Re: first parent, commit graph layout, and pull merge direction

From: Junio C Hamano <gitster@pobox.com>
Date: May 24, 2013, 19:23 UTC
Message-ID: <7vmwrk89mb.fsf@alter.siamese.dyndns.org>
In-Reply-To: <20130524171110.GB9448@inner.h.apk.li>

Andreas Krey <a.krey@gmx.de> writes:

Show 24 quoted lines

> On Thu, 23 May 2013 09:01:15 +0000, Junio C Hamano wrote:
> ...
>> Instead of having a nice "these six commits marked as 'x' were done
>> on a branch forked some time ago, to address only this one issue and
>> to address it fully" history that explains how these commits were
>> related and these commits are the full solution to a single issue:
>> 
>>       x---x---x---x---x---x
>>      /                     \
>>  ---o---o---o---o---o---o---M---o---o---...
>> 
>> they end up with something like this, even with the "flip the heads
>> of a merge" option, by pulling too often:
>> 
>>       x---x   x---x---x   x
>>      /     \ /         \ / \
>>  ---o---o---M---o---o---M---M---o---o---...
>
> Wouldn't that be (you don't want to put your work back into master before
> it's done) the following?
>
>        x---x---M---x---x---M--x
>       /       /           /    \
>   ---o---o---M---o---o---M--o---M---o---o---...

That is what you would get if you "pull from my upstream" with the current software.

And that is what triggered this discussion thread in which some people said that they do not want that shape of the history.

At the leftmost merge M you drew on the upper line (i.e. your topic), the merge pulls in other's commits that are unrelated to each other as if it were a meaningful group of commits on a side branch. They want to see the merge going other way around, pulling your work done on a side branch, integrating into the mainline.

The second illustration you are commenting on were done to explain why such a "when pulling from my upstream, I want the order of parents swapped, so that mainline appears as the first parent" is not solving the whole issue. The time series would go more like this:

(1) While you were working on two 'x's, others have worked to
    advance the mainline:

       x---x  Your 'master'
      /
  ---o---o  Mainline

(2) You cannot push without losing others work, so you pull, but in
    order to avoid the "others work on mixed on a single side
    branch" issue, you use the fictional "flip heads of a merge"
    option, and push the result out.  That becomes the tip of the
    mainline:

       x---x
      /     \
  ---o---o---M

(3) Then you keep working to build more commits on top.

       x---x   x---x---x
      /     \ /
  ---o---o---M

(4) And others also keep working.

       x---x   x---x---x
      /     \ /
  ---o---o---M---o---o