[Python-checkins] GH-98023: Change default child watcher to PidfdChildWatcher on supported systems (#98024)

[gvanrossum]

https://github.com/python/cpython/commit/8ba9378b168ad330c158a001afca03d6c028d39b
commit: 8ba9378b168ad330c158a001afca03d6c028d39b
branch: main
author: Kumar Aditya <59607654+kumaraditya303 at users.noreply.github.com>
committer: gvanrossum <gvanrossum at gmail.com>
date: 2022-10-07T17:29:09-07:00
summary:

GH-98023: Change default child watcher to PidfdChildWatcher on supported systems (#98024)

files:
A Misc/NEWS.d/next/Library/2022-10-07-09-52-37.gh-issue-98023.aliEcl.rst
M Lib/asyncio/unix_events.py
M Lib/test/test_asyncio/test_unix_events.py

diff --git a/Lib/asyncio/unix_events.py b/Lib/asyncio/unix_events.py
index 0f67b4d469f2..7fc75cd17ef7 100644
--- a/Lib/asyncio/unix_events.py
+++ b/Lib/asyncio/unix_events.py
@@ -1403,6 +1403,17 @@ def _do_waitpid(self, loop, expected_pid, callback, args):
 
         self._threads.pop(expected_pid)
 
+def can_use_pidfd():
+    if not hasattr(os, 'pidfd_open'):
+        return False
+    try:
+        pid = os.getpid()
+        os.close(os.pidfd_open(pid, 0))
+    except OSError:
+        # blocked by security policy like SECCOMP
+        return False
+    return True
+
 
 class _UnixDefaultEventLoopPolicy(events.BaseDefaultEventLoopPolicy):
     """UNIX event loop policy with a watcher for child processes."""
@@ -1415,7 +1426,10 @@ def __init__(self):
     def _init_watcher(self):
         with events._lock:
             if self._watcher is None:  # pragma: no branch
-                self._watcher = ThreadedChildWatcher()
+                if can_use_pidfd():
+                    self._watcher = PidfdChildWatcher()
+                else:
+                    self._watcher = ThreadedChildWatcher()
                 if threading.current_thread() is threading.main_thread():
                     self._watcher.attach_loop(self._local._loop)
 
diff --git a/Lib/test/test_asyncio/test_unix_events.py b/Lib/test/test_asyncio/test_unix_events.py
index 5bad21ecbae4..03fb5e649d8e 100644
--- a/Lib/test/test_asyncio/test_unix_events.py
+++ b/Lib/test/test_asyncio/test_unix_events.py
@@ -1702,7 +1702,8 @@ def create_policy(self):
     def test_get_default_child_watcher(self):
         policy = self.create_policy()
         self.assertIsNone(policy._watcher)
-
+        unix_events.can_use_pidfd = mock.Mock()
+        unix_events.can_use_pidfd.return_value = False
         watcher = policy.get_child_watcher()
         self.assertIsInstance(watcher, asyncio.ThreadedChildWatcher)
 
@@ -1710,6 +1711,17 @@ def test_get_default_child_watcher(self):
 
         self.assertIs(watcher, policy.get_child_watcher())
 
+        policy = self.create_policy()
+        self.assertIsNone(policy._watcher)
+        unix_events.can_use_pidfd = mock.Mock()
+        unix_events.can_use_pidfd.return_value = True
+        watcher = policy.get_child_watcher()
+        self.assertIsInstance(watcher, asyncio.PidfdChildWatcher)
+
+        self.assertIs(policy._watcher, watcher)
+
+        self.assertIs(watcher, policy.get_child_watcher())
+
     def test_get_child_watcher_after_set(self):
         policy = self.create_policy()
         watcher = asyncio.FastChildWatcher()
diff --git a/Misc/NEWS.d/next/Library/2022-10-07-09-52-37.gh-issue-98023.aliEcl.rst b/Misc/NEWS.d/next/Library/2022-10-07-09-52-37.gh-issue-98023.aliEcl.rst
new file mode 100644
index 000000000000..1bfd68d4ac7c
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2022-10-07-09-52-37.gh-issue-98023.aliEcl.rst
@@ -0,0 +1 @@
+Change default child watcher to :class:`~asyncio.PidfdChildWatcher` on Linux systems which supports it. Patch by Kumar Aditya.