Working on a Chromium OS Branch

This applies to commits to Chromium OS repositories. For changes to Chromium repository branches, see the information about Drover; for Blink, see experimental branches.

Note: When to update ebuilds when merging changes

This works the same on branches as on trunk: If you're merging a change to code that's built as part of a cros_workon package, or to a .ebuild file for a cros_workon package, the package will be uprev-ed automatically. If you're changing a non-cros_workon package, you must uprev the corresponding .ebuild file on the branch, just as you do when making changes to non-cros_workon packages on trunk.

Merge via Gerrit UI

The Gerrit UI now contains a 'Cherry Pick' button which is currently the easiest way of merging a change which does not conflict, if the change you want to merge to a branch is already on Gerrit, you need only a browser. 

1. Open up the CL in question on Gerrit.
2. Select "Cherry Pick" from the "More" menu at the top right.
3. Enter the full branch name, for instance release-R58-9334.B.  (Use "git branch -a | grep R58" or equivalent to find the full name.)
4. Click "Cherry Pick" at the bottom of the pop up.
5. This should create a new CL against that branch in Gerrit. To land the CL, mark it ready as usual (Code-Review +2, Verified +1, Commit-Queue +1).

Note this only works if the patch applies cleanly on the target branch, if there is a conflict then the merge must be done manually as Gerrit provides no way of dealing with merge conflicts in the UI.

Also note that you can change the commit message in Gerrit, including changing CQ-DEPENDS directives as appropriate.


The second easiest way to create a change from a change you already committed on ToT in Chrome OS is using cros_merge_to_branch. If you're inside the chroot, it'll be in your $PATH.  Otherwise, you can run it directly from ~/chromite/bin/cros_merge_to_branch.

Example usage:

cros_merge_to_branch 32027 32030 32031 R25

This creates Gerrit changes for R25 from CL's 32027, 32030, and 32031 in less than 10 seconds. After running, you can check Gerrit to actually commit the changes (check your open commits for R25-* branch). You can also run these changes through a try-bot (make sure to specify the branch with -b with the remote try-bot). For more advanced usage information, use --help, or ping sosa@ or vapier@.

You should run with --dry-run the first time around to not actually upload your change until you are sure about how to use the tool. Note this tool accepts gerrit change numbers or change-id's, however, since the former is guaranteed to be unique, it is advised you use those instead.

Checking out the whole tree (with repo)

You must have different checkouts (yes, new chroots in a completely new directory) for every branch you are working on. This is to ensure all the prebuilts work automatically for you. You have to pass the -b <branch_name> option to repo during init and you will follow exactly the same workflow described in the Chromium OS Developer Guide (i.e cros_workon + repo start etc)

If you have an existing repo checkout: You can run repo init with --reference to re-use the objects of your existing checkout, to reduce sync time. Note that an absolute path is required (../../foo won't work) and that it must be the topmost directory of the existing repo checkout, i.e. the one that contains the .repo directory.

You can find the exact name of the branch by browsing the manifest repo.

mkdir release-R23-2913.B/
cd release-R23-2913.B/
repo init -u <URL> -b <branchname> [-g minilayout] [--reference /path/to/existing/checkout]
repo sync

NOTE: For older branches (e.g. R22 and earlier), you'll need to specify -m minilayout.xml instead of -g minilayout.

Example (See http://goto/chromeos-building if you are doing an internal build and replace the manifest.git link with the appropriate one)

repo init \
  -u \
  -b release-R23-2913.B \

Next, follow steps in the developer's guide to sync/edit/modify files i.e repo sync, cros_work start, repo start, etc. After you've cherry-picked/made the changes you want, upload the changes for review. If cherry-picking/etc..., make sure to recreate the ChangeID in the log message so that you get a new one -- simply delete the line and git will create a new one for you.

repo upload

Testing w/remote trybot

Before you commit the change, test it! Launch a tryjob to verify it actually builds properly. See Using Remote Trybots for more information.

cd <repo_root>/chromiumos/chromite
git checkout cros/master
cbuildbot --remote -g <review_id> -b <branchname> alex-release-trybot daisy-release-trybot

Checking out a single repository (with repo)

If you don't have a full repo checked out already and want to do a quick one-off merge, you can still check out the much smaller buildtools group:

repo init \
  -u \
  --repo-url \
  -g buildtools

This will get you chromite and all the tools it includes -- i.e. cros_merge_to_branch.  Make sure you pass the --nomirror option so it will fetch the single git repo needed to cherry-pick & upload the CL.

Finally, use cbuildbot with --remote to run remote trybots.  See the section above for more info.

Checking out a single repository (with git)

If you want to push up a few changes without checking out the entire tree, then you can use git to do just that.

You can re-use an existing repo checkout if you like (but make sure you clean up when you're done).  Let's assume you're going to make a new checkout to keep things clean though.

Cloning a new repository

Find the git url you care about.  You can get it by going into your repo checkout and look at .git/config (the url field). You'll need to use the -review variant of the URL to push to the special refs/for/* refs.  Let's demonstrate with the chromite git tree.

git clone

If you want to speed things up, you can use the --reference option to re-use a local tree. 

git clone --reference ~/chromiumos/chromite/.git

Setting your author/committer settings

If your normal user information is not your e-mail, you'll need to set it in the new repo.

git config "Awesome Developer"
git config ${USER}

Start a new branch

Let's assume you want to work on the R23 branch.  You need the full name of the branch, and then create a new local branch to work on with that info.

git branch -a | grep R23

That shows us the full branch name is "remotes/cros/release-R23-2913.B", so we can do:

git checkout -b R23 remotes/cros/release-R23-2913.B

Make your changes

This part is where the real work happens :).  Use git's or repo's cherry-pick feature, or make the changes by hand, or apply patches, or whatever you want.

# When the editor pops, try to change few attributes to help tracking commit history.
# Change Reviewed-on to Previous-Reviewed-on, and
# add a line like 
(cherry picked from commit b9e382afa7e410745ac96b12b49d5a941070db1e).
# For changes in different branches, you can keep same Change-Id; otherwise remove that to get a new unique Change-Id.
git cherry-pick -x -e <SHA1>

Publish your commits to gerrit

Now for the last step.  If you didn't create a new clone, you might have to change "origin" to "cros", or replace it with the full git url.  The "R23" is whatever you called the local branch, and the "release-R23-2913.B" is exactly what the official branch name is called -- make sure it's correct as gerrit will allow you to push to anything.

Note: The rest of the command should be typed verbatim (e.g. the "refs/for/" part).  Only replace components below that are in italics.

git push origin R23:refs/for/release-R23-2913.B

Reusing a single repository in an existing repo checkout

While it is possible to manually checkout a different branch in an existing repo checkout (e.g. checking out release-R69-10895.B in chromite/ when the rest of the manifest is tracking ToT), this is strongly not recommended.

Mixing different branches in git trees in a single repo checkout can easily break existing tools and is not supported. Even if you want to do it as a one off (e.g. checkout a branch, make a change, upload it, and then discard the branch), it's still not recommended as sometimes people forget to clean up when they're done. Depending on the git tree, this can manifest itself days, weeks, or even months later as a weird error in a seemingly unrelated location.

This is why we only support the methods listed above.