Invalid output and infinite redirect loop with adapter-static and trailingSlash: "always"

[mvolfik]

Describe the bug
Only <meta http-equiv="refresh" content="0;url=/test/"> is written to build/test/index.html when using static adapter and always trailing slash.

Logs
None, build runs successfully, maybe just that 301 /test -> /test/ is printed during build.

To Reproduce

Use npm init svelte@next test, choose skeleton project.

Create a page src/routes/test/index.svelte with any content (<p>hello</p>). Have svelte.config.js like this:

import adapter from "@sveltejs/adapter-static";

/** @type {import('@sveltejs/kit').Config} */
const config = {
  kit: {
    // hydrate the <div id="svelte"> element in src/app.html
    target: "#svelte",
    adapter: adapter(),
    trailingSlash: "always",
  },
};

export default config;

Reproduction repo with individual setup steps in https://github.com/mvolfik/probable-disco

Expected behavior
No redirects should be needed at all. build/test/index.html should contain normal html page with <p>hello</p>.

Stacktraces
N/A

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: 292.40 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: 90.0.4430.212
    Firefox: 88.0.1
  npmPackages:
    @sveltejs/adapter-static: next => 1.0.0-next.9 
    @sveltejs/kit: next => 1.0.0-next.107
    svelte: ^3.34.0 => 3.38.2

Severity
Bad bad. Makes adapter-static unusable with trailingSlash: "always".

Additional context
There is one similar issue #1346, but that is apparently caused by Netlify redirects, while this is only sveltekit.

[benmccann]

The value of trailingSlash shouldn't have much impact if you're using adapter-static, so I'd suggest just removing that option for now. adapter-static just outputs .html files which cannot control whether or not they are served with a trailing slash. You may be able to configure that with your hosting provider when generating static files. trailingSlash on the other hand will only be able to be utilized if you're using one of the other adapters which has a server-side component

[mvolfik]

I'm aware of that, though it does have some impact - SvelteKit rewrites the urls client-side upon navigation

[mvolfik]

Yayy, found a workaround - all my pages are interlinked, so I can rely on crawling and set config.kit.prerender.pages = ["/"], and if all links in the app end with the trailing slash, the issue doesn't happen.

[deepmouse]

Probably related to this: I am getting an infinite redirect loop even without trailingSlash: "always" on, all it needs is one link on my site that uses a trailing slash, and this "redirect loop page" is generated.