[CMake] Parallel builds and auto generated header files

[Robert Dailey]

I have a Version.hpp file that I have a custom command tied to which
basically runs CMake in script mode to perform configure_file() on it
during build time. The reason it does this is because it builds
Version.hpp using dynamic information, such as defining a macro with
the current SHA1 being built, etc.

I notice in parallel builds, this header file gets built too late,
because of parallel builds. What if a file includes the header and its
translation unit is built before Version.hpp is generated by the
custom command? Would it be better/simpler to do configure_file() at
configuration time instead of at build time as a custom command?
Here's the logic (some variables are defined elsewhere, but should
give you the gist):


set( generated_source_dir ${CMAKE_CURRENT_BINARY_DIR}/Source )
set( version_file_in  ${CMAKE_CURRENT_SOURCE_DIR}/Version.cpp.in )
set( version_file_out ${generated_source_dir}/Main/Version.cpp )

add_custom_command(
    OUTPUT ${version_file_out}
    COMMAND ${CMAKE_COMMAND}
        -D IN_FILE=${version_file_in}
        -D OUT_FILE=${version_file_out}
        -D GIT_EXECUTABLE=${GIT_EXECUTABLE}
        -D BUILD_VERSION=${ZIOSK_BUILD_VERSION}
        -P ${CMAKE_CURRENT_SOURCE_DIR}/build_version_header.cmake
    WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
)

source_group( Generated FILES ${version_file_out} )
list( APPEND source_files ${version_file_out} )