• Brad King's avatar
    Modules: Include builtin FindPackageHandleStandardArgs directly · c4275592
    Brad King authored
    The FindPackageHandleStandardArgs module was originally created outside
    of CMake.  It was added for CMake 2.6.0 by commit e118a627 (add a macro
    FIND_PACKAGE_HANDLE_STANDARD_ARGS..., 2007-07-18).  However, it also
    proliferated into a number of other projects that at the time required
    only CMake 2.4 and thus could not depend on CMake to provide the module.
    CMake's own find modules started using the module in commit b5f656e0
    (use the new FIND_PACKAGE_HANDLE_STANDARD_ARGS in some of the FindXXX
    modules..., 2007-07-18).
    Then commit d358cf5c (add 2nd, more powerful mode to
    find_package_handle_standard_args, 2010-07-29) added a new feature to
    the interface of the module that was fully optional and backward
    compatible with all existing users of the module.  Later commit 5f183caa
    (FindZLIB: use the FPHSA version mode, 2010-08-04) and others shortly
    thereafter started using the new interface in CMake's own find modules.
    This change was also backward compatible because it was only an
    implementation detail within each module.
    Unforutnately these changes introduced a problem for projects that still
    have an old copy of FindPackageHandleStandardArgs in CMAKE_MODULE_PATH.
    When any such project uses one of CMake's builtin find modules the line
    loads the copy from the project which does not have the new interface!
    Then the including find module tries to use the new interface with the
    old module and fails.
    Whether this breakage can be considered a backward incompatible change
    in CMake is debatable.  The situation is analagous to copying a standard
    library header from one version of a compiler into a project and then
    observing problems when the next version of the compiler reports errors
    in its other headers that depend on its new version of the original
    header.  Nevertheless it is a change to CMake that causes problems for
    projects that worked with previous versions.
    This problem was discovered during the 2.8.3 release candidate cycle.
    It is an instance of a more general problem with projects that provide
    their own versions of CMake modules when other CMake modules depend on
    them.  At the time we resolved this instance of the problem with commit
    b0118402 (Use absolute path to FindPackageHandleStandardArgs.cmake
    everywhere, 2010-09-28) for the 2.8.3 release.
    In order to address the more general problem we introduced policy
    CMP0017 in commit db44848f (Prefer files from CMAKE_ROOT when including
    from CMAKE_ROOT, 2010-11-17).  That change was followed by commit
    ce28737c (Remove usage of CMAKE_CURRENT_LIST_DIR now that we have
    CMP0017, 2010-12-20) which reverted the original workaround in favor of
    using the policy.  However, existing project releases do not set the
    policy behavior to NEW and therefore still exhibit the problem.
    We introduced in commit a364daf1 (Allow users to specify defaults for
    unset policies, 2011-01-03) an option for users to build existing
    projects by adding -DCMAKE_POLICY_DEFAULT_CMP0017=NEW to the command
    line.  Unfortunately this solution still does not allow such projects to
    build out of the box, and there is no good way to suggest the use of the
    new option.
    The only remaining solution to keep existing projects that exhibit this
    problem building is to restore the change originally made in commit
    b0118402 (Use absolute path to FindPackageHandleStandardArgs.cmake
    everywhere, 2010-09-28).  This also avoids policy CMP0017 warnings for
    this particular instance of the problem the policy addresses.
FindGDAL.cmake 3.28 KB