mount: replace mountinfo handling with moby/sys/mountinfo

Trying to reduce duplicated effort in maintaining a mountinfo
parser, this patch replaces the local implementation with the
implementation in github.com/moby/sys, which is actively maintained
and contains various optimizations.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

vendor/github.com/moby/sys/mountinfo/mounted_linux.go

@@ -0,0 +1,58 @@
+package mountinfo
+
+import (
+	"os"
+	"path/filepath"
+
+	"golang.org/x/sys/unix"
+)
+
+// mountedByOpenat2 is a method of detecting a mount that works for all kinds
+// of mounts (incl. bind mounts), but requires a recent (v5.6+) linux kernel.
+func mountedByOpenat2(path string) (bool, error) {
+	dir, last := filepath.Split(path)
+
+	dirfd, err := unix.Openat2(unix.AT_FDCWD, dir, &unix.OpenHow{
+		Flags: unix.O_PATH | unix.O_CLOEXEC,
+	})
+	if err != nil {
+		if err == unix.ENOENT { // not a mount
+			return false, nil
+		}
+		return false, &os.PathError{Op: "openat2", Path: dir, Err: err}
+	}
+	fd, err := unix.Openat2(dirfd, last, &unix.OpenHow{
+		Flags:   unix.O_PATH | unix.O_CLOEXEC | unix.O_NOFOLLOW,
+		Resolve: unix.RESOLVE_NO_XDEV,
+	})
+	_ = unix.Close(dirfd)
+	switch err {
+	case nil: // definitely not a mount
+		_ = unix.Close(fd)
+		return false, nil
+	case unix.EXDEV: // definitely a mount
+		return true, nil
+	case unix.ENOENT: // not a mount
+		return false, nil
+	}
+	// not sure
+	return false, &os.PathError{Op: "openat2", Path: path, Err: err}
+}
+
+func mounted(path string) (bool, error) {
+	// Try a fast path, using openat2() with RESOLVE_NO_XDEV.
+	mounted, err := mountedByOpenat2(path)
+	if err == nil {
+		return mounted, nil
+	}
+	// Another fast path: compare st.st_dev fields.
+	mounted, err = mountedByStat(path)
+	// This does not work for bind mounts, so false negative
+	// is possible, therefore only trust if return is true.
+	if mounted && err == nil {
+		return mounted, nil
+	}
+
+	// Fallback to parsing mountinfo
+	return mountedByMountinfo(path)
+}