Reproduce issue with client error handling + fix

client.go

@@ -95,7 +95,7 @@ func NewClient[Req, Res any](httpClient HTTPClient, url string, options ...Clien
 		response, err := receiveUnaryResponse[Res](conn, config.Initializer)
 		if err != nil {
 			_ = conn.CloseResponse()
-			return nil, err
+			return response, err
 		}
 		return response, conn.CloseResponse()
 	})

client_error_handling_test.go

@@ -0,0 +1,76 @@
+// Copyright 2021-2024 The Connect Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package connect
+
+import (
+	"context"
+	"errors"
+	"net/http"
+	"testing"
+
+	"connectrpc.com/connect/internal/assert"
+	pingv1 "connectrpc.com/connect/internal/gen/connect/ping/v1"
+	"connectrpc.com/connect/internal/memhttp/memhttptest"
+)
+
+var ErrOKToIgnore = errors.New("some error that is ok to ignore")
+
+func TestClientErrorHandling(t *testing.T) {
+	t.Parallel()
+	mux := http.NewServeMux()
+	mux.Handle("/connect.ping.v1.PingService/Ping", NewUnaryHandler(
+		"/connect.ping.v1.PingService/Ping",
+		func(ctx context.Context, r *Request[pingv1.PingRequest]) (*Response[pingv1.PingResponse], error) {
+			return nil, NewError(CodeCanceled, ErrOKToIgnore)
+		},
+	))
+	server := memhttptest.NewServer(t, mux)
+
+	client := NewClient[pingv1.PingRequest, pingv1.PingResponse](
+		server.Client(),
+		server.URL()+"/connect.ping.v1.PingService/Ping",
+		WithInterceptors(
+			errorHidingInterceptor{},
+		),
+	)
+	ctx := context.Background()
+
+	_, err := client.CallUnary(ctx, NewRequest[pingv1.PingRequest](nil))
+	assert.Nil(t, err)
+}
+
+// errorHidingInterceptor is a simple interceptor that hides errors based on
+// some criteria (in this case, if the error is a CodeCanceled error). It is
+// use to reproduce an issue with error handling in the client, where the
+// type information is lost between unaryFunc and client.callUnary.
+type errorHidingInterceptor struct{}
+
+func (e errorHidingInterceptor) WrapStreamingClient(StreamingClientFunc) StreamingClientFunc {
+	panic("unimplemented")
+}
+
+func (e errorHidingInterceptor) WrapStreamingHandler(StreamingHandlerFunc) StreamingHandlerFunc {
+	panic("unimplemented")
+}
+
+func (e errorHidingInterceptor) WrapUnary(next UnaryFunc) UnaryFunc {
+	return func(ctx context.Context, req AnyRequest) (_ AnyResponse, retErr error) {
+		res, err := next(ctx, req)
+		if CodeOf(err) == CodeCanceled { // some criteria for ignored errors
+			return res, nil
+		}
+		return res, err
+	}
+}