Add support for 'image' volume driver

We added the concept of image volumes in 2.2.0, to support
inspecting an image from within a container. However, this is a
strictly read-only mount, with no modification allowed.

By contrast, the new `image` volume driver creates a c/storage
container as its underlying storage, so we have a read/write
layer. This, in and of itself, is not especially interesting, but
what it will enable in the future is. If we add a new command to
allow these image volumes to be committed, we can now distribute
volumes - and changes to them - via a standard OCI image registry
(which is rather new and quite exciting).

Future work in this area:
- Add support for `podman volume push` (commit volume changes and
  push resulting image to OCI registry).
- Add support for `podman volume pull` (currently, we require
  that the image a volume is created from be already pulled; it
  would be simpler if we had a dedicated command that did the
  pull and made a volume from it)
- Add support for scratch images (make an empty image on demand
  to use as the base of the volume)
- Add UOR support to `podman volume push` and
  `podman volume pull` to enable both with non-image volume
  drivers

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

1 files changed, 11 insertions, 0 deletions

diff --git a/libpod/runtime_cstorage.go b/libpod/runtime_cstorage.go
index 372434b49..5917b7931 100644
--- a/libpod/runtime_cstorage.go
+++ b/libpod/runtime_cstorage.go
@@ -86,6 +86,17 @@ func (r *Runtime) RemoveStorageContainer(idOrName string, force bool) error {
 		return fmt.Errorf("refusing to remove %q as it exists in libpod as container %s: %w", idOrName, ctr.ID, define.ErrCtrExists)
 	}
 
+	// Error out if this is an image-backed volume
+	allVols, err := r.state.AllVolumes()
+	if err != nil {
+		return err
+	}
+	for _, vol := range allVols {
+		if vol.config.Driver == define.VolumeDriverImage && vol.config.StorageID == ctr.ID {
+			return fmt.Errorf("refusing to remove %q as it exists in libpod as an image-backed volume %s: %w", idOrName, vol.Name(), define.ErrCtrExists)
+		}
+	}
+
 	if !force {
 		timesMounted, err := r.store.Mounted(ctr.ID)
 		if err != nil {