server/grpc: add MaxConn option to limit max inflight requests (#1247)

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

server/grpc/grpc.go

@@ -24,6 +24,7 @@ import (
 	"github.com/micro/go-micro/v2/util/addr"
 	mgrpc "github.com/micro/go-micro/v2/util/grpc"
 	mnet "github.com/micro/go-micro/v2/util/net"
+	"golang.org/x/net/netutil"
 
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/codes"
@@ -163,13 +164,14 @@ func (g *grpcServer) getGrpcOptions() []grpc.ServerOption {
 }
 
 func (g *grpcServer) getListener() net.Listener {
-	if g.opts.Context != nil {
+	if g.opts.Context == nil {
-		if v := g.opts.Context.Value(netListener{}); v != nil {
+		return nil
-			if l, ok := v.(net.Listener); ok {
-				return l
-			}
-		}
 	}
+
+	if l, ok := g.opts.Context.Value(netListener{}).(net.Listener); ok && l != nil {
+		return l
+	}
+
 	return nil
 }
 
@@ -799,6 +801,12 @@ func (g *grpcServer) Start() error {
 		}
 	}
 
+	if g.opts.Context != nil {
+		if c, ok := g.opts.Context.Value(maxConnKey{}).(int); ok && c > 0 {
+			ts = netutil.LimitListener(ts, c)
+		}
+	}
+
 	log.Infof("Server [grpc] Listening on %s", ts.Addr().String())
 	g.Lock()
 	g.opts.Address = ts.Addr().String()

server/grpc/options.go

@@ -18,6 +18,7 @@ type codecsKey struct{}
 type grpcOptions struct{}
 type netListener struct{}
 type maxMsgSizeKey struct{}
+type maxConnKey struct{}
 type tlsAuth struct{}
 
 // gRPC Codec to be used to encode/decode requests for a given content type
@@ -40,6 +41,11 @@ func AuthTLS(t *tls.Config) server.Option {
 	return setServerOption(tlsAuth{}, t)
 }
 
+// MaxConn specifies maximum number of max simultaneous connections to server
+func MaxConn(n int) server.Option {
+	return setServerOption(maxConnKey{}, n)
+}
+
 // Listener specifies the net.Listener to use instead of the default
 func Listener(l net.Listener) server.Option {
 	return setServerOption(netListener{}, l)