MSC3930: Polls push rules/notifications (#3930)

* Split poll notifications out to their own MSC

* Assign number

* Update proposals/3930-polls-notifications.md

* Change sync behaviour; Add links; Account for room versions

* spelling

* Update proposals/3930-polls-notifications.md

Co-authored-by: Andrew Morgan <[email protected]>

* Clarity around rule order

* spelling

* Use modern push rule practices

* More using of modern push rule practices

---------

Co-authored-by: Andrew Morgan <[email protected]>

proposals/3930-polls-notifications.md

@@ -0,0 +1,143 @@
+# MSC3930: Polls push rules/notifications
+
+[MSC3381](https://github.com/matrix-org/matrix-spec-proposals/pull/3381) describes how chat polls can work,
+though deliberately leaves out details on how push rules or notifications could work for such a feature.
+This proposal aims to address that specific feature gap.
+
+Readers should review MSC3381 to better understand how polls are intended to operate in a room.
+
+Push rules are currently defined [here](https://spec.matrix.org/v1.7/client-server-api/#push-rules) in
+specification.
+
+## Proposal
+
+Polls should behave similar to message events. To achieve this effect, we define the following underride
+push rules which mimic their `m.room.message` partners.
+
+Note that [order matters](https://github.com/matrix-org/matrix-spec/issues/1406) for push rules - these
+underride rules are to be inserted immediately after the `.m.rule.encrypted` underride push rule, in the
+order presented by this MSC.
+
+```jsonc
+{
+  "rule_id": ".m.rule.poll_start_one_to_one",
+  "default": true,
+  "enabled": true,
+  "conditions": [
+    {"kind": "room_member_count", "is": "2"},
+
+    // Note: `.` is escaped once, but for valid JSON we need to escape the escape.
+    {"kind": "event_property_is", "key": "type", "value": "m\\.poll\\.start"}
+  ],
+  "actions": [
+    "notify",
+    {"set_tweak": "sound", "value": "default"}
+  ]
+}
+```
+
+```jsonc
+{
+  "rule_id": ".m.rule.poll_start",
+  "default": true,
+  "enabled": true,
+  "conditions": [
+    // Note: `.` is escaped once, but for valid JSON we need to escape the escape.
+    {"kind": "event_property_is", "key": "type", "value": "m\\.poll\\.start"}
+  ],
+  "actions": [
+    "notify"
+  ]
+}
+```
+
+```jsonc
+{
+  "rule_id": ".m.rule.poll_end_one_to_one",
+  "default": true,
+  "enabled": true,
+  "conditions": [
+    {"kind": "room_member_count", "is": "2"},
+
+    // Note: `.` is escaped once, but for valid JSON we need to escape the escape.
+    {"kind": "event_property_is", "key": "type", "value": "m\\.poll\\.end"}
+  ],
+  "actions": [
+    "notify",
+    {"set_tweak": "sound", "value": "default"}
+  ]
+}
+```
+
+```jsonc
+{
+  "rule_id": ".m.rule.poll_end",
+  "default": true,
+  "enabled": true,
+  "conditions": [
+    // Note: `.` is escaped once, but for valid JSON we need to escape the escape.
+    {"kind": "event_property_is", "key": "type", "value": "m\\.poll\\.end"}
+  ],
+  "actions": [
+    "notify"
+  ]
+}
+```
+
+Additionally, a new override rule is defined to suppress poll responses by default, inserted immediately
+after the `.m.rule.room.server_acl` override rule.
+
+```jsonc
+{
+  "rule_id": ".m.rule.poll_response",
+  "default": true,
+  "enabled": true,
+  "conditions": [
+    // Note: `.` is escaped once, but for valid JSON we need to escape the escape.
+    {"kind": "event_property_is", "key": "type", "value": "m\\.poll\\.response"}
+  ],
+  "actions": []
+}
+```
+
+*Note*: Lack of `actions` means "don't do anything with matching events", or "don't notify".
+
+Typically these rules would be kept in sync with the `m.room.message` rules they are based upon,
+however there is no requirement to do so. A client's approach might be to only keep them in sync
+while setting the values, rather than monitoring for changes to push rules.
+
+Clients might find [MSC3934](https://github.com/matrix-org/matrix-spec-proposals/pull/3934) of value
+for keeping the rules in sync, though this MSC does not require MSC3934.
+
+For the purposes of [MSC3932](https://github.com/matrix-org/matrix-spec-proposals/pull/3932), the
+push rules defined in this proposal are *not* affected by the room version limitations. This is due
+to polls not inherently being room version-specific, unlike other extensible event structures. For
+clarity, this means the push rules described here are treated the same as `.m.rule.master` (for
+example) - they always apply, regardless of room version.
+
+## Potential issues
+
+The push rules for this feature are complex and not ideal. The author believes that it solves a short
+term need while other MSCs work on improving the notifications system. Most importantly, the author
+believes future MSCs which aim to fix notifications for extensible events in general will be a more
+preferred approach over this MSC's (hopefully) short-term solution.
+
+## General considerations
+
+While the order within the MSC is deliberate for the new rules, the positioning relative to other rules
+already in the spec is fairly arbitrary. The new rules are placed at the end of each list (underride and
+override) for simplicity, but could realistically go anywhere in the list.
+
+See ["Dot-separated property paths"](https://spec.matrix.org/v1.7/appendices/#dot-separated-property-paths)
+for more information on why we escape the `event_property_is` condition values.
+
+## Security considerations
+
+None applicable - no new considerations need to be made with this proposal.
+
+## Unstable prefix
+
+While this MSC is not considered stable, implementations should use `org.matrix.msc3930.*` as a prefix
+in place of `m.*` for the push rule IDs. As of writing, polls are only implemented using the legacy
+`org.matrix.msc3381.poll.*` prefix rather than the newer `v2` prefix - implementations of this MSC
+should be aware of which version of MSC3381 they plan to support.