Room link to public room does not work when `via` is missing

[andybalaam]

Steps to reproduce

1. In my new room (public) #stitched-ordering:matrix.org
2. Click Room Info
3. Click Copy Link
4. Click Copy Link
5. Share this link with my friend
6. They click it (in EW, develop.element.io)

(Updated after discussion on matrix-org/matrix.to#385 )

Outcome

What did you expect?

They enter the room.

What happened instead?

They got this message:

If I changed the link to be https://matrix.to/#/#stitched-ordering:matrix.org?via=matrix.org then my friend was able to use the link to join the room.

[t3chguy]

I could not reproduce the issue using https://matrix.to/#/#stitched-ordering:matrix.org -> app.element.io

1. ended up on matrix.to
2. clicked Continue
3. ended up on app.element.io
4. clicked Join
5. ended up joined to the room

No knocking happened.

Trying by manually switching to develop.element.io with only the room alias and no vias I get the same result.

Would need to see logs to know more, but for some reason the client is trying to knock as evidenced in your screenshot. The knocking codepath indeed might have bugs but that's a fairly unmaintained labs feature so bugs are expected

[tulir]

@t3chguy looks like you tested with a server that's already in the room, so a request to join the room by ID will work even without any vias.

The problem is obvious in the browser devtools when clicking a room alias in a message: element resolves the alias, then makes a call to /join with the room ID even though it should use the alias, or at least provide vias

Note: matrix.to links may work as matrix.to seems to be pointlessly adding vias to room alias links when redirecting to element, so click on a link inside a chat to reproduce the bug rather than opening element via matrix.to

This incorrect request worked because the server was in the room:

This similarly incorrect request didn't because the server isn't in the room:

[ledlamp]

Even typing in the room alias like #test:matrix.org is not working.

Element makes request like POST /_matrix/client/v3/join/!aiwVrNhPwbGBNjqlNu%3Amatrix.org

Response is 404 with {"errcode":"M_UNKNOWN","error":"Can't join remote room because no servers that are in the room have been provided."}

This started happening with recent Element update to version 1.11.110.

[tulir]

This change looks rather suspicious in d5a9b3f#diff-7fec003f282a582ef2dde368f5c9272ce6af5954b641f6bd0421ac1844169575R514

[t3chguy]

@tulir good spot on both counts, that change assumed the state of this.state.viaServers would be accurate so joining via ID was safe. Empirically via certain codepaths this is not the case