wireprotov2: establish dedicated classes for input and output streams

wireprotov2: establish dedicated classes for input and output streams

Streams are unidirectional. As part of implementing encoding/decoding
support, it became clear that it didn't make sense for a generic
"stream" class to hold functionality related to both encoding and
decoding. So we create new classes to represent the flavor of
stream.

Differential Revision: https://phab.mercurial-scm.org/D4920

--- a/mercurial/wireprotoframing.py	Thu Oct 04 17:17:57 2018 -0700
+++ b/mercurial/wireprotoframing.py	Thu Oct 04 17:39:16 2018 -0700
@@ -668,6 +668,9 @@
         return makeframe(requestid, self.streamid, streamflags, typeid, flags,
                          payload)
 
+class inputstream(stream):
+    """Represents a stream used for receiving data."""
+
     def setdecoder(self, name, extraobjs):
         """Set the decoder for this stream.
 
@@ -675,6 +678,9 @@
         decoded from the stream encoding settings frame payloads.
         """
 
+class outputstream(stream):
+    """Represents a stream used for sending data."""
+
 def ensureserverstream(stream):
     if stream.streamid % 2:
         raise error.ProgrammingError('server should only write to even '
@@ -799,7 +805,7 @@
                     _('received frame on unknown inactive stream without '
                       'beginning of stream flag set'))
 
-            self._incomingstreams[frame.streamid] = stream(frame.streamid)
+            self._incomingstreams[frame.streamid] = inputstream(frame.streamid)
 
         if frame.streamflags & STREAM_FLAG_ENCODING_APPLIED:
             # TODO handle decoding frames
@@ -1012,7 +1018,7 @@
         streamid = self._nextoutgoingstreamid
         self._nextoutgoingstreamid += 2
 
-        s = stream(streamid)
+        s = outputstream(streamid)
         self._outgoingstreams[streamid] = s
 
         return s
@@ -1372,7 +1378,7 @@
 
         self._nextrequestid = 1
         # We only support a single outgoing stream for now.
-        self._outgoingstream = stream(1)
+        self._outgoingstream = outputstream(1)
         self._pendingrequests = collections.deque()
         self._activerequests = {}
         self._incomingstreams = {}
@@ -1485,7 +1491,8 @@
                                  'without beginning of stream flag set'),
                 }
 
-            self._incomingstreams[frame.streamid] = stream(frame.streamid)
+            self._incomingstreams[frame.streamid] = inputstream(
+                frame.streamid)
 
         if frame.streamflags & STREAM_FLAG_ENCODING_APPLIED:
             raise error.ProgrammingError('support for decoding stream '