From 76a0f34782b5ecc9f48ccab59589ef88a24d51aa Mon Sep 17 00:00:00 2001
From: Kumar Aditya <kumaraditya@python.org>
Date: Mon, 1 Jul 2024 10:17:36 +0530
Subject: [PATCH] gh-87744: fix waitpid race while calling send_signal in
 asyncio (GH-121126)

asyncio earlier relied on subprocess module to send signals to the process, this has some drawbacks one being that subprocess module unnecessarily calls waitpid on child processes and hence it races with asyncio implementation which internally uses child watchers. To mitigate this, now asyncio sends signals directly to the process without going through the subprocess on non windows systems. On Windows it fallbacks to subprocess module handling but on windows there are no child watchers so this issue doesn't exists altogether.
(cherry picked from commit bd473aa598c5161521a7018896dc124728214a6c)

Co-authored-by: Kumar Aditya <kumaraditya@python.org>
---
 Lib/asyncio/base_subprocess.py                | 35 ++++++++++++++-----
 Lib/test/test_asyncio/test_subprocess.py      | 15 ++++++++
 ...4-06-29-05-08-59.gh-issue-87744.rpF6Jw.rst |  1 +
 3 files changed, 42 insertions(+), 9 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2024-06-29-05-08-59.gh-issue-87744.rpF6Jw.rst

diff --git a/Lib/asyncio/base_subprocess.py b/Lib/asyncio/base_subprocess.py
index 6dbde2b696ad1f..9c2ba679ce2bf1 100644
--- a/Lib/asyncio/base_subprocess.py
+++ b/Lib/asyncio/base_subprocess.py
@@ -1,6 +1,9 @@
 import collections
 import subprocess
 import warnings
+import os
+import signal
+import sys
 
 from . import protocols
 from . import transports
@@ -142,17 +145,31 @@ def _check_proc(self):
         if self._proc is None:
             raise ProcessLookupError()
 
-    def send_signal(self, signal):
-        self._check_proc()
-        self._proc.send_signal(signal)
+    if sys.platform == 'win32':
+        def send_signal(self, signal):
+            self._check_proc()
+            self._proc.send_signal(signal)
+
+        def terminate(self):
+            self._check_proc()
+            self._proc.terminate()
+
+        def kill(self):
+            self._check_proc()
+            self._proc.kill()
+    else:
+        def send_signal(self, signal):
+            self._check_proc()
+            try:
+                os.kill(self._proc.pid, signal)
+            except ProcessLookupError:
+                pass
 
-    def terminate(self):
-        self._check_proc()
-        self._proc.terminate()
+        def terminate(self):
+            self.send_signal(signal.SIGTERM)
 
-    def kill(self):
-        self._check_proc()
-        self._proc.kill()
+        def kill(self):
+            self.send_signal(signal.SIGKILL)
 
     async def _connect_pipes(self, waiter):
         try:
diff --git a/Lib/test/test_asyncio/test_subprocess.py b/Lib/test/test_asyncio/test_subprocess.py
index cf1a1985338e40..c822d8045b0c0f 100644
--- a/Lib/test/test_asyncio/test_subprocess.py
+++ b/Lib/test/test_asyncio/test_subprocess.py
@@ -873,6 +873,21 @@ async def main():
 
         self.loop.run_until_complete(main())
 
+    @unittest.skipIf(sys.platform != 'linux', "Linux only")
+    def test_subprocess_send_signal_race(self):
+        # See https://github.com/python/cpython/issues/87744
+        async def main():
+            for _ in range(10):
+                proc = await asyncio.create_subprocess_exec('sleep', '0.1')
+                await asyncio.sleep(0.1)
+                try:
+                    proc.send_signal(signal.SIGUSR1)
+                except ProcessLookupError:
+                    pass
+                self.assertNotEqual(await proc.wait(), 255)
+
+        self.loop.run_until_complete(main())
+
 
 if sys.platform != 'win32':
     # Unix
diff --git a/Misc/NEWS.d/next/Library/2024-06-29-05-08-59.gh-issue-87744.rpF6Jw.rst b/Misc/NEWS.d/next/Library/2024-06-29-05-08-59.gh-issue-87744.rpF6Jw.rst
new file mode 100644
index 00000000000000..c0b4f349fb6dac
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-06-29-05-08-59.gh-issue-87744.rpF6Jw.rst
@@ -0,0 +1 @@
+Fix waitpid race while calling :meth:`~asyncio.subprocess.Process.send_signal` in asyncio. Patch by Kumar Aditya.