Path handling during build with adapter-static

Describe the bug Magic happens. Build fails with weird 404 when paths: { base: "/" } and there’s <a href="/">, and creates a directory buildfavicon.ico instead of nonexistent file build/favicon.ico.

Logs

yarn run v1.22.10
warning package.json: No license field
$ svelte-kit build
vite v2.2.4 building for production...
✓ 14 modules transformed.
.svelte/output/client/_app/manifest.json                    1.18kb
.svelte/output/client/_app/pages/index.svelte-31924b7e.js   0.41kb / brotli: 0.26kb
.svelte/output/client/_app/layout.svelte-f07a7e7f.js        0.49kb / brotli: 0.29kb
.svelte/output/client/_app/assets/start-a8cd1609.css        0.16kb / brotli: 0.10kb
.svelte/output/client/_app/error.svelte-6645eba8.js         1.19kb / brotli: 0.57kb
.svelte/output/client/_app/chunks/vendor-43cd886a.js        5.05kb / brotli: 1.98kb
.svelte/output/client/_app/start-fec3b019.js                16.00kb / brotli: 5.12kb
vite v2.2.4 building SSR bundle for production...
✓ 9 modules transformed.
.svelte/output/server/app.js   8.86kb

Run npm run preview to preview your production build locally.

> Using @sveltejs/adapter-static
> 404  (linked from /)
Error: 404  (linked from /)
    at error (file:///tmp/test/node_modules/@sveltejs/kit/dist/chunks/index5.js:92:11)
    at visit (file:///tmp/test/node_modules/@sveltejs/kit/dist/chunks/index5.js:159:5)
    at async visit (file:///tmp/test/node_modules/@sveltejs/kit/dist/chunks/index5.js:219:6)
    at async prerender (file:///tmp/test/node_modules/@sveltejs/kit/dist/chunks/index5.js:228:5)
    at async Object.prerender (file:///tmp/test/node_modules/@sveltejs/kit/dist/chunks/index5.js:295:5)
    at async adapt (/tmp/test/node_modules/@sveltejs/adapter-static/index.cjs:10:4)
    at async adapt (file:///tmp/test/node_modules/@sveltejs/kit/dist/chunks/index5.js:321:2)
    at async file:///tmp/test/node_modules/@sveltejs/kit/dist/cli.js:590:5
error Command failed with exit code 1.

To Reproduce

$ npm init svelte@next test # create-svelte version 2.0.0-next.64, choose skeleton, no, no, no
$ cd test
$ yarn add -D @sveltejs/adapter-static@next # @next doesn't work, have to edit package.json manually then `yarn` to sync
$ cat > svelte.config.cjs
module.exports = {
  kit: {
    target: "#svelte",
    paths: { base: "/" },
    adapter: require("@sveltejs/adapter-static")(),
  },
};
$ yarn build # fails, HOWEVER: dir buildfavicon.ico/ is created !
$ # now remove favicon link from src/app.html
$ yarn build # builds ok
$ cat > src/routes/index.svelte
<a href="/">home</a>
$ yarn build # the error above

Expected behavior Build correctly, application should assume that everything is served at /

Information about your SvelteKit Installation:

Diagnostics

npx envinfo --system --npmPackages svelte,@sveltejs/kit,vite,@sveltejs/adapter-static --binaries --browsers

  System:
    OS: Linux 5.4 Ubuntu 20.04.2 LTS (Focal Fossa)
    CPU: (4) x64 Intel(R) Core(TM) i3-3110M CPU @ 2.40GHz
    Memory: 291.48 MB / 7.35 GB
    Container: Yes
    Shell: 3.1.0 - /usr/bin/fish
  Binaries:
    Node: 14.16.0 - ~/.local/bin/node
    Yarn: 1.22.10 - ~/.local/bin/yarn
    npm: 7.6.3 - ~/.local/bin/npm
  Browsers:
    Chromium: 90.0.4430.93
    Firefox: 88.0
  npmPackages:
    @sveltejs/adapter-static: next => 1.0.0-next.7 
    @sveltejs/kit: next => 1.0.0-next.96 
    svelte: ^3.34.0 => 3.38.2 
    vite: ^2.2.3 => 2.2.4 

Severity After removing the paths config key everything works correctly, but docs say that base should be root-relative, but the default actually isn’t ('' from docs, idk where to look into code). So this is quite a bad footgun.

[jakemoves]

Didn’t mean to steer this in the wrong direction, was just trying to substantiate the issue and leave a breadcrumb for any other new users running into this.

[Rich-Harris]

In the current version, base: '/' will fail with an error:

config.kit.paths.base option must be a root-relative path that starts but doesn’t end with ‘/’. See https://kit.svelte.dev/docs/configuration#paths

Opened #4713 to clarify that, but I’ll close this first since / is already disallowed.

There’s a lot of other noise in this thread — if there are other issues beyond the OP they’ll need to be opened separately