🌱 Switch link checker to mdbook-linkcheck

[benmoss]

What this PR does / why we need it:
Switches our link checker from https://github.com/raviqqe/liche to https://github.com/Michael-F-Bryan/mdbook-linkcheck/

This tool is directly integrated with mdbook which is nice, since it tests against rendered the rendered Markdown with our preprocessors rather than just scanning the raw files. It also supports adding headers for specific domains, which helps with avoiding rate-limiting by GitHub. It has built-in caching too, so links that have already been verified are faster.

I'm using my fork of https://github.com/japaric/trust/ which just gives us a nice succinct script to download Rust GitHub releases. We could also vendor the script. The upstream trust repo has a problem in that it requires rustc even if you're just using it to download a GitHub release, which I've made a PR to fix here.

Removing verify-book-links as a part of make verify so that it doesn't become PR-blocking. I'm going to make a separate PR to test-infra to make it just a separate optional job, which for now should give us good enough visibility if a PR breaks a link or if a link breaks due to reasons outside our control (external servers changing).

Part of #2703

[benmoss]

We could add GITHUB_TOKEN to the Netlify environment variables to avoid hitting rate-limits https://docs.netlify.com/configure-builds/environment-variables/#deploy-request-notifications but I think we'd be blocking PRs if links fail for unrelated reasons.

I'll remove the link-checking behavior from the netlify deploy for now.

[vincepri]

/milestone v0.3.10

[vincepri]

/milestone Next

[benmoss]

@vincepri any reason we keep pushing this one off? It's catching regressions we're missing

[vincepri]

Happy to merge whenever it's ready

[benmoss]

Should be ready now, I hadn't noticed the netlify failure

[benmoss]

@vincepri should be ready to go now, or at least review 😄

[vincepri]

/approve
/hold cancel
/assign @ncdc @detiber

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[vincepri]

/milestone v0.3.10

[ncdc]

Do we want the deploy/netlify preview to fail if there are broken links? It's not currently because the linkcheck is marked as optional and our netlify configuration doesn't install the link checker as part of the build.

[ncdc]

@detiber WDYT?

[detiber]

I'm a bit torn on failing the build... In one sense it would be nice to get a heads up that things are broken and/or verify any new links added are not broken, but it also seems a bit harsh to penalize a contributor's PR if an existing link has become broken.

Unless there is a way to differentiate the two, then I think it should likely remain optional.

[ncdc]

/lgtm
😄

That's fair. How do we anticipate getting signal that links are broken?

[detiber]

That's fair. How do we anticipate getting signal that links are broken?

For netlify notifications, it looks like it is possible to configure both Slack and email notifications.

For slack, it would require getting a slack token: https://github.com/kubernetes/community/blob/13b3f6a596f3b31c668409c1a52f386f98eab93f/.github/ISSUE_TEMPLATE/slack-request.md