mount: add mount.Lookup for ease of implementing snapshotters

`func Lookup(dir string) (Info, error)` returns the mount
info that corresponds to the dir

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

mount/lookup_unix.go

@@ -0,0 +1,37 @@
+// +build !windows
+
+package mount
+
+import (
+	"fmt"
+	"syscall"
+
+	"github.com/pkg/errors"
+)
+
+// Lookup returns the mount info corresponds to the path.
+func Lookup(dir string) (Info, error) {
+	var dirStat syscall.Stat_t
+	if err := syscall.Stat(dir, &dirStat); err != nil {
+		return Info{}, errors.Wrapf(err, "failed to access %q", dir)
+	}
+
+	mounts, err := Self()
+	if err != nil {
+		return Info{}, err
+	}
+	for _, m := range mounts {
+		// Note that m.{Major, Minor} are generally unreliable for our purpose here
+		// https://www.spinics.net/lists/linux-btrfs/msg58908.html
+		var st syscall.Stat_t
+		if err := syscall.Stat(m.Mountpoint, &st); err != nil {
+			// may fail; ignore err
+			continue
+		}
+		if st.Dev == dirStat.Dev {
+			return m, nil
+		}
+	}
+
+	return Info{}, fmt.Errorf("failed to find the mount info for %q", dir)
+}