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Re: rebase-in-C stability for 2.20

Ævar Arnfjörð Bjarmason <avarab@xxxxxxxxx> writes:

> According to Junio's calendar we're now 2 days from 2.20-rc0. We have
> the js/rebase-autostash-detach-fix bug I reported sitting in "pu" still,
> and then this.
> I see we still have rebase.useBuiltin in the code as an escape hatch,
> but it's undocumented.
> Given that we're still finding regressions bugs in the rebase-in-C
> version should we be considering reverting 5541bd5b8f ("rebase: default
> to using the builtin rebase", 2018-08-08)?
> I love the feature, but fear that the current list of known regressions
> serve as a canary for a larger list which we'd discover if we held off
> for another major release (and would re-enable rebase.useBuiltin=true in
> master right after 2.20 is out the door).
> But maybe I'm being overly paranoid. What do those more familiar with
> this think?

I was hoping that having it early in GfW 2.19 would have smoked out
all the remaining issues, but it seems that those building from the
source and testing have usage patterns different enough to find more
issues.  This is a normal part of the development process, and
hopefully the remaining bugs are minor and can be flushed out in the
-rc testing period---this kind of thing is the whole reason why we
code-freeze and test rcs.

It unfortunately is too late to depend on the rebase.useBuiltin as
an escape hatch, as 'master', 'next', or anywhere else had the
"rebase and rebase -i in C" series merged without it set to false
and used in any seriousness.  Quite honestly, I think we can trust
the rest of the "rebase and rebase -i in C" series much more than
its "use the scripted one even though we have built-in one" part.

So if we have to seriously consider holding back, we may be better
off ripping the whole thing out.  I do not offhand know how involved
such a reversion would be, though, and I am *NOT* looking forward to
having do such a major surgery right before the release.