Clarifications to AS spec, including MSC3905 (#1305)

Primarily this is the spec for MSC3905, but I've also taken the opportunity to clean up the section a bit and move the definition out to a .yaml file.

changelogs/application_service/newsfragments/1305.clarification

@@ -0,0 +1 @@
+Clarify that application services can only register an interest in local users, as per [MSC3905](https://github.com/matrix-org/matrix-spec-proposals/issues/3905).

content/application-service-api.md

@@ -37,14 +37,8 @@ registration for suspicious regex strings.
 {{% /boxes/note %}}
 
 Application services register "namespaces" of user IDs, room aliases and
-room IDs. These namespaces are represented as regular expressions. An
-application service is said to be "interested" in a given event if one
-of the IDs in the event match the regular expression provided by the
-application service, such as the room having an alias or ID in the
-relevant namespaces. Similarly, the application service is said to be
-interested in a given event if one of the application service's
-namespaced users is the target of the event, or is a joined member of
-the room where the event occurred.
+room IDs. An application service is said to be "interested" in a given event
+if it matches any of the namespaces.
 
 An application service can also state whether they should be the only
 ones who can manage a specified namespace. This is referred to as an
@@ -53,28 +47,12 @@ application services from creating/deleting entities in that namespace.
 Typically, exclusive namespaces are used when the rooms represent real
 rooms on another service (e.g. IRC). Non-exclusive namespaces are used
 when the application service is merely augmenting the room itself (e.g.
-providing logging or searching facilities). Namespaces are represented
-by POSIX extended regular expressions and look like:
+providing logging or searching facilities).
 
-    users:
-      - exclusive: true
-        regex: "@_irc_bridge_.*"
-
-Application services may define the following namespaces (with none
-being explicitly required):
-
-| Name     | Description                                                |
-|----------|------------------------------------------------------------|
-| users    | Events which are sent from certain users.                  |
-| aliases  | Events which are sent in rooms with certain room aliases.  |
-| rooms    | Events which are sent in rooms with certain room IDs.      |
-
-Each individual namespace MUST declare the following fields:
+The registration is represented by a series of key-value pairs, which
+is normally encoded as an object in a YAML file. It has the following structure:
 
-| Name       | Description                                                                                                                        |
-|------------|------------------------------------------------------------------------------------------------------------------------------------|
-| exclusive  | **Required** A true or false value stating whether this application service has exclusive access to events within this namespace.  |
-| regex      | **Required** A regular expression defining which values this namespace includes.                                                   |
+{{% definition path="api/application-service/definitions/registration" %}}
 
 Exclusive user and alias namespaces should begin with an underscore
 after the sigil to avoid collisions with other users on the homeserver.
@@ -83,25 +61,10 @@ they represent in the reserved namespace. For example, `@_irc_.*` would
 be a good namespace to register for an application service which deals
 with IRC.
 
-The registration is represented by a series of key-value pairs, which
-this specification will present as YAML. See below for the possible
-options along with their explanation:
-
-
-| Name              | Description                                                                                                                                   |
-|-------------------|-----------------------------------------------------------------------------------------------------------------------------------------------|
-| id                | **Required** A unique, user-defined ID of the application service which will never change.                                                    |
-| url               | **Required** The URL for the application service. May include a path after the domain name. Optionally set to null if no traffic is required. |
-| as_token          | **Required** A unique token for application services to use to authenticate requests to Homeservers.                                          |
-| hs_token          | **Required** A unique token for Homeservers to use to authenticate requests to application services.                                          |
-| sender_localpart  | **Required** The localpart of the user associated with the application service.                                                               |
-| namespaces        | **Required** A list of `users`, `aliases` and `rooms` namespaces that the application service controls.                                       |
-| rate_limited      | Whether requests from masqueraded users are rate-limited. The sender is excluded.                                                             |
-| protocols         | The external protocols which the application service provides (e.g. IRC).                                                                     |
-
 An example registration file for an IRC-bridging application service is
 below:
 
+```yaml
 id: "IRC Bridge"
 url: "http://127.0.0.1:1234"
 as_token: "30c05ae90a248a4188e620216fa72e349803310ec83e2a77b34fe90be6081f46"
@@ -115,6 +78,20 @@ below:
     - exclusive: false
       regex: "#_irc_bridge_.*"
   rooms: []
+```
+
+{{% boxes/note %}}
+For the `users` namespace, application services can only register interest in
+*local* users (i.e., users whose IDs end with the `server_name` of the local
+homeserver). Events affecting users on other homeservers are not sent to an application
+service, even if the user happens to match the one of the `users` namespaces (unless,
+of course, the event affects a room that the application service is interested in
+for another room - for example, because there is another user in the room that the
+application service is interested in).
+
+For the `rooms` and `aliases` namespaces, all events in a matching room will be
+sent to the application service.
+{{% /boxes/note %}}
 
 {{% boxes/warning %}}
 If the homeserver in question has multiple application services, each

data/api/application-service/definitions/namespace_list.yaml

@@ -0,0 +1,28 @@
+# Copyright 2022 The Matrix.org Foundation C.I.C
+#
+# 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.
+
+type: array
+items:
+  type: object
+  title: Namespace
+  properties:
+    regex:
+      type: string
+      description: A POSIX regular expression defining which values this namespace includes.
+    exclusive:
+      type: boolean
+      description: A true or false value stating whether this application service has exclusive access to events within this namespace.
+  required:
+    - regex
+    - exclusive

data/api/application-service/definitions/registration.yaml

@@ -0,0 +1,74 @@
+# Copyright 2022 The Matrix.org Foundation C.I.C
+#
+# 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.
+
+type: object
+title: Registration
+properties:
+  id:
+    type: string
+    description: A unique, user-defined ID of the application service which will never change.
+  url:
+    type: string
+    description: The URL for the application service. May include a path after the domain name. Optionally set to null if no traffic is required.
+  as_token:
+    type: string
+    description: A secret token that the application service will use to authenticate requests to the homeserver.
+  hs_token:
+    type: string
+    description: A secret token that the homeserver will use authenticate requests to the application service.
+  sender_localpart:
+    type: string
+    description: The localpart of the user associated with the application service.
+  namespaces:
+    type: object
+    title: Namespaces
+    description: The namespaces that the application service is interested in.
+    properties:
+      users:
+        allOf:
+          - $ref: namespace_list.yaml
+          - description: |-
+              A list of namespaces defining the user IDs that the application
+              service is interested in. Events will be sent to the AS if a
+              local user matching one of the namespaces is the target of the event,
+              or is a joined member of the room where the event occurred.
+      rooms:
+        allOf:
+          - $ref: namespace_list.yaml
+          - description: |-
+              A list of namespaces defining the room IDs that the application
+              service is interested in. All events sent in a room with an ID
+              which matches one of the namespaces will be sent to the AS.
+      aliases:
+        allOf:
+          - $ref: namespace_list.yaml
+          - description: |-
+              A list of namespaces defining the room aliases that the application
+              service is interested in. All events sent in a room with an alias
+              which matches one of the namespaces will be sent to the AS.
+  rate_limited:
+    type: boolean
+    description: Whether requests from masqueraded users are rate-limited. The sender is excluded.
+  protocols:
+    type: array
+    description: The external protocols which the application service provides (e.g. IRC).
+    items:
+      type: string
+required:
+  - id
+  - url
+  - as_token
+  - hs_token
+  - sender_localpart
+  - namespaces