fix warnings when linkcheck is disabled

[jyn514]

fixes #2808

@apiraino does this fix your problem? i believe those are just warnings and not hard errors, i would expect the output to still show up in book/html.

disclosure: the code and test were generated by an LLM. i wrote the comments.

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[apiraino]

oh, I had forgot about this.

This change just hides the error, which is not exactly what I had in mind. I'd like to investigate why linkcheck2 or the linkcheck.sh script return this error.

[jyn514]

well, i don’t think it’s an error, it’s a warning. what’s happening is:

1. mdbook spawns linkcheck and pipes the chapter to it on stdin
2. linkcheck.sh decides it doesn’t need to check links, since this isn’t in CI. it exits without reading stdin.
3. mdbook gets an EPIPE error and warns that the extension didn’t read stdin.

this makes sense for mdbook to do, the most likely thing that happened (from its perspective) is that linkcheck crashed. but for our use case, the warning is just noise.

[apiraino]

hm, I see.

Also, why did you add an additional CI step building without checking links?

[jyn514]

Also, why did you add an additional CI step building without checking links?

because that's what reproduces this warning. the warning previously only happened when running locally.