Add volume state

We need to be able to track the number of times a volume has been
mounted for tmpfs/nfs/etc volumes. As such, we need a mutable
state for volumes. Add one, with the expected update/save methods
in both states.

There is backwards compat here, in that older volumes without a
state will still be accepted.

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

libpod/in_memory_state.go

@@ -507,6 +507,36 @@ func (s *InMemoryState) RemoveVolume(volume *Volume) error {
 	return nil
 }
 
+// UpdateVolume updates a volume from the database.
+// For the in-memory state, this is a no-op.
+func (s *InMemoryState) UpdateVolume(volume *Volume) error {
+	if !volume.valid {
+		return define.ErrVolumeRemoved
+	}
+
+	if _, ok := s.volumes[volume.Name()]; !ok {
+		volume.valid = false
+		return errors.Wrapf(define.ErrNoSuchVolume, "volume with name %q not found in state", volume.Name())
+	}
+
+	return nil
+}
+
+// SaveVolume saves a volume's state to the database.
+// For the in-memory state, this is a no-op.
+func (s *InMemoryState) SaveVolume(volume *Volume) error {
+	if !volume.valid {
+		return define.ErrVolumeRemoved
+	}
+
+	if _, ok := s.volumes[volume.Name()]; !ok {
+		volume.valid = false
+		return errors.Wrapf(define.ErrNoSuchVolume, "volume with name %q not found in state", volume.Name())
+	}
+
+	return nil
+}
+
 // VolumeInUse checks if the given volume is being used by at least one container
 func (s *InMemoryState) VolumeInUse(volume *Volume) ([]string, error) {
 	if !volume.valid {