[cmake-developers] Fwd: How to handle different cmake versions in extra-cmake-modules ?

[Alexander Neundorf]

On Tuesday 06 December 2011, Alexander Neundorf wrote:
> Hi,
> 
> On Monday 07 November 2011, Brad King wrote:
> > On 11/6/2011 6:12 AM, Stephen Kelly wrote:
> > > ecm_copy_modules(${CMAKE_BINARY_DIR}/modules FindFoo.cmake
> > > 
> > >                                               FindBlub.cmake
> > >                                               ECMDoSomething.cmake)
> > > 
> > > set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} 
> > > ${CMAKE_BINARY_DIR}/modules} )
> > > 
> > > This macro would copy just these needed files into the given directory,
> > > which can then be added to CMAKE_MODULE_PATH.
> > 
> > See below for an idea that may solve cases 2 and 3 together.
> > 
> > > Case 3. I don't really have an idea yet. Add some code at the top of
> > > each file which include()s the file from cmake if the version is
> > > bigger than some specified version ?
> > > 
> > > Something like:
> > > FindBlub.cmake:
> > > 
> > > if(CMAKE_VERSION>  2.8.12)
> > > 
> > >    include(${CMAKE_ROOT}/Modules/FindBlub.cmake)
> > >    return()
> > > 
> > > endif()
> > 
> > We've done the following before:
> >   if(EXISTS ${CMAKE_ROOT}/Modules/FindBlub.cmake)
> >   
> >     include(${CMAKE_ROOT}/Modules/FindBlub.cmake)
> >     return()
> >   
> >   endif()
> > 
> > That way you don't need to know when the module is added.  It is also
> > forward-compatible for any module not yet in CMake but may be later.
> > OTOH the version of the module added to CMake may provide a slightly
> > different interface than the original version.  That leads to the
> > unfortunate situation that a newer CMake breaks an existing build,
> > which looks like CMake's fault but isn't.
> > 
> > Another option is to provide a function that generates forwarding
> > modules.  Instead of ecm_copy_modules, create a similar API that
> > generates short modules that include either the ECM version or the
> > CMake version depending on some conditions.  Do the inclusion by
> > full path so that the actual ECM module dir does not need to be in
> > the CMAKE_MODULE_PATH.
> 
> yesterday in the train I found the time to do start with something on this.
> 
> So, here is the idea:
> 
> * the files containing macros/functions and the find-modules are put into
> two different directories, e.g. modules/macros/ and modules/find/.
> 
> (Since the macros will all have the "ecm_" prefix, a clash with anything
> from cmake is not possible. So we only have to care about the
> Find-modules.) So when doing
> find_package(extra-cmake-modules)
> two variables will be defined, ECM_MACROS_DIR and ECM_FIND_MODULES_DIR.
> 
> If somebody wants everything, he does
> 
> set(CMAKE_MODULES_PATH ${ECM_MACROS_DIR} ${ECM_FIND_MODULES_DIR} )
> 
> If he wants to use only some macros, he does
> set(CMAKE_MODULES_PATH ${ECM_MACROS_DIR} )
> 
> If he wants to use a subset of the find-modules, there is a function
> ecm_use_find_modules:
> ecm_use_find_modules(DIR <destdir>
>                      MODULES FindFoo.cmake FindBar.cmake ...
>                      [NO_OVERRIDE] )
> 
> The listed find-modules will be copied from ${ECM_FIND_MODULES_DIR} to the
> directory given here as destination dir, and this directory has to be added
> to CMAKE_MODULE_PATH.
> If NO_OVERRIDE is used, the files will only be used if they do not exist in
> cmake.
> This way e.g. FindBlub.cmake will be used until it is part of cmake, then
> the cmake version will be used.
> 
> Attached is a draft of the file, just to show the idea, I didn't actually
> try to run it yet.
> 
> 
> Examples:
> 
> 1) use everything from e-c-m:
> 
> find_package(extra-cmake-modules REQUIRED)
> set(CMAKE_MODULE_PATH ${ECM_MACROS_DIR} ${ECM_FIND_MODULES_DIR} )
> 
> 2) make all macros available, but no find-modules:
> 
> find_package(extra-cmake-modules REQUIRED)
> set(CMAKE_MODULE_PATH ${ECM_MACROS_DIR} )
> 
> 3) make all macros available and some find-modules if they do not yet exist
> in cmake:
> 
> find_package(extra-cmake-modules REQUIRED)
> ecm_use_find_modules(DIR ${CMAKE_CURRENT_BINARY_DIR}/ecm/
>                      MODULES FindFoo.cmake NO_OVERRIDE)
> 
> set(CMAKE_MODULE_PATH ${ECM_MACROS_DIR} ${CMAKE_CURRENT_BINARY_DIR}/ecm/ )
> 
> 
> 4) make all macros available and some find-modules as override for the
> cmake ones:
> 
> find_package(extra-cmake-modules REQUIRED)
> ecm_use_find_modules(DIR ${CMAKE_CURRENT_BINARY_DIR}/ecm/
>                      MODULES FindBoost.cmake)
> 
> set(CMAKE_MODULE_PATH ${ECM_MACROS_DIR} ${CMAKE_CURRENT_BINARY_DIR}/ecm/ )
> 
> 
> Does that look like it should cover all use cases, for peopling wanting to
> selectively use some things from e-c-m, and fearing that something would
> break if they simply would make everything available ?

I added this now to extra-cmake-modules:
http://quickgit.kde.org/?p=extra-cmake-
modules.git&a=blob&hb=HEAD&f=modules/ECMUseFindModules.cmake

The find-modules now go into find-modules/, other modules go into modules/.

Alx