[Buildroot] [PATCH v2] Don't build host-cmake if it is available on the build host

[Arnout Vandecappelle <arnout@mind.be>]

On 16-02-16 17:53, Luca Ceresoli wrote:
> Hi Samuel, Arnout,
> 
> On 16/02/2016 16:08, Arnout Vandecappelle wrote:
>>
>>
>> On 16-02-16 14:55, Samuel Martin wrote:
>>> Hi Luca, all,
>>>
>>> On Fri, Feb 5, 2016 at 12:07 PM, Luca Ceresoli <luca@lucaceresoli.net> wrote:
>> [snip]
>>>>> Finally, we do skip adding the dependency on host-cmake for all cmake
>>>>> packages when $(BUILD_HOST_CMAKE) != YES.
>>> Sorry for not spotting this earlier, but:
>>> host-cmake has a dependency on host-pkgconf, which cmake (the host
>>> program) may use at runtime to find the dependencies when it is
>>> configuring a project.
>>> In the BR host tree, pkg-config is a wrapper with environment
>>> variables correctly preset to find stuff in the sysroot tree by
>>> default.
>>>
>>> Here is a test case:
>>>
>>> A is a cmake-package depending on B.
>>> B is a generic-package providing a *.pc file.
>>> CMake uses its FindPkgConfig module to detect B.
>>>
>>> So, currently we got:
>>> - A depends on: B and host-cmake
>>> - host-cmake depends on host-pkgconf
>>> - When configuring A, host-cmake should use host-pkgconf to find B
>>>
>>> With this patch, in the case cmake build is skipped, we get:
>>> - A depends on: B
>>> - When configuring A, cmake (from the host system) may use pkg-config
>>> (from the host system) to find B, but this pkg-config won't have the
>>> env. vars. correctly set to find B in the sysroot. In the best case,
>>> it won't find B, in the worst case it will try to link against B from
>>> the host system.
> 
> Thanks for the insight.
> 
> I wonder whether we can do anything in toolchainfile.cmake to prevent
> cmake from using the pkg-config on the system. Don't the following lines
> already do it? [Disclaimer: I'm in a hurry and checking the docs _now_
> would take too long]
> 
>   set(CMAKE_PROGRAM_PATH "${RELOCATED_HOST_DIR}/usr/bin")
>   set(CMAKE_FIND_ROOT_PATH "${RELOCATED_HOST_DIR}/@@STAGING_SUBDIR@@")
>   set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)
>   set(CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY)
>   set(CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY)
>   set(CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY)

 This will add the host dir to the program path, but not stop it from searching
/usr/bin.

 Maybe we should always put a fake pkg-config script in $(HOST_DIR)/usr/bin that
always returns false. This script will be overwritten by host-pkgconf, so
anything that tries to use it without depending on host-pkgconf will get a
failure, and the pkg-config from /usr/bin/ is completely avoided. Well, this
script should be smart enough to still work for host packages...

>>>
>>> I'm not sure what it the best and clean way to handle this...
>>> Maybe $(BUILD_HOST_CMAKE) != YES should be use to noop the
>>> HOST_CMAKE_{CONFIGURE,BUILD,INSTALLL}_CMDS commands, but we keep the
>>> dependency on host-cmake, so we don't lose host-cmake's dependencies.
>>>
>>> Any thought about this?
>>
>>  The correct approach would be to add explicit dependencies on host-pkgconf for
>> packages that need it, just like we do for autotools packages.
>>
>>  The difficulty is of course finding out which packages need host-pkgconf. It
>> seems it's not as simple as searching for FindPkgConfig in the CMakeLists.txt
>> files of each cmake package, because there are other modules that implicitly use
>> this. So we'd have to make a list of all those modules and check for all of
>> them. And then of course there are the packages that install new cmake modules
>> that use this, but which don't use FindPkgConfig (or even cmake) themselves...
>>
>>  In 3d475ee0 you added the unconditional host-pkgconf dependency to work around
>> this problem. If we remove that dependency, the autobuilders should detect the
>> missing host-pkgconf dependencies even when host-cmake is used.
> 
> The workaround Samuel added in that commit is in package/cmake/cmake.mk:
> 
>   +HOST_CMAKE_DEPENDENCIES = host-pkgconf
> 
> This could probably ported to my patch in package/pkg-cmake.mk:
> 
>   +ifeq ($$(BUILD_HOST_CMAKE),YES)
>    $(2)_DEPENDENCIES += host-cmake
>   +else
>   +$(2)_DEPENDENCIES += host-pkgconf
>   +endif
> 
> But I agree with Arnout that if a package actually uses pkg-conf, it
> should depend on it. Of course this means it must fail when the
> dependency is missing, not use the system pkg-conf.

 Ack that.

 Regards,
 Arnout


-- 
Arnout Vandecappelle                          arnout at mind be
Senior Embedded Software Architect            +32-16-286500
Essensium/Mind                                http://www.mind.be
G.Geenslaan 9, 3001 Leuven, Belgium           BE 872 984 063 RPR Leuven
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