gh-83925: Make asyncio.subprocess communicate similar to non-asyncio (#18650)

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

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``.