Re: feature request - order only deps

[Matěj Týč]

On Ne, 2013-02-03 at 00:45 +0100, Sebastian Pipping wrote:
> On 02.02.2013 18:38, Matěj Týč wrote:
> >> How about something like this?
> >>
> >>   bar_deps = foo1 foo2
> >>
> >>   bar: $(bar_deps)
> >>
> >>   $(bar_deps):
> >>    $(MAKE) cache-foo
> >>    touch $@
> >>
> >>   %:
> >>    touch $@
> > 
> > I have also thought of that, but this can work well reliably only in the
> > case if there is only one make job. If I specify -j4, it may happen that
> > all jobs will attempt to load the same cache simultaneously, which I
> > have to avoid :-(
> 
> If that happens how about replacing
> 
>   $(MAKE) cache-foo
> 
> by something like
> 
>   mkdir .lock 2>/dev/null || exit 0 ; \
>       $(MAKE) cache-foo ; \
>       ret=$$?; \
>       rmdir .lock && exit $${ret}
> 
> The idea is:
> 
>  - mkdir can only succeed once
> 
>  - if "$(MAKE) cache-foo" fails
> 
>     1. the whole should return non-zero
> 
>     2. .lock is not left laying around

Thank you,
but just by looking at it, I think that if .lock exists, 0 is returned
and while one make job is busy loading the cache, the others act like
the cache was loaded.
Writing the makefile and trying it confirms this assumption, so I guess
that it has to behave like that.
I am beginning to think that the most elegant (from the user's point of
view) would be (probably optional) ignore of order-only dependencies if
the target exists and is more up-to-date than its ordinary dependencies.
How difficult could be writing a patch that would enable this?

Matej