Use imageStore.Children instead of whole the Map

daemon cache was getting the whole image map and then iterating through
it to find children. This information is already stored in the image
store.

Prior to this change building the docker repo with a full cache took 30
seconds.
After it takes between 15 seconds or less (As low as 9 seconds).
This is an improvement on docker 1.9.1 which hovered around 17 seconds.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

daemon/daemon.go

@@ -1294,29 +1294,22 @@ func (daemon *Daemon) GetRemappedUIDGID() (int, int) {
 	return uid, gid
 }
 
-// ImageGetCached returns the earliest created image that is a child
+// ImageGetCached returns the most recent created image that is a child
 // of the image with imgID, that had the same config when it was
 // created. nil is returned if a child cannot be found. An error is
 // returned if the parent image cannot be found.
 func (daemon *Daemon) ImageGetCached(imgID image.ID, config *containertypes.Config) (*image.Image, error) {
-	// Retrieve all images
-	imgs := daemon.Map()
-
-	var siblings []image.ID
-	for id, img := range imgs {
-		if img.Parent == imgID {
-			siblings = append(siblings, id)
-		}
-	}
-
 	// Loop on the children of the given image and check the config
+	getMatch := func(siblings []image.ID) (*image.Image, error) {
 		var match *image.Image
 		for _, id := range siblings {
-		img, ok := imgs[id]
+			img, err := daemon.imageStore.Get(id)
-		if !ok {
+			if err != nil {
 				return nil, fmt.Errorf("unable to find image %q", id)
 			}
+
 			if runconfig.Compare(&img.ContainerConfig, config) {
+				// check for the most up to date match
 				if match == nil || match.Created.Before(img.Created) {
 					match = img
 				}
@@ -1325,6 +1318,23 @@ func (daemon *Daemon) ImageGetCached(imgID image.ID, config *containertypes.Conf
 		return match, nil
 	}
 
+	// In this case, this is `FROM scratch`, which isn't an actual image.
+	if imgID == "" {
+		images := daemon.imageStore.Map()
+		var siblings []image.ID
+		for id, img := range images {
+			if img.Parent == imgID {
+				siblings = append(siblings, id)
+			}
+		}
+		return getMatch(siblings)
+	}
+
+	// find match from child images
+	siblings := daemon.imageStore.Children(imgID)
+	return getMatch(siblings)
+}
+
 // tempDir returns the default directory to use for temporary files.
 func tempDir(rootDir string, rootUID, rootGID int) (string, error) {
 	var tmpDir string