Latest @vitejs/plugin-legacy breaks ssr build

[tjk]

Describe the bug

Copied from #4767 (comment):

$ vite build --ssr src/entry-server.ts --outDir dist/ssr

~/dist/ssr/entry-server.js:36
export function __vite_legacy_guard() {
^^^^^^

SyntaxError: Unexpected token 'export'
    at wrapSafe (internal/modules/cjs/loader.js:984:16)
    at Module._compile (internal/modules/cjs/loader.js:1032:27)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1097:10)
    at Module.load (internal/modules/cjs/loader.js:933:32)
    at Function.Module._load (internal/modules/cjs/loader.js:774:14)
    at Module.require (internal/modules/cjs/loader.js:957:19)
    at require (internal/modules/cjs/helpers.js:88:18)
    at Module._compile (internal/modules/cjs/loader.js:1068:30)

Reproduction

https://github.com/tjk/vite-ssr-plugin-legacy-build-issue

System Info

System:
    OS: macOS 11.2.3
    CPU: (8) x64 Intel(R) Core(TM) i7-1068NG7 CPU @ 2.30GHz
    Memory: 198.71 MB / 16.00 GB
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 14.17.0 - ~/.asdf/installs/nodejs/14.17.0/bin/node
    Yarn: 1.22.5 - ~/.asdf/shims/yarn
    npm: 6.14.13 - ~/.asdf/installs/nodejs/14.17.0/bin/npm
  Browsers:
    Chrome: 92.0.4515.159
    Firefox: 86.0
    Safari: 14.0.3
  npmPackages:
    @vitejs/plugin-vue: ^1.0.0 => 1.6.0
    vite: ^2.5.3 => 2.5.3

Used Package Manager

yarn

Logs

No response

Validations

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[patak-dev]

I think this may be surfacing another bug, now that the banner includes the export. But the banner shouldn't be there. We have #4536 to disable plugin-legacy during SSR build, so maybe the solution is to merge it. @tjk would you check it out?
@nulladdict if not, I think the logic to when to add the banner may need to be improved. It is a bit strange we didn't get a report before this though, wouldn't it still be invalid before?

[nulladdict]

Yes, when I wrote this check I kind of assumed there's only one entry and it's the client one. I'm not too familiar with ssr, so I don't know if legacy plugin should or shouldn't be ran during the server build.

I took a look at the bundled entry-server.js in ssr-vue example and I think this didn't surface before because calling import() is still valid inside cjs, but using both export and exports is not.

I think it might be a good idea to add legacy+ssr test, so things like this can be caught in the future, maybe as a part of #4536

[tjk]

I left some random thoughts in there. I currently run the vite --ssr command with an environment variable to distinguish upfront rather than at config resolution, so I can more easily "disable" the legacy-plugin and see if that works...

[tjk]

This should be fixed in @vitejs/plugin-legacy 1.5.3 (after 1f068fc and fdc3212)