Re: Aborting 'rebase main feat' removes unversioned files

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

On Sat, Sep 04 2021, Elijah Newren wrote:

> On Sat, Sep 4, 2021 at 3:19 AM Jeff King <peff@peff.net> wrote:
>>
>> On Sat, Sep 04, 2021 at 11:51:19AM +0200, Fedor Biryukov wrote:
>>
>> > There is no way this could be the intended behavior, but the good news
>> > is that I cannot reproduce it...
>> > Looks like it occurs only in one git version (2.31.0.windows.1, IIRC).
>> > And it does not occur in the latest git version: git version 2.33.0.windows.2.
>>
>> I think it is a bug, and it seems to reproduce easily for me (with both
>> the current tip of master, and with v2.33.0). Here's the recipe you
>> showed, with a little debugging at the end:
>>
>>   set -x
>>   git init repo
>>   cd repo
>>   git commit -m base --allow-empty
>>   git checkout -b feat
>>   echo feat >readme.txt
>>   git add readme.txt
>>   git commit -m txt=feat
>>   git checkout main
>>   echo precious >readme.txt
>>
>>   cat readme.txt
>>   git checkout feat
>>   cat readme.txt
>>   git rebase main feat
>>   cat readme.txt
>>
>> This produces:
>>
>>   + cat readme.txt
>>   precious
>>   + git checkout feat
>>   error: The following untracked working tree files would be overwritten by checkout:
>>         readme.txt
>>   Please move or remove them before you switch branches.
>>   Aborting
>>   + cat readme.txt
>>   precious
>>   + git rebase main feat
>>   Current branch feat is up to date.
>>   + cat readme.txt
>>   feat
>>
>> So git-checkout was not willing to overwrite the untracked content, but
>> rebase was happy to obliterate it.
>>
>> It does the right thing in very old versions. Bisecting, it looks like
>> the problem arrived in 5541bd5b8f (rebase: default to using the builtin
>> rebase, 2018-08-08). So the bug is in the conversion from the legacy
>> shell script to C (which makes sense; the shell version was just calling
>> "git checkout", which we know does the right thing).
>>
>> -Peff
>
> Turns out this is quite a mess.  It's also related to the "don't
> remove empty working directories" discussion we had earlier this
> week[1], because we assumed all relevant codepaths correctly avoided
> deleting untracked files and directories in the way.  But they don't.
> And rebase isn't the only offender, because this is buried in
> unpack_trees.  In fact, it traces back to (and before)
>     fcc387db9b ("read-tree -m -u: do not overwrite or remove untracked
> working tree files.", 2006-05-17)
> which has additional commentary over at
> https://lore.kernel.org/git/7v8xp1jc9h.fsf_-_@assigned-by-dhcp.cox.net/.
> It appears that before this time, git happily nuked untracked files
> and considered them expendable, in basically all cases.  However, this
> patch continued considering them as expendable whenever opts->reset
> was true.  There wasn't much comment about it at the time for the
> reasoning of how opts->reset was handled, though trying to read
> between the lines perhaps Junio was trying to limit the backward
> compatibility concerns of introducing new errors to fewer code paths?
> Anyway, Junio did mention `read-tree --reset` explicitly, but this
> opts->reset usage also occurs in am, checkout, reset -- and anything
> that calls the reset_head() function including: rebase, stash,
> sequencer.c, and add-patch.c.
>
> So, then...should we preserve untracked (and non-ignored) files in all
> these cases?  This rebase case seems clear, but others might be less
> clear.  For example, should "git reset --hard" nuke untracked files
> (what if it's a directory of untracked files getting nuked just to
> place a single file in the location of the directory)?  The
> documentation isn't explicit, but after reading it I would assume that
> untracked files should be preserved.  Since we've had bugs in "git
> reset --hard" before, such as requiring two invocations in order to
> clear out unmerged entries (see [2] and [3]), that also suggests that
> this is just another bug in the same area.  But the bug has been
> around so long that people might be expecting it; our testsuite has
> several cases that incidentally do.  Granted, it's better to throw an
> error and require explicit extra steps than to nuke potentially
> important work, but some folks might be unhappy with a change here.
> Similarly with "git checkout -f".
>
> And stash.c, which operates in that edge case area has tests with
> files nuked from the cache without nuking it from the working tree
> (causing the working tree file to be considered untracked), and then
> attempts to have multiple tests operate on that kind of case.  Those
> cases look a bit buggy to me for other reasons (I'm still digging),
> but those bugs are kind of hidden by the untracked file nuking bugs,
> so fixing the latter requires fixing the former.  And stash.c is a
> mess of shelling out to subcommands.  Ick.
>
> I have some partial patches, but don't know if I'll have anything to
> post until I figure out the stash mess...

I'd just like to applaud this effort, and also suggest that the most
useful result of any such findings would be for us to produce some new
test in t/ showing these various cases of nuking/clobbering and other
"non-precious" edge cases in this logic. See[1] and its linked [2] for
references to some of the past discussions around these cases.

1. https://lore.kernel.org/git/87a6q9kacx.fsf@evledraar.gmail.com/
2. https://lore.kernel.org/git/87ftsi68ke.fsf@evledraar.gmail.com/

> [1] https://lore.kernel.org/git/YS8eEtwQvF7TaLCb@coredump.intra.peff.net/
> [2] 25c200a700 ("t1015: demonstrate directory/file conflict recovery
> failures", 2018-07-31)
> [3] ad3762042a ("read-cache: fix directory/file conflict handling in
> read_index_unmerged()", 2018-07-31)