examples: wait_for_ready (#2503)

* Working example.

* Uses echo server.

* Style fix.

* Changes client streaming implementation.

* Adds README.

* replaces the use of failfast with waitforready.

* Adds package comment.

examples/features/wait_for_ready/README.md

@@ -0,0 +1,11 @@
+# Wait for ready example
+
+This example shows how to enable "wait for ready" in RPC calls.
+
+This code starts a server with a 2 seconds delay. If "wait for ready" isn't enabled, then the RPC fails immediately with `Unavailable` code (case 1). If "wait for ready" is enabled, then the RPC waits for the server. If context dies before the server is available, then it fails with `DeadlineExceeded` (case 3). Otherwise it succeeds (case 2).
+
+## Run the example
+
+```
+go run main.go
+```

examples/features/wait_for_ready/main.go

@@ -0,0 +1,125 @@
+/*
+ *
+ * Copyright 2018 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.
+ *
+ */
+
+// Binary wait_for_ready is an example for "wait for ready".
+package main
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"net"
+	"sync"
+	"time"
+
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/codes"
+	pb "google.golang.org/grpc/examples/features/proto/echo"
+	"google.golang.org/grpc/status"
+)
+
+// server is used to implement EchoServer.
+type server struct{}
+
+func (s *server) UnaryEcho(ctx context.Context, req *pb.EchoRequest) (*pb.EchoResponse, error) {
+	return &pb.EchoResponse{Message: req.Message}, nil
+}
+
+func (s *server) ServerStreamingEcho(req *pb.EchoRequest, stream pb.Echo_ServerStreamingEchoServer) error {
+	return status.Error(codes.Unimplemented, "RPC unimplemented")
+}
+
+func (s *server) ClientStreamingEcho(stream pb.Echo_ClientStreamingEchoServer) error {
+	return status.Error(codes.Unimplemented, "RPC unimplemented")
+}
+
+func (s *server) BidirectionalStreamingEcho(stream pb.Echo_BidirectionalStreamingEchoServer) error {
+	return status.Error(codes.Unimplemented, "RPC unimplemented")
+}
+
+// serve starts listening with a 2 seconds delay.
+func serve() {
+	lis, err := net.Listen("tcp", ":50053")
+	if err != nil {
+		log.Fatalf("failed to listen: %v", err)
+	}
+	s := grpc.NewServer()
+	pb.RegisterEchoServer(s, &server{})
+
+	if err := s.Serve(lis); err != nil {
+		log.Fatalf("failed to serve: %v", err)
+	}
+}
+
+func main() {
+	conn, err := grpc.Dial("localhost:50053", grpc.WithInsecure())
+	if err != nil {
+		log.Fatalf("did not connect: %v", err)
+	}
+	defer conn.Close()
+
+	c := pb.NewEchoClient(conn)
+
+	var wg sync.WaitGroup
+	wg.Add(3)
+
+	// "Wait for ready" is not enabled, returns error with code "Unavailable".
+	go func() {
+		defer wg.Done()
+
+		ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+		defer cancel()
+
+		_, err := c.UnaryEcho(ctx, &pb.EchoRequest{Message: "Hi!"})
+
+		got := status.Code(err)
+		fmt.Printf("[1] wanted = %v, got = %v\n", codes.Unavailable, got)
+	}()
+
+	// "Wait for ready" is enabled, returns nil error.
+	go func() {
+		defer wg.Done()
+
+		ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+		defer cancel()
+
+		_, err := c.UnaryEcho(ctx, &pb.EchoRequest{Message: "Hi!"}, grpc.WaitForReady(true))
+
+		got := status.Code(err)
+		fmt.Printf("[2] wanted = %v, got = %v\n", codes.OK, got)
+	}()
+
+	// "Wait for ready" is enabled but exceeds the deadline before server starts listening,
+	// returns error with code "DeadlineExceeded".
+	go func() {
+		defer wg.Done()
+
+		ctx, cancel := context.WithTimeout(context.Background(), 1*time.Second)
+		defer cancel()
+
+		_, err := c.UnaryEcho(ctx, &pb.EchoRequest{Message: "Hi!"}, grpc.WaitForReady(true))
+
+		got := status.Code(err)
+		fmt.Printf("[3] wanted = %v, got = %v\n", codes.DeadlineExceeded, got)
+	}()
+
+	time.Sleep(2 * time.Second)
+	go serve()
+
+	wg.Wait()
+}