On Thu, 2012-05-17 at 21:18 -0700, Chris Larson wrote: > On Wed, May 16, 2012 at 9:58 AM, Peter Seebach > <peter.seeb...@windriver.com> wrote: > > On Wed, 16 May 2012 07:35:45 +0300 > > Saul Wold <s...@linux.intel.com> wrote: > > > >> My understanding is that a _subtract is fraught with danger, there > >> all sorts of ordering implications. > > > > Yes. > > > > But consider, if you will, the specific case of > > DISTRO_FEATURES_LIBC_DEFAULT, and a libc which is just like eglibc > > except that it lacks RPC. > > > > Anything I do that isn't processed at the tail end of everything, > > around the point where _appends are processed, will be unable to > > cleanly obtain "the value that would have been set by default if > > nothing else happened", and then remove a word from it. I can't set a > > value in advance, because if I do the ?= won't fire and none of those > > words will get set. I can't necessarily set a value later. > > > > Overrides won't work either, because overrides also destroy the > > existing values. > > > > It seems to me that for a subtraction to work, it *must* be the very > > last thing done. > > > > Basically, the purpose of suggesting this as a formal behavior defined > > to be The Very Last Thing is to minimize the complexity of the ordering > > implications. You get exactly what you would have gotten otherwise, > > with these words removed. > > I agree. I think this is the only sane option for implementing this > given bitbake's current behavior. Clearly a -= would be extremely > problematic, and would have clear complications with variable > expansion, as we discussed on IRC. I'd be interested in seeing a > prototype implementation of this, to experiment with it and see how > viable it is.
The _subtract or _remove format of this might be a viable way of adding this functionality. "-=" is the one I'd prefer to avoid as it will end up confusing people which is why we've all effectively been against it but the _xxxx syntax does match the behaviour/ordering of _append and _prepend and hence could be added with less confusion. So yes, I think this could be the best way to address some of these cases. It has always bugged me that its near impossible to counteract an _append other than with an override. I also agree we should audit the system and change to ??= where it makes sense which will reduce the need for this. Cheers, Richard _______________________________________________ Openembedded-core mailing list Openembedded-core@lists.openembedded.org http://lists.linuxtogo.org/cgi-bin/mailman/listinfo/openembedded-core
