feat: allow custom v8 snapshots to be used in the main process and the default snapshot in the renderer process

[ryanthemanuel]

Description of Change

We are using electron-mksnapshot to generate a custom v8 snapshot for our application that is desired to run only in the main/browser process, not the renderer process. This PR will utilize a new fuse (electron/fuses#8) called LoadV8SnapshotFromCustomPath that tells the main/browser process to look for the v8 snapshot file at custom_v8_context_snapshot.bin. Any other process will use the same path as is used today.

Consumers will then be able to build/package their binary as follows:

flipFuses(
  path.join(options.dist, 'Cypress.app', 'Contents', 'MacOS', 'Cypress'), // Returns the path to the electron binary
  {
    version: FuseVersion.V1,
    [FuseV1Options.LoadBrowserProcessSpecificV8Snapshot]: true,
  },
)

Fixes #35170

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• tests are changed or added
• relevant documentation is changed or added
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: Added LoadBrowserProcessSpecificV8Snapshot as a new fuse that will let the main/browser process load its v8 snapshot from a file at browser_v8_context_snapshot.bin. Any other process will use the same path as is used today.

[welcome]

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[ryanthemanuel]

Note for reviewers: Currently, this just works on Macs, but provides an idea of what I'm trying to accomplish. It does not work on Linux because the renderer process is created from a zygote process that presumably is created prior to when the BrowserWindow arguments are specified.

There are a couple of questions I have for this PR before it is ready to go:

1. What approach would be recommended to handle linux? I have a version of code locally that works but it involves disabling the zygote process when the custom-v8-snapshot-file-name argument is present but I'm wondering if there's a better way.
2. Would it be better to make this a first class parameter under webPreferences rather than rely on additionalArguments. This may affect any docs that might be written so I haven't written up any docs just yet.
3. Being new to the code base, I'm curious if there's a good place for a test for this.

[codebytere]

@ryanthemanuel could you speak a little more to your use case here? what would you be trying to accomplish with per-process snapshots?

[ryanthemanuel]

@codebytere, we are using a custom snapshot in our main process to preload node dependencies (similar to what atom does with electron-link). However, we have nodeIntegration disabled in our renderer process and do not want those preloaded dependencies to be available in that context. Thus, we would either want the default snapshot (or no v8 snapshot) to be used in the renderer process.

[brian-mann]

@codebytere does Ryan’s update provide you with more context regarding our use case?

the original issue we opened with more description and context is here: #35170

[MarshallOfSound]

Based on the use case you described I don't think this PR is the right way forward. Allowing custom snapshots via a command line flag in the renderer process is a can of worms and will not function correctly (As I think you've already identified in a few scenarios)

• Sandboxing
• Site isolation
• Delayed renderer creation
• Etc.

additionalArguments is a flawed property that I'm surprised survived the renderer-process-reuse refactor 🤔

I think the correct approach here is to introduce a new fuse that specifically can change which snapshot is loaded in the browser process. The renderer must always use the default snapshot (or at least a modified version of that snapshot) because it contains core blink/v8 builtins. Adding a fuse that when enabled loads a different v8 snapshot in the browser process seems like a minimal lift and slightly more explainable than a command line argument to a web contents (which fundamentally doesn't align with our modern process model)

[ryanthemanuel]

@MarshallOfSound, the fuse was exactly what I was looking for. I originally had explored going down the route of the custom v8 snapshot path being for the browser process, but I couldn't figure out how to make it work without passing in a parameter on launch. I hadn't heard of fuses before but a new one seems like it will solve that exact problem.

I tried it out on mac and linux and it works. The code I used is now attached to this PR; however, I think there are still some discussion points with it before I think it's fully ready for review:

• Right now I'm calling the fuse LoadV8SnapshotFromCustomPath. I was thinking maybe LoadBrowserProcessV8SnapshotFromCustomPath to be even more explicit. Thoughts?
• The custom path for the v8 snapshot will be at custom_v8_context_snapshot.bin not including any of the architecture info in the filename. Does that seem reasonable?
• Do we still want to communicate the custom path via a command line parameter? I have it working that way currently as it was an easy way to test given what I had already done, but wasn't sure if there was a better option. Maybe using the electron_main_delegate as a method to communicate the path?

[ryanthemanuel]

@MarshallOfSound I went ahead and refactored to use the delegate instead of a command line parameter to communicate the custom v8 snapshot path. It seemed to simplify things fairly well. Does this seem like a reasonable approach?

[ryanthemanuel]

Also, I created the corresponding PR to the fuses repo

[ryanthemanuel]

Hi @MarshallOfSound and @codebytere, is there any update on what needs to happen to get this PR moving? Is there any additional information you need from me? Does my current approach seem reasonable?

[welcome]

Congrats on merging your first pull request! 🎉🎉🎉

[release-clerk]

Release Notes Persisted

Added LoadBrowserProcessSpecificV8Snapshot as a new fuse that will let the main/browser process load its v8 snapshot from a file at browser_v8_context_snapshot.bin. Any other process will use the same path as is used today.

[trop]

I have automatically backported this PR to "20-x-y", please check out #35694

[ckerr]

@MarshallOfSound and @ckerr does the no-backport mean that this change will make it to 21.0 or 22.0? We'd really like to consume this as soon as we can and I know 22.0 won't land for another 10 weeks.

I think it was to make the bot happy 🙂

[trop]

I have automatically backported this PR to "21-x-y", please check out #35695