Presets allow illegal option attributes

[slorber]

This issue is for Major League Hacking fellows only.

🐛 Bug Report

Docusaurus aims to be "fail-fast" and validate strictly the configuration to prevent user mistakes.

We have found that some user sites are using wrong preset configs by mistake:
https://github.com/facebookincubator/CG-SQL/blob/master/website/docusaurus.config.js#L129

presets: [
    [
      '@docusaurus/preset-classic',
      {
        // ...
        googleAnalytics: {
          // ...
        },
       // ...
      },
    ],
  ],

The mistake here is that googleAnalytics should be a themeConfig attribute, not a preset option. This lead to a site running in production without Google Analytics.

We should prevent users to do such mistakes and reject such bad configuration.

We have a way for plugins to validate their own options, but we don't have this for presets.
I don't think we need that for presets because the presets just "forward" their options to plugins, and the options end up being validated strictly.

The problem is mostly for "unknown preset attributes" that are silently ignored.

Solution

Keep it simple, no need for a new lifecycle.
Just modify packages/docusaurus-preset-classic/src/index.js

If the preset options object contains unknown keys that are not forwarded to any preset plugin then those keys should be rejected with a user-friendly error message