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Advanced Gerrit Usage
This file gives some examples of Gerrit workflows. This includes uploading patches with Gerrit, using Gerrit dependency chains, and managing local git history.
Managing patches with Gerrit
The instructions in README.md using git cl upload are preferred
for patch management. However, you can interact manually with the Gerrit code
review system via git if you prefer as follows.
Preliminaries
You should install the
Change-Id commit-msg hook.
This adds a Change-Id line to each commit message locally, which Gerrit uses
to track changes. Once installed, this can be toggled with git config gerrit.createChangeId <true|false>.
To download the commit-msg hook for the Open Screen repository, use the following command:
curl -Lo .git/hooks/commit-msg https://chromium-review.googlesource.com/tools/hooks/commit-msg
chmod a+x .git/hooks/commit-msg
Uploading a new patch for review
You should run PRESUBMIT.sh in the root of the repository before pushing for
review (which primarily checks formatting).
There is official Gerrit documentation for this which essentially amounts to:
git push origin HEAD:refs/for/master
Gerrit keeps track of changes using a Change-Id line in each commit.
When there is no Change-Id line, Gerrit creates a new Change-Id for the
commit, and therefore a new change. Gerrit's documentation for
replacing a change
describes this. So if you want to upload a new patchset to an existing review,
it should contain the matching Change-Id line in the commit message.
Adding a new patchset to an existing change
By default, each commit to your local branch will get its own Gerrit change when
pushed, unless it has a Change-Id corresponding to an existing review.
If you need to modify commits on your local branch to ensure they have the
correct Change-Id, you can do one of two things:
After committing to the local branch, run:
git commit --amend
git show
to attach the current Change-Id to the most recent commit. Check that the
correct one was inserted by comparing it with the one shown on
chromium-review.googlesource.com for the existing review.
If you have made multiple local commits, you can squash them all into a single commit with the correct Change-Id:
git rebase -i HEAD~4
git show
where '4' means that you want to squash three additional commits onto an existing commit that has been uploaded for review.
Uploading a new dependent change
If you wish to work on multiple related changes without waiting for them to land, you can do so in Gerrit using dependent changes. There doesn't appear to be any official documentation for this, but the rule seems to be: if the parent of the commit you are pushing has a Change-Id line, that change will also be the current change's parent. This is useful so you can look at only the relative diff between changes instead of looking at all of them relative to master.
To put this into an example, let's say you have a commit for feature A with Change-Id: aaa and this is in the process of being reviewed on Gerrit. Now let's say you want to start more work based on it before it lands on master.
git checkout featureA
git checkout -b featureB
git branch --set-upstream-to featureA
# ... edit some files
# ... git add ...
git commit
The git history then looks something like this:
... ---- master
\
A
\
B <- HEAD
and git log might show:
commit 47525d663586ba09f40e29fb5da1d23e496e0798 (HEAD -> featureB)
Author: btolsch <btolsch@chromium.org>
Date: Fri Mar 23 10:18:01 2018 -0700
Add some better things
commit 167a541e0a2bd3de4710965193213aa1d912f050 (featureA)
Author: btolsch <btolsch@chromium.org>
Date: Thu Mar 22 13:18:09 2018 -0700
Add some good things
Change-Id: aaa
Now you can push B to create a new change for it in Gerrit:
git push origin HEAD:refs/for/master
In Gerrit, there would then be a "relation chain" shown where the feature A change is the parent of the feature B change. If A introduces a new file which B changes, the review for B will only show the diff from A.
Examples for maintaining local git history
D-E --- feature B
/ ^N
A-B-C-F-G-H --- feature A
/ ^M ^O
/
... ---- master
/|
M O
|
N
Consider a local repo with a master branch and two feature branches. Commits M,
N, and O are squash commits that were pushed to Gerrit. The arrow/caret (^)
indicates whence those were created. M, N, and O should all have Change-Id
lines in them (this can be done with the commit-msg
hook).
M and O are separate patchsets in one review (M containing A, B, C and O
containing A, B, C, F, G) and N is the first patchset in a new review that is
dependent on the first patchset of the first review.
Starting without M, N, or O, the commands to create them are as follows:
git checkout C
git checkout -b M
git rebase -i origin/master # squash commits
# Note: make sure a Change-Id line exists on M at this point since N will need
# it. You can git commit --amend with the commit-msg hook active or add it via
# git commit --amend after pushing. Don't git commit --amend after creating N
# though.
git push origin HEAD:refs/for/master
git checkout E
git checkout -b N
git rebase -i C --onto M # squash commits
git push origin HEAD:refs/for/master
git checkout G
git checkout -b O
git rebase -i origin/master # squash commits and copy the Change-Id line from M
git push origin HEAD:refs/for/master
D-E --- feature B
/ ^Q
A-B-C-F-G-H --- feature A
/ ^P
/
... ---- master
/|\
M O P
| |
N Q
The next example shows an additional patchset being uploaded for feature A (commit P) and feature B being rebased onto A, then uploaded to Gerrit as commit Q.
Starting from the endpoint of the previous commands, this point can be reached as follows:
git checkout H
git checkout -b P
git rebase -i origin/master # squash commits, same note as M about Change-Id
git push origin HEAD:refs/for/master
git checkout featureB # E
git rebase # assume featureA is set as featureB's upstream branch
git checkout -b Q
git rebase -i H --onto P
git push origin HEAD:refs/for/master