rfc: Preload Realm for Service Workers

[samuelmaddock]

Reopened from #4 as my fork somehow became unlinked

Preload realms provide an isolated context to run JavaScript prior to code evaluating in another
context. This expands functionality of Electron's preload scripts to provide new context targets.
This proposal intends to target Service Worker contexts with potential plans for new contexts (web
workers, shared workers) in future proposals.

Read rendered document

Recommended prerequisite readings:

• Design of V8 bindings
• ShadowRealms explainer

If you're curious, there's a very rough wip prototype being written in https://github.com/samuelmaddock/electron/tree/feat/preload-realm

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Todo:

• Try to get a better understanding of where ShadowRealm is at in the TC39 process.
• Call out the option of landing the API as experimental until ShadowRealm is stable

[samuelmaddock]

Recent changes:

• ipcMain no longer receives SW IPC messages in favor of handling them on ServiceWorkerMain.ipc.
• "Main world" is now used by contextBridge instead of the added "initiator world" to simplify reusing preload scripts between frames and service workers.

[itsananderson]

Since the ShadowRealms proposal is apparently still in stage 2.7, we should make sure we mark this API as experimental until the web platform API is fully locked in, but otherwise this RFC LGTM

[erickzhao]

RFC LGTM

[jkleinsc]

RFC LGTM

[samuelmaddock]

Regarding ShadowRealm TC39 proposal status, there's been a couple updates this year.

• February: https://github.com/tc39/notes/blob/main/meetings/2024-02/feb-7.md#shadowrealms-update
• June: https://github.com/tc39/notes/blob/main/meetings/2024-06/june-12.md#shadowrealm-update

Before the ShadowRealm proposal is able to move forward, more work is needed on confirming the selection of APIs to be included within the new realm. Also, more test coverage is needed with WPT mentioned.

The good news is that the V8 implementation seems to be stable according to the February slides and I added a WPT test as part of this RFC :)

Assuming no major changes regarding Blink's implementation, we may see more built-in APIs exposed within the ShadowRealm which would be beneficial for Preload Realms as well.