Re: [PATCH v5 20/20] rebase: rename the two primary rebase backends

On Thu, Mar 12, 2020 at 12:12 PM Junio C Hamano [off-list ref] wrote:

Elijah Newren [off-list ref] writes:

quoted

On Thu, Mar 12, 2020 at 8:13 AM Emily Shaffer [off-list ref] wrote:

quoted

...
This broke quite a few upstream users for us today...

One more thing I've been wondering was if we should really have said
"ok, the two known gotchas have been dealt with, so let's ship it".
It is quite likely that users may be hit by remaining differences
between the merge and apply backends that we assume to be merely
subtle and easy-to-work-around ones, and it may be prudent to use
the "let's not change the default just yet, but ask people to test
it in their workflow by advertising the configuration" patch.

Good question, but there's several things to unpack here...

First, note that this particular breakage would have occurred
regardless of the default setting, because the problem was that they
setting rebase.backend to an unrecognized value, not that we used a
different backend than they were used to.

Second, for the particular case of the post-commit hook that they
referenced in their rationale, that item was documented near the end
of the 2.25 cycle and mentioned in previous patchsets[1, 2] so I
figured this case was already considered.

Those two points may obscure the issue, though; your question is still
valid.  I think the bigger question is whether there are other unknown
differences, or even known differences that are a bigger issue than we
currently realize.  That's hard to judge, and it may not be possible
to judge until we've flipped the default.  As such, it's a judgement
call.  I can see the judgement call going either way.  A couple things
to weigh in on how to go:
   - Making rebase.backend default to 'apply' for 2.26 is certainly
the conservative option and may give us more feedback and time to iron
out differences
   - We had multiple complaints this cycle about rebase.backend=apply
merging things incorrectly with the only workaround being to use the
merge backend[3,4]
   - The rebase-backend topic wasn't merged down to master until less
than a week before -rc0.  (For a variety of reasons.)  A big change
like this probably would have been better to merge down earlier in
some cycle.

To be honest, if I was maintainer, I'm not sure which direction I'd
pick.  If you feel safer switching the default backend to apply for
this cycle and then (re-)revert it early next cycle to the merge
backend, I think that's totally reasonable.


[1] https://lore.kernel.org/git/pull.679.v4.git.git.1579155273.gitgitgadget@gmail.com/ (local)
[2] https://lore.kernel.org/git/CABPp-BHONuRyt8VJqRuoCF2rGYZ5EhH9KJXQZ3NO69rYwA5J3g@mail.gmail.com/ (local)
[3] https://lore.kernel.org/git/CABPp-BGiu2nVMQY_t-rnFR5GQUz_ipyEE8oDocKeO+h+t4Mn4A@mail.gmail.com/ (local)
[4] https://lore.kernel.org/git/20200108223557.GE32750@szeder.dev/ (local)