Use mount.Target to specify subdirectory of rootfs mount

- Add Target to mount.Mount.
- Add UnmountMounts to unmount a list of mounts in reverse order.
- Add UnmountRecursive to unmount deepest mount first for a given target, using
moby/sys/mountinfo.

Signed-off-by: Edgar Lee <edgarhinshunlee@gmail.com>

mount/mount.go

@@ -16,6 +16,12 @@
 
 package mount
 
+import (
+	"fmt"
+
+	"github.com/containerd/continuity/fs"
+)
+
 // Mount is the lingua franca of containerd. A mount represents a
 // serialized mount syscall. Components either emit or consume mounts.
 type Mount struct {
@@ -24,12 +30,16 @@ type Mount struct {
 	// Source specifies where to mount from. Depending on the host system, this
 	// can be a source path or device.
 	Source string
+	// Target specifies an optional subdirectory as a mountpoint. It assumes that
+	// the subdirectory exists in a parent mount.
+	Target string
 	// Options contains zero or more fstab-style mount options. Typically,
 	// these are platform specific.
 	Options []string
 }
 
-// All mounts all the provided mounts to the provided target
+// All mounts all the provided mounts to the provided target. If submounts are
+// present, it assumes that parent mounts come before child mounts.
 func All(mounts []Mount, target string) error {
 	for _, m := range mounts {
 		if err := m.Mount(target); err != nil {
@@ -38,3 +48,30 @@ func All(mounts []Mount, target string) error {
 	}
 	return nil
 }
+
+// UnmountMounts unmounts all the mounts under a target in the reverse order of
+// the mounts array provided.
+func UnmountMounts(mounts []Mount, target string, flags int) error {
+	for i := len(mounts) - 1; i >= 0; i-- {
+		mountpoint, err := fs.RootPath(target, mounts[i].Target)
+		if err != nil {
+			return err
+		}
+
+		if err := UnmountAll(mountpoint, flags); err != nil {
+			if i == len(mounts)-1 { // last mount
+				return err
+			}
+		}
+	}
+	return nil
+}
+
+// Mount to the provided target path.
+func (m *Mount) Mount(target string) error {
+	target, err := fs.RootPath(target, m.Target)
+	if err != nil {
+		return fmt.Errorf("failed to join path %q with root %q: %w", m.Target, target, err)
+	}
+	return m.mount(target)
+}