[OE-core] RFC: FOO_subtract, the logical antidote to FOO_append.

[Chris Larson]

On Wed, May 16, 2012 at 9:58 AM, Peter Seebach
<peter.seebach at windriver.com> wrote:
> On Wed, 16 May 2012 07:35:45 +0300
> Saul Wold <sgw at 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.
-- 
Christopher Larson