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neels, 07/12/2016 12:59 PM

Contributing using Gerrit

At OsmoDevCon2016 we discussed problems with our past contribution / patch submission process using mails on the mailing list as well as patchwork. The result is that we want to give Gerrit a try for some time and see if it helps us to have a better process

Gerrit is a review tool that integrates nicely with git and ssh. You can find general information about Gerrit at

The advantages of Gerrit are:
  • patch submission status is automatically tracked, also with several revisions for a patch set.
  • patches are build-tested (and possibly even further tested) by jenkins before they are applied
  • developers + maintainers can formally vote on a patch (developer: -1/0/+1, maintainer: -2/0/+2)
  • once a patch has +2 score, it can be (automatically) merged into master
  • patch sumissions not via git send-email but direcly from git

Osmocom Subprojects using Gerrit

The following projects use Gerrit to contribute changes:

  • libosmocore.git
  • libosmo-abis.git
  • libosmo-netif.git
  • libosmo-sccp.git
  • libsmpp34.git
  • openbsc.git
  • osmo-bts.git
  • osmo-iuh.git
  • osmo-pcu.git
  • cellmgr-ng.git
  • osmo-sip-connector.git

Configuring Gerrit/Account

You will need to sign-up at If you have an Osmocom Redmine account you can use as OpenID provider. If you have no Osmocom redmine account, you can simply create one online at the "Register" link in the upper right corner.

Even without an existing or new redmine account, you should also be able to use any other OpenID provider to authenticate against gerrit (untested).

After the initial sign-up you will need to:

  • Pick a username (can not be changed)
  • Add your public ssh key(s)
  • Add email addresses you intend to use as author/comitter

If you would like to push private branches to the Gerrit repository, you also need to be added to the "known users" group.
Please send a short requesting email to .

Setting up Gerrit for commits and pushing

Note: it is easiest to work with gerrit when gerrit is the only remote in your git clone.
When you clone from and add the gerrit remote, git will have two remotes,
so when you checkout a new branch you have to supply the remote explicitly (cumbersome).
The gerrit repositories and are constantly synced, so it is sufficient
to clone from gerrit only.

Simplest: new clone

  • Create a new clone from gerrit
  • Fetch the commit hook that adds Change-Id to each commit to uniquely identify a commit
    git clone ssh://$$PROJECT.git
    scp -P 29418 $ $PROJECT/.git/hooks/

SSH config

In '~/.ssh/config', add these lines:

Host foo
Port 29418

(replace 'foo' with your favorite shortcut name,
replace '$USERNAME' with your user name as used on the gerrit website)

Then you can shorten above commands to

git clone ssh://foo/$PROJECT.git
scp foo:hooks/commit-msg $PROJECT/.git/hooks/

Add gerrit to an existing clone

  • Add the remote to be able to fetch and push to gerrit
  • Fetch the commit hook that adds Change-Id to each commit to uniquely identify a commit
PROJECT=$(basename $PWD)
git remote add gerrit ssh://$$PROJECT.git
scp -P 29418 $ .git/hooks/
  • In case your local username matches the gerrit username, the setup shortens to
PROJECT=$(basename $PWD)
git remote add gerrit ssh://$PROJECT.git
scp -P 29418 .git/hooks/

Push for review

git push gerrit HEAD:refs/for/master

Push a "private" user branch

Note that you must be a member of the "known users" group, see above.

If gerrit is your only remote, and if your local branch name is of the
form 'yourname/topic', you can just

git push

(and git will tell you what to do)

If you have other remotes as well, do

git push gerrit

To push with a "nonstandard" local branch name, do

git push gerrit HEAD:refs/heads/$USERNAME/topic

List changesets in gerrit
git ls-remote gerrit changes/*

Tips and Tricks

Throw-away branch

If you need to adjust and re-submit patches, it may be handy to create a throw-away branch ("R D" in magit-gerrit in emacs for example),
make your changes/amendments and than send patch(es) back to gerrit while removing temporary branch automatically with "git review -f".

Re-submit a Branch with Amended Commits

On a feature branch, one typically has numerous commits that depend on their preceding commits.
Often, some of the branch commits need to be amended for fixes. But, Gerrit will refuse your branch
re-submission if the first branch commit is unchanged.

To re-submit a branch, make sure to cosmetically tweak the branch's first commit log message
before each re-submission (keep the Change-Id, really make just a cosmetic change).

git rebase -i master
# replace the first line's 'pick' with 'r' (or 'reword'), exit editor
# git presents you with commit log message, make any tiny modification.

The cause: Gerrit refuses to accept a commit with a Change-Id that it already knows and
where the commit hash is identical.

If you just cosmetically tweak the first commit's log message, the commit hash
is changed. Since the following commits contain their predecessor's commit hash, now
all of the branch's commit hashes are modified, and gerrit happily accepts them as a
new patch set. It will still pick up the Change-Ids (which you shouldn't edit) and
notice if commits have remained identical (keeping the votes). But with the minor
commit log tweak, it will no longer thwart your re-submission with an error message.

Note: you could modify all the Change-Ids, but now your branch submission would
open entirely new review entries and you would have to abandon your previous submission.
Comments on the first submission are lost and you cannot diff between patch sets.

Re-submit Previously Abandoned Changes

You have to edit the Change-Ids, on a branch that would be every single commit log message.

cd openbsc
git co my-branch
git rebase -i master
# replace all 'pick' with 'r' (or 'reword'), exit your editor
# git presents each commit log message for editing

Submit a "private" branch for master

If you've pushed a branch to refs/heads/* somewhere, gerrit will already know the Change-Ids on it.
Make sure the option Create a new change for every commit not in the target branch is TRUE for your project,
or gerrit will refuse to accept your submission.

Reasons for Particular Configuration

Rebase if necessary

There are different merge strategies that Gerrit performs to accept patches.
Each project can be configured to a specific merge strategy, but unfortunately you can't
decide on a strategy per patch submission.

It seems that the "Merge if Necessary" strategy is best supported, but it creates non-linear
history with numerous merge commits that are usually not at all necessary.

Instead, the "Cherry Pick" strategy puts each patch onto current master's HEAD to create
linear history. However, this will cause merge failures as soon as one patch depends on
another submitted patch, as typical for a feature branch submission.

So we prefer the "Rebase if Necessary" strategy, which always tries to apply your patches to
the current master HEAD, in sequence with the previous patches on the same branch.
However, some problems still remain, including some bugs in "Rebase if Necessary".

There's a problem with "Rebase if Necessary": If your branch sits at master's HEAD, Gerrit
refuses to accept the submission, because it thinks that no new changes are submitted.
This is a bug in Gerrit, which holger has fixed manually in our Gerrit installation:

Private Branches: Create a new change for every commit...

Say you have an extensive feature in development, and you want to keep it on the
upstream git repository to a) keep it safe and b) collaborate with other devs on it.
So, of course, you have regularly pushed to refs/heads/yoyodyne/feature.

Since you have the gerrit commit hook installed, your feature branch already has
Change-Id tags in all commit log messages.

Now your feature is complete and you would like to submit it to master.
Alas, Gerrit refuses to accept your patch submission for master, because it
knows the Change-Ids are also on a different branch.

Gerrit by default enforces that a Change-Id must be unique across all branches,
so that each submission for review is separate for each branch. Instead, we
want to handle Change-Ids per-branch, so that you can have the same change
submitted to different branches, as separate patch submissions, without having
to cosmetically adjust the Change-Id.

Solution: set the option
Create a new change for every commit not in the target branch to TRUE

Allow content merges

By default, gerrit compares patches only by the files' paths. If two paths are the same,
it immediately shows them as conflicts (path conflicts).

In software development, a conflict usually means an actual content conflict, so if the
edits are in two entirely separate places in the file, we don't consider this a conflict.

By setting Allow content merges to TRUE in the git project config, we tell Gerrit to
perform text merges of the submitted patches and only complain about actual content
conflicts, in the usual software engineering sense.


Adding users to groups

Normally, the gerrit UI auto-completes a user name in the edit field. It has happened
though that an existing user is not auto-completed, as if it didn't exist. In that case,
find out the user ID (seven digit number like 1000123) and just enter that.

The user ID can be found on the user's "Settings" page, or in the database (s.b.).

Querying the database directly

If your user has permission to access the database, you can place SQL queries using the
'gerrit gsql' commands over ssh:

ssh -p 29418 $ 'gerrit gsql --format PRETTY -c "show tables"'
ssh -p 29418 $ 'gerrit gsql --format PRETTY -c "select full_name,account_id from accounts"'

This seems to be the MySQL dialect.

Updated by neels over 6 years ago · 35 revisions

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