Add middleware for debug headers

If the request sends an x-debug header with any value, the response headers will be populated with the build ID and tag.

cmd/adminapi/main.go

@@ -124,6 +124,10 @@ func realMain(ctx context.Context) error {
 	// Install common security headers
 	r.Use(middleware.SecureHeaders(ctx, config.DevMode, "json"))
 
+	// Enable debug headers
+	processDebug := middleware.ProcessDebug(ctx)
+	r.Use(processDebug)
+
 	// Create the renderer
 	h, err := render.New(ctx, "", config.DevMode)
 	if err != nil {

cmd/apiserver/main.go

@@ -137,6 +137,10 @@ func realMain(ctx context.Context) error {
 	// Install common security headers
 	r.Use(middleware.SecureHeaders(ctx, config.DevMode, "json"))
 
+	// Enable debug headers
+	processDebug := middleware.ProcessDebug(ctx)
+	r.Use(processDebug)
+
 	// Create the renderer
 	h, err := render.New(ctx, "", config.DevMode)
 	if err != nil {

cmd/server/main.go

@@ -169,6 +169,10 @@ func realMain(ctx context.Context) error {
 	// Install common security headers
 	r.Use(middleware.SecureHeaders(ctx, config.DevMode, "html"))
 
+	// Enable debug headers
+	processDebug := middleware.ProcessDebug(ctx)
+	r.Use(processDebug)
+
 	// Install the CSRF protection middleware.
 	configureCSRF := middleware.ConfigureCSRF(ctx, config, h)
 	r.Use(configureCSRF)

pkg/controller/middleware/debug.go

@@ -0,0 +1,40 @@
+// Copyright 2020 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package middleware
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/google/exposure-notifications-verification-server/pkg/buildinfo"
+	"github.com/gorilla/mux"
+)
+
+// ProcessDebug adds additional debugging information to the response if the
+// request included the "X-Debug" header with any value.
+func ProcessDebug(ctx context.Context) mux.MiddlewareFunc {
+	header := "x-debug"
+
+	return func(next http.Handler) http.Handler {
+		return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			if r.Header.Get(header) != "" {
+				w.Header().Set("x-build-id", buildinfo.BuildID)
+				w.Header().Set("x-build-tag", buildinfo.BuildTag)
+			}
+
+			next.ServeHTTP(w, r)
+		})
+	}
+}