snapshot: add Usage method to Snapshotter

To allow the querying of usage for snapshots, we define a new method on
the snapshotter to query the resources in use by a single snapshot.
Conversely, it can be said that if the snapshot was deleted, the
reported amount of usage would be recovered.

There are few problems with this model in the implementation of btrfs
that need to be worked out. In btrfs, it is hard to resolve the amount
of data usage with the use of quotas but these may report valuables that
are incompatible with the model.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

snapshot/naive/naive.go

@@ -61,7 +61,35 @@ func (o *snapshotter) Stat(ctx context.Context, key string) (snapshot.Info, erro
 		return snapshot.Info{}, err
 	}
 	defer t.Rollback()
-	return storage.GetInfo(ctx, key)
+	_, info, _, err := storage.GetInfo(ctx, key)
+	if err != nil {
+		return snapshot.Info{}, err
+	}
+
+	return info, nil
+}
+
+func (o *snapshotter) Usage(ctx context.Context, key string) (snapshot.Usage, error) {
+	ctx, t, err := o.ms.TransactionContext(ctx, false)
+	if err != nil {
+		return snapshot.Usage{}, err
+	}
+	defer t.Rollback()
+
+	id, info, usage, err := storage.GetInfo(ctx, key)
+	if err != nil {
+		return snapshot.Usage{}, err
+	}
+
+	if info.Kind == snapshot.KindActive {
+		du, err := fs.DiskUsage(o.getSnapshotDir(id))
+		if err != nil {
+			return snapshot.Usage{}, err
+		}
+		usage = snapshot.Usage(du)
+	}
+
+	return usage, nil
 }
 
 func (o *snapshotter) Prepare(ctx context.Context, key, parent string) ([]containerd.Mount, error) {
@@ -94,7 +122,18 @@ func (o *snapshotter) Commit(ctx context.Context, name, key string) error {
 	if err != nil {
 		return err
 	}
-	if _, err := storage.CommitActive(ctx, key, name); err != nil {
+
+	id, _, _, err := storage.GetInfo(ctx, key)
+	if err != nil {
+		return err
+	}
+
+	usage, err := fs.DiskUsage(o.getSnapshotDir(id))
+	if err != nil {
+		return err
+	}
+
+	if _, err := storage.CommitActive(ctx, key, name, snapshot.Usage(usage)); err != nil {
 		if rerr := t.Rollback(); rerr != nil {
 			log.G(ctx).WithError(rerr).Warn("Failure rolling back transaction")
 		}