Re: [PATCH 2/2] fast-import: add special '-' blob reference to use the previous one.

> Junio C Hamano <gitster@pobox.com> wrote:
>> "Shawn O. Pearce" <spearce@spearce.org> writes:
>>
>> > Hmph, so if create a new path with a blob of "-" the repository
>> > will be corrupt because the zero id was used and error was produced.
>> >
>> > Actually I think you have the same bug in the prior patch with the
>> > mode being inherited.  I wonder if we shouldn't put error checking
>> > in too to validate that versions[0] describes a file entry.
>>
>> Why are these patches necessary?

>> The proposed commit message describes what it does, but does not give hint
>> to even guess being able to use this new feature helps in what situation.
>> As far as I can see, these changes allow the exporter to say "this aspect
>> of the new data is the same as the previous one", but I thought that the
>> way in which fast-import works already revolves around "you have this
>> tree, and the next tree is different from it in this and that way."  Why
>> does one need be able to mention "this is the same as the previous one"
>> explicitly in the first place?
>
> Hmm.  Actually, imagine you were dumping from git-diff output style
> stream into a fast-import stream.
>
> If a file changes only content, the mode is shown in the index line.
> Yay us.  But what if the index line wasn't present in the diff? You
> don't know the prior mode of the file, but you do have its content.
>
> If a file changes only mode, we get no content hints in the diff.
> How do you send that into fast-import without making the frontend
> keep track of every path's current mode?
>
> Though I agree, these details should be described in the commit
> messages, not left as an exercise for the maintainer to make up.