From bd473aa598c5161521a7018896dc124728214a6c 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 (#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.
---
 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 6dbde2b696a..9c2ba679ce2 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 @@ class BaseSubprocessTransport(transports.SubprocessTransport):
         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 terminate(self):
+            self._check_proc()
+            self._proc.terminate()
 
-    def kill(self):
-        self._check_proc()
-        self._proc.kill()
+        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.send_signal(signal.SIGTERM)
+
+        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 23987c70ca7..54501300a29 100644
--- a/Lib/test/test_asyncio/test_subprocess.py
+++ b/Lib/test/test_asyncio/test_subprocess.py
@@ -864,6 +864,21 @@ class SubprocessMixin:
 
         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 00000000000..c0b4f349fb6
--- /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.