support SharedArrayBuffers with crypto.getRandomValues

[vapier]

(forking off of #213)

trying to use crypto.getRandomValues on TypedArrays backed by a SharedArrayBuffer fails. example from Chrome JS console:

// works fine
> ab = new ArrayBuffer(10); u8 = new Uint8Array(ab); crypto.getRandomValues(u8);
Uint8Array(10) [134, 194, 41, 240, 231, 151, 173, 248, 154, 113, buffer: ArrayBuffer(10), byteLength: 10, byteOffset: 0, length: 10, Symbol(Symbol.toStringTag): 'Uint8Array']

// fails
> ab = new SharedArrayBuffer(10); u8 = new Uint8Array(ab); crypto.getRandomValues(u8);
VM253:1 Uncaught TypeError: Failed to execute 'getRandomValues' on 'Crypto': The provided ArrayBufferView value must not be shared.

supporting SharedArrayBuffers is important for me for 2 reasons:

• using threads with WASI requires the WASM module be backed by a SharedArrayBuffer
• message passing between main thread & Worker thread has lower overhead with SharedArrayBuffer

it can obviously be worked around with creating a temporary ArrayBuffer and then copying between them, but seems better to not waste the CPU cycles.

ab = new ArrayBuffer(10)
sab = new SharedArrayBuffer(10)
u8 = new Uint8Array(ab)
su8 = new Uint8Array(sab)
crypto.getRandomValues(u8)
su8.set(u8)

I'm mostly interested in crypto.getRandomValues, but if there's other APIs that would benefit, throw them in the pile I guess.

[Frosne]

Hi :)
Thanks for bringing this up. I was wondering if you could share a bit more context on why you think this belongs in the standard itself?

Do you have insight into how many developers might need this? Understanding the expected demand would be really helpful :)

[vapier]

anyone wanting to do threads and use crypto APIs. if you use WASI, their getrandom APIs are all based on this function (which is how I noticed it).

if it isn't in the standard, then i'm not sure how it'd work otherwise. implementers aren't going to support this if the standard doesn't say it should work.

when these APIs only support ArrayBuffers and not SharedArrayBuffers, it feels like an oversight rather than intentional.