Add CORS preflight support (#5177)

* Add CORS preflight support

* lint

* clarify description

beacon-chain/flags/base.go

@@ -57,6 +57,12 @@ var (
 		Name:  "grpc-gateway-port",
 		Usage: "Enable gRPC gateway for JSON requests",
 	}
+	// GPRCGatewayCorsDomain serves preflight requests when serving gRPC JSON gateway.
+	GPRCGatewayCorsDomain = &cli.StringFlag{
+		Name: "grpc-gateway-corsdomain",
+		Usage: "Comma separated list of domains from which to accept cross origin requests " +
+			"(browser enforced). This flag has no effect if not used with --grpc-gateway-port.",
+	}
 	// MinSyncPeers specifies the required number of successful peer handshakes in order
 	// to start syncing with external peers.
 	MinSyncPeers = &cli.IntFlag{

beacon-chain/gateway/BUILD.bazel

@@ -4,6 +4,7 @@ load("@io_bazel_rules_go//go:def.bzl", "go_library")
 go_library(
     name = "go_default_library",
     srcs = [
+        "cors.go",
         "gateway.go",
         "handlers.go",
         "log.go",
@@ -16,6 +17,7 @@ go_library(
     deps = [
         "//shared:go_default_library",
         "@com_github_prysmaticlabs_ethereumapis//eth/v1alpha1:go_grpc_gateway_library",
+        "@com_github_rs_cors//:go_default_library",
         "@com_github_sirupsen_logrus//:go_default_library",
         "@grpc_ecosystem_grpc_gateway//runtime:go_default_library",
         "@org_golang_google_grpc//:go_default_library",

beacon-chain/gateway/cors.go

@@ -0,0 +1,20 @@
+package gateway
+
+import (
+	"net/http"
+
+	"github.com/rs/cors"
+)
+
+func newCorsHandler(srv http.Handler, allowedOrigins []string) http.Handler {
+	if len(allowedOrigins) == 0 {
+		return srv
+	}
+	c := cors.New(cors.Options{
+		AllowedOrigins: allowedOrigins,
+		AllowedMethods: []string{http.MethodPost, http.MethodGet},
+		MaxAge:         600,
+		AllowedHeaders: []string{"*"},
+	})
+	return c.Handler(srv)
+}

beacon-chain/gateway/gateway.go

@@ -19,13 +19,14 @@ var _ = shared.Service(&Gateway{})
 // Gateway is the gRPC gateway to serve HTTP JSON traffic as a proxy and forward
 // it to the beacon-chain gRPC server.
 type Gateway struct {
-	conn        *grpc.ClientConn
-	ctx         context.Context
-	cancel      context.CancelFunc
-	gatewayAddr string
-	remoteAddr  string
-	server      *http.Server
-	mux         *http.ServeMux
+	conn           *grpc.ClientConn
+	ctx            context.Context
+	cancel         context.CancelFunc
+	gatewayAddr    string
+	remoteAddr     string
+	server         *http.Server
+	mux            *http.ServeMux
+	allowedOrigins []string
 
 	startFailure error
 }
@@ -64,7 +65,7 @@ func (g *Gateway) Start() {
 
 	g.server = &http.Server{
 		Addr:    g.gatewayAddr,
-		Handler: g.mux,
+		Handler: newCorsHandler(g.mux, g.allowedOrigins),
 	}
 	go func() {
 		if err := g.server.ListenAndServe(); err != http.ErrServerClosed {
@@ -105,16 +106,17 @@ func (g *Gateway) Stop() error {
 
 // New returns a new gateway server which translates HTTP into gRPC.
 // Accepts a context and optional http.ServeMux.
-func New(ctx context.Context, remoteAddress, gatewayAddress string, mux *http.ServeMux) *Gateway {
+func New(ctx context.Context, remoteAddress, gatewayAddress string, mux *http.ServeMux, allowedOrigins []string) *Gateway {
 	if mux == nil {
 		mux = http.NewServeMux()
 	}
 
 	return &Gateway{
-		remoteAddr:  remoteAddress,
-		gatewayAddr: gatewayAddress,
-		ctx:         ctx,
-		mux:         mux,
+		remoteAddr:     remoteAddress,
+		gatewayAddr:    gatewayAddress,
+		ctx:            ctx,
+		mux:            mux,
+		allowedOrigins: allowedOrigins,
 	}
 }
 

beacon-chain/gateway/server/main.go

@@ -5,6 +5,7 @@ import (
 	"flag"
 	"fmt"
 	"net/http"
+	"strings"
 
 	joonix "github.com/joonix/log"
 	"github.com/prysmaticlabs/prysm/beacon-chain/gateway"
@@ -13,9 +14,10 @@ import (
 )
 
 var (
-	beaconRPC = flag.String("beacon-rpc", "localhost:4000", "Beacon chain gRPC endpoint")
-	port      = flag.Int("port", 8000, "Port to serve on")
-	debug     = flag.Bool("debug", false, "Enable debug logging")
+	beaconRPC      = flag.String("beacon-rpc", "localhost:4000", "Beacon chain gRPC endpoint")
+	port           = flag.Int("port", 8000, "Port to serve on")
+	debug          = flag.Bool("debug", false, "Enable debug logging")
+	allowedOrigins = flag.String("corsdomain", "", "A comma separated list of CORS domains to allow.")
 )
 
 func init() {
@@ -31,7 +33,7 @@ func main() {
 	}
 
 	mux := http.NewServeMux()
-	gw := gateway.New(context.Background(), *beaconRPC, fmt.Sprintf("0.0.0.0:%d", *port), mux)
+	gw := gateway.New(context.Background(), *beaconRPC, fmt.Sprintf("0.0.0.0:%d", *port), mux, strings.Split(*allowedOrigins, ","))
 	mux.HandleFunc("/swagger/", gateway.SwaggerServer())
 	mux.HandleFunc("/healthz", healthzServer(gw))
 	gw.Start()

beacon-chain/node/node.go

@@ -588,7 +588,8 @@ func (b *BeaconNode) registerGRPCGateway(ctx *cli.Context) error {
 	if gatewayPort > 0 {
 		selfAddress := fmt.Sprintf("127.0.0.1:%d", ctx.Int(flags.RPCPort.Name))
 		gatewayAddress := fmt.Sprintf("0.0.0.0:%d", gatewayPort)
-		return b.services.RegisterService(gateway.New(context.Background(), selfAddress, gatewayAddress, nil /*optional mux*/))
+		allowedOrigins := strings.Split(ctx.String(flags.GPRCGatewayCorsDomain.Name), ",")
+		return b.services.RegisterService(gateway.New(context.Background(), selfAddress, gatewayAddress, nil /*optional mux*/, allowedOrigins))
 	}
 	return nil
 }