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git-rerere - Reuse recorded resolve
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.
GIT
Part of the git(7) suite
CATEGORY