Mike Wilkerson Comments

Mike Wilkerson

•Created by David Alsh on 1/29/2024 in #help

How to call a JS function from Rust?

Something that's still bugging me about this is memory management. call_with_args requires that the arguments are &[Global<Value>]. And in the example code, getting a global seems to involve: 1. creating a local 2. promoting it to global But does that then mean it's not eligible for garbage collection? In my case, this function calling would happen many times in a long-running process, where the range of possible args is unlimited. So if every invocation of the function involves creating globals that won't be garbage collected, doesn't that mean memory leak?

15 replies

DDeno

•Created by David Alsh on 1/29/2024 in #help

How to call a JS function from Rust?

Ok, it looks to me like what I did with lexical scoping is the same solution as what's in those gists. Only those are cleaner overall. The final gist has this:

  let request = {
    let mut scope = &mut runtime.handle_scope();
    let request = serde_v8::to_v8(scope, "asdsd").unwrap();
    Global::new(&mut scope, request)
  };
  let call = runtime.call_with_args(&function, &[request]);

  let request = {
    let mut scope = &mut runtime.handle_scope();
    let request = serde_v8::to_v8(scope, "asdsd").unwrap();
    Global::new(&mut scope, request)
  };
  let call = runtime.call_with_args(&function, &[request]);

The implementation of call_with_args is where the second mutable borrow happens: it also invokes handle_scope. I'd pulled that apart because I thought (not yet understanding scopes), that it should be the same scope used in both places. So I could clean mine up to look more like that, but in principle, both have these two features: 1. using lexical scopes to fix the double mutable borrow 2. invoking handle_scope() twice I think I want to consider that confirmation ...?

15 replies

DDeno

•Created by David Alsh on 1/29/2024 in #help

How to call a JS function from Rust?

For now, I got it to build and run by enclosing the first mutable borrow within a lexical scope and having it return the Future from the call_with_args. Then in a second lexical scope, do the run_event_loop. Like this:

let fut = {
   let mut scope = worker.js_runtime.handle_scope();
   // ...
   JsRuntime::scoped_call_with_args(&mut scope, &fn_global, &args)
};

let string_value = {
  worker.run_event_loop(false).await?;
  let result = fut.await?;
  let mut scope = worker.js_runtime.handle_scope();
  let local = deno_core::v8::Local::new(&mut scope, result);
  // The function I'm calling returns a `String`.
  deno_core::serde_v8::from_v8::<String>(&mut scope, local)?
};

let fut = {
   let mut scope = worker.js_runtime.handle_scope();
   // ...
   JsRuntime::scoped_call_with_args(&mut scope, &fn_global, &args)
};

let string_value = {
  worker.run_event_loop(false).await?;
  let result = fut.await?;
  let mut scope = worker.js_runtime.handle_scope();
  let local = deno_core::v8::Local::new(&mut scope, result);
  // The function I'm calling returns a `String`.
  deno_core::serde_v8::from_v8::<String>(&mut scope, local)?
};

This does get the result I want. But I don't yet understand what's happening with scopes well enough to reason about the fact that a separate scope is used in the second block. It seems like that might be a problem, but I'm not sure what that would be. Still learning...

15 replies

DDeno

•Created by David Alsh on 1/29/2024 in #help

How to call a JS function from Rust?

Oh, and related, the docs for call_with_args say:

The event loop must be polled seperately for this future to resolve. If the event loop is not polled, the future will never make progress.

I verified that, because when I successfully invoke the function, it hangs forever, not making progress. But calling run_event_loop also involves a mutable borrow of js_runtime. So it results in the same complaint from the borrow checker.

15 replies

DDeno

•Created by David Alsh on 1/29/2024 in #help

How to call a JS function from Rust?

I'm trying to do the same thing and have got this working up to the point of invoking call_with_args. Now the problem is with the borrow checker. That call_with_args requires a mutable borrow of the js_runtime. But it's already been mutably borrowed up here:

let mut scope = js_runtime.handle_scope();

let mut scope = js_runtime.handle_scope();

Any suggestions for how to resolve that?

15 replies

DDeno