Cannot get CMake to handle Fortran-90 modules properly - cmake

Question

I've had a miserable afternoon trying to add a Fortran-90 module to a large multilanguage library set (mostly C++) built with CMake, and am at the point where I am clearly going in circles. There is an amazing dearth of information online that might help me with this, and the CMake documentation is as inscrutable as always.
The library set is too large to boil down properly (so I've been unable to get a good small reproducer) but the essence of is this. Each library lives in its own directory, with a test directory beneath it. I've added an F90 file containing a module to one of the directories, which I'll call low_library. The contents of this directory are something like:
low_library:
CMakeList.txt
file1.cc
file1.hh
file2.cc
file2.hh
my_module.f90
test:
CMakeList.txt
test1.cc
test2.cc
test3.cc
my_module_test.f90
except many more .cc and .hh files. In the CMakeList.txt file for the library directory, I have instructions that include
add_library(low_library STATIC "")
target_sources(low_library
PUBLIC:
file1.hh
file2.hh
PRIVATE:
file1.cc
file2.cc
)
There's more but it's not obviously relevant.
So the first question is where to put my_module.f90. I want my_module.f90.o to be included in the liblow_library.a file but I need the my_module.mod file to be visible to other libraries I'm building (and to users of the library set). So I add the line
set(CMAKE_Fortran_MODULE_DIRECTORY ${CMAKE_INSTALL_PREFIX}/fortran_modules)
towards the top of my CMakeList.txt file, add my_module.f90 to the PRIVATE part of the target_sources() PUBLIC list, and ... sigh ... the build system puts the .mod file in the build directory, and not in ${CMAKE_INSTALL_PREFIX}/fortran_modules. The other libraries can't see it and fail to build.
So now I put my_module.f90 in the PUBLIC part of target_sources() and try again. This time the module files goes where I expected it, my test directory can see it and builds successfully, and I'm happy until I move on to building the next library, in directory high_library which depends on low_library. The build comes up with the truly bizarre error:
f951: Fatal Error: Can't rename module file â/scratch/kgbudge/develop/install/fortran_modules/cta_mesh_generator.mod0â to â/scratch/kgbudge/develop/install/fortran_modules/cta_mesh_generator.modâ: No such file or directory
Looking at the rest of the trace, it appears that with my_module.g90 in the public interface of low_library, the build for high_library ignores what's already in my ${CMAKE_INSTALL_PREFIX}/fortran_modules and builds its own copy of the d-mned .mod file in ${CMAKE_INSTALL_PREFIX}/fortran_modules. Which, since we have high-powered build servers and always build in parallel (make -j16 is typical) means a race condition between competing build jobs.
Okay, I know. I needed to include the line
target_include_directories(low_library PUBLIC ${CMAKE_INSTALL_PREFIX}/fortran_modules)
near start of the CMakeLists.txt file for high_library so it knows where to find the modules and wont' try to build its own. Except this doesn't help.
I can find nothing online that gives me any clues what is going on or how to resolve it.

Answer 1

It looks like the problem is that I had my
set(CMAKE_Fortran_MODULE_DIRECTORY ${CMAKE_INSTALL_PREFIX}/fortran_modules)
only in the libraries that produced or consumed the new modules. But other libraries used the library containing the module and needed to see a copy of the module even though they weren't actually using the module. I think.
Anyway, moving this line into the root CMakeLists.txt allows me to build the whole library successfully. I still seem some warnings:
f951: Warning: Nonexistent include directory â/srcâ [-Wmissing-include-dirs]
but at least it all builds.