[AUDIO_WORKLET] Add compatible spinlock tests

[cwoffenden]

Fix for #22962. The fixes include:

• adds tests for various spinlock cases
• locks fixed so they work in audio worklets
• test for atomic wait exposed as a public function
• test renamed to match its new use
• test converted to C-only (like the other tests, plus it wasn't using C++ features)

(The git mv lost the old to new mapping at some point)

[sbc100]

I'm not really sure we need this comment, and I think we we should probably just convert everything to emscripten_get_now.

If you do go with a comment how about "Use emscripten_get_now over emscripten_performance_now since the former is available in all contexts and uses performance.now under the hood where available.".

[cwoffenden]

All the other lock types used waits, so it was a limited set of functions that could work. There’s no current benefit, unless I’m missing something, to changing all uses over to plain now.

[cwoffenden]

I thought it useful to leave a comment in case someone later thinks to change it.

It does need docs adding in the web audio page to explain what locks can be used. I’ll do this, and finish the tests, in the next days.

[sbc100]

I guess i just don't see the point of emscripten_performance_now at all. I'm not sure why it was ever added. Why wouldn't all callsites simply prefer emscripten_get_now? The works in more places (i.e. all places) and lowers the same thing when performance.now is available.

[cwoffenden]

I've had time to think about this: emscripten_performance_now() when supported is very lightweight (it maps directly to the browser's call). emscripten_get_now() has a bit of indirection, and for high performance code I'd prefer the option to know I'm avoiding indirection.

For this PR I've kept the change to the parts that need changing to prove out a set of lock calls work.

[sbc100]

FWIW emscripten_get_now() maps directly to performance.now() whenever `performance.now() is available. See

emscripten/src/library.js

Lines 1393 to 1396 in 7342ee8

	// Modern environment where performance.now() is supported:
	// N.B. a shorter form "_emscripten_get_now = performance.now;" is
	// unfortunately not allowed even in current browsers (e.g. FF Nightly 75).
	emscripten_get_now: () => {{{ getPerformanceNow() }}}(),

. There is no indirection.

[cwoffenden]

Good to know, thanks.