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neels, 03/20/2017 01:24 AM


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 https://www.gerritcodereview.com/

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 https://gerrit.osmocom.org/login/. If you have an Osmocom Redmine account you can use https://osmocom.org/openid as OpenID provider.

careful: enter 'https' to ensure that your openid credentials are passed on encryptedly.
pitfall: if you're logged in on 'projects.osmocom.org', you should also use the openid provider:

https://projects.osmocom.org/openid

note: gerrit will create a distinct user for each openid URL you pass. If you logged in successfully but your user seems to have lost permissions, you may have created an evil twin user: contact us on the mailing list so we can fix it in the user database.

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 git.osmocom.org and add the gerrit remote, git will have two remotes,
so when you first checkout a branch you have to supply the remote explicitly (cumbersome).
The gerrit repositories and git.osmocom.org 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://$USERNAME@gerrit.osmocom.org:29418/$PROJECT.git
scp -P 29418 $USERNAME@gerrit.osmocom.org:hooks/commit-msg $PROJECT/.git/hooks/

SSH config

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

Host go
Hostname gerrit.osmocom.org
Port 29418
User $USERNAME

('go' means gerrit.osmocom, replace 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://go/$PROJECT.git
cd $PROJECT
scp go:hooks/commit-msg .git/hooks/

Committer must match

Your email address on gerrit and the email address git places in your
commits must match, or you will get rejected with an error message like
"invalid commiter". You can add email addresses on the gerrit web UI.

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
USERNAME=gerrit_user_name
PROJECT=$(basename $PWD)
git remote add gerrit ssh://$USERNAME@gerrit.osmocom.org:29418/$PROJECT.git
scp -P 29418 $USERNAME@gerrit.osmocom.org:hooks/commit-msg .git/hooks/

Push for review

Checkout the revision or branch that you want to submit for review, then

git push gerrit HEAD:refs/for/master

You can optionally add a topic name with

git push gerrit HEAD:refs/for/master/my_topic

Merge patch to master

A patch can be merged when it has CR+2 and V+1 votes, and if, in case of a
series of patches pushed from a branch, when its ancestor patches can also be
merged.

Sometimes the reviewer that gives CR+2 also hits the "Submit" button right away
to merge the patch to master. Sometimes it is left up to the owner of the patch
to decide when to hit "Submit" (who needs to be in the "Known Users" group).

The V+1 vote means "build is verified" and is usually given by our jenkins
gerrit builds: https://jenkins.osmocom.org/jenkins/view/Jenkins-Gerrit/

The CR+2 vote means "code reviewed and ready for merge to master branch".
Accounts with the "Reviewer" role for a given project are allowed to give CR+2
votes. Others are allowed to give CR+1 (and CR-1). CR votes don't add up.

Sometimes hitting the "Submit" button results in an error message saying
"Change is New", which is a bug related to a private branch with the same
patches being present. Can be fixed e.g. by an admin's manual push to master.

Push a "private" user branch

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

If your local branch name is of the form 'your_name/topic', you can just

git push

and git will tell you what to do.

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

git push gerrit HEAD:refs/heads/user/$USERNAME/branch_name

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 then send patch(es) back to gerrit while removing temporary branch automatically with "git review -f".

Fetch a patch from gerrit

This script (I called it P) makes fetching a patch set from gerrit a breeze:

#!/bin/sh
# fetch gerrit patch into new branch named like the patch number.
#
# Usage: go to a git clone and pass a patch number:
#
#   cd openbsc
#   P 973
# or
#   P 973/2
#
# Will create new local branches '973_4' (if 4 is the latest patch set)
# or '973_2', respectively.

patch="$1" 

if [ -z "$patch" ]; then
  echo "Usage: P 1234[/5]" 
  exit 1
fi

if [ -z "$(echo "$patch" | grep '/')" ]; then
  patch="/$patch/" 
fi

if [ -z "$(echo "$patch" | grep '^/')" ]; then
  patch="/$patch" 
fi

last_set="$(git ls-remote origin "changes/*" | grep "$patch" | sed 's#.*/\([^/]*\)$#\1 &#' | sort -n | tail -n 1)" 
if [ -z "$last_set" ]; then
  echo "Not found: $patch" 
  exit 1
fi

change_name="$(echo "$last_set" | sed 's/.*\(refs.*\)/\1/')" 
branch_name="$(echo "$change_name" | sed 's#refs/changes/../\([0-9]*\)/\([0-9]*\)#\1_\2#')" 

set -x
git fetch origin "$change_name" 
git co -b "$branch_name" FETCH_HEAD

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. You can re-submit changes to
patches on your branch by pushing in the same way that you first submitted the branch.

Note: if you modify the Change-Ids in the commit logs, your push 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.

(There used to be a bug in gerrit that required editing the first patch to be able to
re-submit a branch, but that's fixed.)

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.

502 Bad Gateway

When getting a "Bad Gateway" error message upon trying to login on gerrit, you probably just need to restart your web browser. The reason is not clear.

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:

https://bugs.chromium.org/p/gerrit/issues/detail?id=4158

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.

Admin

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

(see ~/.ssh/config above for the 'go' shortcut)

This seems to be the MySQL dialect.

Fix evil twin users

If differing openid URLs have lead to evil twin users shadowing the same email address just without the permissions, you can fix it like this:

ssh go "gerrit gsql --format PRETTY -c \"select * from account_external_ids where email_address like '%foo%'\"" 
# ACCOUNT_ID | EMAIL_ADDRESS   | PASSWORD | EXTERNAL_ID
# -----------+-----------------+----------+----------------------------------
# 100004     | foo@example.com | NULL     | https://osmocom.org/openid/user/777
# 100021     | foo@example.com | NULL     | https://projects.osmocom.org/openid/user/777

ssh go "gerrit gsql --format PRETTY -c \"update account_external_ids set account_id = 100004 where email_address like '%foo%'\"" 

ssh go "gerrit gsql --format PRETTY -c \"select * from account_external_ids where email_address like '%foo%'\"" 
# ACCOUNT_ID | EMAIL_ADDRESS   | PASSWORD | EXTERNAL_ID
# -----------+-----------------+----------+----------------------------------
# 100004     | foo@example.com | NULL     | https://osmocom.org/openid/user/777
# 100004     | foo@example.com | NULL     | https://projects.osmocom.org/openid/user/777