Store context as singleton with global reference

[jeffcharles]

The purpose of this change is to enable future work to access data in the context in the host environment which won't have a reference to the context. But as a nice side effect of making the context a singleton, consumers of the API can't accidentally hang a function invocation by trying to create multiple contexts.

As far as I can tell, we don't have a need for multiple contexts. If we ever find we do, the logic in here can be updated to hold a collection of created contexts instead of one context.

[adampetro]

As far as I can tell, we don't have a need for multiple contexts. If we ever find we do, the logic in here can be updated to hold a collection of created contexts instead of one context.

We do have a need for multiple contexts in tests. For example, we create one each time run_function_with_input is called in the shopify_function Rust crate here

[jeffcharles]

We do have a need for multiple contexts in tests. For example, we create one each time run_function_with_input is called in the shopify_function Rust crate here

I should've been more specific, we don't need multiple contexts per thread. The new_with_input invocation can reset the state in current thread's context.

[adampetro]

It wouldn't shock me if someone ended up calling new_with_input multiple times in the same thread and possibly holding onto the values at the same time, but I think it's probably unlikely so we could punt and add support for that later

[jeffcharles]

Yeah I'm not sure when it would make sense to do that. FWIW, I've figured out a different way to handle logging that doesn't involve a context. But while investigating that approach, I've also realized there is very likely a way I could apply the same approach for function output (since I'll have to update that approach to replace the write to stdout in any case). I'll put together a document if prototyping that goes well.