case "windows":
		return "USERPROFILE"
	case "plan9":
		return "home"
	default:
		return "HOME"
	}
}

// tempEnvName returns the environment variable used by os.TempDir
// to locate the default directory for temporary files.
func tempEnvName() string {
	switch runtime.GOOS {
	case "windows":
		return "TMP"
	case "plan9":

	// with a signal they might fail to clean up some temp files, and we don't
	// want that to cause an "unexpected files" failure at the end of the run.
	cmd.Env = append(slices.Clip(tg.env), tempEnvName()+"="+t.TempDir())

	cmd.SysProcAttr = &syscall.SysProcAttr{
		Setpgid: true,
	}
	cmd.Cancel = func() error {
		pgid := cmd.Process.Pid
		return syscall.Kill(-pgid, syscall.SIGINT)
	}

	must(os.MkdirAll(telemetryDir, 0777))
	must(s.Setenv("TEST_TELEMETRY_DIR", filepath.Join(work, "telemetry")))

	must(os.MkdirAll(filepath.Join(work, "tmp"), 0777))
	must(s.Setenv(tempEnvName(), filepath.Join(work, "tmp")))

	gopath := filepath.Join(work, "gopath")
	must(s.Setenv("GOPATH", gopath))
	gopathSrc := filepath.Join(gopath, "src")
	must(os.MkdirAll(gopathSrc, 0777))
	must(s.Chdir(gopathSrc))

	if err != nil {
		log.Fatal(err)
	}
	if !*testWork {
		defer removeAll(topTmpdir)
	} else {
		fmt.Fprintf(os.Stderr, "TESTWORK: preserving top level tempdir %s\n", topTmpdir)
	}
	os.Setenv(tempEnvName(), topTmpdir)

	dir, err := os.MkdirTemp(topTmpdir, "tmpdir")
	if err != nil {
		log.Fatal(err)
	}
	testTmpDir = dir
	if !*testWork {
		defer removeAll(testTmpDir)
	}