[CMake] Apply FIND_PACKAGE_HANDLE_STANDARD_ARGS() on COMPONENTS

[S Roderick]

On Apr 22, 2010, at 06:21 , Michael Hertling wrote:

> On 04/21/2010 09:29 PM, S Roderick wrote:
>> On Apr 21, 2010, at 15:13 , Alexander Neundorf wrote:
>> 
>>> On Tuesday 20 April 2010, Michael Hertling wrote:
>>>> Dear CMake community, dear CMake developers,
>>>> 
>>> ...
>>>> There's another aspect related to this I'd like to comment on: During
>>>> the abovementioned considerations on the bug tracker and the mailing
>>>> list, the question has arisen if it's reasonable to set XXX_FOUND to
>>>> FALSE if any of the requested components aren't found. As for myself,
>>>> I'd say: No, it isn't. Let's have a look at the following scenario:
>>>> 
>>>> Package XXX normally provides components YY1 and YY2, but for some
>>>> reason, only YY1 is installed. Moreover, XXX provides a config file
>>>> XXXConfig.cmake. Now, a project's CMake script requests both YY1/2 by
>>>> FIND_PACKAGE(XXX COMPONENTS YY1 YY2). As Brad King has pointed out in
>>>> <http://www.mail-archive.com/cmake@cmake.org/msg15952.html>, finding a
>>>> config file results in XXX_FOUND to be set to TRUE automatically. Thus,
>>>> the absence of YY2 does not mean the absence of XXX as a whole in any
>>>> case, and, notwithstanding, the requesting CMake script should have a
>>>> chance to proceed even if YY2 isn't available, i.e. the following seems
>>>> reasonable: XXX_YY1_FOUND=TRUE, XXX_YY2_FOUND=FALSE *but* XXX_FOUND=TRUE.
>>> 
>>> I think I don't agree here.
>>> If I say
>>> find_package(XXX COMPONENTS YY1 YY2 REQUIRED)
>>> I think it makes a lot of sense to interpret this as "search package XXX, and 
>>> I need YY1 and YY2 from it".
>>> What other reason would I have to give YY1 and YY2 there otherwise ?
>>> If it still succeeds if they are not found, why should I list them then ?
>> 
>> 
>> +1
>> 
>> find_package(XXX COMPONENTS YY1 YY2 REQUIRED)
>> 
>> means to me, I _require_ both YY1 and YY2 from XXX. Any other YYx I don't care about.
> 
> Yes, of course, the REQUIRED option is definitive; it's
> COMPONENTS without REQUIRED which raises the questions.
> 
>> The first version above translates to me to be
>> 
>> find_package(XXX COMPONENTS YY1 REQUIRED)
>> 
>> I only _require_ YY1.
> 
> As I have replied recently to AN, I sometimes dislike FIND_PACKAGE() to
> look for unrequested components, so I need to request them even if they
> are optional, but this is just my personal preference and, also, not my
> concern here.
> 
> My concern is that FIND_PACKAGE(XXX COMPONENTS YY) will always return
> XXX_FOUND=TRUE if it's driven by XXXConfig.cmake, regardless if YY is
> found or not, while the same command could return XXX_FOUND=FALSE if
> driven by FindXXX.cmake since the latter has XXX_FOUND in its hands.
> Thus, IMHO, it should be reconsidered how XXX_FOUND is interpreted
> w.r.t. components because I would expect both variants - XXXConfig
> and FindXXX - to behave the same.

That seems to make sense. YY isn't listed as required, therefore it is optional. Having XXX_FOUND=TRUE seems reasonable if CMake found some portion of XXX, regardless of whether it did, or did not, find YY. If we required YY, then we should have added REQUIRED.

Now the fact that COMPONENTS and REQUIRED are mutually exclusive is not right IMHO. I'd love to hear Kitware's take on why it was done this way.
Stephen