Re: Bugs with detection of renames that are also overwrites

[Jon Seymour <jon.seymour@gmail.com>]

I came across a similar case recently while testing my rebasing ideas
which lead me to wonder - is there a way to suppress the rename and
overwrite detection? For my purposes, I don't care if the diff is
bulky, I just want it to apply reliably.

jon

On Tue, Feb 23, 2010 at 11:22 PM, Jeff King <peff@peff.net> wrote:
> On Sat, Feb 13, 2010 at 06:46:55PM -0500, Anders Kaseorg wrote:
>
>> One can create a rename that also overwrites an existing file, for example
>> with ‘git mv -f’:
>>
>> $ git init
>> $ seq 100 200 > a; seq 300 400 > b; git add a b
>> $ git commit -m foo; git tag foo
>> $ git mv -f a b
>> $ git commit -m bar; git tag bar
>>
>> Git does not ordinarily detect this as a rename, even with -M.
>
> Right. Git looks at only a subset of the files when looking for renames.
> For straight renames, the set of possible sources is the list of deleted
> files, and the possible destinations are the new files.
>
> Finding copies ("-C") is similar, except that we also consider files
> that were modified but not deleted. And --find-copies-harder will look
> at _all_ files as sources, not just modified ones.
>
> But what you are asking for is to expand the possible destination list
> to include files that were modified but are not new. I don't think there
> is currently a way to do that explicitly.
>
> As you discovered, though, if either the source or destination file has
> changed significantly, we should break them apart using "-B":
>
>> But it can (sometimes*) be forced to detect the rename with -M -B.
>>
>> $ git diff --stat -M -B foo bar
>>  a => b |    0
>>  1 files changed, 0 insertions(+), 0 deletions(-)
>
> In which case the rename engine sees the deletion and addition
> separately (they just happen to have the same path name), and therefore
> the path gets added to the source and destination lists.
>
>> However, the resulting patch incorrectly omits the deletion of the
>> overwritten file!
>>
>> $ git diff -M -B foo bar
>> diff --git a/a b/b
>> similarity index 100%
>> rename from a
>> rename to b
>>
>> Therefore, the patch can’t be applied to its own source tree.
>>
>> $ git checkout foo
>> $ git diff -M -B foo bar | git apply
>> error: b: already exists in working directory
>
> Hmm. I think this is a conflict between what the user wants to see and
> what apply wants to see. From the user's perspective, thinking about the
> diff representing a change, "b" was not really deleted. It was simply
> overwritten. But from apply's perspective, the diff is a set of
> instructions, and those instructions do not mention that "b" previously
> existed and was overwritten. So it is right to be cautious and declare a
> conflict.
>
> I'm not sure just throwing a "delete" line in there would be the best
> thing, though. The instructions for apply really need to be "if 'b' has
> this sha1, then it is safe to delete and rename a into place. Otherwise
> it is a conflict". And I'm not sure how we would represent that (I guess
> with a full diff and an "index" line).
>
>> Also, a question: is it possible to get ‘git merge’ to recognize this
>> rename?
>
> No, I don't think there is a way currently. You would need to patch git
> to set opts.break_opt in merge-recursive.c:get_renames, I think.
>
>> (* I say “sometimes” because -B only detects the rewrite if it deems the
>> renamed file to be sufficiently different than the overwritten file.  If
>> you use 190 and 390 instead of 200 and 400 above, the rewrite and rename
>> will not be detected, even though the rename would be detected if it was
>> not an overwrite.  This also feels like a bug.)
>
> I think you are getting into a philosophical discussion of what is a
> rename, here. If "a" and "b" are very similar, "a" is removed, and "b"
> has the same (or similar) content as "a", was it a rename from "a", or
> was it simply that "b" was changed, possibly to incorporate the
> duplicated items in "a"?
>
> So I don't think it is a bug. But that isn't to say you can't come up
> with a case where it would be nice, during a diff or a merge, to show
> things that way. I mentioned at the beginning of the message that what
> you wanted was to expand the list of destination possibilities. You
> could have a "--find-overwrites" which puts all of the modified files
> into the destination list. You would not want it on by default, though,
> I think, as it would add a lot of computation time to find a somewhat
> rare case.
>
> -Peff
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