1357862 - (same-compartment-realms) [meta] Support multiple globals per compartment (same-compartment realms)

Status: RESOLVED FIXED

(Core :: JavaScript Engine, enhancement, P2)

Description

[Jason Orendorff [:jorendorff]]

Currently a compartment contains exactly one realm (standard-ese for a global and all objects and code associated with it). An object in one realm cannot have a direct reference to an object in another realm. References across realm boundaries are represented as cross-compartment wrappers (CCWs).

The cross-compartment wrapper overhead is sometimes bad, so we're planning to merge compartments that are in the same tab group and are same-origin. Objects in a page will tend to be same-compartment with those in child iframes. The new nesting will be runtimes > zone groups > zones > compartments > realms.

In the new order, the main responsibility of compartments will be security. Security wrappers will continue to exist. Since only same-origin compartments are being merged, the only CCWs removed will be the transparent wrappers we currently insert at the boundary between same-origin compartments.

This will mean changing the implementation and some Web-observable behavior of Window and Location objects and (especially) document.domain.

Many things in various specs require there to be a "current global", so we'll continue to have a cx->global() that's non-null whenever JS could run. JSAutoCompartment will be changed to require a global, not just a JSCompartment*.

Comment 1

[Jan de Mooij [:jandem]]

This has been enabled for chrome code in bug 1512029 and bug 1514210 (Firefox 66) and for more devtools code in bug 1517210 (Firefox 67). We are using this for content since bug 1523843 (Firefox 67). Fingers crossed but so far these changes have stuck.

There has been some follow-up discussion on doing more compartment sharing for content, but that's a much smaller change and shouldn't affect SpiderMonkey (much). In any case, we've disentangled globals and compartments so this bug is FIXED.