[CMake] Using find_package fails to find related subprojects that are exported to the package registry

[Timothy Wrona]

Hi fellow CMake programmers! I've got a bit of a tricky situation and I
wonder if anyone has ever dealt with this before.

I am trying to configure a project that has a structure as follows

myproject/
    executable1/  <-- depends on common_lib1
    my_common_libs/
        common_lib1/
        common_lib2/

Note: Before I begin - this is a legacy system and is MUCH larger than what
I am showing here so restructuring the whole project is not really an
option. Let me begin by describing the CMakeLists.txt structure.

In myproject/CMakeLists.txt subdirectories are added as follows:

file(GLOB entries *)
foreach(entry ${entries})
  if (IS_DIRECTORY ${entry} AND EXISTS ${entry}/CMakeLists.txt)
    add_subdirectory(${entry})
  endif()
endforeach()

my_common_libs/CMakeLists.txt does the same...

Then my_common_libs/common_lib1/CMakeLists.txt does what is necessary to
define the library and export the target to the local package registry:

add_library(common_lib1 ...)

export(TARGETS common_lib1 NAMESPACE common_lib1:: FILE
common_lib1-targets.cmake)
file(COPY common_lib1-config.cmake DESTINATION ${CMAKE_CURRENT_BINARY_DIR})
export(PACKAGE common_lib1)

my_common_libs/common_lib2/CMakeLists.txt does the same and both have
"common_libx-config.cmake" files that properly load the corresponding
"-targets.cmake" files.

In executable1/CMakeLists.text I do what is necessary to find the package
and use it:

add_executable(executable1 ...)

find_package(common_lib1 REQUIRED) <-- this is where I am having the problem
target_link_libraries(executable1
    common_lib1::common_lib1
)

Now this is where things get hairy...

If I go into common_lib1 and manually build that first, it puts it in the
package registry and then I can go to "executable1" and build that and it
finds the package, and builds, and links fine.
BUT... if I clean the project and then try to build it from the top level
it immediately fails and says it can't locate the package "common_lib1".

I understand I could probably replace the GLOB commands with explicit
"add_subdirectory" calls and then carefully reorder all of the
"add_subdirectory" commands to add things in dependency order, but that
would be really tedious with how large the project is and is really not the
way the whole rest of the system is done... It feels somewhat brittle
because then I am essentially declaring the dependencies from outside of
the modules themselves.

Is there any way to force CMake to do all of the "export" commands up front
to essentially initialize the package registry so it will be aware of
common_lib1 when I compile from the "myproject" level? Or is there another
solution I haven't even considered that could solve this same problem?

Thanks,
Tim
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