fix(v2): fix too strict markdown frontmatter validation

[johnnyreilly]

Motivation

PR completed by @slorber:

• Add better markdown validation infra
• Convert numbers to strings automatically (2.0.0-alpha.73 can't handle tags which could be parsed as numbers  #4642)
• Relax too strict string validation options (allow empty title/description...)

---

Original description:

To either get a meaningful error message when invalid tags are supplied, or allow them to be parsed successfully (numbers are not) - this relates to #4642

Hey @slorber,

I've started work on this and have some questions. Whilst I can get meaningful error messages using the mechanism you suggest (see changes in blogFrontMatter.ts) the suggested:

    Joi.attempt(frontMatter, BlogFrontMatterSchema, {convert: true});

does not seem to support parsing numbers to strings automatically as suggested. Also, even if it did, it won't actually change the type of the tags in the frontMatter object. It's just an assertion - no mutation takes place. We could put something in the markdownParser here:

https://github.com/facebook/docusaurus/blob/master/packages/docusaurus-utils/src/markdownParser.ts#L68-L79

that migrates numbers to strings.... Or alternatively, maybe it just throwing a meaningful error message is fine. I think the meaningful error message is the most significant thing from my perspective.

What do you think?

Have you read the [Contributing Guidelines on pull requests]

Yes

Test Plan

Automated tests!

Related PRs

N/A

[netlify]

[V2]

Built with commit fbfa345

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	61
🟢 Accessibility	96
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-4654--docusaurus-2.netlify.app/

[slorber]

Thanks @johnnyreilly , I see the problem

I'd like to do a small bugfix release soon, and have a few issues reported regarding this frontmatter validation.

As this is likely to create conflicts (like with #4655), I'll take your PR as a base and complete it so that I can release the fix asap!

I changed the assertion to actually return the value. I will try to see if I can convert to number from there

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[V1]

Built with commit fbfa345

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