feat(ssr): support external true

[bluwy]

Description

Support ssr.external: true to default all packages to external even if they are linked.

I found this handy in tests in monorepos where we want to emulate the usual case of all packages externaled, or when using ssrLoadModule to load an arbitrary file and we want to always external packages as we don't care about HMR.

Additional context

cc @brillout which brought up something similar some time ago #9367 (reply in thread)

re whether we want to change the default, perhaps we should see how SSR optimize deps help first in Vite 5 as we want to keep Vite 4 less breaking.

Notes on test: I made __tests__ a workspace package to unit test ssrExternal. Feels a bit weird but this avoids adding tests as e2e 😬

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[bluwy]

Removed const { ssr } = config as it's already define above. Removed if (ssr) { as it's always true.

[brillout]

re whether we want to change the default, perhaps we should see how SSR optimize deps help first in Vite 5 as we want to keep Vite 4 less breaking.

👍 Sounds like a good plan. As always, I strongly recommend to always externalize SSR dependencies by default. (Because SSR dependencies often crash when processed by a bundler. For example when they use import(someDynamicImportPath) which bundlers cannot statically analyze.)

[benmccann]

Just a note (mostly to myself): this is a bit different than defaulting everything to external because external: true causes noExternal to have no effect. That has historically meant you wouldn't be able to use external: true with vite-plugin-svelte since it needed to add Svelte libraries to noExternal. That may be different now that we've turned on prebundling, but you'd want to be careful not to both set external: true and prebundleSvelteLibraries: false

[bluwy]

external: true causes noExternal to have no effect

I don't think this is what the PR does. noExternal will always have priority over external, unless if a package is explicitly defined in both external and noExternal (for some reason), external will take priority.

Explicitly defined means ssr.external: ['some-lib'] only, not ssr.external: true. If ssr.external: true and ssr.noExternal: true (for some reason), noExternal will still take priority.