Delete tag if its history is empty

During a run of prune images, history of a tag may get emptied. When an
image with the same tag is re-pushed, the operation will fail because
the history list is not expected to be empty.

This patch checks the history list for non-emptiness before returning a
valid tag event and ensures that tag is deleted from ImageStream's
status.tags  when the last item has been removed from its history during
a prune.

Signed-off-by: Michal Minar <miminar@redhat.com>

pkg/image/api/helper.go

@@ -300,6 +300,9 @@ func LatestTaggedImage(stream *ImageStream, tag string) *TagEvent {
 	// find the most recent tag event with an image reference
 	if stream.Status.Tags != nil {
 		if history, ok := stream.Status.Tags[tag]; ok {
+			if len(history.Items) == 0 {
+				return nil
+			}
 			return &history.Items[0]
 		}
 	}

pkg/image/prune/imagepruner.go

@@ -668,7 +668,12 @@ func pruneStreams(g graph.Graph, imageNodes []*imagegraph.ImageNode, streamPrune
 						updatedTags.Insert(tag)
 					}
 				}
-				stream.Status.Tags[tag] = newHistory
+				if len(newHistory.Items) == 0 {
+					glog.V(4).Infof("Removing tag %q from status.tags of ImageStream %s/%s", tag, stream.Namespace, stream.Name)
+					delete(stream.Status.Tags, tag)
+				} else {
+					stream.Status.Tags[tag] = newHistory
+				}
 			}
 
 			updatedStream, err := streamPruner.PruneImageStream(stream, imageNode.Image, updatedTags.List())