Support "external" buffers when creating a SharedArrayBuffer

[saghul]

When creating a SAB using JS_NewArrayBuffer, the alloc_flag is always set to FALSE, which means we assume A SAB was previously allocated and we are just going to "dup" it.

This doesn't work when all we want to is to maneuver a chunk of memory (I have this use case with FFI) since we didn't allocate the SAB in the first place.

I reckon we might want some JS_NewSharedArrayBuffer function to provide extra flexibility. In addition JSFreeArrayBufferDataFunc doesn't apply for a SAB so we'd make it more specific.

I worked this around on txiki.js by adding a magic number to the SAB header, this way I can detect if we allocated the SAB or if it's just external memory, but it's a hack at best.

Thoughts @chqrlie / @bnoordhuis ?

[bnoordhuis]

There's likely some overlap with the resizable (Shared)ArrayBuffer proposal?

[saghul]

Hola! Not sure what that overlap would be, at a glance. Can you elaborate what you have in mind?

[bnoordhuis]

Your ask is for support of unmanaged buffers, right, where the embedder is responsible for allocating and releasing the buffer?

With resizable buffers, resizing also needs to be delegated to the embedder.

[saghul]

Oh I see now, thanks!

[saghul]

Nevermind, turns out some of the ffi code in txiki.js was using a SAB by mistake. A simple AB is enough with a NULL dealloc function.

I'll revisit if need be.