resources: aws: ec2: Use custom listener for snsServer

This patch replaces the call to Server.ListenAndServe() with
Server.Serve(listener) in order to make sure the listener is up
and running before we subscribe to the topic in a future patch.

resources/aws_ec2.go

@@ -23,6 +23,7 @@ import (
 	"fmt"
 	"io/ioutil"
 	"log"
+	"net"
 	"net/http"
 	"strconv"
 	"sync"
@@ -510,9 +511,17 @@ func (obj *AwsEc2Res) snsWatch() error {
 	var exit *error
 	defer obj.wg.Wait()
 	defer close(obj.closeChan)
-	// set up the sns endpoint
-	snsServer := obj.snsServer()
-	// shutdown the sns endpoint when we're done
+	// create the sns listener
+	// closing is handled by http.Server.Shutdown in the defer func below
+	listener, err := obj.snsListener(obj.WatchListenAddr)
+	if err != nil {
+		return errwrap.Wrapf(err, "error creating listener")
+	}
+	// set up the sns server
+	snsServer := &http.Server{
+		Handler: http.HandlerFunc(obj.snsPostHandler),
+	}
+	// close the listener and shutdown the sns server when we're done
 	defer func() {
 		ctx, cancel := context.WithTimeout(context.TODO(), SnsServerShutdownTimeout*time.Second)
 		defer cancel()
@@ -525,11 +534,11 @@ func (obj *AwsEc2Res) snsWatch() error {
 		}
 	}()
 	obj.wg.Add(1)
-	// start the endpoint
+	// start the sns server
 	go func() {
 		defer obj.wg.Done()
 		defer close(obj.awsChan)
-		if err := snsServer.ListenAndServe(); err != nil {
+		if err := snsServer.Serve(listener); err != nil {
 			// when we shut down
 			if err == http.ErrServerClosed {
 				log.Printf("%s: Stopped SNS Endpoint", obj)
@@ -802,18 +811,18 @@ func (obj *AwsEc2Res) prependName() string {
 	return AwsPrefix + obj.GetName()
 }
 
-// snsServer returns an http server used to listen for sns messages.
-func (obj *AwsEc2Res) snsServer() *http.Server {
-	addr := obj.WatchListenAddr
-	// if addr is a port
-	if _, err := strconv.Atoi(obj.WatchListenAddr); err == nil {
-		addr = fmt.Sprintf(":%s", obj.WatchListenAddr)
+// snsListener returns a listener bound to listenAddr.
+func (obj *AwsEc2Res) snsListener(listenAddr string) (net.Listener, error) {
+	addr := listenAddr
+	// if listenAddr is a port
+	if _, err := strconv.Atoi(listenAddr); err == nil {
+		addr = fmt.Sprintf(":%s", listenAddr)
 	}
-	handler := http.HandlerFunc(obj.snsPostHandler)
-	return &http.Server{
-		Addr:    addr,
-		Handler: handler,
+	listener, err := net.Listen("tcp", addr)
+	if err != nil {
+		return nil, err
 	}
+	return listener, nil
 }
 
 // snsPostHandler listens for posts on the SNS Endpoint.