[cfe-dev] [llvm-dev] [Github] RFC: linear history vs merge commits

[via cfe-dev]

> -----Original Message-----
> From: cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] On Behalf Of David
> Greene via cfe-dev
> Sent: Wednesday, January 30, 2019 3:52 PM
> To: Jeremy Lakeman
> Cc: llvm-dev; LLDB Dev; cfe-dev; openmp-dev (openmp-dev at lists.llvm.org)
> Subject: Re: [cfe-dev] [llvm-dev] [Github] RFC: linear history vs merge
> commits
> 
> Jeremy Lakeman via llvm-dev <llvm-dev at lists.llvm.org> writes:
> 
> > 4. Each reviewed bug / feature must be rebased onto the current "known
> > good" commit, merged into a "probably good" commit, tested by build
> > bots, and only then pushed to trunk. Keeping trunk's history more
> > usable, with most bad patches reworked and resubmitted instead of
> > reverted.
> 
> If you mean having a submitted PR trigger builds and only allow merging
> if all builds pass, that may be doable.  Of course by the time it's
> merged it will be against some later commit (so it should be rebased)
> and there's no guarantee it will build against *that* commit.  But it
> will tend to filter out most problems.

Systems that I've seen will funnel all submitted PRs into a single queue
which *does* guarantee that the trial builds are against HEAD and there
are no "later commits" that can screw it up. If the trial build passes,
the PR goes in and becomes the new HEAD.
By the time a PR reaches the front of the build queue, it might not apply
cleanly, in which case it gets bounced just like a failed build would.

But this would be a radical redesign of the LLVM bot system, and would
have to wait until we're done with the GitHub migration and can spend
a couple of years debating the use of PRs. :-)
--paulr