/*
 *
 * 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.
 *
 */

// The server demonstrates how to consume and validate OAuth2 tokens provided by
// clients for each RPC.
package main

import (
	"context"
	"crypto/tls"
	"flag"
	"fmt"
	"log"
	"net"
	"strings"

	"google.golang.org/grpc"
	"google.golang.org/grpc/codes"
	"google.golang.org/grpc/credentials"
	"google.golang.org/grpc/examples/data"
	"google.golang.org/grpc/metadata"
	"google.golang.org/grpc/status"

	pb "google.golang.org/grpc/examples/features/proto/echo"
)

var (
	errMissingMetadata = status.Errorf(codes.InvalidArgument, "missing metadata")
	errInvalidToken    = status.Errorf(codes.Unauthenticated, "invalid token")
)

var port = flag.Int("port", 50051, "the port to serve on")

func main() {
	flag.Parse()
	fmt.Printf("server starting on port %d...\n", *port)

	cert, err := tls.LoadX509KeyPair(data.Path("x509/server_cert.pem"), data.Path("x509/server_key.pem"))
	if err != nil {
		log.Fatalf("failed to load key pair: %s", err)
	}
	opts := []grpc.ServerOption{
		// The following grpc.ServerOption adds an interceptor for all unary
		// RPCs. To configure an interceptor for streaming RPCs, see:
		// https://godoc.org/google.golang.org/grpc#StreamInterceptor
		grpc.UnaryInterceptor(ensureValidToken),
		// Enable TLS for all incoming connections.
		grpc.Creds(credentials.NewServerTLSFromCert(&cert)),
	}
	s := grpc.NewServer(opts...)
	pb.RegisterEchoServer(s, &ecServer{})
	lis, err := net.Listen("tcp", fmt.Sprintf(":%d", *port))
	if err != nil {
		log.Fatalf("failed to listen: %v", err)
	}
	if err := s.Serve(lis); err != nil {
		log.Fatalf("failed to serve: %v", err)
	}
}

type ecServer struct {
	pb.UnimplementedEchoServer
}

func (s *ecServer) UnaryEcho(ctx context.Context, req *pb.EchoRequest) (*pb.EchoResponse, error) {
	return &pb.EchoResponse{Message: req.Message}, nil
}

// valid validates the authorization.
func valid(authorization []string) bool {
	if len(authorization) < 1 {
		return false
	}
	token := strings.TrimPrefix(authorization[0], "Bearer ")
	// Perform the token validation here. For the sake of this example, the code
	// here forgoes any of the usual OAuth2 token validation and instead checks
	// for a token matching an arbitrary string.
	return token == "some-secret-token"
}

// ensureValidToken ensures a valid token exists within a request's metadata. If
// the token is missing or invalid, the interceptor blocks execution of the
// handler and returns an error. Otherwise, the interceptor invokes the unary
// handler.
func ensureValidToken(ctx context.Context, req interface{}, info *grpc.UnaryServerInfo, handler grpc.UnaryHandler) (interface{}, error) {
	md, ok := metadata.FromIncomingContext(ctx)
	if !ok {
		return nil, errMissingMetadata
	}
	// The keys within metadata.MD are normalized to lowercase.
	// See: https://godoc.org/google.golang.org/grpc/metadata#New
	if !valid(md["authorization"]) {
		return nil, errInvalidToken
	}
	// Continue execution of handler after ensuring a valid token.
	return handler(ctx, req)
}