[CMake] CPack per-component value for CPACK_PACKAGING_INSTALL_PREFIX when using RPM generator

[Sam Lunt]

I am trying to set a per-component value for
CPACK_PACKAGING_INSTALL_PREFIX when using the RPM generator, but I
haven't been able to do so.

I would like to be able to:
1. Install using "make install" (or cmake --build ${BUILD_DIR}
--target install) and have CMAKE_INSTALL_PREFIX control the install
location
2. Generate an rpm file for each component such that the rpm is
relocatable (i.e. --prefix and --relocate are supported) and each
component has a different default installation location

The documentation for CPACK_RPM_<COMPONENT>_PACKAGE_PREFIX seems to
indicate that it is the correct way to set a per-component install
prefix, since it says that CPACK_RPM_<COMPONENT>_PACKAGE_PREFIX "May
be used to set per component CPACK_PACKAGING_INSTALL_PREFIX for
relocatable RPM packages." However, I am only able to successfully use
this if I provide an absolute path to the install command, but that
inhibits the use of CMAKE_INSTALL_PREFIX.

Here is a minimal example that illustrates the problem:

## start CMakeLists.txt
cmake_minimum_required(VERSION 3.8)

project(FooBar
    VERSION 0.1.0
)

add_executable(foo foo.cpp)
add_executable(bar bar.cpp)

include(GNUInstallDirs)
install(TARGETS foo
    DESTINATION "${CMAKE_INSTALL_BINDIR}"
    COMPONENT Foo_Comp
)

install(TARGETS bar
    DESTINATION "${CMAKE_INSTALL_BINDIR}"
    COMPONENT Bar_Comp
)

set(CPACK_PACKAGE_VERSION_MAJOR ${FooBar_VERSION_MAJOR})
set(CPACK_PACKAGE_VERSION_MINOR ${FooBar_VERSION_MINOR})
set(CPACK_PACKAGE_VERSION_PATCH ${FooBar_VERSION_PATCH})

set(CPACK_PACKAGE_RELOCATABLE ON)
set(CPACK_PACKAGING_INSTALL_PREFIX "/tmp_dir/foobar")

set(CPACK_RPM_COMPONENT_INSTALL ON)
set(CPACK_RPM_FOO_COMP_PACKAGE_PREFIX "/tmp_dir/foo")
set(CPACK_RPM_BAR_COMP_PACKAGE_PREFIX "/tmp_dir/bar")

include(CPack)
## end CMakeLists.txt

When the command "cmake ${SOURCE_DIR} -G "Unix Makefiles" && cmake
--build . && cpack -G RPM", the call to cpack generates the warnings:

CMake Warning (dev) at /usr/share/cmake/CPackRPM.cmake:999 (message):
  CPackRPM:Warning: Path /tmp_dir/foobar/bin/bar is not on one of the
  relocatable paths! Package will be partially relocatable.
Call Stack (most recent call first):
  /usr/share/cmake/Modules/CPackRPM.cmake:1841
(cpack_rpm_prepare_relocation_paths)
  /usr/share/cmake/Modules/CPackRPM.cmake:2690 (cpack_rpm_generate_package)
This warning is for project developers.  Use -Wno-dev to suppress it.

CPackRPM: Will use GENERATED spec file:
/var/tmp/test_pack_prefix/build/_CPack_Packages/Linux/RPM/SPECS/foobar-Bar_Comp.spec
CMake Warning (dev) at /usr/share/cmake/Modules/CPackRPM.cmake:999 (message):
  CPackRPM:Warning: Path /tmp_dir/foobar/bin/foo is not on one of the
  relocatable paths! Package will be partially relocatable.
Call Stack (most recent call first):
  /usr/share/cmake/Modules/CPackRPM.cmake:1841
(cpack_rpm_prepare_relocation_paths)
  /usr/share/cmake/Modules/CPackRPM.cmake:2690 (cpack_rpm_generate_package)
This warning is for project developers.  Use -Wno-dev to suppress it.

CPackRPM: Will use GENERATED spec file:
/var/tmp/test_pack_prefix/build/_CPack_Packages/Linux/RPM/SPECS/foobar-Foo_Comp.spec

The rpm installs the files to CPACK_PACKAGING_INSTALL_PREFIX if I
remove these lines (which prevents a per-component install location):
set(CPACK_RPM_FOO_COMP_PACKAGE_PREFIX "/tmp_dir/foo")
set(CPACK_RPM_BAR_COMP_PACKAGE_PREFIX "/tmp_dir/bar")

I can also prevent the warning by changing the install commands (but
then CMAKE_INSTALL_PREFIX is useless):
install(TARGETS foo
    DESTINATION "/tmp_dir/foo/${CMAKE_INSTALL_BINDIR}"
    COMPONENT Foo_Comp
)

install(TARGETS bar
    DESTINATION "/tmp_dir/bar/${CMAKE_INSTALL_BINDIR}"
    COMPONENT Bar_Comp
)

Is there any way to do what I'm trying to?