Using cmake to create protobuf / grpc cc files - cmake

Question

If I want to recreate the following protoc command in cmake:
protoc -I ../proto/ --cpp_out=. service.proto
I use the following lines in cmake:
file(GLOB ProtoFiles "${CMAKE_CURRENT_SOURCE_DIR}/*.proto")
PROTOBUF_GENERATE_CPP(ProtoSources ProtoHeaders ${ProtoFiles})
If I instead want to recreate the protoc command below:
protoc -I ../proto/ --grpc_out=. --plugin=protoc-gen-grpc=`which grpc_cpp_plugin` service.proto
In the case above I am not able to determine how to change the cmake file, please help!
The Question is how do I address the:
--plugin=EXECUTABLE Specifies a plugin executable to use.
Normally, protoc searches the PATH for
plugins, but you may specify additional
executables not in the path using this flag.
Additionally, EXECUTABLE may be of the form
NAME=PATH, in which case the given plugin name
is mapped to the given executable even if
the executable's own name differs.
I have been reading the PROTOBUF_GENERATE_CPP documentation, but did not find an answer!

Answer 1

Module findProtobuf.cmake defines functions-wrappers only for common protoc calls: PROTOBUF_GENERATE_CPP - for --cpp_out and PROTOBUF_GENERATE_PYTHON - for --py_out. But you can implement your own function-wrapper for needed plugin. Code below is based on PROTOBUF_GENERATE_CPP implementation.
find_program(GRPC_CPP_PLUGIN grpc_cpp_plugin) # Get full path to plugin
function(PROTOBUF_GENERATE_GRPC_CPP SRCS HDRS)
if(NOT ARGN)
message(SEND_ERROR "Error: PROTOBUF_GENERATE_GRPC_CPP() called without any proto files")
return()
endif()
if(PROTOBUF_GENERATE_CPP_APPEND_PATH) # This variable is common for all types of output.
# Create an include path for each file specified
foreach(FIL ${ARGN})
get_filename_component(ABS_FIL ${FIL} ABSOLUTE)
get_filename_component(ABS_PATH ${ABS_FIL} PATH)
list(FIND _protobuf_include_path ${ABS_PATH} _contains_already)
if(${_contains_already} EQUAL -1)
list(APPEND _protobuf_include_path -I ${ABS_PATH})
endif()
endforeach()
else()
set(_protobuf_include_path -I ${CMAKE_CURRENT_SOURCE_DIR})
endif()
if(DEFINED PROTOBUF_IMPORT_DIRS)
foreach(DIR ${Protobuf_IMPORT_DIRS})
get_filename_component(ABS_PATH ${DIR} ABSOLUTE)
list(FIND _protobuf_include_path ${ABS_PATH} _contains_already)
if(${_contains_already} EQUAL -1)
list(APPEND _protobuf_include_path -I ${ABS_PATH})
endif()
endforeach()
endif()
set(${SRCS})
set(${HDRS})
foreach(FIL ${ARGN})
get_filename_component(ABS_FIL ${FIL} ABSOLUTE)
get_filename_component(FIL_WE ${FIL} NAME_WE)
list(APPEND ${SRCS} "${CMAKE_CURRENT_BINARY_DIR}/${FIL_WE}.grpc.pb.cc")
list(APPEND ${HDRS} "${CMAKE_CURRENT_BINARY_DIR}/${FIL_WE}.grpc.pb.h")
add_custom_command(
OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/${FIL_WE}.grpc.pb.cc"
"${CMAKE_CURRENT_BINARY_DIR}/${FIL_WE}.grpc.pb.h"
COMMAND ${Protobuf_PROTOC_EXECUTABLE}
ARGS --grpc_out=${CMAKE_CURRENT_BINARY_DIR}
--plugin=protoc-gen-grpc=${GRPC_CPP_PLUGIN}
${_protobuf_include_path} ${ABS_FIL}
DEPENDS ${ABS_FIL} ${Protobuf_PROTOC_EXECUTABLE}
COMMENT "Running gRPC C++ protocol buffer compiler on ${FIL}"
VERBATIM)
endforeach()
set_source_files_properties(${${SRCS}} ${${HDRS}} PROPERTIES GENERATED TRUE)
set(${SRCS} ${${SRCS}} PARENT_SCOPE)
set(${HDRS} ${${HDRS}} PARENT_SCOPE)
endfunction()
Usage is same as for PROTOBUF_GENERATE_CPP:
file(GLOB ProtoFiles "${CMAKE_CURRENT_SOURCE_DIR}/*.proto")
PROTOBUF_GENERATE_GRPC_CPP(ProtoGRPCSources ProtoGRPCHeaders ${ProtoFiles})

Answer 2

Starting at version 3.12, protobuf_generate supports a PLUGIN argument
https://github.com/protocolbuffers/protobuf/blob/v3.12.0/cmake/protobuf-config.cmake.in#L46
so you could try something along the line:
PROTOBUF_GENERATE_CPP(ProtoSources ProtoHeaders ${ProtoFiles} PLUGIN protoc-gen-grpc=${GRPC_CPP_PLUGIN_PATH})

Answer 3

For me the blogpost https://www.falkoaxmann.de/dev/2020/11/08/grpc-plugin-cmake-support.html lead to success because it provides a full example (thanks #powerpete).
I'm putting the code here so it is available as an answer and not just as a comment:
project(my-service VERSION 1.0 LANGUAGES CXX C)
find_package(protobuf CONFIG REQUIRED)
find_package(gRPC CONFIG REQUIRED)
find_package(Threads)
set(PROTO_FILES
MyService.proto
)
# protobuf source files go into the lib just like any other CPP source file
add_library(my-service ${PROTO_FILES})
target_link_libraries(my-service
PUBLIC
protobuf::libprotobuf
gRPC::grpc
gRPC::grpc++
)
target_include_directories(my-service
PUBLIC
${CMAKE_CURRENT_BINARY_DIR}
)
get_target_property(grpc_cpp_plugin_location gRPC::grpc_cpp_plugin LOCATION)
# compile the message types
protobuf_generate(TARGET my-service LANGUAGE cpp)
# compile the GRPC services
protobuf_generate(
TARGET
my-service
LANGUAGE
grpc
GENERATE_EXTENSIONS
.grpc.pb.h
.grpc.pb.cc
PLUGIN
"protoc-gen-grpc=${grpc_cpp_plugin_location}"
)