Cellular Network Infrastructure

h1. Contributing using Gerrit

{{>toc}}

At [[OpenBSC: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

h2. 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

h2. 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. 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

h2. Setting up Gerrit for commits and pushing

* 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

<pre>

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/

</pre>

* In case your local username matches the gerrit username, the setup shortens to

<pre>

PROJECT=$(basename $PWD)

git remote add gerrit ssh://gerrit.osmocom.org:29418/$PROJECT.git

scp -P 29418 gerrit.osmocom.org:hooks/commit-msg .git/hooks/

</pre>

Then

* Push for review

<pre>

git push gerrit HEAD:refs/for/master

</pre>

* Push a user branch

<pre>

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

</pre>

* Directly push to master if you are allowed to

<pre>

git push gerrit HEAD:refs/heads/master

</pre>

* List changesets in gerrit

<pre>

git ls-remote gerrit changes/*

</pre>

h1. Tips and Tricks

h2. 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".

h2. Submitting Branches

On a feature branch, one typically has numerous commits that depend on their preceding commits.

Unfortunately, Gerrit has some pitfalls with those. Here are solutions and details:

h3. Re-submit a Branch with Amended Commits

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).

<pre>

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.

</pre>

(For a detailed explanation, see below under [[Gerrit#Amending-a-Submitted-Branch-First-Commit-Unchanged|Amending a Submitted Branch, First Commit Unchanged]])

h3. Re-submit Previously Abandoned Changes

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

<pre>

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

</pre>

h3. 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 [[Gerrit#Private-Branches-Create-a-new-change-for-every-commit|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.

h1. Reasons for Particular Configuration

h2. 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".

These are problems with "Rebase if Necessary", with different solutions:

h3. Fast-Forward

If your branch sits at master's HEAD, Gerrit may refuse 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

h3. Amending a Submitted Branch, First Commit Unchanged

Say you have submitted a branch with 5 commits, and the third commit has gotten a -1 vote.

You have fixed the problem in your local git and now would like to re-submit it. Of course

you want to keep the Change-Ids identical so that Gerrit shows them as amends.

Gerrit will refuse to accept your patch submission if the first commit on the branch is

still identical.

The quickest work-around: slightly reword the first branch commit's commit log message.

Yes, that's enough! All commits that are identical on the branch are now ignored,

it's only the first branch commit that Gerrit will complain about.

The cause: Gerrit refuses to accept a commit with a Change-Id that it already knows and

where the commit hash is identical.

Some details: 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.

Now, 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.

h2. 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_

h2. 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.