[libvirt PATCH 000/351] port libvirt to Meson build system

[Pavel Hrdina]

On Mon, Jul 27, 2020 at 06:11:11PM +0200, Peter Krempa wrote:
> On Fri, Jul 17, 2020 at 15:02:10 +0100, Daniel Berrange wrote:
> > On Thu, Jul 16, 2020 at 03:44:25PM +0200, Pavel Hrdina wrote:
> > > On Thu, Jul 16, 2020 at 01:59:00PM +0100, Daniel P. Berrangé wrote:
> > > > Personally I'd really like to avoid squashing them, because splitting
> > > > up big patches is not merely to benefit the initial pre-merge review,
> > > > but to also benefit people who need to debug stuff that's already
> > > > merged and understand the scope of the intended change. So being able
> > > > to look back at the changes in isolation after commit is still a big
> > > > plus point.
> > > 
> > > I would like to avoid squashing the patches as well and in most cases I
> > > would object to it as well. I only suggested that to not break git
> > > bisect.
> > > 
> > > If we don't care about git bisect and the fact that we would not be able
> > > to build libvirt correctly within these patches I'm OK with pushing it
> > > without squashing.
> > 
> > git bisect reliabity is key, so I reluctantly think we'll need to
> > squash. I don't want to hit a pathc in this series with a bisect
> > and be unable to continue the bisect due to inability to build the
> > code.
> 
> I agree. It's definitely necessary that the build is complete at any
> point in time.
> 
> I'm reluctantly willing to accept that the build fails with an
> appropriate error message until the build system is able to build
> everything if we opt for commiting a patchset for simplicity. What's
> off-limits is if build "succeeds", but is incomplete due to missing
> steps in the implementation. I'm not going to want to guess which part
> is already built or which isn't.
> 
> Given that the rewrite is a singularity anyways it doesn't really matter
> that we will not be able to bisect problems caused by the build system
> across the boundary.

So based on all the comments we have these options for pushing this
series:

    1) Squash it into single commit.

       Pros: - no issues with git bisect

       Cons: - we will not have the history of changes

    2) Keep the patches as they are and running meson build & ninja will
       not fail.

       Pros: - full history of changes where each commit removes the
               relevant bits from autotools

             - git bisect is not broken as compilation will not fail

       Cons: - meson build && ninja will not produce complete libvirt
               binaries and there is no autogen.sh

             - script used for git bisect will have to detect if tested
               binaries are compiled or use git bisect skip

    3) Keep the patches as they are but error out in meson until the
       conversion is complete. The error can be used to detect if git
       bisect is withing the meson rewrite.

       Pros: - full history of changes where each commit removes the     
               relevant bits from autotools

             - git bisect is not broken if failed compilation is not an
               issue and marked as git bisect skip

       Cons: - meson build will fail and there is no autogen.sh so no
               way how to compile libvirt even partially

             - script used for git bisect will have to skip failed
               compilation with an option to check for specific error

    4) Rework the series to have patches adding meson bits without
       removing anything from autotools and drop the autotools files in
       a single commit once the meson rewrite is complete

       Pros: - full history of changes

             - git bisect not broken because autogen.sh && make will
               work the whole time until meson build && ninja is ready

       Cons: - no reference of the meson changes to autotools code

             - additional work for me to redo the patches


There is a possible workaround as Martin suggested that when running git
bisect the script/developer would test libvirt before meson rewrite and
right after it to figure out if the issue is caused by meson rewrite or
not and then continue purely with meson or autotools to look for the
broken commit. This would make sense for option 2) and 3).

Looking forward to your ideas and comments or suggestion with other
options.

Pavel
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