make it possible to pass options to a transport constructor

config/reflection_magic.go

@@ -1,6 +1,7 @@
 package config
 
 import (
+	"errors"
 	"fmt"
 	"reflect"
 	"runtime"
@@ -97,11 +98,38 @@ func makeArgumentConstructors(fnType reflect.Type, argTypes map[reflect.Type]con
 	return out, nil
 }
 
+func getConstructorOpts(t reflect.Type, opts ...interface{}) ([]reflect.Value, error) {
+	if !t.IsVariadic() {
+		if len(opts) > 0 {
+			return nil, errors.New("constructor doesn't accept any options")
+		}
+		return nil, nil
+	}
+	if len(opts) == 0 {
+		return nil, nil
+	}
+	// variadic parameters always go last
+	wantType := t.In(t.NumIn() - 1).Elem()
+	values := make([]reflect.Value, 0, len(opts))
+	for _, opt := range opts {
+		val := reflect.ValueOf(opt)
+		if opt == nil {
+			return nil, errors.New("expected a transport option, got nil")
+		}
+		if val.Type() != wantType {
+			return nil, fmt.Errorf("expected option of type %s, got %s", wantType, reflect.TypeOf(opt))
+		}
+		values = append(values, val.Convert(wantType))
+	}
+	return values, nil
+}
+
 // makes a transport constructor.
 func makeConstructor(
 	tpt interface{},
 	tptType reflect.Type,
 	argTypes map[reflect.Type]constructor,
+	opts ...interface{},
 ) (func(host.Host, *tptu.Upgrader, connmgr.ConnectionGater) (interface{}, error), error) {
 	v := reflect.ValueOf(tpt)
 	// avoid panicing on nil/zero value.
@@ -121,19 +149,24 @@ func makeConstructor(
 	if err != nil {
 		return nil, err
 	}
+	optValues, err := getConstructorOpts(t, opts...)
+	if err != nil {
+		return nil, err
+	}
 
 	return func(h host.Host, u *tptu.Upgrader, cg connmgr.ConnectionGater) (interface{}, error) {
-		arguments := make([]reflect.Value, len(argConstructors))
+		arguments := make([]reflect.Value, 0, len(argConstructors)+len(opts))
 		for i, makeArg := range argConstructors {
 			if arg := makeArg(h, u, cg); arg != nil {
-				arguments[i] = reflect.ValueOf(arg)
+				arguments = append(arguments, reflect.ValueOf(arg))
 			} else {
 				// ValueOf an un-typed nil yields a zero reflect
 				// value. However, we _want_ the zero value of
 				// the _type_.
-				arguments[i] = reflect.Zero(t.In(i))
+				arguments = append(arguments, reflect.Zero(t.In(i)))
 			}
 		}
+		arguments = append(arguments, optValues...)
 		return callConstructor(v, arguments)
 	}, nil
 }

config/transport.go

@@ -36,14 +36,14 @@ var transportArgTypes = argTypes
 //
 // And returns a type implementing transport.Transport and, optionally, an error
 // (as the second argument).
-func TransportConstructor(tpt interface{}) (TptC, error) {
+func TransportConstructor(tpt interface{}, opts ...interface{}) (TptC, error) {
 	// Already constructed?
 	if t, ok := tpt.(transport.Transport); ok {
 		return func(_ host.Host, _ *tptu.Upgrader, _ connmgr.ConnectionGater) (transport.Transport, error) {
 			return t, nil
 		}, nil
 	}
-	ctor, err := makeConstructor(tpt, transportType, transportArgTypes)
+	ctor, err := makeConstructor(tpt, transportType, transportArgTypes, opts...)
 	if err != nil {
 		return nil, err
 	}

config/transport_test.go

@@ -5,11 +5,39 @@ import (
 
 	"github.com/libp2p/go-libp2p-core/peer"
 	"github.com/libp2p/go-libp2p-core/transport"
+	tptu "github.com/libp2p/go-libp2p-transport-upgrader"
+	"github.com/libp2p/go-tcp-transport"
+
+	"github.com/stretchr/testify/require"
 )
 
 func TestTransportVariadicOptions(t *testing.T) {
 	_, err := TransportConstructor(func(_ peer.ID, _ ...int) transport.Transport { return nil })
-	if err != nil {
-		t.Fatal(err)
-	}
+	require.NoError(t, err)
+}
+
+func TestConstructorWithoutOptsCalledWithOpts(t *testing.T) {
+	_, err := TransportConstructor(func(_ *tptu.Upgrader) transport.Transport {
+		return nil
+	}, 42)
+	require.EqualError(t, err, "constructor doesn't accept any options")
+}
+
+func TestConstructorWithOptsTypeMismatch(t *testing.T) {
+	_, err := TransportConstructor(func(_ *tptu.Upgrader, opts ...int) transport.Transport {
+		return nil
+	}, 42, "foo")
+	require.EqualError(t, err, "expected option of type int, got string")
+}
+
+func TestConstructorWithOpts(t *testing.T) {
+	var options []int
+	c, err := TransportConstructor(func(_ *tptu.Upgrader, opts ...int) transport.Transport {
+		options = opts
+		return tcp.NewTCPTransport(nil)
+	}, 42, 1337)
+	require.NoError(t, err)
+	_, err = c(nil, nil, nil)
+	require.NoError(t, err)
+	require.Equal(t, []int{42, 1337}, options)
 }

options.go

@@ -125,8 +125,8 @@ func Muxer(name string, tpt interface{}) Option {
 // * Public Key
 // * Address filter (filter.Filter)
 // * Peerstore
-func Transport(tpt interface{}) Option {
-	tptc, err := config.TransportConstructor(tpt)
+func Transport(tpt interface{}, opts ...interface{}) Option {
+	tptc, err := config.TransportConstructor(tpt, opts...)
 	err = traceError(err, 1)
 	return func(cfg *Config) error {
 		if err != nil {