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Diffstat (limited to 'python/qemu/aqmp/message.py')
| -rw-r--r-- | python/qemu/aqmp/message.py | 209 |
1 files changed, 209 insertions, 0 deletions
diff --git a/python/qemu/aqmp/message.py b/python/qemu/aqmp/message.py new file mode 100644 index 0000000000..f76ccc9074 --- /dev/null +++ b/python/qemu/aqmp/message.py @@ -0,0 +1,209 @@ +""" +QMP Message Format + +This module provides the `Message` class, which represents a single QMP +message sent to or from the server. +""" + +import json +from json import JSONDecodeError +from typing import ( + Dict, + Iterator, + Mapping, + MutableMapping, + Optional, + Union, +) + +from .error import ProtocolError + + +class Message(MutableMapping[str, object]): + """ + Represents a single QMP protocol message. + + QMP uses JSON objects as its basic communicative unit; so this + Python object is a :py:obj:`~collections.abc.MutableMapping`. It may + be instantiated from either another mapping (like a `dict`), or from + raw `bytes` that still need to be deserialized. + + Once instantiated, it may be treated like any other MutableMapping:: + + >>> msg = Message(b'{"hello": "world"}') + >>> assert msg['hello'] == 'world' + >>> msg['id'] = 'foobar' + >>> print(msg) + { + "hello": "world", + "id": "foobar" + } + + It can be converted to `bytes`:: + + >>> msg = Message({"hello": "world"}) + >>> print(bytes(msg)) + b'{"hello":"world","id":"foobar"}' + + Or back into a garden-variety `dict`:: + + >>> dict(msg) + {'hello': 'world'} + + + :param value: Initial value, if any. + :param eager: + When `True`, attempt to serialize or deserialize the initial value + immediately, so that conversion exceptions are raised during + the call to ``__init__()``. + """ + # pylint: disable=too-many-ancestors + + def __init__(self, + value: Union[bytes, Mapping[str, object]] = b'{}', *, + eager: bool = True): + self._data: Optional[bytes] = None + self._obj: Optional[Dict[str, object]] = None + + if isinstance(value, bytes): + self._data = value + if eager: + self._obj = self._deserialize(self._data) + else: + self._obj = dict(value) + if eager: + self._data = self._serialize(self._obj) + + # Methods necessary to implement the MutableMapping interface, see: + # https://docs.python.org/3/library/collections.abc.html#collections.abc.MutableMapping + + # We get pop, popitem, clear, update, setdefault, __contains__, + # keys, items, values, get, __eq__ and __ne__ for free. + + def __getitem__(self, key: str) -> object: + return self._object[key] + + def __setitem__(self, key: str, value: object) -> None: + self._object[key] = value + self._data = None + + def __delitem__(self, key: str) -> None: + del self._object[key] + self._data = None + + def __iter__(self) -> Iterator[str]: + return iter(self._object) + + def __len__(self) -> int: + return len(self._object) + + # Dunder methods not related to MutableMapping: + + def __repr__(self) -> str: + if self._obj is not None: + return f"Message({self._object!r})" + return f"Message({bytes(self)!r})" + + def __str__(self) -> str: + """Pretty-printed representation of this QMP message.""" + return json.dumps(self._object, indent=2) + + def __bytes__(self) -> bytes: + """bytes representing this QMP message.""" + if self._data is None: + self._data = self._serialize(self._obj or {}) + return self._data + + # Conversion Methods + + @property + def _object(self) -> Dict[str, object]: + """ + A `dict` representing this QMP message. + + Generated on-demand, if required. This property is private + because it returns an object that could be used to invalidate + the internal state of the `Message` object. + """ + if self._obj is None: + self._obj = self._deserialize(self._data or b'{}') + return self._obj + + @classmethod + def _serialize(cls, value: object) -> bytes: + """ + Serialize a JSON object as `bytes`. + + :raise ValueError: When the object cannot be serialized. + :raise TypeError: When the object cannot be serialized. + + :return: `bytes` ready to be sent over the wire. + """ + return json.dumps(value, separators=(',', ':')).encode('utf-8') + + @classmethod + def _deserialize(cls, data: bytes) -> Dict[str, object]: + """ + Deserialize JSON `bytes` into a native Python `dict`. + + :raise DeserializationError: + If JSON deserialization fails for any reason. + :raise UnexpectedTypeError: + If the data does not represent a JSON object. + + :return: A `dict` representing this QMP message. + """ + try: + obj = json.loads(data) + except JSONDecodeError as err: + emsg = "Failed to deserialize QMP message." + raise DeserializationError(emsg, data) from err + if not isinstance(obj, dict): + raise UnexpectedTypeError( + "QMP message is not a JSON object.", + obj + ) + return obj + + +class DeserializationError(ProtocolError): + """ + A QMP message was not understood as JSON. + + When this Exception is raised, ``__cause__`` will be set to the + `json.JSONDecodeError` Exception, which can be interrogated for + further details. + + :param error_message: Human-readable string describing the error. + :param raw: The raw `bytes` that prompted the failure. + """ + def __init__(self, error_message: str, raw: bytes): + super().__init__(error_message) + #: The raw `bytes` that were not understood as JSON. + self.raw: bytes = raw + + def __str__(self) -> str: + return "\n".join([ + super().__str__(), + f" raw bytes were: {str(self.raw)}", + ]) + + +class UnexpectedTypeError(ProtocolError): + """ + A QMP message was JSON, but not a JSON object. + + :param error_message: Human-readable string describing the error. + :param value: The deserialized JSON value that wasn't an object. + """ + def __init__(self, error_message: str, value: object): + super().__init__(error_message) + #: The JSON value that was expected to be an object. + self.value: object = value + + def __str__(self) -> str: + strval = json.dumps(self.value, indent=2) + return "\n".join([ + super().__str__(), + f" json value was: {strval}", + ]) |