Re: [BUG] merge-recursive overly aggressive when skipping updating the working tree

[Ben Peart <peartben@gmail.com>]

On 7/20/2018 7:02 PM, Elijah Newren wrote:
> On Fri, Jul 20, 2018 at 3:05 PM, Junio C Hamano <gitster@pobox.com> wrote:
>> Elijah Newren <newren@gmail.com> writes:
>>
>>> Ah, okay, that's helpful.  So, if there are conflicts, it should be
>>> free to clear the skip_worktree flag.  Since merge-recursive calls
>>> add_cacheinfo() for all entries it needs to update, which deletes the
>>> old cache entry and just makes new ones, we get that for free.
>>
>> Correct.
>>
>>> And conversely, if a file-level merge succeeds without conflicts then
>>> it clearly doesn't "need to materialize a working tree file", so it
>>> should NOT clear the skip_worktree flag for that path.
>>
>> That is not at all implied by what I wrote, though.
>>
>> If it can be done without too much effort, then it certainly is
>> nicer to keep the sparseness when we do not have to materialize the
>> working tree file.  But at least in my mind, if it needs too many
>> special cases, hacks, and conditionals, then it is not worth the
>> complexity---if it is easier to write a correct code by allowing Git
>> to populate working tree files, it is perfectly fine to do so.
>>
>> In a sense, the sparse checkout "feature" itself is a hack by
>> itself, and that is why I think this part should be "best effort" as
>> well.
> 
> That's good to know, but I don't think we can back out easily:
>    - Clearing the skip_worktree bit: no big deal, as you mention above
>    - Avoiding working tree updates when merge doesn't change them: very
> desirable[1]
>    - Doing both: whoops
> 
> [1] https://public-inbox.org/git/CA+55aFzLZ3UkG5svqZwSnhNk75=fXJRkvU1m_RHBG54NOoaZPA@mail.gmail.com/
> 
> 
> I don't want to regress the bug Linus reported, so to fix Ben's issue,
> when we detect that a path's contents/mode won't be modified by the
> merge, we can either:
>    - Update the working tree file if the original cache entry had the
> skip_worktree flag set
>    - Mark the new cache entry as skip_worktree if the original cache
> entry had the skip_worktree flag set
> 
> Both should be about the same amount of work; the first seems weird
> and confusing for future readers of the code.  The second makes sense,
> but probably should be accompanied with a note in the code about how
> there are other codepaths that could consider skip_worktree too.
> 
> I'll see if I can put something together, but I have family flying in
> tomorrow, and then am out on vacation Mon-Sat next week, so...
> 

I agree with the priorities around proposed behavior with this scenario.

It would be preferred that the skip worktree bit be preserved but the 
behavior in 2.17 of clearing it and writing the file to the working 
directory is much better than the current 2.18 behavior that makes the 
user think they had somehow deleted the file just by doing the merge.

At this point, it isn't clear to the user what they should do to recover 
without causing harm to the repo.

$ git status
HEAD detached at df2a63d
You are currently cherry-picking commit 7e6d412.
   (all conflicts fixed: run "git cherry-pick --continue")
   (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes not staged for commit:
   (use "git add/rm <file>..." to update what will be committed)
   (use "git checkout -- <file>..." to discard changes in working directory)

         deleted:    foo