bpo-35621: Support running subprocesses in asyncio when loop is executed in non-main thread (#13630)

Author: asvetlov

Lib/asyncio/unix_events.py

@@ -2,6 +2,7 @@
 
 import errno
 import io
+import itertools
 import os
 import selectors
 import signal
@@ -29,7 +30,9 @@
 __all__ = (
     'SelectorEventLoop',
     'AbstractChildWatcher', 'SafeChildWatcher',
-    'FastChildWatcher', 'DefaultEventLoopPolicy',
+    'FastChildWatcher',
+    'MultiLoopChildWatcher', 'ThreadedChildWatcher',
+    'DefaultEventLoopPolicy',
 )
 
 
@@ -184,6 +187,13 @@ async def _make_subprocess_transport(self, protocol, args, shell,
                                          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, "
+                                   "subproccess support is not installed.")
             waiter = self.create_future()
             transp = _UnixSubprocessTransport(self, protocol, args, shell,
                                               stdin, stdout, stderr, bufsize,
@@ -838,6 +848,15 @@ def close(self):
         """
         raise NotImplementedError()
 
+    def is_active(self):
+        """Watcher status.
+
+        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 +868,20 @@ def __exit__(self, a, b, c):
         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 +891,9 @@ def __init__(self):
     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 +934,6 @@ def _sig_chld(self):
                 '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 +957,6 @@ def __exit__(self, a, b, c):
         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 +992,7 @@ def _do_waitpid(self, expected_pid):
                 # 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 +1053,6 @@ def __exit__(self, a, b, c):
     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 +1085,7 @@ def _do_waitpid_all(self):
                     # A child process is still alive.
                     return
 
-                returncode = self._compute_returncode(status)
+                returncode = _compute_returncode(status)
 
             with self._lock:
                 try:
@@ -1101,6 +1114,177 @@ def _do_waitpid_all(self):
                 callback(pid, returncode, *args)
 
 
+class MultiLoopChildWatcher(AbstractChildWatcher):
+    # 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):
+    # The watcher uses a thread per process
+    # for waiting for the process finish.
+    # It doesn't require subscription on POSIX signal
+
+    def __init__(self):
+        self._pid_counter = itertools.count(0)
+
+    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 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)
+        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)
+
+
 class _UnixDefaultEventLoopPolicy(events.BaseDefaultEventLoopPolicy):
     """UNIX event loop policy with a watcher for child processes."""
     _loop_factory = _UnixSelectorEventLoop
@@ -1112,7 +1296,7 @@ def __init__(self):
     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 +1318,7 @@ def set_event_loop(self, loop):
     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()

Lib/test/test_asyncio/test_subprocess.py

@@ -633,6 +633,7 @@ async def execute():
 
         self.assertIsNone(self.loop.run_until_complete(execute()))
 
+
 if sys.platform != 'win32':
     # Unix
     class SubprocessWatcherMixin(SubprocessMixin):
@@ -648,7 +649,24 @@ def setUp(self):
             watcher = self.Watcher()
             watcher.attach_loop(self.loop)
             policy.set_child_watcher(watcher)
-            self.addCleanup(policy.set_child_watcher, None)
+
+        def tearDown(self):
+            super().setUp()
+            policy = asyncio.get_event_loop_policy()
+            watcher = policy.get_child_watcher()
+            policy.set_child_watcher(None)
+            watcher.attach_loop(None)
+            watcher.close()
+
+    class SubprocessThreadedWatcherTests(SubprocessWatcherMixin,
+                                         test_utils.TestCase):
+
+        Watcher = unix_events.ThreadedChildWatcher
+
+    class SubprocessMultiLoopWatcherTests(SubprocessWatcherMixin,
+                                          test_utils.TestCase):
+
+        Watcher = unix_events.MultiLoopChildWatcher
 
     class SubprocessSafeWatcherTests(SubprocessWatcherMixin,
                                      test_utils.TestCase):
@@ -670,5 +688,25 @@ def setUp(self):
             self.set_event_loop(self.loop)
 
 
+class GenericWatcherTests:
+
+    def test_create_subprocess_fails_with_inactive_watcher(self):
+
+        async def execute():
+            watcher = mock.create_authspec(asyncio.AbstractChildWatcher)
+            watcher.is_active.return_value = False
+            asyncio.set_child_watcher(watcher)
+
+            with self.assertRaises(RuntimeError):
+                await subprocess.create_subprocess_exec(
+                    support.FakePath(sys.executable), '-c', 'pass')
+
+            watcher.add_child_handler.assert_not_called()
+
+        self.assertIsNone(self.loop.run_until_complete(execute()))
+
+
+
+
 if __name__ == '__main__':
     unittest.main()