refactor(server): Introduce a state type to carry runtime state

The state type contains things such as the bucket handle and Nixery's
configuration which need to be passed around in the builder.

This is only added for convenience.

tools/nixery/server/main.go

@@ -122,10 +122,8 @@ func writeError(w http.ResponseWriter, status int, code, message string) {
 }
 
 type registryHandler struct {
-	cfg    *config.Config
-	ctx    *context.Context
-	bucket *storage.BucketHandle
-	cache  *builder.LocalCache
+	ctx   *context.Context
+	state *builder.State
 }
 
 func (h *registryHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
@@ -141,7 +139,7 @@ func (h *registryHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		imageTag := manifestMatches[2]
 		log.Printf("Requesting manifest for image %q at tag %q", imageName, imageTag)
 		image := builder.ImageFromName(imageName, imageTag)
-		buildResult, err := builder.BuildImage(h.ctx, h.cfg, h.cache, &image, h.bucket)
+		buildResult, err := builder.BuildImage(h.ctx, h.state, &image)
 
 		if err != nil {
 			writeError(w, 500, "UNKNOWN", "image build failure")
@@ -172,7 +170,7 @@ func (h *registryHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	layerMatches := layerRegex.FindStringSubmatch(r.RequestURI)
 	if len(layerMatches) == 3 {
 		digest := layerMatches[2]
-		url, err := constructLayerUrl(h.cfg, digest)
+		url, err := constructLayerUrl(&h.state.Cfg, digest)
 
 		if err != nil {
 			log.Printf("Failed to sign GCS URL: %s\n", err)
@@ -197,16 +195,14 @@ func main() {
 
 	ctx := context.Background()
 	bucket := prepareBucket(&ctx, cfg)
-	cache := builder.NewCache()
+	state := builder.NewState(bucket, *cfg)
 
 	log.Printf("Starting Nixery on port %s\n", cfg.Port)
 
 	// All /v2/ requests belong to the registry handler.
 	http.Handle("/v2/", &registryHandler{
-		cfg:    cfg,
-		ctx:    &ctx,
-		bucket: bucket,
-		cache:  &cache,
+		ctx:   &ctx,
+		state: &state,
 	})
 
 	// All other roots are served by the static file server.