SharedArrayBuffer integration

[annevk]

It seems to me that without modification IDB would end up storing a pointer to mutable memory if whatwg/html#2361 landed and no changes to IDB were made. @domenic claims it's not true but won't tell me why unless I open a separate issue...

[domenic]

Let's walk through the spec. For example, https://w3c.github.io/IndexedDB/#dom-idbobjectstore-put. It uses StructuredClone into a user-agent defined Realm in step 10. I guess we'd want to specify that that user-agent defined Realm is inside the same agent cluster as the source realm, otherwise it'll error in some way.

OK, now it moves on and uses that clone, which contains a SAB whose [[ArrayBufferData]] contains all the bytes. (Not a pointer; SABs don't have pointers. It contains all the bytes. They're the same bytes contained by other SABs, but they're still bytes, not pointers.)

We get to https://w3c.github.io/IndexedDB/#steps-for-storing-a-record-into-an-object-store. The relevant step is:

Store a record in store containing key as its key and value as its value.

which is the idea of storing things on disk. Storing a SAB whose [[ArrayBufferData]] is a bunch of bytes on disk works fine, just like storing an ArrayBuffer whose [[ArrayBufferData]] is a bunch of bytes.

[annevk]

[[ArrayBufferData]] points to a block that is shared, no?

[annevk]

I mean, please explain it in terms of https://tc39.github.io/ecma262/#sec-data-blocks.

[domenic]

No, it's a data block, not a pointer to a data block.

[annevk]

Okay, I guess then I'd argue it matters how storing on disk is defined, since a naive implementation might copy the bytes lazily which would not work well for SharedArrayBuffer whereas it doesn't matter for ArrayBuffer.

[domenic]

I mean, if you're assuming that level of willful misinterpretation, it definitely matters for ArrayBuffer too. E.g. you could just modify it from the same thread, or perhaps even cause it to be GCed.

[annevk]

@domenic no, because the ArrayBuffer is cloned, including its bytes.

[inexorabletash]

It's a spec fiction that there's still a SAB at the point of "Store a record in store...". At the point where the clone is made during the put() (or add() or update()) call, implementations will have done a serialization and the value will be a list of (non-shared) bytes. Changes to the SAB data after that point would not be serialized. This would be observable by issuing a put() call in a transaction that has definitely not started (i.e. it is queued behind an unfinished transaction).

The spec is currently written "as if" the clone is retained in some abstract realm (in practice, then serialized to disk), and cloned back into an actual realm when retrieved.

[annevk]

(I also still don't understand the non-pointer concept. How can that work with multiple objects sharing the same data?)

[annevk]

@inexorabletash what requires those bytes to be non-shared at that point? Sounds like implementations are relying on the only-sometimes-share semantics we're trying to disallow.

[inexorabletash]

I can't speak to all implementations, but those I'm aware of need the entire value to be serialized into a buffer. In Blink that's then immediately sent across process boundaries, where it waits for the request to actually be processed, and ends up being the bytes written to disk. (Aside: Blobs are a special case, but they're also immutable.) There's no later step at which to then go fetch the actual bytes from the SAB.

[inexorabletash]

If HTML had Serialize/Deserialize variants of StructuredClone (as @domenic has suggested somewhere...) then IDB would use that instead.

(put() and friends would actually need to Serialize and then potentially Deserialize immediately to compute keys against an unobservable, side-effect-free copy. That's what implementations must do today, internally.)

[annevk]

Serialize for SharedArrayBuffer can't do a clone of the bytes though, right? Those bytes are shared. If you also copy them that seems wasteful.

[inexorabletash]

Either we copy or we throw. I'm fine with either.

[annevk]

I guess I still don't understand the setup then. @domenic claims this works as specified, but in Chrome it relies on StructuredClone copying the bytes, so it wouldn't work if StructuredClone didn't copy the bytes (as per @domenic's change to HTML). Something is amiss.

(I'd also still love to hear how shared memory works without pointers.)