cmake target created with ADD_CUSTOM_TARGET() generated multiple times in parallel mode - cmake

Question

I have project name libtld where I first create a tool¹:
project(tld_parser)
add_executable(${PROJECT_NAME}
../tools/tldc.cpp
tld_compiler.cpp
tld_file.cpp
tld_strings.c
)
Then I use that tool to generate the tld_data.c file, which is a table with all the TLDs (in my newer version of the library, this is a copy of the RIFF/TLDS binary file which is to be compiled internally as a fallback). Here is the add_custom_command() I use to generate said file:
project(tld_data)
set(TLD_DATA_C ${PROJECT_BINARY_DIR}/tld_data.c)
file(GLOB_RECURSE TLD_FILES ${CMAKE_SOURCE_DIR}/conf/tlds/*.ini)
add_custom_command(
OUTPUT ${TLD_DATA_C}
COMMAND "tld_parser"
"--source"
"${CMAKE_SOURCE_DIR}/conf/tlds"
"--verify"
"--output-json"
"--include-offsets"
"--c-file"
"${TLD_DATA_C}"
"${PROJECT_BINARY_DIR}/tlds.tld"
WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
MAIN_DEPENDENCY tld_parser
DEPENDS ${TLD_FILES}
)
add_custom_target(${PROJECT_NAME} ALL DEPENDS ${TLD_DATA_C})
define_property(SOURCE
PROPERTY GENERATED
BRIEF_DOCS "The tld_data.c file is a table of all the TLDs defined in conf/tlds/... .ini files."
FULL_DOCS "In the new version, the tld_data.c is always generated on the fly since it can be done with just C/C++."
)
Next I want to generate the tld dynamic and static libraries.
set(LIBTLD_SOURCES
tld.cpp
tld_compiler.cpp
${TLD_DATA_C}
tld_domain_to_lowercase.c
tld_emails.cpp
tld_file.cpp
tld_object.cpp
tld_strings.c
)
##
## TLD library
##
project(tld)
configure_file(
tld.h.in
${PROJECT_BINARY_DIR}/tld.h
)
add_library(${PROJECT_NAME} SHARED
${LIBTLD_SOURCES}
)
set_target_properties(${PROJECT_NAME} PROPERTIES
VERSION ${LIBTLD_VERSION_MAJOR}.${LIBTLD_VERSION_MINOR}
SOVERSION ${LIBTLD_VERSION_MAJOR}
)
install(
TARGETS ${PROJECT_NAME}
LIBRARY DESTINATION lib
COMPONENT runtime
)
install(
FILES ${PROJECT_BINARY_DIR}/tld.h
DESTINATION include
COMPONENT development
)
##
## TLD static library
##
project(tld_static)
add_library(${PROJECT_NAME} STATIC
${LIBTLD_SOURCES}
)
add_dependencies(${PROJECT_NAME}
tld
)
# We need the -fPIC to use this library as extension of PHP, etc.
set_target_properties(tld_static PROPERTIES COMPILE_FLAGS -fPIC)
install(
TARGETS ${PROJECT_NAME}
ARCHIVE DESTINATION lib
COMPONENT development
)
I added the add_dependencies() to the tld_static project to depend on tld thinking that would help my case, but I still see two run of the tld_parser...
I also had a test that directly includes the tld_data.c file (that way I can directly verify that the tld() function returns what I would expect from the exact source data).
Adding the add_dependencies() to the test worked as expected. Instead of getting the tld_data.c file created three times, now it's only twice.
project(tld_internal_test)
add_executable(${PROJECT_NAME}
tld_internal_test.cpp
)
add_dependencies(${PROJECT_NAME}
tld
)
add_test(
NAME ${PROJECT_NAME}
COMMAND ${PROJECT_NAME}
WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
)
By killing the parallel build feature, it works as expected, however, there should be no good reasons for me to do that (see this cmake question for reference).
What I'm wondering, though, is:
How do you debug such an issue?
cmake's Makefile's are enormous, so reading those is quite a killer. If I knew what to search for, I suppose I could at least look at specific targets instead of trying to understand all that's happening in there (most of which has nothing to do with my issue).
¹ The code is in a branch at the moment. I'm hoping to have it all wrapped up soon at which point it will replace the main branch. The tld() function interface will not have changed very much. The implementation, however, will be quite different, more flexible, dynamically updatable.