Add read markers

This is the spec for https://github.com/matrix-org/matrix-doc/issues/910

Fixes https://github.com/matrix-org/matrix-doc/issues/910

Some reverse engineering was required to work out the complete details as to how this works. In particular, the 405 for setting account data and the behaviour of m.read.

References:
* 405 for account data: d69decd5c7/synapse/rest/client/v2_alpha/account_data.py (L85-L90)
* m.read behaviour: d69decd5c7/synapse/rest/client/v2_alpha/read_marker.py (L45-L52)

api/client-server/account-data.yaml

@@ -67,6 +67,17 @@ paths:
         200:
           description:
             The account_data was successfully added.
+        405:
+          description:
+            The account data event type cannot be set here. This generally indicates
+            that another API is available for managing the data contained in the event.
+          schema:
+            $ref: "definitions/errors/error.yaml"
+          examples: 
+            application/json: {
+              "errcode": "M_UNKNOWN",
+              "error": "Cannot set m.fully_read through this API. Use /rooms/{roomId}/read_markers"
+            }
       tags:
         - User data
   "/user/{userId}/rooms/{roomId}/account_data/{type}":
@@ -116,5 +127,16 @@ paths:
         200:
           description:
             The account_data was successfully added.
+        405:
+          description:
+            The account data event type cannot be set here. This generally indicates
+            that another API is available for managing the data contained in the event.
+          schema:
+            $ref: "definitions/errors/error.yaml"
+          examples: 
+            application/json: {
+              "errcode": "M_UNKNOWN",
+              "error": "Cannot set m.fully_read through this API. Use /rooms/{roomId}/read_markers"
+            }
       tags:
         - User data

api/client-server/read_markers.yaml

@@ -0,0 +1,79 @@
+# Copyright 2018 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.
+swagger: '2.0'
+info:
+  title: "Matrix Client-Server Read Marker API"
+  version: "1.0.0"
+host: localhost:8008
+schemes:
+  - https
+  - http
+basePath: /_matrix/client/%CLIENT_MAJOR_VERSION%
+consumes:
+  - application/json
+produces:
+  - application/json
+securityDefinitions:
+  $ref: definitions/security.yaml
+paths:
+  "/rooms/{roomId}/read_markers":
+    post:
+      summary: Set the position of the read marker for a room.
+      description: |-
+        Sets the position of the read marker for a given room, and optionally
+        the read receipt's location.
+      operationId: setReadMarker
+      security:
+        - accessToken: []
+      parameters:
+        - in: path
+          type: string
+          name: roomId
+          description: The room ID to set the read marker in for the user.
+          required: true
+          x-example: "!somewhere:domain.com"
+        - in: body
+          name: body
+          description: The read marker and optional read receipt locations.
+          required: true
+          schema:
+            type: object
+            properties:
+              "m.fully_read":
+                type: string
+                description: |-
+                  The event ID the read marker should be located at. The
+                  event MUST belong to the room.
+                example: "$somewhere:domain.com"
+              "m.read":
+                type: string
+                description: |-
+                  The event ID to set the read receipt location at. This is
+                  equivalent to calling ``/receipt/m.read/$elsewhere:domain.com``
+                  and is provided here to save that extra call.
+                example: "$elsewhere:domain.com"
+            required: ['m.fully_read']
+      responses:
+        200:
+          description: |-
+            The read marker, and read receipt if provided, have been updated.
+          schema:
+            type: object
+            properties: {}
+        429:
+          description: This request was rate-limited.
+          schema:
+            "$ref": "definitions/errors/rate_limited.yaml"
+      tags:
+        - Read Markers

event-schemas/examples/m.fully_read

@@ -0,0 +1,8 @@
+{
+  "$ref": "core/event.json",
+  "type": "m.fully_read",
+  "room_id": "!somewhere:domain.com",
+  "content": {
+    "event_id": "$someplace:domain.com"
+  }
+}

event-schemas/schema/m.fully_read

@@ -0,0 +1,29 @@
+{
+    "type": "object",
+    "title": "Read Marker Location Event",
+    "description": "The current location of the user's read marker in a room. This event appears in the user's room account data for the room the marker is applicable for.",
+    "allOf": [{
+        "$ref": "core-event-schema/event.yaml"
+    }],
+    "properties": {
+        "content": {
+            "type": "object",
+            "properties": {
+                "event_id": {
+                    "type": "string",
+                    "description": "The event the user's read marker is located at in the room."
+                }
+            },
+            "required": ["event_id"]
+        },
+        "type": {
+            "type": "string",
+            "enum": ["m.fully_read"]
+        },
+        "room_id": {
+            "type": "string",
+            "description": "The room ID the read marker applies to."
+        }
+    },
+    "required": ["type", "room_id", "content"]
+}

specification/modules/read_markers.rst

@@ -0,0 +1,70 @@
+.. Copyright 2018 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.
+
+Read Markers
+============
+
+.. _module:read-markers:
+
+A "read marker" is a bookmark in the room's history where the user has last
+read a message. Read receipts cover the most recently read message, and have
+slightly different semantics where they are to be sent when the user has most
+likely seen the message, but not necessarily read it. A read marker tracks
+the user's last read message, therefore making the messages between the read
+marker and read receipt "potentially read" and messages before the read marker
+as "read". Messages after the read receipt should be considered as "unread",
+therefore the read marker should always be earlier or equal to the location of
+the read receipt.
+
+Events
+------
+The user's read marker is kept as an event in the room's `account data`_. The
+event may be read to determine the user's current read marker location in the
+room, and just like other account data events the event will be pushed down
+the event stream when updated.
+
+The read marker is kept under an ``m.fully_read`` event. If the event does
+not exist on the user's account data, the read marker should be considered
+to be the user's read receipt location.
+
+{{m_fully_read_event}}
+
+Client behaviour
+----------------
+The client cannot update read markers by directly modifying the ``m.fully_read``
+account data event. Instead, the client must make use of the read markers API
+to change the values.
+
+The read markers API can additionally update the user's read receipt (``m.read``)
+location in the same operation as setting the read marker location. This is
+because read receipts and read markers are commonly updated at the same time,
+and therefore the client might wish to save an extra HTTP call. Providing an
+``m.read`` location performs the same task as a request to ``/receipts/m.read/$event:domain.com``.
+
+{{read_markers_cs_http_api}}
+
+Server behaviour
+----------------
+The server MUST prevent clients from setting ``m.fully_read`` directly in
+room account data. The server must additionally ensure that it treats the
+presence of ``m.read`` in the ``/read_markers`` request the same as how it
+would for a request to ``/receipts/m.read/$event:domain.com``.
+
+Upon updating the ``m.fully_read`` event due to a request to ``/read_markers``,
+the server MUST send the updated account data event through to the client via
+the event stream (eg: ``/sync``), provided any applicable filters are also
+satisfied.
+
+
+.. _`account data`: #client-config

specification/targets.yaml

@@ -45,6 +45,7 @@ groups:  # reusable blobs of files when prefixed with 'group:'
     - modules/voip_events.rst
     - modules/typing_notifications.rst
     - modules/receipts.rst
+    - modules/read_markers.rst
     - modules/presence.rst
     - modules/content_repo.rst
     - modules/send_to_device.rst