fix(core): prevent useBaseUrl returning /base/base when on /base

[Josh-Cena]

Motivation

Fix #6315. It's a pretty trivial fix; not sure if a better solution exists. Since we have this duplicate base URL detection anyways, this fix only exists to make the behavior more consistent. Related to #6294

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Added two tests

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	53
🟢 Accessibility	100
🟢 Best practices	92
🟢 SEO	100
🟢 PWA	90

Lighthouse ran on https://deploy-preview-6993--docusaurus-2.netlify.app/

[github-actions]

Size Change: +36 B (0%)

Total Size: 805 kB

Filename	Size	Change
website/build/assets/js/main.********.js	612 kB	+36 B (0%)

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	49.9 kB
website/build/assets/css/styles.********.css	105 kB
website/build/index.html	38.3 kB

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[slorber]

I'm not sure to understand how this fixes the other issue.

To me the problem is that if user use baseUrl: "/docusaurus/" and routeBasePath: "docusaurus", then we want:

expect(mockUseBaseUrl('/docusaurus')).toBe('/docusaurus/docusaurus');

(which we prevented on purpose as a workaround/security)

instead of

expect(mockUseBaseUrl('/docusaurus')).toBe('/docusaurus/');

Can we build our own site with baseUrl: "/docs" and the current fix? That's the relevant test here

[Josh-Cena]

No, this one fixes the other issue, which is exactly that the base URL is prepended twice, because we only check url.startsWidth(baseUrl), but in this case, we are on /base instead of /base/ because of trailingSlash: false

[slorber]

oh I see, it's not a fix for https://github.com/facebook/docusaurus/issues/6294🤦‍♂️

[slorber]

👍 that looks good yes

[slorber]

But in practice does it really solve the problem?

Because afaik the problem is when accessing the site on /docusaurus instead of /docusaurus/, not clicking a link

Have you tested this with a host that serves Docusaurus with baseUrl: "/docusaurus/" under /docusaurus?

We may need a client-side redirect or something

[Josh-Cena]

This is only about the canonical and nothing else (not about content hoster serving the right file or redirecting). The problem is that if you are on /base, the SiteMetadata component would call useBaseUrl("/base") and thus add a canonical of "/base/base". This PR would correct that to adding a canonical of "/base/".

[slorber]

👍 agree, this fixes the issue above

Although serving the site through /docusaurus instead of /docusaurus/ might still trigger other issues in the future (cf #6315 (comment)), at least it's a bit more resilient now for the reported case