MSC4010: Push rules and account data (#4010)

* Clarify how m.push_rules works with account_data.

* Typo fix.

* Fix typo.

Co-authored-by: Richard van der Hoff <[email protected]>

* Add info on MSC3391.

* Clarify potential issues.

* Clarify returned value

* Fix typo.

Co-authored-by: David Vo <[email protected]>

* Add information on Conduit.

Co-authored-by: Alexey Rusakov <[email protected]>

* More consistent wording.

---------

Co-authored-by: Richard van der Hoff <[email protected]>
Co-authored-by: David Vo <[email protected]>
Co-authored-by: Alexey Rusakov <[email protected]>

proposals/4010-push-rules-and-account-data.md

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+# MSC4010: Push rules and account data
+
+Push rules have a [bespoke API](https://spec.matrix.org/v1.6/client-server-api/#push-rules-api)
+which clients use to retrieve, add, modify, and remove push rules. Any modifications
+made using this API are sent to all clients via a `m.push_rules` event in the
+`account_data` section in the
+[the next `/sync` response](https://spec.matrix.org/v1.6/client-server-api/#push-rules-events).
+
+The client-server API does not have any special behavior around using the
+[account data APIs](https://spec.matrix.org/v1.6/client-server-api/#client-behaviour-17)
+with the `m.push_rules` type leading to different behavior on different homeservers:
+
+* Synapse will accept the data and shadow the real push rules in `/sync`, but
+  *won't use the data when evaluating push rules*.
+* Dendrite will return an HTTP 403 if you attempt to set `m.push_rules` via
+  `/account_data`.
+* Conduit has no protections for special account data. It will accept `m.push_rules` via
+  `/account_data` *and* use those when evaluating push rules.
+
+The [fully read marker](https://spec.matrix.org/v1.6/client-server-api/#fully-read-markers)
+operates in a similar way and
+[servers must reject updating `m.fully_read` via `/account_data`](https://spec.matrix.org/v1.6/client-server-api/#server-behaviour-10).
+
+Note that when attempting to set `m.fully_read` via `/account_data` the following
+behavior is observed:
+
+* Synapse will reject it with a 405 error (only for room account data).
+* Dendrite will reject it with an HTTP 403 error.
+
+## Proposal
+
+To make push rules data consistent with fully read markers, the following
+clarifications are offered:
+
+* The `m.push_rules` account data type becomes protected and cannot be set using
+  the `/account_data` API, similarly to `m.fully_read`.
+* "Rejected" means to use the 405 error response as
+  [documented](https://spec.matrix.org/v1.6/client-server-api/#put_matrixclientv3useruseridaccount_datatype):
+
+  > This `type` of account data is controlled by the server; it cannot be modified
+  > by clients. Errcode: `M_BAD_JSON`.
+* `m.push_rules` and `m.fully_read` should be rejected for both global and room
+  account data.
+* Reading `m.push_rules` and `m.fully_read` should be allowed (although note that
+  currently `m.push_rules` only makes sense for global account data and `m.fully_read`
+  only makes sense for room account data). The format should match what is currently
+  [returned via `/sync`](https://spec.matrix.org/v1.6/client-server-api/#push-rules-events).
+
+The above rules shall also apply when deleting account data if [MSC3391](https://github.com/matrix-org/matrix-spec-proposals/pull/3391)
+is merged before this MSC.
+
+## Potential issues
+
+It is possible that a client is currently storing data in the `m.push_rules`
+(or global `m.fully_read`) account data. After this change it could no longer
+be updated, deleted, or retrieved. It seems unlikely that the data stored here
+is done purposefully (and is likely the result of undefined behavior that this
+MSC is attempting to fix).
+
+## Alternatives
+
+An alternative would be to remove the current push rules API and have clients
+store all push rules in bulk. This would be subject to race conditions between
+clients.
+
+A slight variation of the above would be to *additionally* define the `/account_data/m.push_rules`
+endpoint as bulk modifying the push rules data. This could be seen as an alternative
+to [MSC3934](https://github.com/matrix-org/matrix-spec-proposals/pull/3934).
+
+## Security considerations
+
+None foreseen.
+
+## Unstable prefix
+
+This is mostly clarifications and does not add any event types or new endpoints.
+
+## Dependencies
+
+N/A