MSC3987: Push actions clean-up (#3987)

proposals/3987-push-actions-clean-up.md

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+# MSC3987: Push actions clean-up
+
+There are [two defined push rule actions](https://spec.matrix.org/v1.6/client-server-api/#actions)
+which are of dubious use:
+
+* `coalesce` is defined, but unused in the spec and not implemented on any homeservers [^1]
+  or clients [^2] beyond validating it or treating it identically to `notify`.
+* `dont_notify` is a no-op, but frequently used as a way to denote do nothing.
+  It is clearer to provide an empty list of actions to avoid a situation where
+  additional actions are given with it, e.g. `["notify", "dont_notify"]` is
+  currently valid, but does not make sense. [^3]
+
+## Proposal
+
+The `coalesce` push rule action is removed from the Matrix specification.
+
+The `dont_notify` push rule action is deprecated. Homeservers and clients should
+ignore it. Any [pre-defined rules](https://spec.matrix.org/v1.6/client-server-api/#actions)
+which include the `dont_notify` action are redefined to have an empty list of actions:
+
+* `.m.rule.master`
+* `.m.rule.suppress_notices`
+* `.m.rule.member_event`
+
+It is recommended that homeservers continue accepting the `coalesce` and `dont_notify`
+actions, but ignore them during processing. (Treating them as no-ops.) A future
+Matrix spec version should remove them completely.
+
+## Potential issues
+
+A client might attempt to create a push rule with a `coalesce` or `dont_notify`
+action that homeservers will reject as an unknown action.
+
+## Alternatives
+
+Do nothing and continue propagating confusion.
+
+## Security considerations
+
+None.
+
+## Dependencies
+
+None.
+
+[^1]: [Dendrite](https://github.com/search?q=repo%3Amatrix-org%2Fdendrite+CoalesceAction+NOT+path%3A%2F_test.go%24%2F&type=code),
+[Synapse](https://github.com/search?q=repo%3Amatrix-org%2Fsynapse+coalesce+language%3ARust&type=code&l=Rust),
+[Conduit](https://gitlab.com/search?search=coalesce&nav_source=navbar&project_id=22083768&group_id=4616224&search_code=true&repository_ref=next),
+[Construct](https://github.com/matrix-construct/construct/blob/4ecf1ef037ecc1a5d1e3a1049d9a63cb0a6f3455/matrix/push.cc#L739-L740)..
+
+[^2]: [matrix-js-sdk](https://github.com/search?q=repo%3Amatrix-org/matrix-js-sdk%20Coalesce&type=code),
+[matrix-ios-sdk](https://github.com/search?q=repo%3Amatrix-org%2Fmatrix-ios-sdk%20coalesce&type=code),
+[matrix-rust-sdk](https://github.com/matrix-org/matrix-rust-sdk/commit/59edc22a35c4ef162ea0a8cafccdf25e37ab1070),
+[matrix-android-sdk2](https://github.com/search?q=repo%3Amatrix-org/matrix-android-sdk2%20ACTION_COALESCE&type=code),
+[Ruma](https://github.com/search?q=repo%3Aruma/ruma%20Coalesce&type=code).
+
+[^3]: It has been noted on recent MSCs that new rules should not use `dont_notify`,
+see [MSC3786](https://github.com/matrix-org/matrix-spec-proposals/pull/3786#discussion_r864607531),
+[MSC2153](https://github.com/matrix-org/matrix-spec-proposals/pull/2153#discussion_r450188777) /
+[MSC2677](https://github.com/matrix-org/matrix-spec-proposals/pull/2677#discussion_r879701007).