Re: Removing useless merge commit with "filter-branch"

Hi

On Thu, Mar 8, 2012 at 3:30 PM, Junio C Hamano [off-list ref] wrote:

Anatol Pomozov [off-list ref] writes:

quoted

|
o      - merge commit that previously merged feature X
|\
| \
|  \
o  |   - real commit
|   |
|  /
|/
|

It is unclear how many commits are drawn in the above picture and
what "feature X" is about in the above picture.  Care to redraw the
commit DAG to explain what you are trying to do a bit better?

The way I read it is that you start from a history like this (note
that when we draw an ascii art history we often write it sideways,
time flows from left to right):

   ---A-----B-----M---
       \         /
        C-------D

where a side branch to implement "feature X" that has C and D forked
at A, and it was merged at M after somebody else committed B on the
mainline.  When you filtered out some parts of the tree, it turns
out that C and D are totally unintereseting because their changes
touch parts outside of your interest, i.e. the history is:

   ---A-----B-----M---
       \         /
        o-------o

where 'o' are now no-op.

Is that what you are talking about?

Yes, in fact --prune-empty flag removes empty commits so the history looks like

-----A-------B-------M--------
       \               /
        --------------


So M is a merge that has 2 parents A and B. I would like to remove
this merge M and leave the history as

-----A-----B-----

as only these commits have changes in my library that I am trying to extract.

I think some trickery with "git filter-branch --parent-filter" should help here.

First one runs filter-branch with --parent-filter and removes useless
parents from merges (in this example with will be parent A---M), this
converts such merges to regular empty commits

then run filter-branch one more time with --prune-empty - it removes
empty commits.

I think "log --simplify-merges A..M -- path" may already has logic
that deals with this, so it may help if you study what it does and
how it does what it does.