Weird goings on with make::empty()

[Enji Cooper]

> On Sep 4, 2019, at 07:58, Ian Lepore <ian at freebsd.org> wrote:
> 
>> On Wed, 2019-09-04 at 09:33 -0500, Kyle Evans wrote:
>> On Wed, Sep 4, 2019 at 9:27 AM Enji Cooper <yaneurabeya at gmail.com>
>> wrote:
>>> 
>>> 
>>>> On Sep 4, 2019, at 04:59, Poul-Henning Kamp <phk at phk.freebsd.dk>
>>>> wrote:
>>>> 
>>>> On:
>>>> 
>>>>   Repository Root: svn+ssh://repo.freebsd.org/base
>>>>   Repository UUID: ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
>>>>   Revision: 351809
>>>> 
>>>> I built a kernel, but drm-current-kmod did not get compiled
>>>> from the new world order in /usr/local/sys/modules
>>>> 
>>>> Debugging I ended up doing this to src/sys/conf/kern.post.mk:
>>>> 
>>>>   Index: sys/conf/kern.post.mk
>>>>   ==============================================================
>>>> =====
>>>>   --- sys/conf/kern.post.mk    (revision 351809)
>>>>   +++ sys/conf/kern.post.mk    (working copy)
>>>>   @@ -77,12 +77,14 @@
>>>>           ${target:S/^reinstall$/install/:S/^clobber$/cleandir/}
>>>>    .endif
>>>>    .for module in ${LOCAL_MODULES}
>>>>   -.if !empty(module)
>>>>   +    true "XXX A $(module) 2 ${LOCALBASE} 3 ${LOCAL_MODULES} 4
>>>> ${MODULES_WITH_WORLD}"
>>>>   +#.if !empty(module)
>>>>   +    true "XXX B $(module) 2 ${LOCALBASE} 3 ${LOCAL_MODULES} 4
>>>> ${MODULES_WITH_WORLD}"
>>>>       @${ECHODIR} "===> ${module}
>>>> (${target:S/^reinstall$/install/:S/^clobber$/cleandir/})"
>>>>       @cd ${LOCAL_MODULES_DIR}/${module}; ${MKMODULESENV}
>>>> ${MAKE} \
>>>>           DIRPRFX="${module}/" \
>>>>           ${target:S/^reinstall$/install/:S/^clobber$/cleandir/}
>>>>   -.endif
>>>>   +#.endif
>>>>    .endfor
>>>>    .endif
>>>>    .endfor
>>>> 
>>>> This gives me the expected output from buildkernel:
>>>> 
>>>>   true "XXX A drm-current-kmod 2 /usr/local 3 drm-current-kmod 4
>>>> "
>>>>   true "XXX B drm-current-kmod 2 /usr/local 3 drm-current-kmod 4
>>>> "
>>>> 
>>>> If I leave in the ".if !empty(module)" line in, I only get:
>>>> 
>>>>   true "XXX A drm-current-kmod 2 /usr/local 3 drm-current-kmod 4
>>>> "
>>>> 
>>>> suggestions welcome...
>>> 
>>> (CCing Kyle)
>>> 
>>> This behavior change is probably caused by r351799.
>>> 
>>> I personally think the code before Kyle’s change and after it was
>>> buggy. It’s not word splitting LOCAL_MODULES before iterating over
>>> it.
>>> 
>> 
>> I've backed out r351799 since it breaks usage of LOCAL_MODULES
>> (though
>> I really don't understand how empty works, apparently, and that makes
>> me sad)... please advise on a correct path forward, because it's not
>> clear to me.
>> 
>> Thanks,
>> 
>> Kyle Evans
>> 
> 
> I'm not positive, but I suspect this has to do with when variables are
> expanded.  In particular, the loop variable for a .for is expanded on
> each loop iteration, but doesn't yet exist during parsing.  I suspect
> that the .if is evaluated earlier, during parsing.  For example, this
> makefile:
> 
>   all:
>   .for x in a "" b
>   .if empty(x)
>       @echo empty
>   .endif
>       @echo ${x}
>   .endfor
>       @echo done
> 
> gives this output:
> 
>   revolution > make -f /tmp/Makefile 
>   empty
>   a
>   empty
> 
>   empty
>   b
>   done
> 
> The way I interpret that is that empty(x) is true during parsing, so
> the for loop is generated to contain "@echo empty" and "@echo ${x}",
> then the for loop actually runs and prints both "empty" and the value
> of ${x} on each iteration.

Correct. Word splitting is the key. I’ll take a stab at this later on today.
Thanks,
-Enji