testscript: ensure that temp dir isn't a symlink

[leitzler]

I ran into issues in govim with the testscript matcher since it uses real paths in the logs. When running macOS ioutil.TempDir() returns a sym-linked directory by default.

This PR ensures that the path used is a real directory by evaluating the root test temp dir that acts as base for $WORK.

See full description in govim/govim#308

[myitcv]

This LGTM but we'll wait for @rogpeppe to opine. We can't test this adequately because the macOS Travis environment is not recent enough.

[mvdan]

When running macOS ioutil.TempDir() returns a sym-linked directory by default.

If that's the case, add a test to prove what this is fixing? The project is already tested on Mac via Travis.

[myitcv]

If that's the case, add a test to prove what this is fixing? The project is already tested on Mac via Travis.

I think (but @leitzler will be able to confirm) this is only surfaced on Mojave.

[leitzler]

I've only tested on Mojave (10.14.6) but I don't think the current tests would catch it anyway since it's a bit of an edge case.

go-internal as well as the original go source always handles the symlink and therefor won't be affected, but as govim is acting from within vim as context where it resolves the actual path it becomes a problem.

This would end up with different paths on macOS:

package main

import (
	"fmt"
	"io/ioutil"
	"os"
)

func main() {
	tmpd, err := ioutil.TempDir("", "foo")
	if err != nil {
		panic(err)
	}
	fmt.Println(tmpd)

	os.Chdir(tmpd)
	cwd, err := os.Getwd()
	if err != nil {
		panic(err)
	}
	fmt.Println(cwd)
}

e.g.:

/var/folders/xm/ls208xd174v95pgd20rn_qbh0000gn/T/foo872692457
/private/var/folders/xm/ls208xd174v95pgd20rn_qbh0000gn/T/foo872692457

I'll see if I can create a simple testscript that details it further.

[leitzler]

Here is an example repo: https://github.com/leitzler/testscript-symlink

It's a bit hard to create a fair example since govim has a custom testdriver with custom commands to compare logs, but I hope this makes more sense.

[rogpeppe]

LGTM with one suggestion, thanks!

[myitcv]

Comment looks good to me @leitzler - I think we should include the test, no?

https://github.com/leitzler/testscript-symlink/blob/master/scripts/foo.txt

[leitzler]

Good idea, I added it.

[myitcv]

Thanks @leitzler

Please can you squash the commits down and provide a descriptive commit message?

As you can see from https://github.com/rogpeppe/go-internal/commits/master we do not use merge commits, instead relying on the commit message in the PR. When there are multiple commits in a PR (like now) GitHub simply does a concatenation of the messages... which leaves us with something less useful :)