python/aqmp: add AsyncProtocol._readline() method

This is added as a courtesy: many protocols are line-based, including
QMP. Putting it in AsyncProtocol lets us keep the QMP class
implementation just a pinch more abstract.

(And, if we decide to add a QTEST implementation later, it will need
this, too. (Yes, I have a QTEST implementation.))

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-id: 20210915162955.333025-13-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

python/qemu/aqmp/protocol.py

@@ -794,6 +794,35 @@ class AsyncProtocol(Generic[T]):
         self.logger.debug("<-- %s", str(msg))
         return msg
 
+    @upper_half
+    @bottom_half
+    async def _readline(self) -> bytes:
+        """
+        Wait for a newline from the incoming reader.
+
+        This method is provided as a convenience for upper-layer
+        protocols, as many are line-based.
+
+        This method *may* return a sequence of bytes without a trailing
+        newline if EOF occurs, but *some* bytes were received. In this
+        case, the next call will raise `EOFError`. It is assumed that
+        the layer 5 protocol will decide if there is anything meaningful
+        to be done with a partial message.
+
+        :raise OSError: For stream-related errors.
+        :raise EOFError:
+            If the reader stream is at EOF and there are no bytes to return.
+        :return: bytes, including the newline.
+        """
+        assert self._reader is not None
+        msg_bytes = await self._reader.readline()
+
+        if not msg_bytes:
+            if self._reader.at_eof():
+                raise EOFError
+
+        return msg_bytes
+
     @upper_half
     @bottom_half
     async def _do_recv(self) -> T: