(adapter-netlify): _redirects file gets overwritten instead of changed

[roschaefer]

Describe the bug
The documentation clearly says:

During compilation a required "catch all" redirect rule is automatically appended to your _redirects file. (If it doesn't exist yet, it will be created.)

See: https://github.com/sveltejs/kit/tree/master/packages/adapter-netlify#using-netlify-redirect-rules

This is not quite true. When I put my own _redirects into either the root directory or build/ folder, the file gets overwritten.

To Reproduce

1. Create a file build/_redirects and add some content:

/workshops    kategorie=:kategorie /.netlify/functions/render 200!

2. Run svelte-kit build
3. Content of build/ is:

/* /.netlify/functions/render 200

If possible, we recommend creating a small repo that illustrates the problem.

You can clone my repository and checkout my commit: roschaefer/tanzimpulse@984ba91

Expected behavior

Expected content of build/_redirects should be:

/workshops    kategorie=:kategorie /.netlify/functions/render 200!

/* /.netlify/functions/render 200

Information about your SvelteKit Installation:

Diagnostics

• The output of npx envinfo --system --npmPackages svelte,@sveltejs/kit,vite --binaries --browsers

$ npx envinfo --system --npmPackages svelte,@sveltejs/kit,vite --binaries --browsers
Need to install the following packages:
  envinfo
Ok to proceed? (y) y

  System:
    OS: Linux 5.12 Arch Linux
    CPU: (12) x64 AMD Ryzen 5 PRO 4650U with Radeon Graphics
    Memory: 18.48 GB / 30.65 GB
    Container: Yes
    Shell: 3.2.2 - /usr/bin/fish
  Binaries:
    Node: 14.15.4 - ~/.asdf/installs/nodejs/lts/bin/node
    Yarn: 1.22.10 - ~/.asdf/installs/nodejs/lts/.npm/bin/yarn
    npm: 6.14.10 - ~/.asdf/installs/nodejs/lts/bin/npm
  Browsers:
    Chromium: 91.0.4472.77
    Firefox: 88.0.1
  npmPackages:
    @sveltejs/kit: next => 1.0.0-next.109
    svelte: ^3.38.2 => 3.38.2

• Your browser
  Irrelevant

• Your adapter (e.g. Node, static, Vercel, Begin, etc...)
  It's adapter-netlify

Severity

A bit unfortunate but not a blocking issue. For my use case, this would allow to prerender sites (+performance) while catering noscript-browsers(+accessiblity).

Additional context
I asked at Discord: https://discord.com/channels/457912077277855764/819723698415599626/848204698808352788

Hey everyone 👋 does anybody know if it's possible to configure netlify-adapter in a way that /workhops gets served by a pre-rendered page but /workshops?filter=something hits the serverless function?

I figured it out myself! In build/_redirects:

/workshops    kategorie=:kategorie /.netlify/functions/render 200!

/* /.netlify/functions/render 200

The relevant documentation is here: https://docs.netlify.com/routing/redirects/rewrites-proxies/#shadowing

Suggestions to Fix

I'm quite sure it's because the entire build/ folder gets deleted here:

kit/packages/adapter-netlify/index.js

Line 15 in 028abd9

utils.rimraf(publish);

appendFileSync would never find an existing file:

kit/packages/adapter-netlify/index.js

Line 42 in 028abd9

appendFileSync(`${publish}/_redirects`, '\n\n/* /.netlify/functions/render 200');

Furthermore, I would suggest that users can put a _redirects file into the project root folder instead of the build/ folder. This would allow users to .gitignore the build/ folder.

[kvn-shn]

I believe there is a small misunderstanding. :) The _redirects file belongs into the project's static folder (not build nor your project's root).

[roschaefer]

@kvn-shn you're right! Should the documentation be updated to avoid further confusion? Or is it just me and we can assume that people understand that they have to put netlify configuration files into /static?

[roschaefer]

I'm going to close this non-issue. Feel free to ask me if I should send a PR to update docs.