Guidance around imlementing callbacks with custom deno embedding

[Yohe-Am]

Hey, so we're embedding deno in our program with extensions added and have need for a callback system. I.e. the typescript code registers a bunch of functions that are supposed to be invoked by the native section. Calling a simple function is simple enough but some of the callbacks are to be async so it needs to support waiting on a promise.

We were able to find an example of how to get this done within the deno repositories, the watch_promise construct from deno_core, and it generally works but with the exception of exception processing. The code adjacent to watch_promise relies on a lot of private machinery to handle this. I be I can hack around this with enough gumption but it seems like there should be an easier way to do this.

TLDR: what is the recommended way to implement callbacks or await js promises from rust side with support for catching exceptions?

[bartlomieju]

There's no built-in mechanism for this right now. When a promise rejects, it goes through machinery in deno_core that allows the unhandled rejection to be handled by callbacks like onunhandledrejection or process.on("unhandledRejection").

[Yohe-Am]

Hey, thanks for the quick reply. I suppose I can use a javascript function to catch any unhandled errors and explicitly pipe that to the native side.

But yeah, am I barking up the right tree with the watch_promise approach? I was not able to find any examples in the Deno extensions that relied on such a callback pattern. There seem to be a few methods on the JsRuntime type itself but they appear to coarse for this purpose.

[bartlomieju]

Yeah, watch_promise is the function you want to use and indeed you can use hooks in JS to catch unhandled promises to pipe it back to native side.