C++ foxglove websocket server: ambiguous handler invoke timing

[paulsohn]

Hey guys. Thanks for wonderful products and code repositories.
I'm using C++ foxglove websocket server, and found out some features doesn't work out as I expected.

Namely, my server stores the last received message per topic, and if a subscription request from client comes then it emits the saved message of the requested topic immediately. The problem is that server->sendMessage() inside of subscribeHandler() doesn't work.

I figured out that sendMessage() filters out unsubscribed topics, but when sendMessage() is called (inside of subscribeHandler() the request topic is not yet recognized as a subscribed topic so the call itself has no effect. This is the relevant line:

https://github.com/foxglove/ws-protocol/blob/main/cpp/foxglove-websocket/include/foxglove/websocket/websocket_server.hpp#L1270

In fact, I block-scoped L1271~1272 and move L1270 after the block, then I could get the desired behavior.

Based on its python and typescript counterparts, I suppose that the C++ callback subscribeHandler() should be called AFTER the topic has been added to internal subscribed topic table as well. How do you think?

Note: Here are the handlers I have checked so far which call timing should be fixed.

• subscribe
• unsubscribe
• client advertise
• client unadvertise
• subscribe connection graph

[defunctzombie]

Thanks for the detailed write-up. I've ticketed this internally (https://linear.app/foxglove/issue/FG-9181/c-foxglove-websocket-server-ambiguous-handler-invoke-timing) and we'll try to get to it over the next few weeks. If you feel inspired to make a PR for this before we get around to it we'll happily review.

[paulsohn]

Sure, here you go. foxglove/ws-protocol#850