Add new ctr option for discarding unpacked layers

Add a new ctr cli option, allowing the garbage collector to discard any
unpacked layers after importing an image. This new option is
incompatible with the no-unpack ctr import option.

Signed-off-by: James Jenkins <James.Jenkins@ibm.com>

import.go

@@ -38,6 +38,7 @@ type importOpts struct {
 	allPlatforms    bool
 	platformMatcher platforms.MatchComparer
 	compress        bool
+	discardLayers   bool
 }
 
 // ImportOpt allows the caller to specify import specific options
@@ -105,6 +106,15 @@ func WithImportCompression() ImportOpt {
 	}
 }
 
+// WithDiscardUnpackedLayers allows the garbage collector to clean up
+// layers from content store after unpacking.
+func WithDiscardUnpackedLayers(discard bool) ImportOpt {
+	return func(c *importOpts) error {
+		c.discardLayers = discard
+		return nil
+	}
+}
+
 // Import imports an image from a Tar stream using reader.
 // Caller needs to specify importer. Future version may use oci.v1 as the default.
 // Note that unreferenced blobs may be imported to the content store as well.
@@ -195,7 +205,11 @@ func (c *Client) Import(ctx context.Context, reader io.Reader, opts ...ImportOpt
 	}
 
 	handler = images.FilterPlatforms(handler, platformMatcher)
-	handler = images.SetChildrenLabels(cs, handler)
+	if iopts.discardLayers {
+		handler = images.SetChildrenMappedLabels(cs, handler, images.ChildGCLabelsFilterLayers)
+	} else {
+		handler = images.SetChildrenLabels(cs, handler)
+	}
 	if err := images.WalkNotEmpty(ctx, handler, index); err != nil {
 		return nil, err
 	}