Fixed zero exit code when plugin not found

[segfaultsourcery]

In response to issue #893.

[azerupi]

From the tests, I see that missing backends were purposefully allowed. Before merging this, it would be great to check the reason and if someone relies on this behavior.

Otherwise ✔️

[segfaultsourcery]

I noticed that too, but I'm honestly not sure who I should ask.

[azerupi]

I am not sure either. @Michael-F-Bryan do you have an idea?

[segfaultsourcery]

I guess an alternative could be to hide it behind a flag, like --abort-on-missing-plugin, or so, but I also know that excessive flags are (rightfully) frowned upon.

[Michael-F-Bryan]

I'm surprised that missing backends are allowed. I guess one reason you may want that is if the book.toml enables link checking but you don't have mdbook-linkcheck installed. I could see how that might be annoying.

Instead of a command line flag, you could add an extra item to the [build] section of book.toml. For example missing-plugin = "ignore" (or "warn" or "fatal", where the default is "warn"). That'd involve creating a MissingPluginBehaviour enum and using that to dictate what happens if a backend/preprocessor doesn't exist.

[segfaultsourcery]

It surprised me as well. It seems like a design decision that's definitely going to bite someone at some point (and it seems like it did). I like the idea with the [build] item. I'm tempted to write it now, but I think I'll wait for someone to decide for me.

Maybe @ehuss wants to decide? :)

[segfaultsourcery]

Or maybe @Dylan-DPC ?

[Dylan-DPC-zz]

looks good to me. .will wait for a review from @ehuss

[ehuss]

What about adding an optional key to the output table? Similar to preprocessor.*.renderers as a way to control whether or not the renderer is used.

[output.linkcheck]
optional = true

Or maybe ignore-if-missing if optional is too vague?

build.missing-plugin is also fine, but doesn't give much fine-grain control.

[Michael-F-Bryan]

optional = true seems like an intuitive solution.

We'd need to update the docs so 3rd party backends (and preprocessors?) know about the convention and don't try to use the key as part of their config, although I doubt that happens in practice at the moment anyway.

[segfaultsourcery]

Thank you for your input. How does this look? The default behavior is to require the backend and to stop if it's missing. Adding optional = true to its configuration allows it to be skipped.

[ehuss]

Seems ok. Can you add documentation? And rebase on master.

[segfaultsourcery]

I wrote a little blurb in the docs, but I'm not sure how to go about rebasing to master.

[ehuss]

Thanks! I made a few wording tweaks, and squashed the changes.