[ADDED] CustomDialer interface/option setter

We recently added a Dialer option, however, we made it a *net.Dialer,
which is very restrictive (since *net.Dialer is a struct, not an
interface).
One could imagine creating a Dialer that would create pipes between
the client and an embedded server. Since Dial() returns a net.Conn,
which is an interface, everything would work the same.

Resolves #329

Author: kozlovic

nats.go

@@ -123,6 +123,12 @@ type asyncCB func()
 // Option is a function on the options for a connection.
 type Option func(*Options) error
 
+// CustomDialer can be used to specify any dialer, not necessarily
+// a *net.Dialer.
+type CustomDialer interface {
+	Dial(network, address string) (net.Conn, error)
+}
+
 // Options can be used to create a customized connection.
 type Options struct {
 
@@ -225,9 +231,14 @@ type Options struct {
 	// Token sets the token to be used when connecting to a server.
 	Token string
 
-	// Dialer allows a custom Dialer when forming connections.
+	// Dialer allows a custom net.Dialer when forming connections.
+	// DEPRECATED: should use CustomDialer instead.
 	Dialer *net.Dialer
 
+	// CustomDialer allows to specify a custom dialer (not necessarily
+	// a *net.Dialer).
+	CustomDialer CustomDialer
+
 	// UseOldRequestStyle forces the old method of Requests that utilize
 	// a new Inbox and a new Subscription for each request.
 	UseOldRequestStyle bool
@@ -586,13 +597,24 @@ func Token(token string) Option {
 
 // Dialer is an Option to set the dialer which will be used when
 // attempting to establish a connection.
+// DEPRECATED: Should use CustomDialer instead.
 func Dialer(dialer *net.Dialer) Option {
 	return func(o *Options) error {
 		o.Dialer = dialer
 		return nil
 	}
 }
 
+// SetCustomDialer is an Option to set a custom dialer which will be
+// used when attempting to establish a connection. If both Dialer
+// and CustomDialer are specified, CustomDialer takes precedence.
+func SetCustomDialer(dialer CustomDialer) Option {
+	return func(o *Options) error {
+		o.CustomDialer = dialer
+		return nil
+	}
+}
+
 // UseOldRequestyStyle is an Option to force usage of the old Request style.
 func UseOldRequestStyle() Option {
 	return func(o *Options) error {
@@ -877,7 +899,13 @@ func (nc *Conn) createConn() (err error) {
 		cur.lastAttempt = time.Now()
 	}
 
-	dialer := nc.Opts.Dialer
+	// CustomDialer takes precedence. If not set, use Opts.Dialer which
+	// is set to a default *net.Dialer (in Connect()) if not explicitly
+	// set by the user.
+	dialer := nc.Opts.CustomDialer
+	if dialer == nil {
+		dialer = nc.Opts.Dialer
+	}
 	nc.conn, err = dialer.Dial("tcp", nc.url.Host)
 	if err != nil {
 		return err

test/conn_test.go

@@ -1266,6 +1266,15 @@ func TestNoRaceOnLastError(t *testing.T) {
 	}
 }
 
+type customDialer struct {
+	ch chan bool
+}
+
+func (cd *customDialer) Dial(network, address string) (net.Conn, error) {
+	cd.ch <- true
+	return nil, fmt.Errorf("on purpose")
+}
+
 func TestUseCustomDialer(t *testing.T) {
 	s := RunDefaultServer()
 	defer s.Shutdown()
@@ -1310,6 +1319,51 @@ func TestUseCustomDialer(t *testing.T) {
 	if nc3.Opts.Dialer.Timeout != nats.DefaultTimeout {
 		t.Fatalf("Expected DialTimeout to be set to %v, got %v", nats.DefaultTimeout, nc.Opts.Dialer.Timeout)
 	}
+
+	// Create custom dialer that return error on Dial().
+	cdialer := &customDialer{ch: make(chan bool, 1)}
+
+	// When both Dialer and CustomDialer are set, CustomDialer
+	// should take precedence. That means that the connection
+	// should fail for these two set of options.
+	options := []*nats.Options{
+		&nats.Options{Dialer: dialer, CustomDialer: cdialer},
+		&nats.Options{CustomDialer: cdialer},
+	}
+	for _, o := range options {
+		o.Servers = []string{nats.DefaultURL}
+		nc, err := o.Connect()
+		// As of now, Connect() would not return the actual dialer error,
+		// instead it returns "no server available for connections".
+		// So use go channel to ensure that custom dialer's Dial() method
+		// was invoked.
+		if err == nil {
+			if nc != nil {
+				nc.Close()
+			}
+			t.Fatal("Expected error, got none")
+		}
+		if err := Wait(cdialer.ch); err != nil {
+			t.Fatal("Did not get our notification")
+		}
+	}
+	// Same with variadic
+	foptions := [][]nats.Option{
+		[]nats.Option{nats.Dialer(dialer), nats.SetCustomDialer(cdialer)},
+		[]nats.Option{nats.SetCustomDialer(cdialer)},
+	}
+	for _, fos := range foptions {
+		nc, err := nats.Connect(nats.DefaultURL, fos...)
+		if err == nil {
+			if nc != nil {
+				nc.Close()
+			}
+			t.Fatal("Expected error, got none")
+		}
+		if err := Wait(cdialer.ch); err != nil {
+			t.Fatal("Did not get our notification")
+		}
+	}
 }
 
 func TestDefaultOptionsDialer(t *testing.T) {