text/template: new keywords (e.g. break, continue) breaks older working programs

[bep]

E.g.

package main

import (
	"os"
	"text/template"
)

func main() {

	t := `{{ break 32 }}`

	echo := func(in any) any {
		return in
	}

	funcs := template.FuncMap{
		"break": echo,
	}

	tmpl, err := template.New("test").Funcs(funcs).Parse(t)
	if err != nil {
		panic(err)
	}

	err = tmpl.Execute(os.Stdout, nil)
	if err != nil {
		panic(err)
	}
}

Note that the above program is a little constructed; I, personally, am not concerned about break/continue. But I notice that the upcoming new return keyword (#53015) uses the same approach. And an app I'm maintaining (Hugo) does have a return template func.

Go 1.18:

go1.18 run main.go                                                              
32

Go 1.19.3:

go version && go run main.go                                                       
go version go1.19.3 darwin/arm64
panic: template: test:1: unexpected "32" in {{break}}
goroutine 1 [running]:
main.main()
	/Users/bep/dev/go/bep/temp/main.go:24 +0x258
exit status 2

[seankhliao]

cc @robpike

[bcmills]

FWIW, the change could be made non-breaking by only giving the new behavior to new keywords if they do not have a corresponding entry in the FuncMap.

[bep]

FWIW, the change could be made non-breaking by only giving the new behavior to new keywords if they do not have a corresponding entry in the FuncMap.

Yes, and that was also the original implementation (note that break and continue was introduced in Go 1.18, this is a Go 1.19 regression).

[bcmills]

this is a Go 1.19 regression

Any idea when specifically it regressed?

https://go.dev/cl/410414 (for #53234) looks like a likely culprit to me (attn @eliben; CC @mvdan)

[eliben]

Just a note that I can reproduce this with tip as well (go version devel go1.20-2af48cbb7d).

@robpike significantly refactored the template lexer for 1.20 (https://go-review.googlesource.com/c/go/+/421883) but this doesn't affect the issue (it works in 1.18 but not 1.19 or 1.20)

[eliben]

Indeed, https://go-review.googlesource.com/c/go/+/410414 is the culprit as @bcmills suspected.

The fix is straightforward; I want to spend a bit of time devising tests, since it's annoying that the package's tests didn't catch this bug.

[gopherbot]

Change https://go.dev/cl/447775 mentions this issue: text/template/parse: fix interaction of break/continue keywords and functions