git-rerere.txt

git-rerere(1)
=============

NAME
----
git-rerere - Reuse recorded resolve

SYNOPSIS
--------
'git-rerere'


DESCRIPTION
-----------

In a workflow that employs relatively long lived topic branches,
the developer sometimes needs to resolve the same conflict over
and over again until the topic branches are done (either merged
to the "release" branch, or sent out and accepted upstream).

This command helps this process by recording conflicted
automerge results and corresponding hand-resolve results on the
initial manual merge, and later by noticing the same automerge
results and applying the previously recorded hand resolution.

[NOTE]
You need to create `$GIT_DIR/rr-cache` directory to enable this
command.

DISCUSSION
----------

When your topic branch modifies overlapping area that your
master branch (or upstream) touched since your topic branch
forked from it, you may want to test it with the latest master,
even before your topic branch is ready to be pushed upstream:

------------
 o---*---o topic
 /
 o---o---o---*---o---o master
------------

For such a test, you need to merge master and topic somehow.
One way to do it is to pull master into the topic branch:

------------
$ git checkout topic
$ git pull . master

 o---*---o---+ topic
 / /
 o---o---o---*---o---o master
------------

The commits marked with `*` touch the same area in the same
file; you need to resolve the conflicts when creating the commit
marked with `+`. Then you can test the result to make sure your
work-in-progress still works with what is in the latest master.

After this test merge, there are two ways to continue your work
on the topic. The easiest is to build on top of the test merge
commit `+`, and when your work in the topic branch is finally
ready, pull the topic branch into master, and/or ask the
upstream to pull from you. By that time, however, the master or
the upstream might have been advanced since the test merge `+`,
in which case the final commit graph would look like this:

------------
$ git checkout topic
$ git pull . master
$ ... work on both topic and master branches
$ git checkout master
$ git pull . topic

 o---*---o---+---o---o topic
 / / \
 o---o---o---*---o---o---o---o---+ master
------------

When your topic branch is long-lived, however, your topic branch
would end up having many such "Merge from master" commits on it,
which would unnecessarily clutter the development history.
Readers of the Linux kernel mailing list may remember that Linus
complained about such too frequent test merges when a subsystem
maintainer asked to pull from a branch full of "useless merges".

As an alternative, to keep the topic branch clean of test
merges, you could blow away the test merge, and keep building on
top of the tip before the test merge:

------------
$ git checkout topic
$ git pull . master
$ git reset --hard HEAD^ ;# rewind the test merge
$ ... work on both topic and master branches
$ git checkout master
$ git pull . topic

 o---*---o-------o---o topic
 / \
 o---o---o---*---o---o---o---o---+ master
------------

This would leave only one merge commit when your topic branch is
finally ready and merged into the master branch. This merge
would require you to resolve the conflict, introduced by the
commits marked with `*`. However, often this conflict is the
same conflict you resolved when you created the test merge you
blew away. `git-rerere` command helps you to resolve this final
conflicted merge using the information from your earlier hand
resolve.

Running `git-rerere` command immediately after a conflicted
automerge records the conflicted working tree files, with the
usual conflict markers `<<<<<<<`, `=======`, and `>>>>>>>` in
them. Later, after you are done resolving the conflicts,
running `git-rerere` again records the resolved state of these
files. Suppose you did this when you created the test merge of
master into the topic branch.

Next time, running `git-rerere` after seeing a conflicted
automerge, if the conflict is the same as the earlier one
recorded, it is noticed and a three-way merge between the
earlier conflicted automerge, the earlier manual resolution, and
the current conflicted automerge is performed by the command.
If this three-way merge resolves cleanly, the result is written
out to your working tree file, so you would not have to manually
resolve it. Note that `git-rerere` leaves the index file alone,
so you still need to do the final sanity checks with `git diff`
(or `git diff -c`) and `git update-index` when you are
satisfied.

As a convenience measure, `git-merge` automatically invokes
`git-rerere` when it exits with a failed automerge, which
records it if it is a new conflict, or reuses the earlier hand
resolve when it is not. `git-commit` also invokes `git-rerere`
when recording a merge result. What this means is that you do
not have to do anything special yourself (Note: you still have
to create `$GIT_DIR/rr-cache` directory to enable this command).

In our example, when you did the test merge, the manual
resolution is recorded, and it will be reused when you do the
actual merge later with updated master and topic branch, as long
as the earlier resolution is still applicable.

The information `git-rerere` records is also used when running
`git-rebase`. After blowing away the test merge and continuing
development on the topic branch:

------------
 o---*---o-------o---o topic
 /
 o---o---o---*---o---o---o---o master

$ git rebase master topic

 o---*---o-------o---o topic
 /
 o---o---o---*---o---o---o---o master
------------

you could run `git rebase master topic`, to keep yourself
up-to-date even before your topic is ready to be sent upstream.
This would result in falling back to three-way merge, and it
would conflict the same way the test merge you resolved earlier.
`git-rerere` is run by `git rebase` to help you resolve this
conflict.


Author
------
Written by Junio C Hamano <junkio@cox.net>

GIT
---
Part of the gitlink:git[7] suite