gh-83925: make asyncio.subprocess communicate similar to non-asyncio

Doc/library/asyncio-subprocess.rst

@@ -207,8 +207,9 @@ their completion.
       Interact with process:
 
       1. send data to *stdin* (if *input* is not ``None``);
-      2. read data from *stdout* and *stderr*, until EOF is reached;
-      3. wait for process to terminate.
+      2. closes *stdin*;
+      3. read data from *stdout* and *stderr*, until EOF is reached;
+      4. wait for process to terminate.
 
       The optional *input* argument is the data (:class:`bytes` object)
       that will be sent to the child process.
@@ -229,6 +230,10 @@ their completion.
       Note, that the data read is buffered in memory, so do not use
       this method if the data size is large or unlimited.
 
+      .. versionchanged:: 3.12
+
+         *stdin* gets closed when `input=None` too.
+
    .. method:: send_signal(signal)
 
       Sends the signal *signal* to the child process.

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 the subprocess's stdin even when called with ``input=None``.