Missing thread_id in m.receipt for events recieved over federation

[germain-gg]

Description

I simulated a conversation between two users in a thread. When a user sees the other party message, it will send a read receipt

POST https://matrix-client.matrix.org/_matrix/client/r0/rooms/[ROOM_ID]/receipt/m.read/[EVENT_ID]

With a body

{ "thread_id": THREAD_ROOT_ID }

When this is processed, the other user will receive the m.receipt event down /sync. However I notice that the thread_id that has been passed in the original request body is missing (highlighted in red below).

{
  "type": "m.receipt",
  "content": {
    "[EVENT_ID]": {
      "m.read": {
        "@germaingermain:matrix.org": {
          "ts": 1668591503180
-         "thread_id": THREAD_ROOT_ID
        }
      }
    }
  }
}

Steps to reproduce

• Send a read receipt from one client
• Inspect the sync response of the other party

The issue does not occur when the two users are using the threads dev server we have for internal testing.
However the issue seems to happen on matrix.org/element.io, I am unsure whether this happens only over federation or not

Homeserver

matrix.org

Synapse Version

1.71.0rc2

Installation Method

No response

Platform

n/a

Relevant log output

n/a

Anything else that would be useful to know?

No response

[clokep]

The issue does not occur when the two users are using the threads dev server we have for internal testing.
However the issue seems to happen on matrix.org/element.io, I am unsure whether this happens only over federation or not

Can you expand on this a bit -- are you only seeing the issue if the users are on different servers?

[germain-gg]

I'm unsure whether the issue is with federation, or whether the problem is only on matrix.org and element.io.
I do not have a good way to validate that.

What I can say is that the same Element Web client works when testing against the threads dev server we have, and is not working when inspecting what comes down /sync on my matrix.org account, or element.io account.

[clokep]

I'm unsure whether the issue is with federation, or whether the problem is only on matrix.org and element.io.
I do not have a good way to validate that.

From discussion with @gsouquet, this is seen with an account on matrix.org and one on element.io.

[clokep]

From investigating this it seems that we're not properly sending the thread ID information over federation. I'm not sure if this was just never implemented properly or if it got lost in the rebasing of work at some point. Sad times.