Make DummySock() look more like an actual socket

This forces DummySock() to look like a properly connected socket where
there is a buffer that is read from by the remote, and a buffer that is
written to by the remote.

The local side does the opposite, this way data written by the local
side can be read by the remote without operating on the same buffer.

tests/test_channel.py

@@ -18,7 +18,7 @@ def _makeOneWithMap(self, adj=None):
         map = {}
         inst = self._makeOne(sock, "127.0.0.1", adj, map=map)
         inst.outbuf_lock = DummyLock()
-        return inst, sock, map
+        return inst, sock.local(), map
 
     def test_ctor(self):
         inst, _, map = self._makeOneWithMap()
@@ -218,7 +218,7 @@ def test_write_soon_nonempty_byte(self):
         def send(_):
             return 0
 
-        sock.send = send
+        sock.remote.send = send
 
         wrote = inst.write_soon(b"a")
         self.assertEqual(wrote, 1)
@@ -236,7 +236,7 @@ def test_write_soon_filewrapper(self):
         def send(_):
             return 0
 
-        sock.send = send
+        sock.remote.send = send
 
         outbufs = inst.outbufs
         wrote = inst.write_soon(wrapper)
@@ -270,7 +270,7 @@ def test_write_soon_rotates_outbuf_on_overflow(self):
         def send(_):
             return 0
 
-        sock.send = send
+        sock.remote.send = send
 
         inst.adj.outbuf_high_watermark = 3
         inst.current_outbuf_count = 4
@@ -286,7 +286,7 @@ def test_write_soon_waits_on_backpressure(self):
         def send(_):
             return 0
 
-        sock.send = send
+        sock.remote.send = send
 
         inst.adj.outbuf_high_watermark = 3
         inst.total_outbufs_len = 4
@@ -315,7 +315,7 @@ def send(_):
             inst.connected = False
             raise Exception()
 
-        sock.send = send
+        sock.remote.send = send
 
         inst.adj.outbuf_high_watermark = 3
         inst.total_outbufs_len = 4
@@ -345,7 +345,7 @@ def send(_):
             inst.connected = False
             raise Exception()
 
-        sock.send = send
+        sock.remote.send = send
 
         wrote = inst.write_soon(b"xyz")
         self.assertEqual(wrote, 3)
@@ -376,7 +376,7 @@ def test_handle_write_no_notify_after_flush(self):
         inst.total_outbufs_len = len(inst.outbufs[0])
         inst.adj.send_bytes = 1
         inst.adj.outbuf_high_watermark = 2
-        sock.send = lambda x, do_close=True: False
+        sock.remote.send = lambda x, do_close=True: False
         inst.will_close = False
         inst.last_activity = 0
         result = inst.handle_write()
@@ -400,7 +400,7 @@ def test__flush_some_full_outbuf_socket_returns_nonzero(self):
 
     def test__flush_some_full_outbuf_socket_returns_zero(self):
         inst, sock, map = self._makeOneWithMap()
-        sock.send = lambda x: False
+        sock.remote.send = lambda x: False
         inst.outbufs[0].append(b"abc")
         inst.total_outbufs_len = sum(len(x) for x in inst.outbufs)
         result = inst._flush_some()
@@ -907,7 +907,8 @@ class DummySock:
     closed = False
 
     def __init__(self):
-        self.sent = b""
+        self.local_sent = b""
+        self.remote_sent = b""
 
     def setblocking(self, *arg):
         self.blocking = True
@@ -925,14 +926,44 @@ def close(self):
         self.closed = True
 
     def send(self, data):
-        self.sent += data
+        self.remote_sent += data
         return len(data)
 
     def recv(self, buffer_size):
-        result = self.sent[:buffer_size]
-        self.sent = self.sent[buffer_size:]
+        result = self.local_sent[:buffer_size]
+        self.local_sent = self.local_sent[buffer_size:]
         return result
 
+    def local(self):
+        outer = self
+
+        class LocalDummySock:
+            def send(self, data):
+                outer.local_sent += data
+                return len(data)
+
+            def recv(self, buffer_size):
+                result = outer.remote_sent[:buffer_size]
+                outer.remote_sent = outer.remote_sent[buffer_size:]
+                return result
+
+            def close(self):
+                outer.closed = True
+
+            @property
+            def sent(self):
+                return outer.remote_sent
+
+            @property
+            def closed(self):
+                return outer.closed
+
+            @property
+            def remote(self):
+                return outer
+
+        return LocalDummySock()
+
 
 class DummyLock:
     notified = False