How to design opendal c binding's async api

[asukaminato0721]

c users have some choices. The main problem is c doesn't have async/await/yield keyword.

• use callback with libuv, libevent
• Manual State Machines
• Macro-based "Coroutines" / Protothreads (something like https://dunkels.com/adam/pt/ or modern version https://github.com/naasking/async.h)
• Coroutine Libraries (https://github.com/semistrict/libcoro or https://github.com/hnes/libaco)

all seems complicated...

the async io part seems also complicated...

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Refer to #6072 for more context.

[asukaminato0721]

2 ideas:

1. Callback-based API (very common, but call back hell)
2. Event Loop Integration (integrate into libuv, libevent or other framework)

[yuchanns]

Cool. I'm looking forward to it. Go binding can be beneficial from this.

[asukaminato0721]

I made a "callback" version in the PR with test and example, feed back welcome.

[Xuanwo]

Some other things to consider:

• How should we handle the existing tokio code?
• Should we consider splitting the library into a blocking version and an async version?

I can imagine that users who want to use the async API don't want to have tokio included.

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Or maybe the biggest question is: Where are our C users, and what do they want?