testscript: add go:version condition

[twpayne]

Changes in Go's standard library can affect program behavior. For example, Go 1.18's text/template changes the behavior of the and and or to lazily evaluate their arguments. This can impact application behavior.

This PR adds a go:version condition that allows testscripts to be conditional on the Go version being used.

[mvdan]

Are you aware that you can use build tags for this, such as [go1.18] or [!go1.17]?

[twpayne]

Are you aware that you can use build tags for this, such as [go1.18] or [!go1.17]?

No, I was not aware. Are you sure that this is the case? The word "tag" does appear anywhere in the testscript/ subdirectory of this repo, and when I try a simple test I get an "unknown condition" error:

$ go test ./testscript
--- FAIL: TestScripts (0.00s)
    --- FAIL: TestScripts/goversion (0.00s)
        testscript.go:397: 
            > [go1.16] exec sh -c 'exit 0'
            FAIL: testdata/goversion.txt:4: unknown condition "go1.16"
            
FAIL
FAIL    github.com/rogpeppe/go-internal/testscript      0.846s
FAIL

[mvdan]

Ah, I was slightly wrong. We do already support this via conditions, but only if you opt into gotooltest:

go-internal/gotooltest/setup.go

Lines 115 to 122 in 115ce09

	if goVersionRegex.MatchString(cond) {
	for _, v := range build.Default.ReleaseTags {
	if cond == v {
	return true, nil
	}
	}
	return false, nil
	}

[mvdan]

So I think you could classify this as "gotooltest is not particularly well documented" :) Though, then again, its code is barely 150 lines.

[twpayne]

Ah, thank you. In my case I'm looking to test an application built with Go rather than the go command itself, so I don't think using gotooltest is appropriate.

Using build tags is a much nicer solution that what's in this PR. What do you think about replacing this PR with a PR that implements the tag:foo condition that returns true iff the build tag foo is set?

[mvdan]

Yeah, perhaps using gotooltest would be a bit overkill.

An alternative might be to promote those conditionals to the testscript package. I really don't see why they are specific to testing Go tools. You might want to write conditionals based on the Go version for the standard library changes, like you say. That seems to me like a smaller change overall, since we wouldn't end up with two ways to write the same conditional.

I'm also not sure your proposed fix is right, because it seems to only look at ReleaseTags, so I'm pretty sure it won't work for any arbitrary build tag like -tags=foobar.

[twpayne]

I'm also not sure your proposed fix is right, because it seems to only look at ReleaseTags, so I'm pretty sure it won't work for any arbitrary build tag like -tags=foobar.

Are the actual build tags like foobar available to runtime Go code? I didn't even know about go/build.Default.ReleaseTags until I saw it :) In the short term I'll update the doc for #150 to mention release tags only.

[twpayne]

Closing in favor of #150.