Add errorf template func

[jetwash]

I'm working on a heavily data-driven hugo site; virtually all the content is in front matter, and it's all relatively structured. As a result I tend to have a lot of templates, and they're more complicated than a blog or something where you mostly just need {{ .Content }}.

One thing I've struggled a bit with is making sure the input data is formatted correctly. I'd like to add a bit of logic to my templates to verify that required properties exist and, if not, emit an error message. I was thinking something like errorf, which would be just like printf but output to the console, and cause the build to fail. That way I could do something like

{{ if not .Params.foo }}{{ errorf "foo parameter missing for %s" .URL }}{{ end }}

Without this, sometimes Hugo will crash (e.g., if I try to dereference a null pointer) or emit an error message (e.g., passing an unset value to jsonify results in an error about an incorrect number of arguments), but often Hugo will just quietly keep going and I end up with bad data.

I know doing this in the template isn't really ideal, but the only other thing I can think of would be validating against a schema, but I'm not aware of anything for markdown (which is the front matter format I'm using)… Maybe a way to automatically convert front matter to JSON then validate it against a JSON schema?

[bep]

I think an errorf func would be a great addition.

/cc @moorereason

[digitalcraftsman]

Should errorf print to stdout like the regular log infos or to stderr?

[bep]

Should errorf to stdout like the regular log infos or to stderr?

It should log to the defined ERROR logger.

[bep]

@bmon no panics/exits here. What you need to is just to log to ns.Deps.Logger.ERROR (or something).

We have some log counting magic going on behind the scenes, so any ERROR logging will exit -1 at the end.

[bep]

@bmon thinking a little about this, template errors can be a bit spammy, so use the "distinct error logger":

hugo/helpers/general.go

Line 221 in 3054a46

var DistinctErrorLog = NewDistinctErrorLogger()

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