From 5d1013256c133b61587b6a80a0f9d509ac11d123 Mon Sep 17 00:00:00 2001
From: Chris Jerdonek <chris.jerdonek@gmail.com>
Date: Sat, 16 May 2020 15:57:27 -0700
Subject: [PATCH 1/5] bpo-38323: Fix rare MultiLoopChildWatcher hangs.
This changes asyncio.MultiLoopChildWatcher's attach_loop() method
to call loop.add_signal_handler() instead of calling only
signal.signal(). This should eliminate some rare hangs since
loop.add_signal_handler() calls signal.set_wakeup_fd(). Without
this, the main thread sometimes wasn't getting awakened if a
signal occurred during an await.
---
Doc/library/asyncio-eventloop.rst | 4 ++-
Doc/library/asyncio-policy.rst | 13 ++++++-
Lib/asyncio/unix_events.py | 34 ++++++++++++++-----
Lib/test/test_asyncio/test_subprocess.py | 3 +-
.../2020-05-16-17-50-10.bpo-38323.Ar35np.rst | 2 ++
5 files changed, 44 insertions(+), 12 deletions(-)
create mode 100644 Misc/NEWS.d/next/Library/2020-05-16-17-50-10.bpo-38323.Ar35np.rst
--- Python-3.9.2/Lib/asyncio/unix_events.py.orig 2021-02-19 13:31:44.000000000 +0000
+++ Python-3.9.2/Lib/asyncio/unix_events.py 2021-02-27 22:27:23.974509830 +0000
@@ -78,6 +78,8 @@
def add_signal_handler(self, sig, callback, *args):
"""Add a handler for a signal. UNIX only.
+ This method can only be called from the main thread.
+
Raise ValueError if the signal number is invalid or uncatchable.
Raise RuntimeError if there is a problem setting up the handler.
"""
@@ -1234,10 +1236,15 @@
return
handler = signal.getsignal(signal.SIGCHLD)
- if handler != self._sig_chld:
+ # add_signal_handler() sets the handler to _sighandler_noop.
+ if handler != _sighandler_noop:
logger.warning("SIGCHLD handler was changed by outside code")
else:
+ loop = self._loop
+ # This clears the wakeup file descriptor if necessary.
+ loop.remove_signal_handler(signal.SIGCHLD)
signal.signal(signal.SIGCHLD, self._saved_sighandler)
+
self._saved_sighandler = None
def __enter__(self):
@@ -1265,6 +1272,11 @@
# 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 loop is None or self._saved_sighandler is not None:
+ return
+
+ self._loop = loop
+ self._saved_sighandler = signal.getsignal(signal.SIGCHLD)
if self._saved_sighandler is not None:
return
@@ -1274,8 +1286,14 @@
"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)
+ if self._callbacks:
+ warnings.warn(
+ 'A loop is being detached '
+ 'from a child watcher with pending handlers',
+ RuntimeWarning)
+
+ # This also sets up the wakeup file descriptor.
+ loop.add_signal_handler(signal.SIGCHLD, self._sig_chld)
def _do_waitpid_all(self):
for pid in list(self._callbacks):
@@ -1318,7 +1336,7 @@
expected_pid, returncode)
loop.call_soon_threadsafe(callback, pid, returncode, *args)
- def _sig_chld(self, signum, frame):
+ def _sig_chld(self, *args):
try:
self._do_waitpid_all()
except (SystemExit, KeyboardInterrupt):
diff --git a/Lib/test/test_asyncio/test_subprocess.py b/Lib/test/test_asyncio/test_subprocess.py
index 6657a88e657c2..b11a31a34a2c6 100644
--- a/Lib/test/test_asyncio/test_subprocess.py
+++ b/Lib/test/test_asyncio/test_subprocess.py
@@ -672,12 +672,13 @@ def setUp(self):
policy.set_child_watcher(watcher)
def tearDown(self):
- super().tearDown()
policy = asyncio.get_event_loop_policy()
watcher = policy.get_child_watcher()
policy.set_child_watcher(None)
watcher.attach_loop(None)
watcher.close()
+ # Since setUp() does super().setUp() first, do tearDown() last.
+ super().tearDown()
class SubprocessThreadedWatcherTests(SubprocessWatcherMixin,
test_utils.TestCase):
diff --git a/Misc/NEWS.d/next/Library/2020-05-16-17-50-10.bpo-38323.Ar35np.rst b/Misc/NEWS.d/next/Library/2020-05-16-17-50-10.bpo-38323.Ar35np.rst
new file mode 100644
index 0000000000000..556e08c69d7a5
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2020-05-16-17-50-10.bpo-38323.Ar35np.rst
@@ -0,0 +1,2 @@
+Fix rare cases with ``MultiLoopChildWatcher`` where the event loop can
+fail to awaken in response to a :py:data:`SIGCHLD` signal.
From 9618884446dc4a72e401b0f05b2992e34e39d700 Mon Sep 17 00:00:00 2001
From: Chris Jerdonek <chris.jerdonek@gmail.com>
Date: Sat, 16 May 2020 18:49:59 -0700
Subject: [PATCH 2/5] Add docstring.
---
Lib/asyncio/unix_events.py | 5 +++++
1 file changed, 5 insertions(+)
diff --git a/Lib/asyncio/unix_events.py b/Lib/asyncio/unix_events.py
index d2a32cb879b6b..17614c23c984c 100644
--- a/Lib/asyncio/unix_events.py
+++ b/Lib/asyncio/unix_events.py
@@ -1266,6 +1266,11 @@ def remove_child_handler(self, pid):
return False
def attach_loop(self, loop):
+ """
+ This registers the SIGCHLD signal handler.
+
+ This method can only be called from the main thread.
+ """
# 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.
From 4d4c147b9bfe4ce7bb51aa4745ead8a422e98c14 Mon Sep 17 00:00:00 2001
From: Chris Jerdonek <chris.jerdonek@gmail.com>
Date: Fri, 16 Oct 2020 16:37:11 -0700
Subject: [PATCH 3/5] Address a couple review comments.
---
Doc/library/asyncio-policy.rst | 2 +-
.../next/Library/2020-05-16-17-50-10.bpo-38323.Ar35np.rst | 4 ++--
2 files changed, 3 insertions(+), 3 deletions(-)
diff --git a/Misc/NEWS.d/next/Library/2020-05-16-17-50-10.bpo-38323.Ar35np.rst b/Misc/NEWS.d/next/Library/2020-05-16-17-50-10.bpo-38323.Ar35np.rst
index 556e08c69d7a5..e9401d6a2e486 100644
--- a/Misc/NEWS.d/next/Library/2020-05-16-17-50-10.bpo-38323.Ar35np.rst
+++ b/Misc/NEWS.d/next/Library/2020-05-16-17-50-10.bpo-38323.Ar35np.rst
@@ -1,2 +1,2 @@
-Fix rare cases with ``MultiLoopChildWatcher`` where the event loop can
-fail to awaken in response to a :py:data:`SIGCHLD` signal.
+Fix rare cases with :class:`asyncio.MultiLoopChildWatcher` where the event
+loop can fail to awaken in response to a :py:data:`SIGCHLD` signal.
From 14f6cfc20e77a349a22ced05352afd3ee200b403 Mon Sep 17 00:00:00 2001
From: Chris Jerdonek <chris.jerdonek@gmail.com>
Date: Fri, 16 Oct 2020 16:46:49 -0700
Subject: [PATCH 4/5] Revert tearDown() change.
---
Lib/test/test_asyncio/test_subprocess.py | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)
diff --git a/Lib/test/test_asyncio/test_subprocess.py b/Lib/test/test_asyncio/test_subprocess.py
index b11a31a34a2c6..6657a88e657c2 100644
--- a/Lib/test/test_asyncio/test_subprocess.py
+++ b/Lib/test/test_asyncio/test_subprocess.py
@@ -672,13 +672,12 @@ def setUp(self):
policy.set_child_watcher(watcher)
def tearDown(self):
+ super().tearDown()
policy = asyncio.get_event_loop_policy()
watcher = policy.get_child_watcher()
policy.set_child_watcher(None)
watcher.attach_loop(None)
watcher.close()
- # Since setUp() does super().setUp() first, do tearDown() last.
- super().tearDown()
class SubprocessThreadedWatcherTests(SubprocessWatcherMixin,
test_utils.TestCase):