Add helpers for common tasks in action handlers

This patch also changes internal server error message and action
handlers tests to unify all error messages.

errors.go

@@ -0,0 +1,8 @@
+package main
+
+import "errors"
+
+var (
+	// ErrRelayNotFound is returned when Relay is not found.
+	ErrRelayNotFound = errors.New("relay not found")
+)

handlers.go

@@ -25,22 +25,36 @@ func RelayIndex(w http.ResponseWriter, r *http.Request, _ httprouter.Params) {
 // RelayShow handles the relays show action (GET /relays/:id).
 func RelayShow(w http.ResponseWriter, r *http.Request, params httprouter.Params) {
 	id := params.ByName("id")
+	relay, err := findRelay(id)
+	if err != nil {
+		writeError(w, http.StatusNotFound, err.Error())
+		return
+	}
+	writeResponse(w, http.StatusOK, &JsonResponse{Data: relay})
+}
+
+// findRelay locates Relay with the given ID on the module.
+func findRelay(id string) (*Relay, error) {
 	relay, ok := module[id]
 	if !ok {
 		// No relay with the ID given in the URL has been found
-		apiError := &ApiError{Status: http.StatusNotFound, Title: "Relay not found"}
-		writeResponse(w, http.StatusNotFound, &JsonErrorResponse{Error: apiError})
-		return
+		return nil, ErrRelayNotFound
 	}
-	writeResponse(w, http.StatusOK, &JsonResponse{Data: relay})
+	return relay, nil
+}
+
+// writeError wraps writing API error response.
+func writeError(w http.ResponseWriter, status int, title string) {
+	apiError := &ApiError{Status: status, Title: title}
+	writeResponse(w, status, &JsonErrorResponse{Error: apiError})
 }
 
 // writeResponse writes standard JSON API response with status code.
 func writeResponse(w http.ResponseWriter, status int, response interface{}) {
 	w.Header().Set("Content-Type", "application/json; charset=UTF-8")
 	w.WriteHeader(status)
 	if err := json.NewEncoder(w).Encode(response); err != nil {
-		apiError := &ApiError{Status: http.StatusInternalServerError, Title: "Internal server error"}
+		apiError := &ApiError{Status: http.StatusInternalServerError, Title: "internal server error"}
 		writeResponse(w, http.StatusInternalServerError, apiError)
 	}
 }

handlers_test.go

@@ -45,7 +45,7 @@ var _ = Describe("Main", func() {
 	})
 
 	It("should return an error on show request for nonexistent relay", func() {
-		expected := "{\"error\":{\"status\":404,\"title\":\"Relay not found\"}}\n"
+		expected := "{\"error\":{\"status\":404,\"title\":\"relay not found\"}}\n"
 		actual := call("GET", "/relays/I0", "/relays/:id", RelayShow)
 		Expect(actual.Code).To(Equal(http.StatusNotFound))
 		Expect(actual.Body.String()).To(MatchJSON(expected))