cross realm signals

[mhofman]

What is the expected interactions of signals across realms, whether the callable boundary of ShadowRealms is involved or not.

In particular:

• Would a watcher from one realm attempting to read during its notify callback a signal from another realm throw an error like it would for a signal from the same realm? Aka is it a cross realm brand check on signals?
• What would subtle.currentComputed() return while executing a function in a realm different from where the "current computed" originated.
  • In particular, if the function is from a ShadowRealm, the callable boundary precludes observing the identity of objects from other realms. Similarly if the current computed is from a ShadowRealm, the callable boundary precludes observing its identity from another other realms, regardless whether that other realm is a ShadowRealm or "legacy" realm.
• It's technically possibly to construct dependency graphs where a computed signal from one realm depends on a signal from another realm, even if one of the realm is a ShadowRealm (you pass through the callable boundary a get function "bound" to its signal). How would these dependencies be exposed without violating the callable boundary invariants.

[littledan]

My first thought is, to stay conservative, we should make tracked reads throw when the current computed is from a different Realm. This will ensure that Realms do not have entangled dependency graphs (which, in the case of ShadowRealms, would give us something a bit unworkable when it comes to introspection).