How to specify a dependency graph when generating files with CMake? - c++

Question

I'm integrating protobuf files compilation in my project's CMake-based build process. Currently I'm using the following CMake script:
set (TARGET_NAME ProtobufMessages)
file (GLOB_RECURSE PROTOBUF_FILES "protobuf/*.proto")
set (HEADER_FILES "")
set (SOURCE_FILES "")
foreach (FILE ${PROTOBUF_FILES})
string (REPLACE ${CMAKE_CURRENT_SOURCE_DIR} ${CMAKE_CURRENT_BINARY_DIR} FILE ${FILE})
string (REPLACE ".proto" ".pb.h" HEADER_FILE ${FILE})
string (REPLACE ".proto" ".pb.cc" SOURCE_FILE ${FILE})
list (APPEND HEADER_FILES ${HEADER_FILE})
list (APPEND SOURCE_FILES ${SOURCE_FILE})
endforeach ()
add_library(${TARGET_NAME} STATIC
${HEADER_FILES}
${SOURCE_FILES}
${PROTOBUF_FILES}
)
set_target_properties (${TARGET_NAME} PROPERTIES
FOLDER ${COMMON_PROJECTS_FOLDER_NAME})
set (PROTOBUF_COMPILE_COMMAND
${CONAN_BIN_DIRS_PROTOBUF_RELEASE}/protoc
--cpp_out=${CMAKE_CURRENT_BINARY_DIR}/protobuf
-I${CMAKE_CURRENT_SOURCE_DIR}/protobuf
${PROTOBUF_FILES}
)
add_custom_command(OUTPUT ${HEADER_FILES} ${SOURCE_FILES}
COMMAND ${CMAKE_COMMAND} -E make_directory ${CMAKE_CURRENT_BINARY_DIR}/protobuf
COMMAND ${PROTOBUF_COMPILE_COMMAND}
DEPENDS ${PROTOBUF_FILES}
COMMENT "Compiling protocol buffer files ..."
)
source_group (TREE ${CMAKE_CURRENT_SOURCE_DIR} FILES
${PROTOBUF_FILES}
)
set_target_properties (${TARGET_NAME} PROPERTIES
FOLDER ${COMMON_PROJECTS_FOLDER_NAME}
)
target_link_libraries (${TARGET_NAME}
CONAN_PKG::protobuf
)
It works fine, but the problem with this approach is that when a single proto file is changed all the files are regenerated.
A simple modification of this approach with calling add_custom_command in a loop partially solves the problem:
set (PROTOBUF_COMPILE_COMMAND
${CONAN_BIN_DIRS_PROTOBUF_RELEASE}/protoc
--cpp_out=${CMAKE_CURRENT_BINARY_DIR}/protobuf
-I${CMAKE_CURRENT_SOURCE_DIR}/protobuf
)
foreach (PROTOBUF_FILE ${PROTOBUF_FILES})
string (REPLACE ${CMAKE_CURRENT_SOURCE_DIR} ${CMAKE_CURRENT_BINARY_DIR} TEMP_FILE ${PROTOBUF_FILE})
string (REPLACE ".proto" ".pb.h" HEADER_FILE ${TEMP_FILE})
string (REPLACE ".proto" ".pb.cc" SOURCE_FILE ${TEMP_FILE})
add_custom_command(OUTPUT ${HEADER_FILE} ${SOURCE_FILE}
COMMAND ${CMAKE_COMMAND} -E make_directory ${CMAKE_CURRENT_BINARY_DIR}/protobuf
COMMAND ${PROTOBUF_COMPILE_COMMAND} ${PROTOBUF_FILE}
DEPENDS ${PROTOBUF_FILE}
COMMENT "Compiling ${PROTOBUF_FILE} ..."
)
endforeach ()
But there is still a problem, because protobuf supports including one protobuf file into another:
import "myproject/base.proto";
In this case when base.proto is changed the files including it also must be recompiled. Actually the dependencies can form an arbitrary DAG. Although the current problem is arising specifically with protobuf files compilation this is relevant to all code generation steps when some of the input files can depend on some of the other ones.
Is it possible to solve this problem with CMake?