added e2e greeter server (matthewstevenson88#58)

* created e2e test server file

* added localhost to handshakerserviceaddress

* fixed flag names, and removed localidentities

* changed var names, listen address

* fixed comment

security/s2a/examples/e2e_server/main.go

@@ -0,0 +1,72 @@
+/*
+ *
+ * Copyright 2020 gRPC authors.
+ *
+ * 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.
+ *
+ */
+
+// Runs a Greeter service that uses S2A to establish secure connections with 
+// greeter clients.
+package main
+
+import (
+	"context"
+	"flag"
+	"fmt"
+	"log"
+	"net"
+
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/security/s2a"
+	pb "google.golang.org/grpc/security/s2a/examples/helloworld"
+)
+
+var (
+	serverAddr    = flag.String("server_address", "localhost:50051", "The address of the gRPC server.")
+	s2aServerAddr = flag.String("s2a_server_address", "localhost:61365", "S2A server address.")
+)
+
+// server is used to implement helloworld.GreeterServer.
+type server struct {
+	pb.UnimplementedGreeterServer
+}
+
+// SayHello implements helloworld.GreeterServer
+func (s *server) SayHello(ctx context.Context, in *pb.HelloRequest) (*pb.HelloReply, error) {
+	log.Printf("Received: %v", in.GetName())
+	return &pb.HelloReply{Message: "Hello, " + in.GetName() + "!"}, nil
+}
+
+func main() {
+	flag.Parse()
+
+	// Set up server-side S2A transport credentials.
+	serverOpts := &s2a.ServerOptions{
+		HandshakerServiceAddress: *s2aServerAddr,
+	}
+	creds, err := s2a.NewServerCreds(serverOpts)
+	if err != nil {
+		log.Fatalf("NewServerCreds(%v) failed: %v", serverOpts, err)
+	}
+
+	lis, err := net.Listen("tcp", fmt.Sprintf("%s", *serverAddr))
+	if err != nil {
+		log.Fatalf("failed to listen on address %s: %v", *serverAddr, err)
+	}
+	s := grpc.NewServer(grpc.Creds(creds))
+	pb.RegisterGreeterServer(s, &server{})
+	if err := s.Serve(lis); err != nil {
+		log.Fatalf("failed to serve: %v", err)
+	}
+}