Add debug server to support pprof and expvar

If configured, a debug http server will be started to serve default registered
endpoints, such as pprof and expvar. The endpoint should be secured carefully
and not available to external traffic. It is disabled by default but the
development config has been modified to make it available on localhost.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

cmd/registry/config.yml

@@ -5,3 +5,6 @@ storage:
         rootdirectory: /tmp/registry-dev
 http:
     addr: :5000
+    secret: asecretforlocaldevelopment
+    debug:
+        addr: localhost:5001

cmd/registry/main.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	_ "expvar"
 	"flag"
 	"fmt"
 	"net/http"
@@ -47,6 +48,10 @@ func main() {
 	handler = handlers.CombinedLoggingHandler(os.Stdout, handler)
 	log.SetLevel(logLevel(config.Loglevel))
 
+	if config.HTTP.Debug.Addr != "" {
+		go debugServer(config.HTTP.Debug.Addr)
+	}
+
 	if config.HTTP.TLS.Certificate == "" {
 		log.Infof("listening on %v", config.HTTP.Addr)
 		if err := http.ListenAndServe(config.HTTP.Addr, handler); err != nil {
@@ -142,3 +147,13 @@ func configureReporting(app *registry.App) http.Handler {
 
 	return handler
 }
+
+// debugServer starts the debug server with pprof, expvar among other
+// endpoints. The addr should not be exposed externally. For most of these to
+// work, tls cannot be enabled on the endpoint, so it is generally separate.
+func debugServer(addr string) {
+	log.Infof("debug server listening %v", addr)
+	if err := http.ListenAndServe(addr, nil); err != nil {
+		log.Fatalf("error listening on debug interface: %v", err)
+	}
+}