Handler: add SetCacheControl

This interface allows for returning a Cache-Control header that can vary
depending on HTTP 404 vs. HTTP 200. The interface could be expanded in
future.

handler.go

@@ -25,10 +25,12 @@ func New(packfile string) (*Handler, error) {
 	}
 
 	h := &Handler{
-		mapped:    mapped.Data,
-		dir:       mapped.Directory.Files,
-		headers:   make(map[string]string),
-		startTime: time.Now(),
+		mapped:          mapped.Data,
+		dir:             mapped.Directory.Files,
+		headers:         make(map[string]string),
+		cacheControl200: "no-cache",
+		cacheControl404: "no-cache",
+		startTime:       time.Now(),
 	}
 
 	// https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Frame-Options
@@ -42,11 +44,13 @@ func New(packfile string) (*Handler, error) {
 
 // Handler implements http.Handler and allows options to be set.
 type Handler struct {
-	mapped    []byte
-	dir       map[string]*packed.File
-	headers   map[string]string
-	startTime time.Time
-	notFound  *packed.File
+	mapped          []byte
+	dir             map[string]*packed.File
+	headers         map[string]string
+	cacheControl200 string
+	cacheControl404 string
+	startTime       time.Time
+	notFound        *packed.File
 }
 
 // SetHeader allows a custom header to be set on HTTP responses. These are
@@ -107,6 +111,19 @@ func (h *Handler) SetNotFound(notFound string) error {
 	return nil
 }
 
+// SetCacheControl allows control over the cache-control header. If not called,
+// "Cache-Control: no-cache" is the default.
+//
+// The first and second arguments are required, and specify the header to use
+// for 200 responses and 404 responses respectively.
+//
+// TODO: could be expanded to take extra varargs for more specific control over
+// particular assets, like different expiry per content-type.
+func (h *Handler) SetCacheControl(hdr200, hdr404 string) {
+	h.cacheControl200 = hdr200
+	h.cacheControl404 = hdr404
+}
+
 // ServeHTTP handles requests for files. It supports GET and HEAD methods, with
 // anything else returning a 405. Exact path matches are required, else a 404 is
 // returned.
@@ -127,6 +144,7 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 
 	info := h.dir[path.Clean(req.URL.Path)]
 	if info == nil {
+		w.Header().Set("Cache-Control", h.cacheControl404)
 		if h.notFound == nil {
 			http.NotFound(w, req)
 			return
@@ -135,6 +153,7 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 	}
 
 	// set standard headers
+	w.Header().Set("Cache-Control", h.cacheControl200)
 	w.Header().Set("Vary", "Accept-Encoding")
 	w.Header().Set("Etag", info.Etag)
 	w.Header().Set("Content-Type", info.ContentType)