Missing transaction_id in syncs for remote-echo of messages for guests

[t3chguy]

See element-hq/element-web#13706 for details

[clokep]

I've unfortunately been unable to reproduce this. I've been unable to get Element Web into a room as a guest. Can you provide more specific steps to reproduce?

[t3chguy]

It is only possible if Synapse joins them into that room, e.g using auto_join

[clokep]

I was able to confirm this:

1. Configure synapse to allow guests and to auto-join rooms (not sure this is all necessary), this uses the room #public:localhost:8480, which I created with a separate user:

allow_guest_access: true
auto_join_rooms:
  - "#public:localhost:8480"
auto_join_rooms_for_guests: true

2. Setup an Element Web that points to localhost:8480 by default

   1. Download the latest Element Web release
   2. Unpack it.
   3. cp config.sample.json config.json
   4. Modify the default_server_config key

3. Serve Element Web locally: python -m http.server from the directory you unpacked Element Web in

4. Go to http://localhost:8000/#/room/#public:localhost:8480

5. Type something and you'll see copies of your own messages stuck at the bottom.

[clokep]

This is a bit of an edge-case and it is unclear how much impact this is having on end-users. At the moment we're not going to prioritize this, but if anyone is keen to investigate further they can join #synapse-dev:matrix.org for help! On the surface it seems to be something with the serialize_event method.

[grinapo]

I've unfortunately been unable to reproduce this. I've been unable to get Element Web into a room as a guest. Can you provide more specific steps to reproduce?

See element-hq/element-web#15567 and try the link there.

[grinapo]

This is a bit of an edge-case and it is unclear how much impact this is having on end-users.

Basically it affects everyone coming to a specific matrix room from a click on a webpage, as a guest user, to simply chat, without even being aware that it's "something called matrix". As it is now it makes plainly impossible to use since own messages accumulate at the bottom and new messages appear from the middle to top, possibly without even noticed and the user simply gives up and leaves.

[erikjohnston]

This looks to be because guests use macaroons, which don't have an associated token_id:

synapse/synapse/api/auth.py

Lines 357 to 364 in 34a5696

	ret = {
	"user": user,
	"is_guest": True,
	"shadow_banned": False,
	"token_id": None,
	# all guests get the same device id
	"device_id": GUEST_DEVICE_ID,
	}

So when we serialize the event we don't include the transaction ID:

synapse/synapse/events/utils.py

Lines 305 to 309 in 34a5696

	if token_id is not None:
	if token_id == getattr(e.internal_metadata, "token_id", None):
	txn_id = getattr(e.internal_metadata, "txn_id", None)
	if txn_id is not None:
	d["unsigned"]["transaction_id"] = txn_id

To fix this we may want to pass in both a user_id and token_id and changing the guard to be if user_id is not None: .... I think only a couple of places call serialize_event while specifying a token_id so it shouldn't be too invasive