Document that incoming invite events will reach a homeserver twice - once over /invite and again over /send #2062
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improvement
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IIRC, invite events are supposed to be signed by the invited homeserver, before they are sent out over You could argue that sending it again over |
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Ah, yes. That is necessary in order for the invite to happen. I agree that opting to exclude sending out the invite event for just the invited homeserver sounds fragile. Perhaps instead the spec for invite endpoints should include a notice to implementations that the invite event will reach the homeserver once again over the |
anoadragon453
changed the title
PUT /_matrix/federation/v1/invite/{roomId}/{eventId} on a remote homeserver is redundant if they're already in the room
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A clarification sounds very sensible to me |
MadLittleMods
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Jan 27, 2025
This also happens for rejecting an invite. Basically, any out-of-band membership transition where we first get the membership as an `outlier` and then rely on federation filling us in to de-outlier it. This PR mainly addresses automated test flakiness, bots/scripts, and options within Synapse like [`auto_accept_invites`](https://element-hq.github.io/synapse/v1.122/usage/configuration/config_documentation.html#auto_accept_invites) that are able to react quickly (before federation is able to push us events), but also helps in generic scenarios where federation is lagging. I initially thought this might be a Synapse consistency issue (see issues labeled with [`Z-Read-After-Write`](https://github.com/matrix-org/synapse/labels/Z-Read-After-Write)) but it seems to be an event auth logic problem. Workers probably do increase the number of possible race condition scenarios that make this visible though (replication and cache invalidation lag). Fix #15012 (probably fixes matrix-org/synapse#15012 (#15012)) Related to matrix-org/matrix-spec#2062 Problems: 1. We don't consider [out-of-band membership](https://github.com/element-hq/synapse/blob/develop/docs/development/room-dag-concepts.md#out-of-band-membership-events) (outliers) in our `event_auth` logic even though we expose them in `/sync`. 1. (This PR doesn't address this point) Perhaps we should consider authing events in the persistence queue as events already in the queue could allow subsequent events to be allowed (events come through many channels: federation transaction, remote invite, remote join, local send). But this doesn't save us in the case where the event is more delayed over federation. ### What happened before? I wrote some Complement test that stresses this exact scenario and reproduces the problem: matrix-org/complement#757 ``` COMPLEMENT_ALWAYS_PRINT_SERVER_LOGS=1 COMPLEMENT_DIR=../complement ./scripts-dev/complement.sh -run TestSynapseConsistency ``` We have `hs1` and `hs2` running in monolith mode (no workers): 1. `@charlie1:hs2` is invited and joins the room: 1. `hs1` invites `@charlie1:hs2` to a room which we receive on `hs2` as `PUT /_matrix/federation/v1/invite/{roomId}/{eventId}` (`on_invite_request(...)`) and the invite membership is persisted as an outlier. The `room_memberships` and `local_current_membership` database tables are also updated which means they are visible down `/sync` at this point. 1. `@charlie1:hs2` decides to join because it saw the invite down `/sync`. Because `hs2` is not yet in the room, this happens as a remote join `make_join`/`send_join` which comes back with all of the auth events needed to auth successfully and now `@charlie1:hs2` is successfully joined to the room. 1. `@charlie2:hs2` is invited and and tries to join the room: 1. `hs1` invites `@charlie2:hs2` to the room which we receive on `hs2` as `PUT /_matrix/federation/v1/invite/{roomId}/{eventId}` (`on_invite_request(...)`) and the invite membership is persisted as an outlier. The `room_memberships` and `local_current_membership` database tables are also updated which means they are visible down `/sync` at this point. 1. Because `hs2` is already participating in the room, we also see the invite come over federation in a transaction and we start processing it (not done yet, see below) 1. `@charlie2:hs2` decides to join because it saw the invite down `/sync`. Because `hs2`, is already in the room, this happens as a local join but we deny the event because our `event_auth` logic thinks that we have no membership in the room ❌ (expected to be able to join because we saw the invite down `/sync`) 1. We finally finish processing the `@charlie2:hs2` invite event from and de-outlier it. - If this finished before we tried to join we would have been fine but this is the race condition that makes this situation visible. Logs for `hs2`: ``` 🗳️ on_invite_request: handling event <FrozenEventV3 event_id=$PRPCvdXdcqyjdUKP_NxGF2CcukmwOaoK0ZR1WiVOZVk, type=m.room.member, state_key=@user-2-charlie1:hs2, membership=invite, outlier=False> 🔦 _store_room_members_txn update room_memberships: <FrozenEventV3 event_id=$PRPCvdXdcqyjdUKP_NxGF2CcukmwOaoK0ZR1WiVOZVk, type=m.room.member, state_key=@user-2-charlie1:hs2, membership=invite, outlier=True> 🔦 _store_room_members_txn update local_current_membership: <FrozenEventV3 event_id=$PRPCvdXdcqyjdUKP_NxGF2CcukmwOaoK0ZR1WiVOZVk, type=m.room.member, state_key=@user-2-charlie1:hs2, membership=invite, outlier=True> 📨 Notifying about new event <FrozenEventV3 event_id=$PRPCvdXdcqyjdUKP_NxGF2CcukmwOaoK0ZR1WiVOZVk, type=m.room.member, state_key=@user-2-charlie1:hs2, membership=invite, outlier=True> ✅ on_invite_request: handled event <FrozenEventV3 event_id=$PRPCvdXdcqyjdUKP_NxGF2CcukmwOaoK0ZR1WiVOZVk, type=m.room.member, state_key=@user-2-charlie1:hs2, membership=invite, outlier=True> 🧲 do_invite_join for @user-2-charlie1:hs2 in !sfZVBdLUezpPWetrol:hs1 🔦 _store_room_members_txn update room_memberships: <FrozenEventV3 event_id=$bwv8LxFnqfpsw_rhR7OrTjtz09gaJ23MqstKOcs7ygA, type=m.room.member, state_key=@user-1-alice:hs1, membership=join, outlier=True> 🔦 _store_room_members_txn update room_memberships: <FrozenEventV3 event_id=$oju1ts3G3pz5O62IesrxX5is4LxAwU3WPr4xvid5ijI, type=m.room.member, state_key=@user-2-charlie1:hs2, membership=join, outlier=False> 📨 Notifying about new event <FrozenEventV3 event_id=$oju1ts3G3pz5O62IesrxX5is4LxAwU3WPr4xvid5ijI, type=m.room.member, state_key=@user-2-charlie1:hs2, membership=join, outlier=False> ... 🗳️ on_invite_request: handling event <FrozenEventV3 event_id=$O_54j7O--6xMsegY5EVZ9SA-mI4_iHJOIoRwYyeWIPY, type=m.room.member, state_key=@user-3-charlie2:hs2, membership=invite, outlier=False> 🔦 _store_room_members_txn update room_memberships: <FrozenEventV3 event_id=$O_54j7O--6xMsegY5EVZ9SA-mI4_iHJOIoRwYyeWIPY, type=m.room.member, state_key=@user-3-charlie2:hs2, membership=invite, outlier=True> 🔦 _store_room_members_txn update local_current_membership: <FrozenEventV3 event_id=$O_54j7O--6xMsegY5EVZ9SA-mI4_iHJOIoRwYyeWIPY, type=m.room.member, state_key=@user-3-charlie2:hs2, membership=invite, outlier=True> 📨 Notifying about new event <FrozenEventV3 event_id=$O_54j7O--6xMsegY5EVZ9SA-mI4_iHJOIoRwYyeWIPY, type=m.room.member, state_key=@user-3-charlie2:hs2, membership=invite, outlier=True> ✅ on_invite_request: handled event <FrozenEventV3 event_id=$O_54j7O--6xMsegY5EVZ9SA-mI4_iHJOIoRwYyeWIPY, type=m.room.member, state_key=@user-3-charlie2:hs2, membership=invite, outlier=True> 📬 handling received PDU in room !sfZVBdLUezpPWetrol:hs1: <FrozenEventV3 event_id=$O_54j7O--6xMsegY5EVZ9SA-mI4_iHJOIoRwYyeWIPY, type=m.room.member, state_key=@user-3-charlie2:hs2, membership=invite, outlier=False> 📮 handle_new_client_event: handling <FrozenEventV3 event_id=$WNVDTQrxy5tCdPQHMyHyIn7tE4NWqKsZ8Bn8R4WbBSA, type=m.room.member, state_key=@user-3-charlie2:hs2, membership=join, outlier=False> ❌ Denying new event <FrozenEventV3 event_id=$WNVDTQrxy5tCdPQHMyHyIn7tE4NWqKsZ8Bn8R4WbBSA, type=m.room.member, state_key=@user-3-charlie2:hs2, membership=join, outlier=False> because 403: You are not invited to this room. synapse.http.server - 130 - INFO - POST-16 - <SynapseRequest at 0x7f460c91fbf0 method='POST' uri='/_matrix/client/v3/join/%21sfZVBdLUezpPWetrol:hs1?server_name=hs1' clientproto='HTTP/1.0' site='8080'> SynapseError: 403 - You are not invited to this room. 📨 Notifying about new event <FrozenEventV3 event_id=$O_54j7O--6xMsegY5EVZ9SA-mI4_iHJOIoRwYyeWIPY, type=m.room.member, state_key=@user-3-charlie2:hs2, membership=invite, outlier=False> ✅ handled received PDU in room !sfZVBdLUezpPWetrol:hs1: <FrozenEventV3 event_id=$O_54j7O--6xMsegY5EVZ9SA-mI4_iHJOIoRwYyeWIPY, type=m.room.member, state_key=@user-3-charlie2:hs2, membership=invite, outlier=False> ```
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Original title: Calling
PUT /_matrix/federation/v1/invite/{roomId}/{eventId}on a remote homeserver is redundant if they're already in the roomPUT /_matrix/federation/v1/invite/{roomId}/{eventId}is useful, and its own endpoint, as homeservers that aren't yet participating in a room need to be directly notified when one of their users has been invited.However, if a homeserver is already participating in a room, when one of their user is invited they'll receive both:
m.room.memberinviteevent over thePUT /_matrix/federation/v1/send/{txnId}endpoint.PUT /_matrix/federation/v1/invite/{roomId}/{eventId}.Both of these tell the homeserver the same thing - and in practice developers tend to only implement event handling based on one of these. Typically
/invite.This also created subtle implementation bugs such as element-hq/synapse#18075 in Synapse - where Synapse notifies clients that they've been invited based on
/invite- but doesn't actually consider the user invited until the correspondingm.room.memberevent comes in. If the user tries to join the room in this gap - they'll get an error that they haven't been invited.It's also just unnecessary work - we don't need to notify a remote homeserver about an invite twice.
(credit to @MadLittleMods for originally pointing this out in the Synapse PR linked about.)
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