[tlively]

Since #144, we've specified that memory.atomic.notify is allowed to be executed on unshared memories, but always returns 0 since it is impossible for there to be anything waiting on an unshared memory since all variations of the wait instructions in Wasm and JS trap or throw exceptions on unshared memories. However, I realized recently that there is no technical reason why Atomics.waitAsync shouldn't be able to wait on an unshared memory. Because Atomics.waitAsync returns a promise rather than blocking, it can in principle be used to synchronize between multiple asynchronous tasks in flight concurrently in the same JS context, and now that JSPI is becoming more real, this actually might be a useful thing to do. In particular Atomics.waitAsync could be used to build suspending versions of mutexes and condition variables to synchronize between threads running as different asynchronous tasks within a single JS context.

Actually realizing this vision of asynchronous threading with JSPI on top of Atomics.waitAsync requires changes on the JS side to allow Atomics.waitAsync and Atomics.notify to be used on unshared ArrayBuffers, but in the meantime we can prepare for that change on the Wasm side by softening the language stating that memory.atomic.notify unconditionally returns 0 for unshared memories.

cc @syg

[conrad-watt]

Makes sense, thanks!

[conrad-watt]

(I'd also be interested in @syg's take on how likely this is on the JS side)