Disable SharedArrayBuffer for BroadcastChannel?

[annevk]

@mystor raised a good point that supporting SharedArrayBuffer in BroadcastChannel puts pressure on really getting the "agent cluster" definition correct. E.g., if you put two same-origin tabs in the same agent cluster, they can suddenly share memory, even if they cannot synchronously access each other. That seems bad and given that it's rather error prone, perhaps we're better off disabling SharedArrayBuffer there?

[domenic]

E.g., if you put two same-origin tabs in the same agent cluster, they can suddenly share memory, even if they cannot synchronously access each other.

How would this happen? It doesn't seem to fit with the current definition of agent cluster.

[annevk]

So the example that came up is if we add the isolation header and you have Isolated A -> B -> Isolated A'. Are A and A' in different clusters? It'd only be observable through this. I guess we could continue to allow it, if we're very careful, but the fact that you have to specifically think about BroadcastChannel reach besides stuff that can directly reach each other (or not) seems dangerous.

[domenic]

I'm not too worried myself. I think we should indeed strive to get the agent cluster definition correct, but if we don't, the natural process boundaries in implementations will lead us to discover the mismatch pretty quickly. (As long as we maintain the usual discipline for writing tests and such.)

[domenic]

Closing this as some people are being misled into thinking that it's an open question how BroadcastChannel and SharedArrayBuffers interact.