Clarify the behavior of `wgpuInstanceWaitAny` called from a callback

[eliemichel]

Question about WGPUFutures: Is it legal to call wgpuInstanceWaitAny from within a callback, and if so, what completion status is reported for the WGPUFuture of the callback we are currently in?

(EDIT: parent issue #199)

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FTR, replies got on matrix:

From @Kangz IMHO it should work, but it might be difficult to implement

From @kainino0x I think it should work, it's implementable in theory but YMMV with actual implementations The (now outdated, but mostly correct) design doc says that the completed field on your first WaitAny should have already been set before the callback is called. However at least at that time, I left the answer to your question undefined: https://docs.google.com/document/d/1qJRTJRY318ZEqhK6ryw4P-91FumYQfOnNP6LpANYy6A/edit#bookmark=id.8zqwnqgij106 Until recently Dawn would actually assert-crash in debug mode if you did that, but we took it out

[kainino0x]

Thinking on this further, I think it's necessary to make this safe. Sure, we could say it's unsafe to WaitAny(f1) during f1's own callback inside another WaitAny(f1).

• But what if you WaitAny(f1) while f1's callback is being called by ProcessEvents? If that's unsafe too, it becomes hazardous to use WaitAny on any AllowProcessEvents-mode future in a multithreaded program.

  • What if you WaitAny(f1) while f1's callback is being called spontaneously? You effectively can't WaitAny on any Spontaneous-mode future at all, even in a single-threaded application, because the implementation can be using background threads. (Also, the application can't just add its own locking in the callback - the critical section is larger than the whole callback.)

If those things have to be safe then nested WaitAny may as well have to be re-entrancy-safe too (for both single-threaded nesting and for multithreading).

[kainino0x]

If those things have to be safe

OTOH they don't necessarily have to be safe. There just would be no way to do them safely.

[kainino0x]

I formalized the less safe version in #416. This might be too pessimistic though.

cc @lokokung

[lokokung]

FWIW, I think WaitAny inside a callback should work in our current implementation... (I haven't explicitly tried it, but I do remember ensuring that we release any Instance related locks before calling callbacks which should in turn make it fine.)

However, we still have issues with re-entrancy for device level stuff because of the default global Device lock... If/when we have a separate thread for doing stuff, I think we can defer certain spontaneous callbacks, i.e. Device lost, and that might allow re-entrancy? Would need to think a little more about whether that could work though.

[kainino0x]

Ah, I didn't think about the fact that there can be things that are safe in multithreaded code but will cause deadlocks in nested code....