Issue #25441: asyncio: Raise error from drain() when socket is closed.

Lib/test/test_asyncio/test_streams.py

@@ -2,8 +2,10 @@
 
 import gc
 import os
+import queue
 import socket
 import sys
+import threading
 import unittest
 from unittest import mock
 try:
@@ -632,6 +634,47 @@ os.close(fd)
         protocol = asyncio.StreamReaderProtocol(reader)
         self.assertIs(protocol._loop, self.loop)
 
+    def test_drain_raises(self):
+        # See http://bugs.python.org/issue25441
+
+        # This test should not use asyncio for the mock server; the
+        # whole point of the test is to test for a bug in drain()
+        # where it never gives up the event loop but the socket is
+        # closed on the  server side.
+
+        q = queue.Queue()
+
+        def server():
+            # Runs in a separate thread.
+            sock = socket.socket()
+            sock.bind(('localhost', 0))
+            sock.listen(1)
+            addr = sock.getsockname()
+            q.put(addr)
+            clt, _ = sock.accept()
+            clt.close()
+        
+        @asyncio.coroutine
+        def client(host, port):
+            reader, writer = yield from asyncio.open_connection(host, port, loop=self.loop)
+            while True:
+                writer.write(b"foo\n")
+                yield from writer.drain()
+
+        # Start the server thread and wait for it to be listening.
+        thread = threading.Thread(target=server)
+        thread.setDaemon(True)
+        thread.start()
+        addr = q.get()
+
+        # Should not be stuck in an infinite loop.
+        with self.assertRaises((ConnectionResetError, BrokenPipeError)):
+            self.loop.run_until_complete(client(*addr))
+
+        # Clean up the thread.  (Only on success; on failure, it may
+        # be stuck in accept().)
+        thread.join()
+
     def test___repr__(self):
         stream = asyncio.StreamReader(loop=self.loop)
         self.assertEqual("<StreamReader>", repr(stream))