asyncio, tulip issue 193: Convert StreamWriter.drain() to a classic coroutine

Replace also _make_drain_waiter() function with a classic _drain_helper()
coroutine.

Lib/asyncio/streams.py

@@ -141,15 +141,14 @@ class FlowControlMixin(protocols.Protocol):
     resume_reading() and connection_lost().  If the subclass overrides
     these it must call the super methods.
 
-    StreamWriter.drain() must check for error conditions and then call
-    _make_drain_waiter(), which will return either () or a Future
-    depending on the paused state.
+    StreamWriter.drain() must wait for _drain_helper() coroutine.
     """
 
     def __init__(self, loop=None):
         self._loop = loop  # May be None; we may never need it.
         self._paused = False
         self._drain_waiter = None
+        self._connection_lost = False
 
     def pause_writing(self):
         assert not self._paused
@@ -170,6 +169,7 @@ class FlowControlMixin(protocols.Protocol):
                 waiter.set_result(None)
 
     def connection_lost(self, exc):
+        self._connection_lost = True
         # Wake up the writer if currently paused.
         if not self._paused:
             return
@@ -184,14 +184,17 @@ class FlowControlMixin(protocols.Protocol):
         else:
             waiter.set_exception(exc)
 
-    def _make_drain_waiter(self):
+    @coroutine
+    def _drain_helper(self):
+        if self._connection_lost:
+            raise ConnectionResetError('Connection lost')
         if not self._paused:
-            return ()
+            return
         waiter = self._drain_waiter
         assert waiter is None or waiter.cancelled()
         waiter = futures.Future(loop=self._loop)
         self._drain_waiter = waiter
-        return waiter
+        yield from waiter
 
 
 class StreamReaderProtocol(FlowControlMixin, protocols.Protocol):
@@ -247,6 +250,8 @@ class StreamWriter:
     def __init__(self, transport, protocol, reader, loop):
         self._transport = transport
         self._protocol = protocol
+        # drain() expects that the reader has a exception() method
+        assert reader is None or isinstance(reader, StreamReader)
         self._reader = reader
         self._loop = loop
 
@@ -278,26 +283,20 @@ class StreamWriter:
     def get_extra_info(self, name, default=None):
         return self._transport.get_extra_info(name, default)
 
+    @coroutine
     def drain(self):
-        """This method has an unusual return value.
+        """Flush the write buffer.
 
         The intended use is to write
 
           w.write(data)
           yield from w.drain()
-
-        When there's nothing to wait for, drain() returns (), and the
-        yield-from continues immediately.  When the transport buffer
-        is full (the protocol is paused), drain() creates and returns
-        a Future and the yield-from will block until that Future is
-        completed, which will happen when the buffer is (partially)
-        drained and the protocol is resumed.
         """
-        if self._reader is not None and self._reader._exception is not None:
-            raise self._reader._exception
-        if self._transport._conn_lost:  # Uses private variable.
-            raise ConnectionResetError('Connection lost')
-        return self._protocol._make_drain_waiter()
+        if self._reader is not None:
+            exc = self._reader.exception()
+            if exc is not None:
+                raise exc
+        yield from self._protocol._drain_helper()
 
 
 class StreamReader: