CMake: target not in export set - cmake

Question

I have a library foo which depends on bar. bar provides a nice barConfig.cmake upon installation.
cmake_minimum_required(VERSION 3.7)
project(foo VERSION 1.0)
find_package(bar)
add_library(foo SHARED foo.c)
target_link_libraries(foo PUBLIC bar)
install(TARGETS foo EXPORT fooTargets LIBRARY DESTINATION lib)
export(EXPORT fooTargets FILE fooTargets.cmake)
install(EXPORT fooTargets FILE fooTargets.cmake DESTINATION lib/cmake)
This works great. We simply apt install bar-dev then compile foo.
But since these packages are both developed by us and are related, my team would like to develop them in the same IDE session and compile them at the same time. I want to allow that, but I don't want to change the fact that these are already deployed as seperate packages and can be built independently.
if (DIRECTORY_EXISTS ../bar)
set(BAR_NO_INSTALL ON)
set(BAR_SKIP_TESTS ON)
add_subdirectory(../bar ${CMAKE_CURRENT_BINARY_DIR}/bar)
else()
find_package(bar)
endif()
If ../bar/ exists, then this results in:
CMake Error: install(EXPORT "fooTargets" ...) includes target "foo" which
requires target "bar" that is not in the export set.
How can I prevent the need to export bar in the foo package?
I'm trying to figure out why find_package(bar) works, but add_subdirectory(bar) doesn't. barConfig.cmake via barTargets.cmake defines bar like so:
add_library(bar SHARED IMPORTED)
set_property(TARGET bar APPEND PROPERTY IMPORTED_CONFIGURATIONS NOCONFIG)
set_target_properties(bar PROPERTIES
INTERFACE_INCLUDE_DIRECTORIES "../bar/include"
IMPORTED_LOCATION_NOCONFIG "build/libbar.so.1.0.13574"
IMPORTED_SONAME_NOCONFIG "libbar.so.1"
)
My guess was the IMPORTED property that is used when bar is created in the autogenerated barConfig.cmake. I tried adding setting property (below), but saw no differences:
set_target_properties(bar PROPERTIES IMPORTED TRUE)
fooConfig.cmake does include the following which creates target bar (not bar::bar).
find_depdendency(bar)
I tried the following aliasing, hoping that an alias' inability to be exported would help me. But it still expects me to export bar.
add_library(barImported ALIAS bar)
target_link_libraries(foo PUBLIC barImported)
I tried linking only the build interface, hoping that bar would not need to be exported when installed. But that also had no effect.
target_link_libraries(foo PUBLIC $<BUILD_INTERFACE:bar>)

Answer 1

How can I prevent the need to export bar in the foo package?
You can't, at least not without hacks (see below).
I'm trying to figure out why find_package(bar) works, but add_subdirectory(bar) doesn't. barConfig.cmake via barTargets.cmake defines bar like so:
Because in one case, the target is imported (which can only be set on creation) and in the other case, the target is normal. Normal targets must be installed if normal targets that depend on them are installed.
The semantic issue with add_subdirectory is that CMake believes that any code you add this way is first-party. I have found that add_subdirectory and its relative FetchContent are more trouble than they're worth for installed dependencies. They're useful for build-or-test-only dependencies that do not need to be shipped.
One way to work around this is to use the COMPONENT / NAMELINK_COMPONENT features of the install() command. Then you would set the CPACK_COMPONENTS_ALL variable to include only the components from foo and not those from bar. When specifying components, you should avoid generic names like development or runtime, and instead prefix those names with the project name: foo_development, bar_runtime, etc.
Unfortunately, this does not affect the cmake --build . --target install command. You'll have to go through cmake --install . --component <comp> or CPack.
A hack is to add the bar dependency to foo as $<BUILD_INTERFACE:...> and then re-attach it to foo in fooConfig.cmake after include-ing your generated target export file and finding bar via find_dependency.

Answer 2

Here's the solution I came up with. This is for an arbitrary library foo (dynamic and static versions plus tests) which could also be a dependency in the same manner that I import bar:
cmake_minimum_required(VERSION 3.7)
include(GNUInstallDirs)
project(foo VERSION 1.0)
# Options set to
# ON explicitly for build machines,
# OFF by default for developers who build dependencies inline and only build for local use
option(FOO_EXPORT_TARGETS "Generate fooConfig.cmake for packaging." OFF)
option(FOO_BUILD_TESTS "Build and run tests. Default off for aircraft. Turn on in your IDE if developing foo" OFF)
# This is best implemented as a macro/function from an included cmake
set(BAR_ROOT "/noexist" CACHE PATH "Location of BAR for in-source building")
if (DIRECTORY_EXISTS "${BAR_ROOT}")
add_subdirectory(${BAR_ROOT})
else()
find_package(bar)
endif()
set(fooSources foo.c)
set(fooPublicHeaders include/foo.h)
add_library(fooObjects OBJECT ${fooSources})
target_include_directories(fooObjects
PUBLIC
"$<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>"
"$<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>"
PRIVATE
$<TARGET_PROPERTY:bar,INTERFACE_INCLUDE_DIRECTORIES>
)
add_library(foo SHARED $<TARGET_OBJECTS:fooObjects>)
add_library(fooStatic STATIC $<TARGET_OBJECTS:fooObjects>)
target_link_libraries(foo PUBLIC fooObjects bar)
target_link_libraries(fooStatic PUBLIC fooObjects barStatic)
set_target_properties(fooObjects foo fooStatic
PROPERTIES
POSITION_INDEPENDENT_CODE ON
)
if (NOT FOO_EXPORT_TARGETS)
install(TARGETS foo LIBRARY DESTINATION lib)
else()
set_target_properties(foo fooStatic
PROPERTIES
VERSION ${foo_VERSION}
SOVERSION ${foo_VERSION_MAJOR}
PUBLIC_HEADER "${fooPublicHeaders}"
)
install(
TARGETS foo fooStatic fooObjects
EXPORT fooTargets
RUNTIME DESTINATION ${CMAKE_INSTALL_FULL_BINDIR}
LIBRARY DESTINATION ${CMAKE_INSTALL_FULL_LIBDIR}
ARCHIVE DESTINATION ${CMAKE_INSTALL_FULL_LIBDIR}
PUBLIC_HEADER DESTINATION ${CMAKE_INSTALL_FULL_INCLUDEDIR}/foo
)
include(CMakePackageConfigHelpers)
set(configDir "${CMAKE_INSTALL_FULL_LIBDIR}/cmake/foo-${foo_VERSION_MAJOR}")
configure_package_config_file(
"${CMAKE_CURRENT_LIST_DIR}/fooConfig.cmake.in"
"${CMAKE_CURRENT_BINARY_DIR}/fooConfig.cmake"
INSTALL_DESTINATION ${configDir}
)
write_basic_package_version_file(
"${CMAKE_CURRENT_BINARY_DIR}/fooConfigVersion.cmake"
VERSION ${foo_VERSION}
COMPATIBILITY SameMajorVersion
)
install(FILES
"${CMAKE_CURRENT_BINARY_DIR}/fooConfig.cmake"
"${CMAKE_CURRENT_BINARY_DIR}/fooConfigVersion.cmake"
DESTINATION ${configDir}
)
export (EXPORT fooTargets FILE fooTargets.cmake)
install(EXPORT fooTargets FILE fooTargets.cmake DESTINATION ${configDir})
endif()
if (FOO_BUILD_TESTS)
add_executable(test_foo test.c)
target_link_libraries(test_foo foo)
enable_testing()
add_test(NAME test_foo COMMAND test_foo WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/test)
endif()
With the following fooConfig.cmake.in:
include(CMakeFindDependencyMacro)
#PACKAGE_INIT#
find_dependency(bar)
if(NOT TARGET foo)
include("${CMAKE_CURRENT_LIST_DIR}/fooTargets.cmake")
endif()
The build machines create a debian package out of this. This is what the debian/rules files look like to ensure it is compiled without any in-source building, with packaging, and with tests:
#!/usr/bin/make -f
#export DH_VERBOSE = 1
%:
dh $#
override_dh_auto_configure:
dh_auto_configure -- \
-DCMAKE_LIBRARY_PATH=$(DEB_HOST_MULTIARCH) \
-DFOO_BUILD_TESTS=ON \
-DFOO_EXPORT_TARGETS=ON \
-DBAR_ROOT=/noexist