Re: Git commit generation numbers

> On Fri, 22 Jul 2011, David Lang <david@lang.hm> wrote:
>> On Fri, 22 Jul 2011, Nicolas Pitre wrote:
>>> On Fri, 22 Jul 2011, Jakub Narebski wrote:
>>>
>>>> BTW. with storing generation number in commit header there is a problem
>>>> what would old version of git, one which does not understand said header,
>>>> do during rebase.  Would it strip unknown headers, or would it copy
>>>> generation number verbatim - which means that it can be incorrect?
>>>
>>> They would indeed be copied verbatim and become incorrect.
>>
>> how would they become incorrect?
>
> Let's assume that the following history was created with new git, one
> that correcly adds generation number header to commits:
>
>
>  A(1)---B(2)---C(3)---D(4)---E(5)       <-- master
>          \
>           \----x(3)---y(4)---z(5)       <-- foo
>
> The numbers are generation numbers in commit object.
>
> Let's assume that this repository is fetched into repository instance
> that is managed by older git, one that doesn't understand generation
> header.
>
> Then, if we do
>
>  [old]$ git rebase master foo
>
> and if old git _copies_ generation number header _verbatim_, we would
> get:
>
>  A(1)---B(2)---C(3)---D(4)---E(5)                         <-- master
>                               \
>                                \---x'(3)--y'(4)--z'(5)    <-- foo
>
> Those generation numbers are *incorrect*; they should be:
>
>  A(1)---B(2)---C(3)---D(4)---E(5)                         <-- master
>                               \
>                                \---x'(6)--y'(7)--z'(8)    <-- foo
>
>
> That is IF unknown headers are copied verbatim during rebase.  For
> "encoding" header this is a good thing, for "generation" it isn't.