Re: [PATCH v1] rebase -i: stop checking out the tip of the branch to rebase

[Alban Gruin <alban.gruin@gmail.com>]

Hi Junio,

Le 22/01/2020 à 21:47, Junio C Hamano a écrit :
> Alban Gruin <alban.gruin@gmail.com> writes:
> 
>> One of the first things done by the interactive rebase is to make a todo
>> list.  This requires knowledge of the commit range to rebase.  To get
>> the oid of the last commit of the range, the tip of the branch to rebase
>> is checked out with prepare_branch_to_be_rebased(), then the oid of the
>> HEAD is read.  On big repositories, it's a performance penalty: the user
>> may have to wait before editing the todo list while git is extracting the
>> branch silently (because git-checkout is silenced here).  After this,
>> the head of the branch is not even modified.
> 
> Hmph.  One curious thing in the above is why this is specific to
> "rebase -i". The need to know the commit range to rebase is shared
> across any rebase backend, and it would be the most natural to parse
> the optional second argument (i.e. the branch or the commit to
> rebase) before builtin/rebase.c dispatches to a specific rebase
> backend, wouldn't it?  So, the question is why a normal "rebase"
> does not need the same fix?
> 

That's a problem shared by all rebases using the sequencer, so -m and -r
are also affected by this.  `am' is not.

> If the answer is "rebase in general was fine without extra checkout,
> but 'rebase -i' was doing an unnecessary checkout" (or any other
> answer) that is something that would help future readers to record
> in the commit log message.
> 

So yes, the answer is that the am backend does not perform this
checkout, unlike all others rebases.

I will resend this patch very soon.

> Thanks.
> 
> 

Cheers,
Alban