.__class__} has registered but not finished child processes",
+ ResourceWarning,
+ source=self)
+
+ def add_child_handler(self, pid, callback, *args):
+ loop = events.get_running_loop()
+ thread = threading.Thread(target=self._do_waitpid,
+ name=f"waitpid-{next(self._pid_counter)}",
+ args=(loop, pid, callback, args),
+ daemon=True)
+ self._threads[pid] = thread
+ thread.start()
+
+ def remove_child_handler(self, pid):
+ # asyncio never calls remove_child_handler() !!!
+ # The method is no-op but is implemented because
+ # abstract base classe requires it
+ return True
+
+ def attach_loop(self, loop):
+ pass
+
+ def _do_waitpid(self, loop, expected_pid, callback, args):
+ assert expected_pid > 0
+
+ try:
+ pid, status = os.waitpid(expected_pid, 0)
+ except ChildProcessError:
+ # The child process is already reaped
+ # (may happen if waitpid() is called elsewhere).
+ pid = expected_pid
+ returncode = 255
+ logger.warning(
+ "Unknown child process pid %d, will report returncode 255",
+ pid)
+ else:
+ returncode = _compute_returncode(status)
+ if loop.get_debug():
+ logger.debug('process %s exited with returncode %s',
+ expected_pid, returncode)
+
+ if loop.is_closed():
+ logger.warning("Loop %r that handles pid %r is closed", loop, pid)
+ else:
+ loop.call_soon_threadsafe(callback, pid, returncode, *args)
+
+ self._threads.pop(expected_pid)
+
+
class _UnixDefaultEventLoopPolicy(events.BaseDefaultEventLoopPolicy):
"""UNIX event loop policy with a watcher for child processes."""
_loop_factory = _UnixSelectorEventLoop
@@ -1112,7 +1327,7 @@
def _init_watcher(self):
with events._lock:
if self._watcher is None: # pragma: no branch
- self._watcher = SafeChildWatcher()
+ self._watcher = ThreadedChildWatcher()
if isinstance(threading.current_thread(),
threading._MainThread):
self._watcher.attach_loop(self._local._loop)
@@ -1134,7 +1349,7 @@
def get_child_watcher(self):
"""Get the watcher for child processes.
- If not yet set, a SafeChildWatcher object is automatically created.
+ If not yet set, a ThreadedChildWatcher object is automatically created.
"""
if self._watcher is None:
self._init_watcher()