python/aqmp: add well-known QMP object models

The QMP spec doesn't define very many objects that are iron-clad in
their format, but there are a few. This module makes it trivial to
validate them without relying on an external third-party library.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-id: 20210915162955.333025-15-jsnow@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>

python/qemu/aqmp/models.py

@@ -0,0 +1,133 @@
+"""
+QMP Data Models
+
+This module provides simplistic data classes that represent the few
+structures that the QMP spec mandates; they are used to verify incoming
+data to make sure it conforms to spec.
+"""
+# pylint: disable=too-few-public-methods
+
+from collections import abc
+from typing import (
+    Any,
+    Mapping,
+    Optional,
+    Sequence,
+)
+
+
+class Model:
+    """
+    Abstract data model, representing some QMP object of some kind.
+
+    :param raw: The raw object to be validated.
+    :raise KeyError: If any required fields are absent.
+    :raise TypeError: If any required fields have the wrong type.
+    """
+    def __init__(self, raw: Mapping[str, Any]):
+        self._raw = raw
+
+    def _check_key(self, key: str) -> None:
+        if key not in self._raw:
+            raise KeyError(f"'{self._name}' object requires '{key}' member")
+
+    def _check_value(self, key: str, type_: type, typestr: str) -> None:
+        assert key in self._raw
+        if not isinstance(self._raw[key], type_):
+            raise TypeError(
+                f"'{self._name}' member '{key}' must be a {typestr}"
+            )
+
+    def _check_member(self, key: str, type_: type, typestr: str) -> None:
+        self._check_key(key)
+        self._check_value(key, type_, typestr)
+
+    @property
+    def _name(self) -> str:
+        return type(self).__name__
+
+    def __repr__(self) -> str:
+        return f"{self._name}({self._raw!r})"
+
+
+class Greeting(Model):
+    """
+    Defined in qmp-spec.txt, section 2.2, "Server Greeting".
+
+    :param raw: The raw Greeting object.
+    :raise KeyError: If any required fields are absent.
+    :raise TypeError: If any required fields have the wrong type.
+    """
+    def __init__(self, raw: Mapping[str, Any]):
+        super().__init__(raw)
+        #: 'QMP' member
+        self.QMP: QMPGreeting  # pylint: disable=invalid-name
+
+        self._check_member('QMP', abc.Mapping, "JSON object")
+        self.QMP = QMPGreeting(self._raw['QMP'])
+
+
+class QMPGreeting(Model):
+    """
+    Defined in qmp-spec.txt, section 2.2, "Server Greeting".
+
+    :param raw: The raw QMPGreeting object.
+    :raise KeyError: If any required fields are absent.
+    :raise TypeError: If any required fields have the wrong type.
+    """
+    def __init__(self, raw: Mapping[str, Any]):
+        super().__init__(raw)
+        #: 'version' member
+        self.version: Mapping[str, object]
+        #: 'capabilities' member
+        self.capabilities: Sequence[object]
+
+        self._check_member('version', abc.Mapping, "JSON object")
+        self.version = self._raw['version']
+
+        self._check_member('capabilities', abc.Sequence, "JSON array")
+        self.capabilities = self._raw['capabilities']
+
+
+class ErrorResponse(Model):
+    """
+    Defined in qmp-spec.txt, section 2.4.2, "error".
+
+    :param raw: The raw ErrorResponse object.
+    :raise KeyError: If any required fields are absent.
+    :raise TypeError: If any required fields have the wrong type.
+    """
+    def __init__(self, raw: Mapping[str, Any]):
+        super().__init__(raw)
+        #: 'error' member
+        self.error: ErrorInfo
+        #: 'id' member
+        self.id: Optional[object] = None  # pylint: disable=invalid-name
+
+        self._check_member('error', abc.Mapping, "JSON object")
+        self.error = ErrorInfo(self._raw['error'])
+
+        if 'id' in raw:
+            self.id = raw['id']
+
+
+class ErrorInfo(Model):
+    """
+    Defined in qmp-spec.txt, section 2.4.2, "error".
+
+    :param raw: The raw ErrorInfo object.
+    :raise KeyError: If any required fields are absent.
+    :raise TypeError: If any required fields have the wrong type.
+    """
+    def __init__(self, raw: Mapping[str, Any]):
+        super().__init__(raw)
+        #: 'class' member, with an underscore to avoid conflicts in Python.
+        self.class_: str
+        #: 'desc' member
+        self.desc: str
+
+        self._check_member('class', str, "string")
+        self.class_ = self._raw['class']
+
+        self._check_member('desc', str, "string")
+        self.desc = self._raw['desc']