GH-100133: fix asyncio subprocess losing stderr and stdout output

Lib/asyncio/base_subprocess.py

@@ -215,9 +215,15 @@ def _process_exited(self, returncode):
             # object. On Python 3.6, it is required to avoid a ResourceWarning.
             self._proc.returncode = returncode
         self._call(self._protocol.process_exited)
-        for p in self._pipes.values():
-            if p is not None:
-                p.pipe.close()
+        # See https://github.com/python/cpython/issues/100133
+        # The pipes should not be closed otherwise some data may be lost.
+        # If the pipe is closed here then _UnixReadPipeTransport will remove the
+        # reader prematurely and the data will be lost, instead of doing that
+        # the pipe will be closed when the process is finished via _pipe_connection_lost
+        # followed by _try_finish.
+        # for p in self._pipes.values():
+        #     if p is not None:
+        #         p.pipe.close()
 
         self._try_finish()
 

Lib/test/test_asyncio/test_subprocess.py

@@ -686,6 +686,23 @@ async def execute():
 
         self.assertIsNone(self.loop.run_until_complete(execute()))
 
+    def test_subprocess_communicate_stdout(self):
+        # See https://github.com/python/cpython/issues/100133
+        async def get_command_stdout(cmd, *args):
+            proc = await asyncio.create_subprocess_exec(
+                cmd, *args, stdout=asyncio.subprocess.PIPE,
+            )
+            stdout, _ = await proc.communicate()
+            return stdout.decode().strip()
+
+        async def main():
+            outputs = [f'foo{i}' for i in range(10)]
+            res = await asyncio.gather(*[get_command_stdout(sys.executable, '-c',
+                                        f'import sys; print({out!r})') for out in outputs])
+            self.assertEqual(res, outputs)
+
+        self.loop.run_until_complete(main())
+
 
 if sys.platform != 'win32':
     # Unix

Misc/NEWS.d/next/Library/2022-12-10-08-36-07.gh-issue-100133.g-zQlp.rst

@@ -0,0 +1 @@
+Fix bug in :mod:`asyncio` where a subprocess would sometimes lose data received from pipe. Patch by Kumar Aditya.