Re: deprecation of in-tree builds

[BALATON Zoltan]

On Tue, 31 Mar 2020, Daniel P. Berrangé wrote:
> On Tue, Mar 31, 2020 at 02:33:46PM +0200, BALATON Zoltan wrote:
> > > We will have to ask developers to change habits anyway when we switch to
> > > Meson.  I agree with Daniel's recommendation to delay changes requiring
> > > habit-changes until then.  However, telling people to stay clear of the
> > > unloved and brittle in-tree build is simply good advice we should not
> > > withhold.
> >
> > Can someone please explain why is it brittle and cannot be supported? It has
> > worked well so far apart from some breakage due to being untested which is
> > also not a techincal necessity just a decision by some maintiners to not
> > test it. Adding a CI job to keep it working would also not be difficult or
> > much complexity.
>
> Writing make rules to correctly handle both src-dir and build-dir scenarios
> is a non-negligible maint burden. If you look back through QEMU's history
> we have a steady stream of patches which have broken one or the other
> build scenarios.

That's probably becuase there are no clear rules (such as always prefix 
files in source dir with $(SRC_PATH), generated files with some BUILDDIR, 
etc.) and these are not documented so every time someone touches it has to 
explore and debug it again. This could be avoided if these were written 
down once but instead of trying to document and clean up the build system 
the chosen direction is to just throw it out and replace it with something 
more complex and with more dependencies (Meson) and then to save 
"additional complexity" it also breaks people's workflow and demand them 
to adapt themselves. I'm not saying there should be no changes but if 
there's a way to make them less painful it could be considered if it's not 
much extra work and in this case it does not seem to be.

> Developers will often not test both scenarios, just the one they prefer
> to use. This results in a maint burden on the subsystem maintainers who
> merge patches and then find they break & have to back them out. Sometimes
> even the subsystem maintainer gets it wrong and burden falls on Peter to
> find & reject it.
>
> Even if we have CI to test both, it is still a burden on developers to
> debug failures reported by the CI and figure out what needs fixing. With
> the number of builds we do & the time for a single CI cycle it gets very
> time consuming. I've personally wasted many many hours debugging src-dir
> vs build-dir problems in QEMU's makefiles - probably more than an entire
> day was lost when I did the patches to split the trace.h header file.
>
> This all serves to divert time away from useful work on QEMU. If there
> was some critically important functional thing that src-dir builds offer
> that can't be achieved by build-dir builds, then the extra maint work
> could be justified. I don't think that's the case though.

OK, so then only supporting out-of-tree builds but adding convenience 
function and Makefile to still allow people to run configure; make from 
source dir would solve this without also needing people to change what 
they always did so why is that solution not acceptable?

Regards,
BALATON Zoltan