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

@@ -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:

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

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.