Fixed receive_some ignoring max_bytes when reading from buffer

Fixes nedbat#31.

anyio/_networking.py

@@ -222,7 +222,7 @@ def buffered_data(self) -> bytes:
 
     async def receive_some(self, max_bytes: Optional[int]) -> bytes:
         if self._buffer:
-            data, self._buffer = self._buffer, b''
+            data, self._buffer = self._buffer[:max_bytes], self._buffer[max_bytes:]
             return data
 
         return await self._socket.recv(max_bytes)

tests/test_networking.py

@@ -30,6 +30,24 @@ async def server():
 
         assert response == b'halb'
 
+    @pytest.mark.anyio
+    async def test_receive_some_from_cache(self):
+        async def server():
+            async with await stream_server.accept() as stream:
+                await stream.receive_until(b'a', 10)
+                request = await stream.receive_some(1)
+                await stream.send_all(request + b'\n')
+
+        received = None
+        async with create_task_group() as tg:
+            async with await create_tcp_server(interface='localhost') as stream_server:
+                await tg.spawn(server)
+                async with await connect_tcp('localhost', stream_server.port) as client:
+                    await client.send_all(b'abc')
+                    received = await client.receive_until(b'\n', 3)
+
+        assert received == b'b'
+
     @pytest.mark.parametrize('method_name, params', [
         ('receive_until', [b'\n', 100]),
         ('receive_exactly', [5])