Re-optimize Svelte libraries when compile options change

[benmccann]

Describe the problem

Because the optimized Svelte libraries depend on some Svelte compile options, whenever the Svelte options change we want to trigger a re-optimize when using the prebundleSvelteLibraries option.

Describe the proposed solution

This info is currently kept in a _svelte_metadata.json in the Vite cache dir. So whenever that file doesn’t exist or if we compare and it’s stale, we would re-optimize.

As part of this, we should check if this metadata json is correctly saved in the right place, and not accidentally purged by Vite 3's new prebundling flow, eg the .vite/deps_temp thing.

Alternatives considered

Ideally we may want Vite to expose an API to say “mark optimized deps as stale and re-optimize on next run”

Importance

nice to have

[dominikg]

We already restart the devserver on svelte config change here:

vite-plugin-svelte/packages/vite-plugin-svelte/src/utils/watch.ts

Line 57 in c5d5354

server.restart();

and server.restart has a flag to force reoptimization https://github.com/vitejs/vite/blob/main/packages/vite/src/node/server/index.ts#L265

So to implement this, we have to check if the project has svelte dependencies and add that information to the restart call. Re-optimize always has to be done in full, as there could be dependencies between optimized dependencies that need to be updated as well.

const forcePrebundle = hasSvelteDeps();
server.restart(forcePrebundle);

Question is how to get that information there.

[benmccann]

server.restart has a flag to force reoptimization

That link is outdated, so here's a permalink for reference: https://github.com/vitejs/vite/blob/27e28328235d2f5c6a44129a7635b697097ec50a/packages/vite/src/node/server/index.ts#L271

Ok, so the challenge is we call watcher.on and need to know if the project has svelte dependencies from within the watcher. I think as a shortcut we could always force reoptimization. This would probably be fair to do because the vast majority of projects of any reasonable complexity will have a svelte dependency and changing the Svelte config is relatively rare. If a project doesn't have any Svelte dependencies then it's probably cheaper on average to reoptimize. While we could certainly do better in that case I think it'd be okay for us to take the easy way out as a quick way to get the prebundling feature turned on

[dominikg]

Always reoptimizing would also affect non-svelte libraries. At the very least we should only do it when prebundleSvelteLibraries is active

[bluwy]

Not sure if I miss something, but I think this is already implemented in the buildStart hook:

vite-plugin-svelte/packages/vite-plugin-svelte/src/index.ts

Lines 91 to 94 in 74fd16d

	// Force Vite to optimize again. Although we mutate the config here, it works because
	// Vite's optimizer runs after `buildStart()`.
	// TODO: verify this works in vite3
	viteConfig.optimizeDeps.force = true;

It is sort-of a hack but it prevents unnecessary restarts.

[benmccann]

ah, thanks! closing this then