path/filepath: Clean removes ending slash for volume on Windows in Go 1.21.4

[calmh]

Go 1.21.4 fixed a problem in filepath.Clean regarding \??\ paths. However, it also removed the ending slash for volumes in \\?\ paths. Is this expected? The docs still state

The returned path ends in a slash only if it represents a root directory, such as "/" on Unix or C:\ on Windows.

The following test passes on Go 1.21.3.

func TestFilepathClean(t *testing.T) {
	cases := []struct {
		in  string
		out string
	}{
		{`C:\`, `C:\`},
		{`\\?\C:\`, `\\?\C:\`},
	}
	for _, c := range cases {
		if out := filepath.Clean(c.in); out != c.out {
			t.Errorf("filepath.Clean(%s) => %s expected %s", c.in, out, c.out)
		}
	}
}

On Go 1.21.4 this happens:

PS C:\Users\jb\dev\syncthing\lib\fs> go1.21.4 test -run FilepathClean
--- FAIL: TestFilepathClean (0.00s)
    basicfs_windows_test.go:30: filepath.Clean(\\?\C:\) => \\?\C: expected \\?\C:\
FAIL
exit status 1
FAIL    github.com/syncthing/syncthing/lib/fs   0.062s

[qmuntal]

Thanks for reporting. Seems like a regression.

@golang/windows @neild

[bcmills]

@gopherbot, please backport to Go 1.21 and maybe 1.20. This appears to be a regression introduced in a security release.

[gopherbot]

Backport issue(s) opened: #64040 (for 1.20), #64041 (for 1.21).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.

[gopherbot]

Change https://go.dev/cl/541175 mentions this issue: path/filepath: consider \\?\c: as a volume on Windows

[neild]

This falls out from an inadvertent change to what we consider the "volume name" to be in paths starting with \\?\. https://go.dev/cl/541175 restores the previous behavior.