Add DialOption to overwrite :authority pseudo-header

The :authority pseudo-header for a gRPC Client defaults to the host
portion of the dialed target and can only be overwritten by providing a
TransportCredentials. However, there are cases where setting this header
independent of any tranport security is valid. In my particular case,
in order to leverage Envoy for request routing, the cluster/service name
must be provided in the :authority header. This may also be useful in a
testing context.

This patch adds a DialOption to overwrite the authority header,
even if TransportCredentials are provided (I'd imagine you'd only ever
need to specify one or the other).

clientconn.go

@@ -263,6 +263,15 @@ func WithStreamInterceptor(f StreamClientInterceptor) DialOption {
 	}
 }
 
+// WithAuthority returns a DialOption that specifies the value to be used as
+// the :authority pseudo-header. This value overrides the :authority value
+// provided by TransportCredentials if present.
+func WithAuthority(a string) DialOption {
+	return func(o *dialOptions) {
+		o.copts.Authority = a
+	}
+}
+
 // Dial creates a client connection to the given target.
 func Dial(target string, opts ...DialOption) (*ClientConn, error) {
 	return DialContext(context.Background(), target, opts...)
@@ -319,7 +328,9 @@ func DialContext(ctx context.Context, target string, opts ...DialOption) (conn *
 		cc.dopts.bs = DefaultBackoffConfig
 	}
 	creds := cc.dopts.copts.TransportCredentials
-	if creds != nil && creds.Info().ServerName != "" {
+	if cc.dopts.copts.Authority != "" {
+		cc.authority = cc.dopts.copts.Authority
+	} else if creds != nil && creds.Info().ServerName != "" {
 		cc.authority = creds.Info().ServerName
 	} else {
 		colonPos := strings.LastIndex(target, ":")

clientconn_test.go

@@ -85,6 +85,22 @@ func TestTLSServerNameOverwrite(t *testing.T) {
 	}
 }
 
+func TestWithAuthority(t *testing.T) {
+	overwriteServerName := "over.write.server.name"
+	creds, err := credentials.NewClientTLSFromFile(tlsDir+"ca.pem", "transport.security.overwrite")
+	if err != nil {
+		t.Fatalf("Failed to create credentials %v", err)
+	}
+	conn, err := Dial("Non-Existent.Server:80", WithTransportCredentials(creds), WithAuthority(overwriteServerName))
+	if err != nil {
+		t.Fatalf("Dial(_, _) = _, %v, want _, <nil>", err)
+	}
+	conn.Close()
+	if conn.authority != overwriteServerName {
+		t.Fatalf("%v.authority = %v, want %v", conn, conn.authority, overwriteServerName)
+	}
+}
+
 func TestDialContextCancel(t *testing.T) {
 	ctx, cancel := context.WithCancel(context.Background())
 	cancel()

transport/transport.go

@@ -374,6 +374,8 @@ func NewServerTransport(protocol string, conn net.Conn, config *ServerConfig) (S
 type ConnectOptions struct {
 	// UserAgent is the application user agent.
 	UserAgent string
+	// Authority is the :authority pseudo-header to use.
+	Authority string
 	// Dialer specifies how to dial a network address.
 	Dialer func(context.Context, string) (net.Conn, error)
 	// FailOnNonTempDialError specifies if gRPC fails on non-temporary dial errors.