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/builder/state.go

@@ -0,0 +1,24 @@
+package builder
+
+import (
+	"cloud.google.com/go/storage"
+	"github.com/google/nixery/config"
+	"github.com/google/nixery/layers"
+)
+
+// State holds the runtime state that is carried around in Nixery and
+// passed to builder functions.
+type State struct {
+	Bucket *storage.BucketHandle
+	Cache  LocalCache
+	Cfg    config.Config
+	Pop    layers.Popularity
+}
+
+func NewState(bucket *storage.BucketHandle, cfg config.Config) State {
+	return State{
+		Bucket: bucket,
+		Cfg:    cfg,
+		Cache:  NewCache(),
+	}
+}

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.