fix: prevent reactivity loss during/after fork

[hmnd]

fixes #17197, fixes #17304, fixes #17301, fixes #17309, possibly others

Thanks to @dummdidumm's prior digging into this here

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[changeset-bot]

🦋 Changeset detected

Latest commit: 19baf10

The changes in this PR will be included in the next version bump.

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Playground

pnpm add https://pkg.pr.new/svelte@17335

[soniquete0]

I tested this with my repro mentioned in this comment - sveltejs/kit#15059 (comment)
It does fix the hovering a link bug.
There seems to be one more bug related to async as well related to navigation also mentioned in the comment. I am leaving a comment about it here as I thought that you might find it interesting.

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-svelte-17335-svelte.vercel.app/

[paoloricciuti]

I think it makes sense...there's only one thing that it kinda irks me here: we are restoring ALL the branches to CLEAN but wouldn't this lead to over-running?

[hmnd]

Hmm I don't think that's a concern because we only do this for branches that had CLEAN before the fork. I'm pretty sure setting CLEAN doesn't necessarily mean branches will run either, it just allows them to be scheduled.

[Heniker]

I am not sure If I understand fork correctly, but it seems that just using fork(() => {}) can completely hang the app: REPL (offending line is commented)

This seems to be even execution order-dependent.

[hmnd]

@Heniker you're seeing a combination of the bug fixed in this pr, and the bug with deriveds (which {@const} compiles to) in my other pr #17362. Stacking the two prs fixes your repro and doesn't result in a freeze

[hmnd]

@teemingc let me know if you'd prefer for me to combine this PR with #17362. As you can see from the above comment, this PR is sort of dependent on that one to fully fix reactivity, although they touch different parts of the runtime.

[teemingc]

Thanks David. I think it's completely fine to keep the PRs separate as they make reviewing the changes easier. I'd recommend editing the descriptions to mention how the two are linked.

Unfortunately, I'm not too familiar with the Svelte codebase so I'd wait for one of the more experienced maintainers to review the two PRs once everyone's back from their holidays.