[cmake-developers] adding automatic handling of dependencies for swig-generated modules

[Sébastien Barthélémy]

Ok, I got no answer, so I'll try to be more specific, and cross post
to -dev, please answer where suitable:

1) is there a known recipe to deal dependency detection?

2) I found the IMPLICIT_DEPENDS option of add_custom_command which
suggests such support is possible, but implemented directly in cmake
source code. Am I right?

3) is there a way to update a target list of dependencies from a custom command?

I can imagine some tricks such as including from the CMakeLists.txt a
file which is the output of an add_custom_command. It is probably not
possible, is it?

4) can we add a rule to automatically run cmake when certain files are
changed? Better, only when the content changes, not when the mtime
changes.

Thank you for your guidance
Regards,
-- Sebastian

On Mon, Jan 28, 2013 at 3:39 PM, Sébastien Barthélémy
<barthelemy at crans.org> wrote:
> Hello all,
>
> swig supports the -MM option which lists a module dependencies (C/C++
> headers and other ".i" interface files). I'd like to use it in order
> to let cmake know about these dependencies without user intervention.
> In addition to reducing the user burden, that would also help managing
> dependencies on files which are outside of the current cmake project.
> For instance when a swig module extends another one.
>
> Currently, the user has to list these dependencies in a
> SWIG_MODULE_${modulename}_EXTRA_DEPS variable. If a user  changes the
> .i file to add a #include line, he should also edit the CMakeLists.txt
> file to add the corresponding header to the list. Editing the
> CMakeLists.txt will cause make to run cmake again, hence the new
> dependency will be taken into account.
>
> Apparently, there was an attempt already:
> http://public.kitware.com/Bug/view.php?id=4147
>
> Its inner workings are simple:
>
>  * first, in swig_add_module, call swig -MM on the interface file, in
> order to gather the dependencies,
>
>  * then use add_custom_command to generate the wrapper code (call swig
> without -MM) whenever a dependency changes.
>
> I see two shortcomings with this implementation (please correct me if I'm wrong)
>
>  *  swig -MM fails if it cannot find an header, hence it should be
> called *after* the headers generation. Because of this problem, the
> patch was reverted. I fear the only solution here is to let the user
> list these generated dependencies explicitly, and make the swig -MM
> call depend on them. We could use the
> SWIG_MODULE_${modulename}_EXTRA_DEPS variable for this.
>
>  * swig -MM is only called when cmake is run. If the user changes the
> interface file to add a %include, make will call swig to generate the
> wrappers, but won't call swig -MM again. So the new dependency will go
> unnoticed.
>
> This last problem is probably trickier. Maybe we could use a single
> add_custom_command, which would
>
>  * call swig -MM
>  * compare the dependencies it gets against its current ones. If the
> two sets differ, call cmake,
>  * call swig to generate the wrappers
>
> But I'm not sure about how to save the "current dependencies" and
> about the conditional call to cmake.
> Maybe swig --MM could output to a file, whose (content) change would
> trigger cmake?
>
> I'm pretty sure this is a quite common scenario, is there a canonical
> solution? Or another module doing the same kind of dependency analysis
> where I should look for inspiration?
>
> Regards,
> -- Sébastien