Collect room-version variations into one place (#4969)

Collect all the things that make room-versions different to one another into
one place, so that it's easier to define new room versions.

synapse/api/room_versions.py

@@ -0,0 +1,91 @@
+# -*- coding: utf-8 -*-
+# Copyright 2019 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+import attr
+
+
+class EventFormatVersions(object):
+    """This is an internal enum for tracking the version of the event format,
+    independently from the room version.
+    """
+    V1 = 1   # $id:server format
+    V2 = 2   # MSC1659-style $hash format: introduced for room v3
+
+
+KNOWN_EVENT_FORMAT_VERSIONS = {
+    EventFormatVersions.V1,
+    EventFormatVersions.V2,
+}
+
+
+class StateResolutionVersions(object):
+    """Enum to identify the state resolution algorithms"""
+    V1 = 1   # room v1 state res
+    V2 = 2   # MSC1442 state res: room v2 and later
+
+
+class RoomDisposition(object):
+    STABLE = "stable"
+    UNSTABLE = "unstable"
+
+
+@attr.s(slots=True, frozen=True)
+class RoomVersion(object):
+    """An object which describes the unique attributes of a room version."""
+
+    identifier = attr.ib()      # str; the identifier for this version
+    disposition = attr.ib()     # str; one of the RoomDispositions
+    event_format = attr.ib()    # int; one of the EventFormatVersions
+    state_res = attr.ib()       # int; one of the StateResolutionVersions
+
+
+class RoomVersions(object):
+    V1 = RoomVersion(
+        "1",
+        RoomDisposition.STABLE,
+        EventFormatVersions.V1,
+        StateResolutionVersions.V1,
+    )
+    STATE_V2_TEST = RoomVersion(
+        "state-v2-test",
+        RoomDisposition.UNSTABLE,
+        EventFormatVersions.V1,
+        StateResolutionVersions.V2,
+    )
+    V2 = RoomVersion(
+        "2",
+        RoomDisposition.STABLE,
+        EventFormatVersions.V1,
+        StateResolutionVersions.V2,
+    )
+    V3 = RoomVersion(
+        "3",
+        RoomDisposition.STABLE,
+        EventFormatVersions.V2,
+        StateResolutionVersions.V2,
+    )
+
+
+# the version we will give rooms which are created on this server
+DEFAULT_ROOM_VERSION = RoomVersions.V1
+
+
+KNOWN_ROOM_VERSIONS = {
+    v.identifier: v for v in (
+        RoomVersions.V1,
+        RoomVersions.V2,
+        RoomVersions.V3,
+        RoomVersions.STATE_V2_TEST,
+    )
+}   # type: dict[str, RoomVersion]