html/template: Parsing invalid templates with no actions should fail-fast

[josharian]

Run:

package main

import (
	"html/template"
	"io"
	"log"
)

func main() {
	t := template.Must(template.New("errmsg").Parse(`<textarea/>`))
	err := t.Execute(io.Discard, nil)
	log.Print(err)
}

https://go.dev/play/p/oiAIres95vU

Result:

2009/11/10 23:00:00 html/template:errmsg: ends in a non-text context: {stateRCDATA delimNone urlPartNone jsCtxRegexp attrNone elementTextarea <nil>}

That error message reads more like an internal debugging message than something intended for users. It took me a while to narrow down the source of the problem.

cc @empijei @kele

[josharian]

...and on further reflection, since this template has no pipelines at all, it really ought to either fail at Parse time or execute successfully.

[empijei]

Hi,

Thanks for reporting this.

The problem here is twofold:

1. The error message is not human-friendly
2. The template is lazily evaluated, so the error only comes up when the template is executed.

About 1: #30635 is tracking this, and we should definitely address it (yes, I was the original reporter and then forgot about it by the time I started maintaining the package 🤦 ). Since this part of the bug would be a duplicate, I'm changing the title of the bug to only refer to the second point.

About 2: I think this is working as intended. Templates are not necessarily valid when parsed the first time, as they might only become so once sub-templates are added. We could special-case templates that contain no actions as you point out, but I'm not sure this is the best way forward. It would definitely improve UX, but it adds a bit complexity and any unit-test that covers the template at least once would provide the same value.

That said, I am not against such a change, but I'd be curious about @kele's opinion on it before we decide to address it.

[josharian]

Thanks. Makes sense. I’m fine closing this as a dup of 30635, as long as we add this test case there.