Support multiple Duktape contexts with a shared heap about ducc HOT 4 OPEN

Thanks for giving this thought. It's one of those things that never came up in my usage but I knew it'd be important in many other cases. I'll help give this some careful study over the next few days (sorry, just a bit busy here) but please feel free to keep iterating and to continue updating this issue!

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First, I want to say that I love Ducc! It's the Duktape library I didn't know I was waiting for, and it was really easy for me to pick up and use

Anyway, for motivation on this issue: I'm trying to implement a CommonJS/NodeJS-style module system in Duktape. Duktape has an official answer on modules, but I feel like the recommended approach is flawed (their official implementation literally wraps the module code in (function(require, exports, module) { /* module code */ })(...)). I think running each module in a separate context and using a shared heap for the module is a more promising avenue.

I'd be willing to take a stab at implementing this in a PR, but I think there's multiple different ways to tackle this from an API perspective (all of which have trade-offs). A few possible solutions I've thought about:

1. Add a lifetime to Ducc, add new ducc.thread()/ducc.thread_new_global() instance methods that use temporary lifetimes. This would unfortunately be a breaking change since existing code would have to be updated to use Ducc<'static> throughout.
2. Add a new Heap type, update Ducc with an Rc<Heap> field. Ducc::new() stays the same but ducc.thread() just clones Rc<Heap>. This should be a non-breaking change, but using reference counting is not zero-cost.
3. Add a ducc.with_thread() method that takes a closure which would take &Ducc. I think this would feel a bit hacky, but dealing with a closure should make it easier to implement without significantly impacting the rest of the library.

I also don't know much about the safety implications of duk_push_thread so it's hard for me to say if there are any potential unsoundness issues with any particular approach.

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I pushed up a WIP/proof-of-concept branch with a basic implementation for this issue: wip/duktape-threading. Unfortunately it segfaults in one of the tests I added so there's still some work to do before it's actually useable!

I based it on the with_thread style I mentioned above. It seems to work well enough for my use case, but I'm not convinced it's the best API interface

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Minor update: I fixed the segfault (it happened because I hadn't set udata or anymap in the newly-created thread context). So, the branch now works at least. I'll leave it at that for now, but do let me know if you have any thoughts/suggestions/proposals and I can work on prototyping it out! I can make a PR when I have a bit more certainty on what the final form of the API should look like

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