Using Gerrit to submit and review patches

What is Gerrit?

The Impala project uses Gerrit for all our code reviews. Gerrit is a git-based code review tool. The Impala project Gerrit server is here.

Logging in

To authenticate with Impala's Gerrit server, you'll need a Github account. Once you have one, logging in to Gerrit is as easy as clicking 'Github Sign-in' in the top-right corner.

You will need to authorize Gerrit to access some details of your Github account, and then you should be brought back to the Gerrit homepage. Unfortunately, there's a bug in the Gerrit OAuth plugin that will send you to a page that gives a 404 error. You'll need to manually go back to https://gerrit.cloudera.org/, and at that point you should be signed in as 'Anonymous Coward (X)'.

If your Gerrit username is different than your username on the system you want to send patches from, set your "GERRIT_USER" environment variable to your Gerrit username.

Add your public SSH key

Once you are logged in, click your user name in the top-right corner, and go to 'Settings'. Under 'SSH Public Keys' you'll need to paste in your public key:

cat ~/.ssh/id_\{r or d\}sa.pub

While you're in the settings page, please make sure to update your profile with your name, and your e-mail address!

Configuring Gerrit as a Git remote

Gerrit works by keeping a Git repository for all the projects it maintains, including Impala. The source of truth for what is in Impala is the official Apache git server. Gerrit serves as a staging ground for reviewing patches, and once a patch is approved, a sort of waiting room while patches wait for a committer to officially move them to the Apache git repo.

Sending a patch to Gerrit is exactly the same as 'pushing' a sequence of commits to a remote Git repository:

git push --no-thin asf-gerrit HEAD:refs/for/master

To set up Gerrit as a git remote for your Impala repository, do the following:

cd ${IMPALA_HOME}
git remote add asf-gerrit ssh://<your-github-username>@gerrit.cloudera.org:29418/Impala-ASF

If using HTTP to connect to Gerrit, use the following command to add the Gerrit remote is also works:

git remote add asf-gerrit http://<your-github-username>@gerrit.cloudera.org:8080/a/Impala-ASF

Pay attention, when using HTTP to push/pull code, we need to input username/password. Do not use login account directly, we should get username/password from "Settings→HTTP Password" instead.

Sending a patch for review

Install the Gerrit pre-commit hook

Gerrit keeps track of different patches by generating a unique Change-Id that is constant over all iterations of a patch (obviously the Git hash won't do, since that changes every time the patch changes). That Change-Id is appended to every commit message:

IMPALA-1726: Move JNI / Thrift utilities to separate header

To avoid pulling in jni.h in the codegen module (where it does not
compile), this patch moves the methods that depend on both Thrift and
JNI to a separate header where it can be included more precisely.

Change-Id: I4d97f1816b24149d9163dce59f31b2afe2642b11

Gerrit provides a "pre-commit hook" that automates Change-Id generation. Installing it is very easy:

cd ${IMPALA_HOME}
curl -o .git/hooks/commit-msg https://gerrit.cloudera.org/tools/hooks/commit-msg
chmod u+x .git/hooks/commit-msg

After the hook is installed, every commit you make will get its own Change-Id. Without it, Gerrit will reject all patches.

Rebase Your Branch

Commit all outstanding changes:

cd $IMPALA_HOME
git add --update
git commit

Rebase against asf-gerrit/master (good practice to catch conflicts introduced by recent commits)

git fetch asf-gerrit
git rebase asf-gerrit/master

You might have to resolve merge conflicts during 'git rebase'. Follow the instructions, then continue with

git rebase --continue

Squash everything down to a single commit:

git rebase -i asf-gerrit/master

Make sure to pick only the first commit and 's' (squash) following lines, then edit the commit message into something coherent. Run the git log command to see that you're exactly one commit ahead of asf-gerrit/master

branch-master() {
  git fetch asf-gerrit && git checkout -b $1 asf-gerrit/master
}
alias "rebase-master"="git fetch asf-gerrit && git rebase -i asf-gerrit/master"

Sending a patch for review

To send a patch for review, you should push it to a Gerrit-specific 'branch' on the Gerrit remote. That branch has the form refs/for/<branch-name>. That patch will then be visible to code reviewers, who will review your code before it becomes part of Impala. If you want to see what a patch looks like without making it visible to code reviewers, push to the branch refs/drafts/<branch-name>. For the vast majority of patches the target branch will be master, which is where all mainline development happens. During releases, release managers may push commits to a specific release branch, however most new development will happen on master.

In the example below, we're pushing HEAD to be a draft change. Gerrit provides a URL for the review.

$ git push asf-gerrit HEAD:refs/drafts/master
Counting objects: 3, done.
Delta compression using up to 8 threads.
Compressing objects: 100% (3/3), done.
Writing objects: 100% (3/3), 332 bytes | 0 bytes/s, done.
Total 3 (delta 2), reused 0 (delta 0)
remote: Resolving deltas: 100% (2/2)
remote: Processing changes: new: 1, refs: 1, done
remote:
remote: New Changes:
remote:   http://gerrit.cloudera.org:8080/8335 test [DRAFT]
remote:
To ssh://XXXX@gerrit.cloudera.org:29418/Impala-ASF

Visit the URL that Gerrit provided to review the diff before sending it out to reviewers. Check that you've uploaded the right commit and that you've included what you wanted in the diff. When you've reviewed it yourself, hit the "Publish" button () to send the review to the community. This will make the review visible, but it won't be an official part of the Impala code base yet.

Note that Gerrit will send all patches between the HEAD of the target branch and your current branch's HEAD for review. Please therefore make certain that you are only pushing the commits you want reviewed: every separate commit translates to an e-mail in everyone's inbox!

Verifying a patch (open to all Impala contributors)

The following Jenkins jobs are available to all Impala contributors for testing patches they submit to Gerrit.

1. pre-review-test: Use this to compile Impala and run, if desired, tests or test subsets. bin/run-all-tests.sh in the source tree is the best place to understand the meanings to the options of this job.
2. gerrit-verify-dryrun-external (new September 2017): Use this to run the full set of verifications, including multiple compilations of Impala using various build tools, and execution of all backend tests, frontend tests, core end-to-end tests, and Apache RAT verification. This is a wrapper around what a committer will ultimately run for you to verify and submit your patch (gerrit-verify-dryrun).

Neither of these jobs write a comment or Verified bit or attempt to Submit a Gerrit patch set. A submitter ought to monitor his or her job as it progresses. A submitter is encouraged to manually link a successful job in a Gerrit review comment as demonstration of success. When testing a draft, the "Impala Public Jenkins" user needs to be added to the change, so that Jenkins can access it.

Verifying and submitting a patch (Impala committers only)

Once a change has a +2 from a committer and no -1 from any committer, it is ready to be verified and merged to gerrit. Remember that the source of truth for what is in Impala is the official Apache git server. Gerrit serves as a staging ground for reviewing patches, and once a patch is approved, a sort of waiting room while patches wait for a committer to officially move them to the Apache git repo.

To verify a patch, we use one of two different automated processes. Both are restricted to use by committers. If you are not a committer and your patch has been +2ed, please ask a committer to run the verification for you.

First, log in to https://jenkins.impala.io. If you are a committer and forgot your password, ask dev@impala.apache.org for help. Someone will reset your password.

Then, if your commit changes only files in the docs/ folder:

1. Go to https://jenkins.impala.io/job/gerrit-docs-submit/build?delay=0sec. Put your gerrit patch URL in the PATCH_URL field. Leave the other fields with their default values. Press "Build".
2. If all goes well, you will see messages like "Build started: https://jenkins.impala.io/job/gerrit-docs-submit/4/" and "Verified+1" and "Change has been successfully cherry-picked as 84ee40428dbb18146760adec8c5a03559f527ddb by Impala Public Jenkins" on your patch.
3. If something breaks, go to the URL Jenkins posted to your patch. It will look something like this: https://jenkins.impala.io/job/gerrit-docs-submit/10/. Click on the "Build Artifact" and "Console Output" to look at the logs and figure out what went wrong.

If your commit doesn't change any files in the docs/ folder: 

1. Go to https://jenkins.impala.io/job/gerrit-verify-dryrun/build?delay=0sec. Put your gerrit patch URL in the PATCH_URL field. Leave the other fields with their default values. Press "Build".
2. If all goes well, you will see messages like "Build started: https://jenkins.impala.io/job/gerrit-verify-dryrun/151/" and "Verified+1" and "Change has been successfully cherry-picked as 84ee40428dbb18146760adec8c5a03559f527ddb by Impala Public Jenkins" on your patch.
3. If something breaks, go to the URL Jenkins posted to your patch. It will look something like this: https://jenkins.impala.io/view/Gerrit/job/gerrit-verify-dryrun/141/. Click on the "Build Artifact" and "Console Output" to look at the logs and figure out what went wrong.

If your commit touches files both in and out of the docs/ folder, run https://jenkins.impala.io/job/gerrit-verify-dryrun/build?delay=0sec first, following the directions above. Once it succeeds, run https://jenkins.impala.io/job/gerrit-docs-submit/build?delay=0sec.

When Jenkins has completed successfully, your change is in the master gerrit repo, but not yet in the official Apache repo. Apache does not allow non-sentient robots like Jenkins to submit patches. To get your patch to the official Apache repo, a committer needs to push the commit to the Apache git repository, by running:

bin/push_to_asf.py

Creating a branch (Impala committers only) 

According to project bylaws, branch creation is by lazy consensus of the PMC. We recommend that you email the dev mailing list to let people know about your branch so they understand its purpose.

The steps to create a branch are:

git fetch asf-gerrit
git checkout -b the-new-branch asf-gerrit/the-new-branch
git push apache HEAD:the-new-branch

Git and Gerrit etiquette

Contribution guidelines

Before you send a patch, please make sure that you've read the contribution guidelines.

Squash your commits

As described above, Gerrit generates one review request per new commit. It's much easier to review everything related to a patch together in one single commit, so please make sure to squash all your related commits together before sending. If you don't, your review requests will probably get forcibly removed

Other Useful Tools and Information

Searching for all Impala reviews

You might want to bookmark this link, which shows all open reviews for Impala.

git-review (Python Gerrit helper module)

git-review is a Python module that can simplify publishing gerrit reviews. To install it use:

sudo pip install git-review

By default, git-review will look for a remote named 'gerrit'. The usage is:

# Send a code review (or update an existing review). This will also do a git rebase -i asf-gerrit/master.
git review -r asf-gerrit

git-review can be used to apply changes from a review locally and compare patch sets. For a full set of options see: https://pypi.python.org/pypi/git-review

Gerrit documentation

The documentation for Gerrit is here:

http://gerrit.cloudera.org/Documentation/index.html