Pushrules for relations

Signed-off-by: Nicolas Werner <nicolas.werner@hotmail.de>

proposals/3664-notifications-for-relations.md

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+# MSC3664: Notifications for relations
+
+Relations are very powerful and are becoming a platform to build new features
+like replies, edits, reactions, threads, polls and much more on.
+
+On the other hand there is currently no way to control, what you are getting
+notified for. Some people want to get notified, when someone replies to their
+message. Some want to get notified for reactions to their message. Some people
+explicitly do not want that. You might want to be able to mute a thread and you
+may want to get notified for poll responeses or not. Some people like getting
+notified for edits, others prefer to not get notified, when someone fixes typos
+20 times in a row for a long message they sent a week ago.
+
+We should extend push rules so that a server can provide sane defaults and users
+can adjust them to their own wishes.
+
+## Proposal
+
+### New push rule condition: `related_event_match`
+
+Notifications for relation based features need to distinguish, what type of
+relation was used and potentially match on the content of the related-to event.
+
+To do that we introduce a new type of condition: `related_event_match`. This is
+largely similar to the existing `event_match`, but operates on the related-to
+event. Such a condition could look like this:
+
+```json5
+{
+  "kind": "related_event_match",
+  "rel_type": "m.in_reply_to",
+  "key": "sender",
+  "pattern": "@me:my.server"
+}
+```
+
+This condition can be used to notify me whenever someone sends a reply to my
+messages.
+
+- `rel_type` is the relation type. For the sake of compatibility replies
+    should be matched as if they were sent in the relation format from
+    [MSC2674](https://github.com/matrix-org/matrix-doc/pull/2674) with a
+    `rel_type` of `m.in_reply_to`. If the event has any relation of this type,
+    the related event should be matched using `pattern` and `key`.
+- `key` (optional): The dot-separated field of the event to match, e.g. `content.body`
+    or `sender`. If it is not present, the condition should match all events,
+    that have a relation of type `rel_type`.
+- `pattern` (optional): The glob-style pattern to match against. Patterns with
+    no special glob characters should be treated as having asterisks prepended
+    and appended when testing the condition. If this is not present, it should
+    match everything if the specific key is present.
+
+`key` and `pattern` are optional to allow you to enable or suppress all
+notifications for a specific event type. For example one could suppress
+notifications for all events in
+[threads](https://github.com/matrix-org/matrix-doc/pull/3440) and all
+[edits](https://github.com/matrix-org/matrix-doc/pull/2676) with the following
+two conditions:
+
+```json5
+{
+  "kind": "related_event_match",
+  "rel_type": "m.replace"
+}
+```
+
+```json5
+{
+  "kind": "related_event_match",
+  "rel_type": "m.thread"
+}
+```
+
+Without a `key` the push rule can be evaluated without fetching the related to
+event.
+
+### A push rule for replies
+
+To enable notifications for replies without relying on the reply fallback, but
+with similar semantics we also define a new default push rule. The proposed
+push rule differs slightly from the old behaviour, because it only notifies you
+for replies to your events, but it does not notify you for replies to events,
+which contained your display name or matrix ID. The rule should look like this:
+
+```json5
+{
+    "rule_id": ".m.rule.reply",
+    "default": true,
+    "enabled": true,
+    "conditions": [
+        {
+            "kind": "related_event_match"
+            "rel_type": "m.in_reply_to",
+            "key": "sender",
+            "pattern": "[the user's Matrix ID]"
+        }
+    ],
+    "actions": [
+        "notify",
+        {
+            "set_tweak": "sound",
+            "value": "default"
+        },
+        {
+            "set_tweak": "highlight"
+        }
+    ]
+}
+```
+
+This should be an underride rule, since it can't be a content rule. It ensures
+you get notified for replies to all events you sent. The actions are the same as
+for `.m.rule.contains_display_name` and `.m.rule.contains_user_name`.
+
+No other rules are proposed as no other relations are in the specification as of
+writing this MSC to decrease dependencies.
+
+## Potential issues
+
+- Most push rules for relations will need a lookup into a second event. This
+    causes additional implementation complexity and can potentially be
+    expensive. Looking up one event shouldn't be that heavy, but it is overhead,
+    that wasn't there before and it needs to be evaluated for every event, so it
+    clearly is somewhat performance sensitive.
+- If the related to event is not present on the homeserver, evaluating the
+    push rule may be delayed or fail completely. For most rules this should not
+    be an issue. You can assume the event was not sent by a user on your server
+    if the event is not present on your server.
+
+## Alternatives
+
+- One could add an optional `rel_type` key to all existing conditions. This
+    would allow you to also easily match by `contains_display_name`,
+    `sender_notification_permission` and `room_member_count`. Out of those
+    conditions only `contains_display_name` seems to be useful in a related
+    event context. Having a potentially expensive key like `rel_type` available
+    for all conditions would also increase implementation complexity. As such
+    this MSC proposes the minimum amount of conditions to support push rules for
+    most relations, although allowing `rel_type` on `contains_display_name` and
+    `event_match` could be a good alternative.
+
+## Security considerations
+
+- These pushrules could be used to increase load on the homeserver. Apart from
+    that there shouldn't be any potential security issues.
+
+## Unstable prefix
+
+While this proposal is still in progress, implementations should use the
+unstable prefix `im.nheko.msc3664` for the `related_event_match` condition. As
+a result it should be called `im.nheko.msc3664.related_event_match`.
+