Add handler for unknown method

server/server.go

@@ -100,6 +100,9 @@ type Server struct {
 	// Calls are inserted into this queue, to be handled
 	// by a goroutine running handleCalls()
 	callQueue *mpsc.Queue[*Call]
+
+	// Handler for custom behavior of unknown methods
+	HandleUnknownMethod func(m capnp.Method) *Method
 }
 
 // New returns a client hook that makes calls to a set of methods.
@@ -126,6 +129,9 @@ func New(methods []Method, brand any, shutdown Shutdowner) *Server {
 // Send starts a method call.
 func (srv *Server) Send(ctx context.Context, s capnp.Send) (*capnp.Answer, capnp.ReleaseFunc) {
 	mm := srv.methods.find(s.Method)
+	if mm == nil && srv.HandleUnknownMethod != nil {
+		mm = srv.HandleUnknownMethod(s.Method)
+	}
 	if mm == nil {
 		return capnp.ErrorAnswer(s.Method, capnp.Unimplemented("unimplemented")), func() {}
 	}
@@ -150,6 +156,9 @@ func (srv *Server) Send(ctx context.Context, s capnp.Send) (*capnp.Answer, capnp
 // Recv starts a method call.
 func (srv *Server) Recv(ctx context.Context, r capnp.Recv) capnp.PipelineCaller {
 	mm := srv.methods.find(r.Method)
+	if mm == nil && srv.HandleUnknownMethod != nil {
+		mm = srv.HandleUnknownMethod(r.Method)
+	}
 	if mm == nil {
 		r.Reject(capnp.Unimplemented("unimplemented"))
 		return nil

server/server_test.go

@@ -5,8 +5,11 @@ import (
 	"errors"
 	"strings"
 	"sync"
+	"sync/atomic"
 	"testing"
 
+	"github.com/stretchr/testify/require"
+
 	"capnproto.org/go/capnp/v3"
 	air "capnproto.org/go/capnp/v3/internal/aircraftlib"
 	"capnproto.org/go/capnp/v3/server"
@@ -91,6 +94,47 @@ func TestServerCall(t *testing.T) {
 			}
 		}
 	})
+	t.Run("Unimplemented hook", func(t *testing.T) {
+		t.Parallel()
+		var echoText = "are you there?"
+		var proxyReceived atomic.Value
+
+		// start a proxy server with hook
+		srv := server.New(nil, nil, nil)
+		srv.HandleUnknownMethod = func(method capnp.Method) *server.Method {
+			sm := server.Method{
+				Method: method,
+				Impl:   nil,
+			}
+			sm.Impl = func(ctx context.Context, call *server.Call) error {
+				echoArgs := air.Echo_echo_Params(call.Args())
+				inText, err := echoArgs.In()
+				require.NoError(t, err)
+				proxyReceived.Store(inText)
+				// pretend we received an answer
+				echo := air.Echo_echo{Call: call}
+				resp, _ := echo.AllocResults()
+				err = resp.SetOut(inText)
+				return err
+			}
+			return &sm
+		}
+		blankBoot := capnp.NewClient(srv)
+		echoClient := air.Echo(blankBoot)
+		defer echoClient.Release()
+
+		ans, finish := echoClient.Echo(context.Background(), func(p air.Echo_echo_Params) error {
+			err := p.SetIn(echoText)
+			return err
+		})
+		defer finish()
+		resp, err := ans.Struct()
+		answerOut, _ := resp.Out()
+		rxValue := proxyReceived.Load()
+		require.Equal(t, echoText, rxValue)
+		assert.Equal(t, echoText, answerOut)
+		assert.NoError(t, err, "echo.Echo() error != <nil>; want success")
+	})
 }
 
 type callSeq uint32