re: git rerere

16 Feb 2020 | categories: tips

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Have you ever tried to merge two branches only to end up in conflict hell? You fix a bunch of conflicts only to run git merge --continue and be presented with the same conflicts. Repeat this process and after a few iterations you give up because it just isn’t worth the pain and effort.

Would you be surprised to know that there is a git feature specifically for this problem? It’s called rerere and I’m going to enrich your life with it now. (I’m going to talk specifically about merging but I think it also helps rebasing)

rerere stands for Reuse Recorded Resolution. The TL;DR version is you ask git to remember how you’ve resolved hunks in the past, and if the same one comes up for a file in future just redo what you did last time.

To enable this feature just run this lovely command git config --global rerere.enabled true. You can also turn it on by creating this directory in your projects .git/rr-cache, although the global setting is much clearer and will create this directory for you automatically.

I’ll try to take you through an example of how this works, bear with me it might get long.

We have our (tiny) project with only one file in it, which looks like this

.
└── user.rb

0 directories, 1 file

I branch off master to create a branch called dev and I add a line to user.rb. Now I would like to stage this change so I pull down staging and try to merge my dev branch but uh oh, someone has merged a change to staging affecting the same line in user.rb that I am editing.

/tmp/example [staging] » git merge dev
Auto-merging user.rb
CONFLICT (content): Merge conflict in user.rb
Automatic merge failed; fix conflicts and then commit the result.

We’ve all seen this before, a run of the mill conflict message. However if you were to have rerere enabled you would get this output

/tmp/example [staging] » git merge dev
Auto-merging user.rb
CONFLICT (content): Merge conflict in user.rb
Recorded preimage for 'user.rb'
Automatic merge failed; fix conflicts and then commit the result.

You can now see a new line saying Recorded preimage for 'user.rb'. Running git rerere diff right now will give you the current state of the resolution file:

/tmp/example [c013552] » git rerere diff
--- a/user.rb
+++ b/user.rb
@@ -1,5 +1,5 @@
-<<<<<<<
-hello
-=======
+<<<<<<< HEAD
 hi
 ->>>>>>>
 +=======
 +hello
 +>>>>>>> commit from dev

You go about the usual conflict workflow, choose which changes to keep, and commit the result. If you run git rerere diff again, you see the recorded resolution:

/tmp/example [c013552] » git rerere diff
--- a/user.rb
+++ b/user.rb
@@ -1,5 +1 @@
-<<<<<<<
 hello
-=======
-hi
->>>>>>>

Running git merge --continue will apply your commit and tell you about the new resolution for our file:

/tmp/example [c013552] » git merge --continue
Recorded resolution for 'user.rb'.

Let’s undo that merge with git reset --hard HEAD^ and try merging the same branches again, keeping in mind that before we had to manually fix the conflict:

/tmp/example [staging] » git merge dev
Auto-merging user.rb
CONFLICT (content): Merge conflict in user.rb
Resolved 'user.rb' using previous resolution.
Automatic merge failed; fix conflicts and then commit the result.
/tmp/example [staging] » git add .
/tmp/example [staging] » git merge --continue
[staging f4a7d36] Merge branch 'dev' into staging

The important line in this output is Resolved 'user.rb' using previous resolution.. I didn’t need to even look at the file, just commit the result. This worked because git saw the conflict, looked in the rr-cache folder and recognised this hunk from this file from a previous merge and applied your decision from last time!

As always, I hope you found that useful.