bpo-39744: make asyncio.subprocess communicate similar to non-asyncio one

subprocess's communicate(None) closes stdin of the child process, after
sending no (extra) data. Make asyncio variant do the same.
This fixes issues with processes that waits for EOF on stdin before
continuing.

Author: marmarek

Lib/asyncio/subprocess.py

@@ -144,10 +144,11 @@ def kill(self):
 
     async def _feed_stdin(self, input):
         debug = self._loop.get_debug()
-        self.stdin.write(input)
-        if debug:
-            logger.debug(
-                '%r communicate: feed stdin (%s bytes)', self, len(input))
+        if input is not None:
+            self.stdin.write(input)
+            if debug:
+                logger.debug(
+                    '%r communicate: feed stdin (%s bytes)', self, len(input))
         try:
             await self.stdin.drain()
         except (BrokenPipeError, ConnectionResetError) as exc:
@@ -180,7 +181,7 @@ async def _read_stream(self, fd):
         return output
 
     async def communicate(self, input=None):
-        if input is not None:
+        if self.stdin is not None:
             stdin = self._feed_stdin(input)
         else:
             stdin = self._noop()

Lib/test/test_asyncio/test_subprocess.py

@@ -151,6 +151,24 @@ async def run(data):
         self.assertEqual(exitcode, 0)
         self.assertEqual(stdout, b'some data')
 
+    def test_communicate_none_input(self):
+        args = PROGRAM_CAT
+
+        async def run():
+            proc = await asyncio.create_subprocess_exec(
+                *args,
+                stdin=subprocess.PIPE,
+                stdout=subprocess.PIPE,
+            )
+            stdout, stderr = await proc.communicate()
+            return proc.returncode, stdout
+
+        task = run()
+        task = asyncio.wait_for(task, support.LONG_TIMEOUT)
+        exitcode, stdout = self.loop.run_until_complete(task)
+        self.assertEqual(exitcode, 0)
+        self.assertEqual(stdout, b'')
+
     def test_shell(self):
         proc = self.loop.run_until_complete(
             asyncio.create_subprocess_shell('exit 7')

Misc/NEWS.d/next/Library/2020-02-25-00-43-22.bpo-39744.hgK689.rst

@@ -0,0 +1 @@
+Make func:``asyncio.subprocess.Process.communicate`` close subprocess's stdin even when called with input=None