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.

Lib/asyncio/subprocess.py

@@ -145,10 +145,11 @@ class Process:
 
     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:
@@ -181,7 +182,7 @@ class Process:
         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()

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