Provide a generic client-side unary callback API

BUILD

@@ -119,6 +119,7 @@ GRPCXX_SRCS = [
     "src/cpp/client/credentials_cc.cc",
     "src/cpp/client/generic_stub.cc",
     "src/cpp/common/alarm.cc",
+    "src/cpp/common/callback_common.cc",
     "src/cpp/common/channel_arguments.cc",
     "src/cpp/common/channel_filter.cc",
     "src/cpp/common/completion_queue_cc.cc",
@@ -243,6 +244,7 @@ GRPCXX_PUBLIC_HDRS = [
     "include/grpcpp/support/async_unary_call.h",
     "include/grpcpp/support/byte_buffer.h",
     "include/grpcpp/support/channel_arguments.h",
+    "include/grpcpp/support/client_callback.h",
     "include/grpcpp/support/config.h",
     "include/grpcpp/support/proto_buffer_reader.h",
     "include/grpcpp/support/proto_buffer_writer.h",
@@ -1979,7 +1981,9 @@ grpc_cc_library(
         "include/grpcpp/impl/codegen/byte_buffer.h",
         "include/grpcpp/impl/codegen/call.h",
         "include/grpcpp/impl/codegen/call_hook.h",
+        "include/grpcpp/impl/codegen/callback_common.h",
         "include/grpcpp/impl/codegen/channel_interface.h",
+        "include/grpcpp/impl/codegen/client_callback.h",
         "include/grpcpp/impl/codegen/client_context.h",
         "include/grpcpp/impl/codegen/client_unary_call.h",
         "include/grpcpp/impl/codegen/completion_queue.h",

CMakeLists.txt

@@ -564,6 +564,7 @@ add_dependencies(buildtests_cxx check_gcp_environment_linux_test)
 add_dependencies(buildtests_cxx check_gcp_environment_windows_test)
 add_dependencies(buildtests_cxx chttp2_settings_timeout_test)
 add_dependencies(buildtests_cxx cli_call_test)
+add_dependencies(buildtests_cxx client_callback_end2end_test)
 add_dependencies(buildtests_cxx client_channel_stress_test)
 if(_gRPC_PLATFORM_LINUX OR _gRPC_PLATFORM_MAC OR _gRPC_PLATFORM_POSIX)
 add_dependencies(buildtests_cxx client_crash_test)
@@ -2771,6 +2772,7 @@ add_library(grpc++
   src/cpp/client/credentials_cc.cc
   src/cpp/client/generic_stub.cc
   src/cpp/common/alarm.cc
+  src/cpp/common/callback_common.cc
   src/cpp/common/channel_arguments.cc
   src/cpp/common/channel_filter.cc
   src/cpp/common/completion_queue_cc.cc
@@ -2917,6 +2919,7 @@ foreach(_hdr
   include/grpcpp/support/async_unary_call.h
   include/grpcpp/support/byte_buffer.h
   include/grpcpp/support/channel_arguments.h
+  include/grpcpp/support/client_callback.h
   include/grpcpp/support/config.h
   include/grpcpp/support/proto_buffer_reader.h
   include/grpcpp/support/proto_buffer_writer.h
@@ -3014,7 +3017,9 @@ foreach(_hdr
   include/grpcpp/impl/codegen/byte_buffer.h
   include/grpcpp/impl/codegen/call.h
   include/grpcpp/impl/codegen/call_hook.h
+  include/grpcpp/impl/codegen/callback_common.h
   include/grpcpp/impl/codegen/channel_interface.h
+  include/grpcpp/impl/codegen/client_callback.h
   include/grpcpp/impl/codegen/client_context.h
   include/grpcpp/impl/codegen/client_unary_call.h
   include/grpcpp/impl/codegen/completion_queue.h
@@ -3129,6 +3134,7 @@ add_library(grpc++_cronet
   src/cpp/client/credentials_cc.cc
   src/cpp/client/generic_stub.cc
   src/cpp/common/alarm.cc
+  src/cpp/common/callback_common.cc
   src/cpp/common/channel_arguments.cc
   src/cpp/common/channel_filter.cc
   src/cpp/common/completion_queue_cc.cc
@@ -3486,6 +3492,7 @@ foreach(_hdr
   include/grpcpp/support/async_unary_call.h
   include/grpcpp/support/byte_buffer.h
   include/grpcpp/support/channel_arguments.h
+  include/grpcpp/support/client_callback.h
   include/grpcpp/support/config.h
   include/grpcpp/support/proto_buffer_reader.h
   include/grpcpp/support/proto_buffer_writer.h
@@ -3583,7 +3590,9 @@ foreach(_hdr
   include/grpcpp/impl/codegen/byte_buffer.h
   include/grpcpp/impl/codegen/call.h
   include/grpcpp/impl/codegen/call_hook.h
+  include/grpcpp/impl/codegen/callback_common.h
   include/grpcpp/impl/codegen/channel_interface.h
+  include/grpcpp/impl/codegen/client_callback.h
   include/grpcpp/impl/codegen/client_context.h
   include/grpcpp/impl/codegen/client_unary_call.h
   include/grpcpp/impl/codegen/completion_queue.h
@@ -3993,7 +4002,9 @@ foreach(_hdr
   include/grpcpp/impl/codegen/byte_buffer.h
   include/grpcpp/impl/codegen/call.h
   include/grpcpp/impl/codegen/call_hook.h
+  include/grpcpp/impl/codegen/callback_common.h
   include/grpcpp/impl/codegen/channel_interface.h
+  include/grpcpp/impl/codegen/client_callback.h
   include/grpcpp/impl/codegen/client_context.h
   include/grpcpp/impl/codegen/client_unary_call.h
   include/grpcpp/impl/codegen/completion_queue.h
@@ -4171,7 +4182,9 @@ foreach(_hdr
   include/grpcpp/impl/codegen/byte_buffer.h
   include/grpcpp/impl/codegen/call.h
   include/grpcpp/impl/codegen/call_hook.h
+  include/grpcpp/impl/codegen/callback_common.h
   include/grpcpp/impl/codegen/channel_interface.h
+  include/grpcpp/impl/codegen/client_callback.h
   include/grpcpp/impl/codegen/client_context.h
   include/grpcpp/impl/codegen/client_unary_call.h
   include/grpcpp/impl/codegen/completion_queue.h
@@ -4247,6 +4260,7 @@ add_library(grpc++_unsecure
   src/cpp/client/credentials_cc.cc
   src/cpp/client/generic_stub.cc
   src/cpp/common/alarm.cc
+  src/cpp/common/callback_common.cc
   src/cpp/common/channel_arguments.cc
   src/cpp/common/channel_filter.cc
   src/cpp/common/completion_queue_cc.cc
@@ -4392,6 +4406,7 @@ foreach(_hdr
   include/grpcpp/support/async_unary_call.h
   include/grpcpp/support/byte_buffer.h
   include/grpcpp/support/channel_arguments.h
+  include/grpcpp/support/client_callback.h
   include/grpcpp/support/config.h
   include/grpcpp/support/proto_buffer_reader.h
   include/grpcpp/support/proto_buffer_writer.h
@@ -4489,7 +4504,9 @@ foreach(_hdr
   include/grpcpp/impl/codegen/byte_buffer.h
   include/grpcpp/impl/codegen/call.h
   include/grpcpp/impl/codegen/call_hook.h
+  include/grpcpp/impl/codegen/callback_common.h
   include/grpcpp/impl/codegen/channel_interface.h
+  include/grpcpp/impl/codegen/client_callback.h
   include/grpcpp/impl/codegen/client_context.h
   include/grpcpp/impl/codegen/client_unary_call.h
   include/grpcpp/impl/codegen/completion_queue.h
@@ -11297,6 +11314,46 @@ target_link_libraries(cli_call_test
 endif (gRPC_BUILD_TESTS)
 if (gRPC_BUILD_TESTS)
 
+add_executable(client_callback_end2end_test
+  test/cpp/end2end/client_callback_end2end_test.cc
+  third_party/googletest/googletest/src/gtest-all.cc
+  third_party/googletest/googlemock/src/gmock-all.cc
+)
+
+
+target_include_directories(client_callback_end2end_test
+  PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}
+  PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/include
+  PRIVATE ${_gRPC_SSL_INCLUDE_DIR}
+  PRIVATE ${_gRPC_PROTOBUF_INCLUDE_DIR}
+  PRIVATE ${_gRPC_ZLIB_INCLUDE_DIR}
+  PRIVATE ${_gRPC_BENCHMARK_INCLUDE_DIR}
+  PRIVATE ${_gRPC_CARES_INCLUDE_DIR}
+  PRIVATE ${_gRPC_GFLAGS_INCLUDE_DIR}
+  PRIVATE ${_gRPC_ADDRESS_SORTING_INCLUDE_DIR}
+  PRIVATE ${_gRPC_NANOPB_INCLUDE_DIR}
+  PRIVATE third_party/googletest/googletest/include
+  PRIVATE third_party/googletest/googletest
+  PRIVATE third_party/googletest/googlemock/include
+  PRIVATE third_party/googletest/googlemock
+  PRIVATE ${_gRPC_PROTO_GENS_DIR}
+)
+
+target_link_libraries(client_callback_end2end_test
+  ${_gRPC_PROTOBUF_LIBRARIES}
+  ${_gRPC_ALLTARGETS_LIBRARIES}
+  grpc++_test_util
+  grpc_test_util
+  grpc++
+  grpc
+  gpr_test_util
+  gpr
+  ${_gRPC_GFLAGS_LIBRARIES}
+)
+
+endif (gRPC_BUILD_TESTS)
+if (gRPC_BUILD_TESTS)
+
 add_executable(client_channel_stress_test
   ${_gRPC_PROTO_GENS_DIR}/src/proto/grpc/lb/v1/load_balancer.pb.cc
   ${_gRPC_PROTO_GENS_DIR}/src/proto/grpc/lb/v1/load_balancer.grpc.pb.cc

Makefile

@@ -1160,6 +1160,7 @@ check_gcp_environment_linux_test: $(BINDIR)/$(CONFIG)/check_gcp_environment_linu
 check_gcp_environment_windows_test: $(BINDIR)/$(CONFIG)/check_gcp_environment_windows_test
 chttp2_settings_timeout_test: $(BINDIR)/$(CONFIG)/chttp2_settings_timeout_test
 cli_call_test: $(BINDIR)/$(CONFIG)/cli_call_test
+client_callback_end2end_test: $(BINDIR)/$(CONFIG)/client_callback_end2end_test
 client_channel_stress_test: $(BINDIR)/$(CONFIG)/client_channel_stress_test
 client_crash_test: $(BINDIR)/$(CONFIG)/client_crash_test
 client_crash_test_server: $(BINDIR)/$(CONFIG)/client_crash_test_server
@@ -1665,6 +1666,7 @@ buildtests_cxx: privatelibs_cxx \
   $(BINDIR)/$(CONFIG)/check_gcp_environment_windows_test \
   $(BINDIR)/$(CONFIG)/chttp2_settings_timeout_test \
   $(BINDIR)/$(CONFIG)/cli_call_test \
+  $(BINDIR)/$(CONFIG)/client_callback_end2end_test \
   $(BINDIR)/$(CONFIG)/client_channel_stress_test \
   $(BINDIR)/$(CONFIG)/client_crash_test \
   $(BINDIR)/$(CONFIG)/client_crash_test_server \
@@ -1845,6 +1847,7 @@ buildtests_cxx: privatelibs_cxx \
   $(BINDIR)/$(CONFIG)/check_gcp_environment_windows_test \
   $(BINDIR)/$(CONFIG)/chttp2_settings_timeout_test \
   $(BINDIR)/$(CONFIG)/cli_call_test \
+  $(BINDIR)/$(CONFIG)/client_callback_end2end_test \
   $(BINDIR)/$(CONFIG)/client_channel_stress_test \
   $(BINDIR)/$(CONFIG)/client_crash_test \
   $(BINDIR)/$(CONFIG)/client_crash_test_server \
@@ -2302,6 +2305,8 @@ test_cxx: buildtests_cxx
 	$(Q) $(BINDIR)/$(CONFIG)/chttp2_settings_timeout_test || ( echo test chttp2_settings_timeout_test failed ; exit 1 )
 	$(E) "[RUN]     Testing cli_call_test"
 	$(Q) $(BINDIR)/$(CONFIG)/cli_call_test || ( echo test cli_call_test failed ; exit 1 )
+	$(E) "[RUN]     Testing client_callback_end2end_test"
+	$(Q) $(BINDIR)/$(CONFIG)/client_callback_end2end_test || ( echo test client_callback_end2end_test failed ; exit 1 )
 	$(E) "[RUN]     Testing client_channel_stress_test"
 	$(Q) $(BINDIR)/$(CONFIG)/client_channel_stress_test || ( echo test client_channel_stress_test failed ; exit 1 )
 	$(E) "[RUN]     Testing client_crash_test"
@@ -5219,6 +5224,7 @@ LIBGRPC++_SRC = \
     src/cpp/client/credentials_cc.cc \
     src/cpp/client/generic_stub.cc \
     src/cpp/common/alarm.cc \
+    src/cpp/common/callback_common.cc \
     src/cpp/common/channel_arguments.cc \
     src/cpp/common/channel_filter.cc \
     src/cpp/common/completion_queue_cc.cc \
@@ -5329,6 +5335,7 @@ PUBLIC_HEADERS_CXX += \
     include/grpcpp/support/async_unary_call.h \
     include/grpcpp/support/byte_buffer.h \
     include/grpcpp/support/channel_arguments.h \
+    include/grpcpp/support/client_callback.h \
     include/grpcpp/support/config.h \
     include/grpcpp/support/proto_buffer_reader.h \
     include/grpcpp/support/proto_buffer_writer.h \
@@ -5426,7 +5433,9 @@ PUBLIC_HEADERS_CXX += \
     include/grpcpp/impl/codegen/byte_buffer.h \
     include/grpcpp/impl/codegen/call.h \
     include/grpcpp/impl/codegen/call_hook.h \
+    include/grpcpp/impl/codegen/callback_common.h \
     include/grpcpp/impl/codegen/channel_interface.h \
+    include/grpcpp/impl/codegen/client_callback.h \
     include/grpcpp/impl/codegen/client_context.h \
     include/grpcpp/impl/codegen/client_unary_call.h \
     include/grpcpp/impl/codegen/completion_queue.h \
@@ -5585,6 +5594,7 @@ LIBGRPC++_CRONET_SRC = \
     src/cpp/client/credentials_cc.cc \
     src/cpp/client/generic_stub.cc \
     src/cpp/common/alarm.cc \
+    src/cpp/common/callback_common.cc \
     src/cpp/common/channel_arguments.cc \
     src/cpp/common/channel_filter.cc \
     src/cpp/common/completion_queue_cc.cc \
@@ -5905,6 +5915,7 @@ PUBLIC_HEADERS_CXX += \
     include/grpcpp/support/async_unary_call.h \
     include/grpcpp/support/byte_buffer.h \
     include/grpcpp/support/channel_arguments.h \
+    include/grpcpp/support/client_callback.h \
     include/grpcpp/support/config.h \
     include/grpcpp/support/proto_buffer_reader.h \
     include/grpcpp/support/proto_buffer_writer.h \
@@ -6002,7 +6013,9 @@ PUBLIC_HEADERS_CXX += \
     include/grpcpp/impl/codegen/byte_buffer.h \
     include/grpcpp/impl/codegen/call.h \
     include/grpcpp/impl/codegen/call_hook.h \
+    include/grpcpp/impl/codegen/callback_common.h \
     include/grpcpp/impl/codegen/channel_interface.h \
+    include/grpcpp/impl/codegen/client_callback.h \
     include/grpcpp/impl/codegen/client_context.h \
     include/grpcpp/impl/codegen/client_unary_call.h \
     include/grpcpp/impl/codegen/completion_queue.h \
@@ -6392,7 +6405,9 @@ PUBLIC_HEADERS_CXX += \
     include/grpcpp/impl/codegen/byte_buffer.h \
     include/grpcpp/impl/codegen/call.h \
     include/grpcpp/impl/codegen/call_hook.h \
+    include/grpcpp/impl/codegen/callback_common.h \
     include/grpcpp/impl/codegen/channel_interface.h \
+    include/grpcpp/impl/codegen/client_callback.h \
     include/grpcpp/impl/codegen/client_context.h \
     include/grpcpp/impl/codegen/client_unary_call.h \
     include/grpcpp/impl/codegen/completion_queue.h \
@@ -6546,7 +6561,9 @@ PUBLIC_HEADERS_CXX += \
     include/grpcpp/impl/codegen/byte_buffer.h \
     include/grpcpp/impl/codegen/call.h \
     include/grpcpp/impl/codegen/call_hook.h \
+    include/grpcpp/impl/codegen/callback_common.h \
     include/grpcpp/impl/codegen/channel_interface.h \
+    include/grpcpp/impl/codegen/client_callback.h \
     include/grpcpp/impl/codegen/client_context.h \
     include/grpcpp/impl/codegen/client_unary_call.h \
     include/grpcpp/impl/codegen/completion_queue.h \
@@ -6661,6 +6678,7 @@ LIBGRPC++_UNSECURE_SRC = \
     src/cpp/client/credentials_cc.cc \
     src/cpp/client/generic_stub.cc \
     src/cpp/common/alarm.cc \
+    src/cpp/common/callback_common.cc \
     src/cpp/common/channel_arguments.cc \
     src/cpp/common/channel_filter.cc \
     src/cpp/common/completion_queue_cc.cc \
@@ -6771,6 +6789,7 @@ PUBLIC_HEADERS_CXX += \
     include/grpcpp/support/async_unary_call.h \
     include/grpcpp/support/byte_buffer.h \
     include/grpcpp/support/channel_arguments.h \
+    include/grpcpp/support/client_callback.h \
     include/grpcpp/support/config.h \
     include/grpcpp/support/proto_buffer_reader.h \
     include/grpcpp/support/proto_buffer_writer.h \
@@ -6868,7 +6887,9 @@ PUBLIC_HEADERS_CXX += \
     include/grpcpp/impl/codegen/byte_buffer.h \
     include/grpcpp/impl/codegen/call.h \
     include/grpcpp/impl/codegen/call_hook.h \
+    include/grpcpp/impl/codegen/callback_common.h \
     include/grpcpp/impl/codegen/channel_interface.h \
+    include/grpcpp/impl/codegen/client_callback.h \
     include/grpcpp/impl/codegen/client_context.h \
     include/grpcpp/impl/codegen/client_unary_call.h \
     include/grpcpp/impl/codegen/completion_queue.h \
@@ -17076,6 +17097,49 @@ endif
 endif
 
 
+CLIENT_CALLBACK_END2END_TEST_SRC = \
+    test/cpp/end2end/client_callback_end2end_test.cc \
+
+CLIENT_CALLBACK_END2END_TEST_OBJS = $(addprefix $(OBJDIR)/$(CONFIG)/, $(addsuffix .o, $(basename $(CLIENT_CALLBACK_END2END_TEST_SRC))))
+ifeq ($(NO_SECURE),true)
+
+# You can't build secure targets if you don't have OpenSSL.
+
+$(BINDIR)/$(CONFIG)/client_callback_end2end_test: openssl_dep_error
+
+else
+
+
+
+
+ifeq ($(NO_PROTOBUF),true)
+
+# You can't build the protoc plugins or protobuf-enabled targets if you don't have protobuf 3.5.0+.
+
+$(BINDIR)/$(CONFIG)/client_callback_end2end_test: protobuf_dep_error
+
+else
+
+$(BINDIR)/$(CONFIG)/client_callback_end2end_test: $(PROTOBUF_DEP) $(CLIENT_CALLBACK_END2END_TEST_OBJS) $(LIBDIR)/$(CONFIG)/libgrpc++_test_util.a $(LIBDIR)/$(CONFIG)/libgrpc_test_util.a $(LIBDIR)/$(CONFIG)/libgrpc++.a $(LIBDIR)/$(CONFIG)/libgrpc.a $(LIBDIR)/$(CONFIG)/libgpr_test_util.a $(LIBDIR)/$(CONFIG)/libgpr.a
+	$(E) "[LD]      Linking $@"
+	$(Q) mkdir -p `dirname $@`
+	$(Q) $(LDXX) $(LDFLAGS) $(CLIENT_CALLBACK_END2END_TEST_OBJS) $(LIBDIR)/$(CONFIG)/libgrpc++_test_util.a $(LIBDIR)/$(CONFIG)/libgrpc_test_util.a $(LIBDIR)/$(CONFIG)/libgrpc++.a $(LIBDIR)/$(CONFIG)/libgrpc.a $(LIBDIR)/$(CONFIG)/libgpr_test_util.a $(LIBDIR)/$(CONFIG)/libgpr.a $(LDLIBSXX) $(LDLIBS_PROTOBUF) $(LDLIBS) $(LDLIBS_SECURE) $(GTEST_LIB) -o $(BINDIR)/$(CONFIG)/client_callback_end2end_test
+
+endif
+
+endif
+
+$(OBJDIR)/$(CONFIG)/test/cpp/end2end/client_callback_end2end_test.o:  $(LIBDIR)/$(CONFIG)/libgrpc++_test_util.a $(LIBDIR)/$(CONFIG)/libgrpc_test_util.a $(LIBDIR)/$(CONFIG)/libgrpc++.a $(LIBDIR)/$(CONFIG)/libgrpc.a $(LIBDIR)/$(CONFIG)/libgpr_test_util.a $(LIBDIR)/$(CONFIG)/libgpr.a
+
+deps_client_callback_end2end_test: $(CLIENT_CALLBACK_END2END_TEST_OBJS:.o=.dep)
+
+ifneq ($(NO_SECURE),true)
+ifneq ($(NO_DEPS),true)
+-include $(CLIENT_CALLBACK_END2END_TEST_OBJS:.o=.dep)
+endif
+endif
+
+
 CLIENT_CHANNEL_STRESS_TEST_SRC = \
     $(GENDIR)/src/proto/grpc/lb/v1/load_balancer.pb.cc $(GENDIR)/src/proto/grpc/lb/v1/load_balancer.grpc.pb.cc \
     test/cpp/client/client_channel_stress_test.cc \

build.yaml

@@ -1169,7 +1169,9 @@ filegroups:
   - include/grpcpp/impl/codegen/byte_buffer.h
   - include/grpcpp/impl/codegen/call.h
   - include/grpcpp/impl/codegen/call_hook.h
+  - include/grpcpp/impl/codegen/callback_common.h
   - include/grpcpp/impl/codegen/channel_interface.h
+  - include/grpcpp/impl/codegen/client_callback.h
   - include/grpcpp/impl/codegen/client_context.h
   - include/grpcpp/impl/codegen/client_unary_call.h
   - include/grpcpp/impl/codegen/completion_queue.h
@@ -1297,6 +1299,7 @@ filegroups:
   - include/grpcpp/support/async_unary_call.h
   - include/grpcpp/support/byte_buffer.h
   - include/grpcpp/support/channel_arguments.h
+  - include/grpcpp/support/client_callback.h
   - include/grpcpp/support/config.h
   - include/grpcpp/support/proto_buffer_reader.h
   - include/grpcpp/support/proto_buffer_writer.h
@@ -1324,6 +1327,7 @@ filegroups:
   - src/cpp/client/credentials_cc.cc
   - src/cpp/client/generic_stub.cc
   - src/cpp/common/alarm.cc
+  - src/cpp/common/callback_common.cc
   - src/cpp/common/channel_arguments.cc
   - src/cpp/common/channel_filter.cc
   - src/cpp/common/completion_queue_cc.cc
@@ -4465,6 +4469,20 @@ targets:
   - grpc
   - gpr_test_util
   - gpr
+- name: client_callback_end2end_test
+  gtest: true
+  cpu_cost: 0.5
+  build: test
+  language: c++
+  src:
+  - test/cpp/end2end/client_callback_end2end_test.cc
+  deps:
+  - grpc++_test_util
+  - grpc_test_util
+  - grpc++
+  - grpc
+  - gpr_test_util
+  - gpr
 - name: client_channel_stress_test
   gtest: false
   build: test

gRPC-C++.podspec

@@ -111,6 +111,7 @@ Pod::Spec.new do |s|
                       'include/grpcpp/support/async_unary_call.h',
                       'include/grpcpp/support/byte_buffer.h',
                       'include/grpcpp/support/channel_arguments.h',
+                      'include/grpcpp/support/client_callback.h',
                       'include/grpcpp/support/config.h',
                       'include/grpcpp/support/proto_buffer_reader.h',
                       'include/grpcpp/support/proto_buffer_writer.h',
@@ -127,7 +128,9 @@ Pod::Spec.new do |s|
                       'include/grpcpp/impl/codegen/byte_buffer.h',
                       'include/grpcpp/impl/codegen/call.h',
                       'include/grpcpp/impl/codegen/call_hook.h',
+                      'include/grpcpp/impl/codegen/callback_common.h',
                       'include/grpcpp/impl/codegen/channel_interface.h',
+                      'include/grpcpp/impl/codegen/client_callback.h',
                       'include/grpcpp/impl/codegen/client_context.h',
                       'include/grpcpp/impl/codegen/client_unary_call.h',
                       'include/grpcpp/impl/codegen/completion_queue.h',
@@ -187,6 +190,7 @@ Pod::Spec.new do |s|
                       'src/cpp/client/credentials_cc.cc',
                       'src/cpp/client/generic_stub.cc',
                       'src/cpp/common/alarm.cc',
+                      'src/cpp/common/callback_common.cc',
                       'src/cpp/common/channel_arguments.cc',
                       'src/cpp/common/channel_filter.cc',
                       'src/cpp/common/completion_queue_cc.cc',

grpc.gyp

@@ -1381,6 +1381,7 @@
         'src/cpp/client/credentials_cc.cc',
         'src/cpp/client/generic_stub.cc',
         'src/cpp/common/alarm.cc',
+        'src/cpp/common/callback_common.cc',
         'src/cpp/common/channel_arguments.cc',
         'src/cpp/common/channel_filter.cc',
         'src/cpp/common/completion_queue_cc.cc',
@@ -1528,6 +1529,7 @@
         'src/cpp/client/credentials_cc.cc',
         'src/cpp/client/generic_stub.cc',
         'src/cpp/common/alarm.cc',
+        'src/cpp/common/callback_common.cc',
         'src/cpp/common/channel_arguments.cc',
         'src/cpp/common/channel_filter.cc',
         'src/cpp/common/completion_queue_cc.cc',

include/grpcpp/channel.h

@@ -78,8 +78,13 @@ class Channel final : public ChannelInterface,
   bool WaitForStateChangeImpl(grpc_connectivity_state last_observed,
                               gpr_timespec deadline) override;
 
+  CompletionQueue* CallbackCQ() override;
+
   const grpc::string host_;
   grpc_channel* const c_channel_;  // owned
+
+  CompletionQueue* callback_cq_ = nullptr;
+  std::mutex mu_;
 };
 
 }  // namespace grpc

include/grpcpp/generic/generic_stub.h

@@ -19,9 +19,12 @@
 #ifndef GRPCPP_GENERIC_GENERIC_STUB_H
 #define GRPCPP_GENERIC_GENERIC_STUB_H
 
+#include <functional>
+
 #include <grpcpp/support/async_stream.h>
 #include <grpcpp/support/async_unary_call.h>
 #include <grpcpp/support/byte_buffer.h>
+#include <grpcpp/support/status.h>
 
 namespace grpc {
 
@@ -62,6 +65,26 @@ class GenericStub final {
       ClientContext* context, const grpc::string& method, CompletionQueue* cq,
       void* tag);
 
+  /// NOTE: class experimental_type is not part of the public API of this class
+  /// TODO(vjpai): Move these contents to the public API of GenericStub when
+  ///              they are no longer experimental
+  class experimental_type {
+   public:
+    explicit experimental_type(GenericStub* stub) : stub_(stub) {}
+
+    void UnaryCall(ClientContext* context, const grpc::string& method,
+                   const ByteBuffer* request, ByteBuffer* response,
+                   std::function<void(Status)> on_completion);
+
+   private:
+    GenericStub* stub_;
+  };
+
+  /// NOTE: The function experimental() is not stable public API. It is a view
+  /// to the experimental components of this class. It may be changed or removed
+  /// at any time.
+  experimental_type experimental() { return experimental_type(this); }
+
  private:
   std::shared_ptr<ChannelInterface> channel_;
 };

include/grpcpp/impl/codegen/call.h

@@ -608,6 +608,9 @@ class CallOpSetInterface : public CompletionQueueTag {
   /// Fills in grpc_op, starting from ops[*nops] and moving
   /// upwards.
   virtual void FillOps(grpc_call* call, grpc_op* ops, size_t* nops) = 0;
+
+  /// Get the tag to be used at the CQ
+  virtual void* cq_tag() = 0;
 };
 
 /// Primary implementation of CallOpSetInterface.
@@ -627,7 +630,7 @@ class CallOpSet : public CallOpSetInterface,
                   public Op5,
                   public Op6 {
  public:
-  CallOpSet() : return_tag_(this), call_(nullptr) {}
+  CallOpSet() : cq_tag_(this), return_tag_(this), call_(nullptr) {}
   void FillOps(grpc_call* call, grpc_op* ops, size_t* nops) override {
     this->Op1::AddOp(ops, nops);
     this->Op2::AddOp(ops, nops);
@@ -654,7 +657,12 @@ class CallOpSet : public CallOpSetInterface,
 
   void set_output_tag(void* return_tag) { return_tag_ = return_tag; }
 
+  void* cq_tag() override { return cq_tag_; }
+
+  void set_cq_tag(void* cq_tag) { cq_tag_ = cq_tag; }
+
  private:
+  void* cq_tag_;
   void* return_tag_;
   grpc_call* call_;
 };

include/grpcpp/impl/codegen/callback_common.h

@@ -0,0 +1,79 @@
+/*
+ *
+ * Copyright 2018 gRPC 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.
+ *
+ */
+
+#ifndef GRPCPP_IMPL_CODEGEN_CALLBACK_COMMON_H
+#define GRPCPP_IMPL_CODEGEN_CALLBACK_COMMON_H
+
+#include <functional>
+
+#include <grpcpp/impl/codegen/call.h>
+#include <grpcpp/impl/codegen/channel_interface.h>
+#include <grpcpp/impl/codegen/config.h>
+#include <grpcpp/impl/codegen/core_codegen_interface.h>
+#include <grpcpp/impl/codegen/status.h>
+
+// Forward declarations
+namespace grpc_core {
+class CQCallbackInterface;
+};
+
+namespace grpc {
+namespace internal {
+
+class CallbackWithStatusTag {
+ public:
+  // TODO(vjpai): make impl and ops part of this structure to avoid allocation,
+  // ownership transfer, and delete
+  CallbackWithStatusTag(std::function<void(Status)> f, bool self_delete,
+                        CompletionQueueTag* ops);
+  ~CallbackWithStatusTag() { delete ops_; }
+  void* tag() { return static_cast<void*>(impl_); }
+  Status* status_ptr() { return status_; }
+  CompletionQueueTag* ops() { return ops_; }
+
+  // force_run can only be performed on a tag before it can ever be active
+  void force_run(Status s);
+
+ private:
+  grpc_core::CQCallbackInterface* impl_;
+  Status* status_;
+  CompletionQueueTag* ops_;
+};
+
+class CallbackWithSuccessTag {
+ public:
+  // TODO(vjpai): make impl and ops part of this structure to avoid allocation,
+  // ownership transfer, and delete
+  CallbackWithSuccessTag(std::function<void(bool)> f, bool self_delete,
+                         CompletionQueueTag* ops);
+  ~CallbackWithSuccessTag() { delete ops_; }
+  void* tag() { return static_cast<void*>(impl_); }
+  CompletionQueueTag* ops() { return ops_; }
+
+  // force_run can only be performed on a tag before it can ever be active
+  void force_run(bool ok);
+
+ private:
+  grpc_core::CQCallbackInterface* impl_;
+  CompletionQueueTag* ops_;
+};
+
+}  // namespace internal
+}  // namespace grpc
+
+#endif  // GRPCPP_IMPL_CODEGEN_CLIENT_CALLBACK_H

include/grpcpp/impl/codegen/channel_interface.h

@@ -41,6 +41,8 @@ class CallOpSetInterface;
 class RpcMethod;
 template <class InputMessage, class OutputMessage>
 class BlockingUnaryCallImpl;
+template <class InputMessage, class OutputMessage>
+class CallbackUnaryCallImpl;
 template <class R>
 class ClientAsyncReaderFactory;
 template <class W>
@@ -103,6 +105,8 @@ class ChannelInterface {
   friend class ::grpc::internal::ClientAsyncResponseReaderFactory;
   template <class InputMessage, class OutputMessage>
   friend class ::grpc::internal::BlockingUnaryCallImpl;
+  template <class InputMessage, class OutputMessage>
+  friend class ::grpc::internal::CallbackUnaryCallImpl;
   friend class ::grpc::internal::RpcMethod;
   virtual internal::Call CreateCall(const internal::RpcMethod& method,
                                     ClientContext* context,
@@ -115,6 +119,16 @@ class ChannelInterface {
                                        CompletionQueue* cq, void* tag) = 0;
   virtual bool WaitForStateChangeImpl(grpc_connectivity_state last_observed,
                                       gpr_timespec deadline) = 0;
+
+  // EXPERIMENTAL
+  // A method to get the callbackable completion queue associated with this
+  // channel. If the return value is nullptr, this channel doesn't support
+  // callback operations.
+  // TODO(vjpai): Consider a better default like using a global CQ
+  // Returns nullptr (rather than being pure) since this is a new method
+  // and adding a new pure method to an interface would be a breaking change
+  // (even though this is private and non-API)
+  virtual CompletionQueue* CallbackCQ() { return nullptr; }
 };
 }  // namespace grpc
 

include/grpcpp/impl/codegen/client_callback.h

@@ -0,0 +1,92 @@
+/*
+ *
+ * Copyright 2018 gRPC 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.
+ *
+ */
+
+#ifndef GRPCPP_IMPL_CODEGEN_CLIENT_CALLBACK_H
+#define GRPCPP_IMPL_CODEGEN_CLIENT_CALLBACK_H
+
+#include <functional>
+
+#include <grpcpp/impl/codegen/call.h>
+#include <grpcpp/impl/codegen/callback_common.h>
+#include <grpcpp/impl/codegen/channel_interface.h>
+#include <grpcpp/impl/codegen/config.h>
+#include <grpcpp/impl/codegen/core_codegen_interface.h>
+#include <grpcpp/impl/codegen/status.h>
+
+namespace grpc {
+
+class Channel;
+class ClientContext;
+class CompletionQueue;
+
+namespace internal {
+class RpcMethod;
+
+/// Perform a callback-based unary call
+/// TODO(vjpai): Combine as much as possible with the blocking unary call code
+template <class InputMessage, class OutputMessage>
+void CallbackUnaryCall(ChannelInterface* channel, const RpcMethod& method,
+                       ClientContext* context, const InputMessage* request,
+                       OutputMessage* result,
+                       std::function<void(Status)> on_completion) {
+  CallbackUnaryCallImpl<InputMessage, OutputMessage> x(
+      channel, method, context, request, result, on_completion);
+}
+
+template <class InputMessage, class OutputMessage>
+class CallbackUnaryCallImpl {
+ public:
+  CallbackUnaryCallImpl(ChannelInterface* channel, const RpcMethod& method,
+                        ClientContext* context, const InputMessage* request,
+                        OutputMessage* result,
+                        std::function<void(Status)> on_completion) {
+    CompletionQueue* cq = channel->CallbackCQ();
+    GPR_CODEGEN_ASSERT(cq != nullptr);
+
+    // TODO(vjpai): Allocate this as part of the tag's arena
+    auto* ops = new CallOpSet<CallOpSendInitialMetadata, CallOpSendMessage,
+                              CallOpRecvInitialMetadata,
+                              CallOpRecvMessage<OutputMessage>,
+                              CallOpClientSendClose, CallOpClientRecvStatus>;
+
+    // TODO(vjpai): Move to using pre-allocated tags rather than new/self-delete
+    auto* tag = new CallbackWithStatusTag(on_completion, true, ops);
+
+    // TODO(vjpai): Unify code with sync API as much as possible
+    Call call(channel->CreateCall(method, context, cq));
+    Status s = ops->SendMessage(*request);
+    if (!s.ok()) {
+      tag->force_run(s);
+      return;
+    }
+    ops->SendInitialMetadata(context->send_initial_metadata_,
+                             context->initial_metadata_flags());
+    ops->RecvInitialMetadata(context);
+    ops->RecvMessage(result);
+    ops->AllowNoMessage();
+    ops->ClientSendClose();
+    ops->ClientRecvStatus(context, tag->status_ptr());
+    ops->set_cq_tag(tag->tag());
+    call.PerformOps(ops);
+  }
+};
+
+}  // namespace internal
+}  // namespace grpc
+
+#endif  // GRPCPP_IMPL_CODEGEN_CLIENT_CALLBACK_H

include/grpcpp/impl/codegen/client_context.h

@@ -68,6 +68,8 @@ class CallOpClientRecvStatus;
 class CallOpRecvInitialMetadata;
 template <class InputMessage, class OutputMessage>
 class BlockingUnaryCallImpl;
+template <class InputMessage, class OutputMessage>
+class CallbackUnaryCallImpl;
 }  // namespace internal
 
 template <class R>
@@ -389,6 +391,8 @@ class ClientContext {
   friend class ::grpc::ClientAsyncResponseReader;
   template <class InputMessage, class OutputMessage>
   friend class ::grpc::internal::BlockingUnaryCallImpl;
+  template <class InputMessage, class OutputMessage>
+  friend class ::grpc::internal::CallbackUnaryCallImpl;
 
   // Used by friend class CallOpClientRecvStatus
   void set_debug_error_string(const grpc::string& debug_error_string) {

include/grpcpp/impl/codegen/completion_queue.h

@@ -274,6 +274,9 @@ class CompletionQueue : private GrpcLibraryCodegen {
   template <class InputMessage, class OutputMessage>
   friend class ::grpc::internal::BlockingUnaryCallImpl;
 
+  // Friends that need access to constructor for callback CQ
+  friend class ::grpc::Channel;
+
   /// EXPERIMENTAL
   /// Creates a Thread Local cache to store the first event
   /// On this completion queue queued from this thread.  Once

include/grpcpp/support/client_callback.h

@@ -0,0 +1,24 @@
+/*
+ *
+ * Copyright 2015 gRPC 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.
+ *
+ */
+
+#ifndef GRPCPP_SUPPORT_CLIENT_CALLBACK_H
+#define GRPCPP_SUPPORT_CLIENT_CALLBACK_H
+
+#include <grpcpp/impl/codegen/client_callback.h>
+
+#endif  // GRPCPP_SUPPORT_CLIENT_CALLBACK_H

src/core/lib/surface/completion_queue.cc

@@ -1364,9 +1364,11 @@ static void cq_shutdown_callback(grpc_completion_queue* cq) {
   }
   cqd->shutdown_called = true;
   if (gpr_atm_full_fetch_add(&cqd->pending_events, -1) == 1) {
+    gpr_mu_unlock(cq->mu);
     cq_finish_shutdown_callback(cq);
+  } else {
+    gpr_mu_unlock(cq->mu);
   }
-  gpr_mu_unlock(cq->mu);
   GRPC_CQ_INTERNAL_UNREF(cq, "shutting_down (callback cq)");
 }
 

src/cpp/client/channel_cc.cc

@@ -42,8 +42,10 @@
 #include <grpcpp/support/time.h>
 #include "src/core/lib/gpr/env.h"
 #include "src/core/lib/gpr/string.h"
+#include "src/core/lib/gprpp/memory.h"
 #include "src/core/lib/gprpp/thd.h"
 #include "src/core/lib/profiling/timers.h"
+#include "src/core/lib/surface/completion_queue.h"
 
 namespace grpc {
 
@@ -53,7 +55,12 @@ Channel::Channel(const grpc::string& host, grpc_channel* channel)
   g_gli_initializer.summon();
 }
 
-Channel::~Channel() { grpc_channel_destroy(c_channel_); }
+Channel::~Channel() {
+  grpc_channel_destroy(c_channel_);
+  if (callback_cq_ != nullptr) {
+    callback_cq_->Shutdown();
+  }
+}
 
 namespace {
 
@@ -135,8 +142,8 @@ void Channel::PerformOpsOnCall(internal::CallOpSetInterface* ops,
   size_t nops = 0;
   grpc_op cops[MAX_OPS];
   ops->FillOps(call->call(), cops, &nops);
-  GPR_ASSERT(GRPC_CALL_OK ==
-             grpc_call_start_batch(call->call(), cops, nops, ops, nullptr));
+  GPR_ASSERT(GRPC_CALL_OK == grpc_call_start_batch(call->call(), cops, nops,
+                                                   ops->cq_tag(), nullptr));
 }
 
 void* Channel::RegisterMethod(const char* method) {
@@ -185,4 +192,32 @@ bool Channel::WaitForStateChangeImpl(grpc_connectivity_state last_observed,
   return ok;
 }
 
+namespace {
+class ShutdownCallback : public grpc_core::CQCallbackInterface {
+ public:
+  void TakeCQ(CompletionQueue* cq) { cq_ = cq; }
+  void Run(bool) override {
+    delete cq_;
+    grpc_core::Delete(this);
+  }
+
+ private:
+  CompletionQueue* cq_ = nullptr;
+};
+}  // namespace
+
+CompletionQueue* Channel::CallbackCQ() {
+  // TODO(vjpai): Consider using a single global CQ for the default CQ
+  // if there is no explicit per-channel CQ registered
+  std::lock_guard<std::mutex> l(mu_);
+  if (callback_cq_ == nullptr) {
+    auto* shutdown_callback = grpc_core::New<ShutdownCallback>();
+    callback_cq_ = new CompletionQueue(grpc_completion_queue_attributes{
+        GRPC_CQ_CURRENT_VERSION, GRPC_CQ_CALLBACK, GRPC_CQ_DEFAULT_POLLING,
+        shutdown_callback});
+    shutdown_callback->TakeCQ(callback_cq_);
+  }
+  return callback_cq_;
+}
+
 }  // namespace grpc

src/cpp/client/generic_stub.cc

@@ -16,9 +16,11 @@
  *
  */
 
-#include <grpcpp/generic/generic_stub.h>
+#include <functional>
 
+#include <grpcpp/generic/generic_stub.h>
 #include <grpcpp/impl/rpc_method.h>
+#include <grpcpp/support/client_callback.h>
 
 namespace grpc {
 
@@ -60,4 +62,14 @@ std::unique_ptr<GenericClientAsyncResponseReader> GenericStub::PrepareUnaryCall(
           context, request, false));
 }
 
+void GenericStub::experimental_type::UnaryCall(
+    ClientContext* context, const grpc::string& method,
+    const ByteBuffer* request, ByteBuffer* response,
+    std::function<void(Status)> on_completion) {
+  internal::CallbackUnaryCall(
+      stub_->channel_.get(),
+      internal::RpcMethod(method.c_str(), internal::RpcMethod::NORMAL_RPC),
+      context, request, response, on_completion);
+}
+
 }  // namespace grpc

src/cpp/common/callback_common.cc

@@ -0,0 +1,109 @@
+/*
+ *
+ * Copyright 2018 gRPC 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.
+ *
+ */
+
+#include <functional>
+
+#include <grpcpp/impl/codegen/callback_common.h>
+#include <grpcpp/impl/codegen/status.h>
+
+#include "src/core/lib/gprpp/memory.h"
+#include "src/core/lib/surface/completion_queue.h"
+
+namespace grpc {
+namespace internal {
+
+namespace {
+class CallbackWithSuccessImpl : public grpc_core::CQCallbackInterface {
+ public:
+  CallbackWithSuccessImpl(CallbackWithSuccessTag* parent,
+                          std::function<void(bool)> f, bool self_delete)
+      : parent_(parent), func_(f), self_delete_(self_delete) {}
+
+  void Run(bool ok) override {
+    void* ignored = parent_->ops();
+    bool new_ok = ok;
+    GPR_ASSERT(parent_->ops()->FinalizeResult(&ignored, &new_ok));
+    GPR_ASSERT(ignored == parent_->ops());
+    func_(ok);
+    if (self_delete_) {
+      delete parent_;
+      // Must use grpc_core::Delete since base is GRPC_ABSTRACT
+      grpc_core::Delete(this);
+    }
+  }
+
+ private:
+  CallbackWithSuccessTag* parent_;
+  std::function<void(bool)> func_;
+  bool self_delete_;
+};
+
+class CallbackWithStatusImpl : public grpc_core::CQCallbackInterface {
+ public:
+  CallbackWithStatusImpl(CallbackWithStatusTag* parent,
+                         std::function<void(Status)> f, bool self_delete)
+      : parent_(parent), func_(f), status_(), self_delete_(self_delete) {}
+
+  void Run(bool ok) override {
+    void* ignored = parent_->ops();
+
+    GPR_ASSERT(parent_->ops()->FinalizeResult(&ignored, &ok));
+    GPR_ASSERT(ignored == parent_->ops());
+
+    func_(status_);
+    if (self_delete_) {
+      delete parent_;
+      // Must use grpc_core::Delete since base is GRPC_ABSTRACT
+      grpc_core::Delete(this);
+    }
+  }
+  Status* status_ptr() { return &status_; }
+
+ private:
+  CallbackWithStatusTag* parent_;
+  std::function<void(Status)> func_;
+  Status status_;
+  bool self_delete_;
+};
+
+}  // namespace
+
+CallbackWithSuccessTag::CallbackWithSuccessTag(std::function<void(bool)> f,
+                                               bool self_delete,
+                                               CompletionQueueTag* ops)
+    : impl_(grpc_core::New<CallbackWithSuccessImpl>(this, f, self_delete)),
+      ops_(ops) {}
+
+void CallbackWithSuccessTag::force_run(bool ok) { impl_->Run(ok); }
+
+CallbackWithStatusTag::CallbackWithStatusTag(std::function<void(Status)> f,
+                                             bool self_delete,
+                                             CompletionQueueTag* ops)
+    : ops_(ops) {
+  auto* impl = grpc_core::New<CallbackWithStatusImpl>(this, f, self_delete);
+  impl_ = impl;
+  status_ = impl->status_ptr();
+}
+
+void CallbackWithStatusTag::force_run(Status s) {
+  *status_ = s;
+  impl_->Run(true);
+}
+
+}  // namespace internal
+}  // namespace grpc

src/cpp/server/server_cc.cc

@@ -657,7 +657,8 @@ void Server::PerformOpsOnCall(internal::CallOpSetInterface* ops,
   size_t nops = 0;
   grpc_op cops[MAX_OPS];
   ops->FillOps(call->call(), cops, &nops);
-  auto result = grpc_call_start_batch(call->call(), cops, nops, ops, nullptr);
+  auto result =
+      grpc_call_start_batch(call->call(), cops, nops, ops->cq_tag(), nullptr);
   if (result != GRPC_CALL_OK) {
     gpr_log(GPR_ERROR, "Fatal: grpc_call_start_batch returned %d", result);
     grpc_call_log_batch(__FILE__, __LINE__, GPR_LOG_SEVERITY_ERROR,

src/cpp/server/server_context.cc

@@ -61,6 +61,9 @@ class ServerContext::CompletionOp final : public internal::CallOpSetInterface {
     tag_ = tag;
   }
 
+  /// TODO(vjpai): Allow override of cq_tag if appropriate for callback API
+  void* cq_tag() override { return this; }
+
   void Unref();
 
  private:

test/cpp/end2end/BUILD

@@ -98,6 +98,25 @@ grpc_cc_binary(
     ],
 )
 
+grpc_cc_test(
+    name = "client_callback_end2end_test",
+    srcs = ["client_callback_end2end_test.cc"],
+    external_deps = [
+        "gtest",
+    ],
+    deps = [
+        ":test_service_impl",
+        "//:gpr",
+        "//:grpc",
+        "//:grpc++",
+        "//src/proto/grpc/testing:echo_messages_proto",
+        "//src/proto/grpc/testing:echo_proto",
+        "//test/core/util:gpr_test_util",
+        "//test/core/util:grpc_test_util",
+        "//test/cpp/util:test_util",
+    ],
+)
+
 grpc_cc_library(
     name = "end2end_test_lib",
     testonly = True,

test/cpp/end2end/client_callback_end2end_test.cc

@@ -0,0 +1,126 @@
+/*
+ *
+ * Copyright 2018 gRPC 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.
+ *
+ */
+
+#include <functional>
+#include <mutex>
+
+#include <grpcpp/channel.h>
+#include <grpcpp/client_context.h>
+#include <grpcpp/create_channel.h>
+#include <grpcpp/generic/generic_stub.h>
+#include <grpcpp/impl/codegen/proto_utils.h>
+#include <grpcpp/server.h>
+#include <grpcpp/server_builder.h>
+#include <grpcpp/server_context.h>
+#include <grpcpp/support/client_callback.h>
+
+#include "src/proto/grpc/testing/echo.grpc.pb.h"
+#include "test/core/util/test_config.h"
+#include "test/cpp/end2end/test_service_impl.h"
+#include "test/cpp/util/byte_buffer_proto_helper.h"
+
+#include <gtest/gtest.h>
+
+namespace grpc {
+namespace testing {
+namespace {
+
+class ClientCallbackEnd2endTest : public ::testing::Test {
+ protected:
+  ClientCallbackEnd2endTest() {}
+
+  void SetUp() override {
+    ServerBuilder builder;
+    builder.RegisterService(&service_);
+
+    server_ = builder.BuildAndStart();
+    is_server_started_ = true;
+  }
+
+  void ResetStub() {
+    ChannelArguments args;
+    channel_ = server_->InProcessChannel(args);
+    stub_.reset(new GenericStub(channel_));
+  }
+
+  void TearDown() override {
+    if (is_server_started_) {
+      server_->Shutdown();
+    }
+  }
+
+  void SendRpcs(int num_rpcs) {
+    const grpc::string kMethodName("/grpc.testing.EchoTestService/Echo");
+    grpc::string test_string("");
+    for (int i = 0; i < num_rpcs; i++) {
+      EchoRequest request;
+      std::unique_ptr<ByteBuffer> send_buf;
+      ByteBuffer recv_buf;
+      ClientContext cli_ctx;
+
+      test_string += "Hello world. ";
+      request.set_message(test_string);
+      send_buf = SerializeToByteBuffer(&request);
+
+      std::mutex mu;
+      std::condition_variable cv;
+      bool done;
+      stub_->experimental().UnaryCall(
+          &cli_ctx, kMethodName, send_buf.get(), &recv_buf,
+          [&request, &recv_buf, &done, &mu, &cv](Status s) {
+            GPR_ASSERT(s.ok());
+
+            EchoResponse response;
+            EXPECT_TRUE(ParseFromByteBuffer(&recv_buf, &response));
+            EXPECT_EQ(request.message(), response.message());
+            std::lock_guard<std::mutex> l(mu);
+            done = true;
+            cv.notify_one();
+          });
+      std::unique_lock<std::mutex> l(mu);
+      while (!done) {
+        cv.wait(l);
+      }
+    }
+  }
+  bool is_server_started_;
+  std::shared_ptr<Channel> channel_;
+  std::unique_ptr<grpc::GenericStub> stub_;
+  TestServiceImpl service_;
+  std::unique_ptr<Server> server_;
+};
+
+TEST_F(ClientCallbackEnd2endTest, SimpleRpc) {
+  ResetStub();
+  SendRpcs(1);
+}
+
+TEST_F(ClientCallbackEnd2endTest, SequentialRpcs) {
+  ResetStub();
+  SendRpcs(10);
+}
+
+}  // namespace
+}  // namespace testing
+}  // namespace grpc
+
+int main(int argc, char** argv) {
+  grpc_test_init(argc, argv);
+  ::testing::InitGoogleTest(&argc, argv);
+  return RUN_ALL_TESTS();
+}

tools/doxygen/Doxyfile.c++

@@ -945,7 +945,9 @@ include/grpcpp/impl/codegen/async_unary_call.h \
 include/grpcpp/impl/codegen/byte_buffer.h \
 include/grpcpp/impl/codegen/call.h \
 include/grpcpp/impl/codegen/call_hook.h \
+include/grpcpp/impl/codegen/callback_common.h \
 include/grpcpp/impl/codegen/channel_interface.h \
+include/grpcpp/impl/codegen/client_callback.h \
 include/grpcpp/impl/codegen/client_context.h \
 include/grpcpp/impl/codegen/client_unary_call.h \
 include/grpcpp/impl/codegen/completion_queue.h \
@@ -997,6 +999,7 @@ include/grpcpp/support/async_stream.h \
 include/grpcpp/support/async_unary_call.h \
 include/grpcpp/support/byte_buffer.h \
 include/grpcpp/support/channel_arguments.h \
+include/grpcpp/support/client_callback.h \
 include/grpcpp/support/config.h \
 include/grpcpp/support/proto_buffer_reader.h \
 include/grpcpp/support/proto_buffer_writer.h \

tools/doxygen/Doxyfile.c++.internal

@@ -946,7 +946,9 @@ include/grpcpp/impl/codegen/async_unary_call.h \
 include/grpcpp/impl/codegen/byte_buffer.h \
 include/grpcpp/impl/codegen/call.h \
 include/grpcpp/impl/codegen/call_hook.h \
+include/grpcpp/impl/codegen/callback_common.h \
 include/grpcpp/impl/codegen/channel_interface.h \
+include/grpcpp/impl/codegen/client_callback.h \
 include/grpcpp/impl/codegen/client_context.h \
 include/grpcpp/impl/codegen/client_unary_call.h \
 include/grpcpp/impl/codegen/completion_queue.h \
@@ -999,6 +1001,7 @@ include/grpcpp/support/async_stream.h \
 include/grpcpp/support/async_unary_call.h \
 include/grpcpp/support/byte_buffer.h \
 include/grpcpp/support/channel_arguments.h \
+include/grpcpp/support/client_callback.h \
 include/grpcpp/support/config.h \
 include/grpcpp/support/proto_buffer_reader.h \
 include/grpcpp/support/proto_buffer_writer.h \
@@ -1187,6 +1190,7 @@ src/cpp/client/secure_credentials.h \
 src/cpp/codegen/codegen_init.cc \
 src/cpp/common/alarm.cc \
 src/cpp/common/auth_property_iterator.cc \
+src/cpp/common/callback_common.cc \
 src/cpp/common/channel_arguments.cc \
 src/cpp/common/channel_filter.cc \
 src/cpp/common/channel_filter.h \

tools/run_tests/generated/sources_and_headers.json

@@ -3302,6 +3302,25 @@
     "third_party": false, 
     "type": "target"
   }, 
+  {
+    "deps": [
+      "gpr", 
+      "gpr_test_util", 
+      "grpc", 
+      "grpc++", 
+      "grpc++_test_util", 
+      "grpc_test_util"
+    ], 
+    "headers": [], 
+    "is_filegroup": false, 
+    "language": "c++", 
+    "name": "client_callback_end2end_test", 
+    "src": [
+      "test/cpp/end2end/client_callback_end2end_test.cc"
+    ], 
+    "third_party": false, 
+    "type": "target"
+  }, 
   {
     "deps": [
       "gpr", 
@@ -11081,7 +11100,9 @@
       "include/grpcpp/impl/codegen/byte_buffer.h", 
       "include/grpcpp/impl/codegen/call.h", 
       "include/grpcpp/impl/codegen/call_hook.h", 
+      "include/grpcpp/impl/codegen/callback_common.h", 
       "include/grpcpp/impl/codegen/channel_interface.h", 
+      "include/grpcpp/impl/codegen/client_callback.h", 
       "include/grpcpp/impl/codegen/client_context.h", 
       "include/grpcpp/impl/codegen/client_unary_call.h", 
       "include/grpcpp/impl/codegen/completion_queue.h", 
@@ -11147,7 +11168,9 @@
       "include/grpcpp/impl/codegen/byte_buffer.h", 
       "include/grpcpp/impl/codegen/call.h", 
       "include/grpcpp/impl/codegen/call_hook.h", 
+      "include/grpcpp/impl/codegen/callback_common.h", 
       "include/grpcpp/impl/codegen/channel_interface.h", 
+      "include/grpcpp/impl/codegen/client_callback.h", 
       "include/grpcpp/impl/codegen/client_context.h", 
       "include/grpcpp/impl/codegen/client_unary_call.h", 
       "include/grpcpp/impl/codegen/completion_queue.h", 
@@ -11305,6 +11328,7 @@
       "include/grpcpp/support/async_unary_call.h", 
       "include/grpcpp/support/byte_buffer.h", 
       "include/grpcpp/support/channel_arguments.h", 
+      "include/grpcpp/support/client_callback.h", 
       "include/grpcpp/support/config.h", 
       "include/grpcpp/support/proto_buffer_reader.h", 
       "include/grpcpp/support/proto_buffer_writer.h", 
@@ -11409,6 +11433,7 @@
       "include/grpcpp/support/async_unary_call.h", 
       "include/grpcpp/support/byte_buffer.h", 
       "include/grpcpp/support/channel_arguments.h", 
+      "include/grpcpp/support/client_callback.h", 
       "include/grpcpp/support/config.h", 
       "include/grpcpp/support/proto_buffer_reader.h", 
       "include/grpcpp/support/proto_buffer_writer.h", 
@@ -11428,6 +11453,7 @@
       "src/cpp/client/credentials_cc.cc", 
       "src/cpp/client/generic_stub.cc", 
       "src/cpp/common/alarm.cc", 
+      "src/cpp/common/callback_common.cc", 
       "src/cpp/common/channel_arguments.cc", 
       "src/cpp/common/channel_filter.cc", 
       "src/cpp/common/channel_filter.h", 

tools/run_tests/generated/tests.json

@@ -3945,6 +3945,30 @@
     ], 
     "uses_polling": true
   }, 
+  {
+    "args": [], 
+    "benchmark": false, 
+    "ci_platforms": [
+      "linux", 
+      "mac", 
+      "posix", 
+      "windows"
+    ], 
+    "cpu_cost": 0.5, 
+    "exclude_configs": [], 
+    "exclude_iomgrs": [], 
+    "flaky": false, 
+    "gtest": true, 
+    "language": "c++", 
+    "name": "client_callback_end2end_test", 
+    "platforms": [
+      "linux", 
+      "mac", 
+      "posix", 
+      "windows"
+    ], 
+    "uses_polling": true
+  }, 
   {
     "args": [], 
     "benchmark": false,