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Eric Eastwood
625b331768
### Background This PR was originally just trying to remove the flawed [MSC4311](https://github.com/matrix-org/matrix-spec-proposals/pull/4311) partial implementation as client side API's like `/sync` should still use stripped events. But it turns out we were just re-using the client logic for the federation side and things might break if we didn't include the full `m.room.create` event so this PR now introduces MSC4311 support to use full PDU's in the `invite_room_state`/`knock_room_state` in the federation API's. The flawed implementation was originally introduced in https://github.com/element-hq/synapse/commit/0eb7252a230811e59679cc7e55b92dac26532efc (no PR I assume because part of Hydra security fix) which was part of [Synapse v1.136.0](https://github.com/element-hq/synapse/blob/7530874a1250d6ad975b39582a784c594d29a505/CHANGES.md#synapse-11360-2025-08-12). Spawning from reviewing https://github.com/element-hq/synapse/pull/19722 and noticing that we have [`TestMSC4311FullCreateEventOnStrippedState`](https://github.com/matrix-org/complement/blob/1e2e12eebc1edb27bbf12108ec849a8254b6ddcd/tests/v12_test.go#L1341-L1376) in Complement which already passes even though that test looks [flawed](https://github.com/matrix-org/complement/pull/791#discussion_r3132468346): > I think this test is mixing up what [MSC4311](https://github.com/matrix-org/matrix-spec-proposals/pull/4311) proposes. Perhaps these were changes to the MSC that came after? > > For the client API's like `/sync`, it only proposes that `m.room.create` is a required *stripped* state event. > > For the federation API's, alongside requiring `m.room.create`, it also mandates using the full event PDU format for all events in the `invite_room_state`/`knock_room_state` on `m.room.member` events (in `unsigned`) ### What does this PR do? 1. Always use stripped state for client API's 1. Remove flawed [MSC4311](https://github.com/matrix-org/matrix-spec-proposals/pull/4311) partial implementation (as explained above) 1. Sanitize stripped state when we receive events over federation 1. Use full PDU's when sending `invite_room_state`/`knock_room_state` over federation 1. Validate PDU's and warn when receiving `invite_room_state`/`knock_room_state` over federation 1. In the future, we will strictly validate and reject Complement tests: https://github.com/matrix-org/complement/pull/796 --- Part of https://github.com/element-hq/synapse/issues/19414