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Gerrit » History » Version 14

Version 13 (neels, 06/17/2016 03:41 PM) → Version 14/112 (neels, 06/17/2016 03:55 PM)

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>

h2. Tips and Tricks

If you need to adjust and re-submit patches the easiest way is to create throw-away branch ("R D" in magit-gerrit in emacs for example), make you 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 issues with those, which boil down to these rules:

1. Do not push your branch "privately" to the gerrit repository. You can, but Gerrit will force
you to remove all Change-Ids from commit logs once you want to submit it to master.

2. 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 just a cosmetic change).

How to edit commit logs:

<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. Why is this is necessary?

There are different merge strategies that Gerrit performs to accept your 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 are in general three problems with "Rebase if Necessary", with different solutions:

h4. 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

h4. 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.

h4. Private Branches

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/users/me/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.

IMHO this is a very clumsy non-feature of Gerrit. It enforces that a Change-Id
must be unique across all branches, so that each submission for review is
separate for each branch. Instead, it should 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.

Solutions:

Currently the best solution is to *not* push private branches to the gerrit
repository. This makes collaborating publicly on branches currently impossible.
Since our git.osmocom.org now are non-pushable, the only openly public place
would be the gerrit repos; intead, you need a separate, "private" git repository
to collaborate. IMHO we must still find a good solution to this problem.

If you have already pushed your Change-Ids to refs/heads/* somewhere, the only
way to submit to master is to, yes, edit every single commit log message on
your branch to remove the Change-Id, to let the commit hook re-add a new one.
Of course you can't push your modified Change-Ids back to the private branch on
the gerrit repos, because then Gerrit will have the same problem all over.
That means you can't easily edit branch commits and keep them public until they
are through review, because between pushes to your private branch and pushes for
review, you have to switch the Change-Id for the proper branch. Note: If you remove
and re-create the Change-Ids over and over, you will create new review items instead
of amending previous patch submissions with fixes.

A third solution might be to not have Change-Ids on your private branch, but that
means that once you want to submit for review, you also need to edit every single
commit log message on your branch, and then you're again stuck with the same problem.

My wish would be to teach Gerrit to ignore all of our private branches
and simply not look for Change-Id tags in those, so we can push them for review
while also keeping them on a private branch at the same time.

An easier way might be to have a public collaboration repository, e.g. to allow
pushing to git.osmocom.org repositories again and only handle for/master reviews
via the Gerrit repos. This won't work if the gerrit repos fetches upstream branches,
so we might need yet another place for repositories -- we already have git.osmocom.org
and gerrit, so it would be confusing to have even a third set of public repositories.

Let's hope to find nicer solutions...

h2. Project Config Details

There are more non-obvious configuration items with Gerrit.

h3. 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.

h3. Create a new change for every commit not in the target branch

If this is set to FALSE, Gerrit makes re-submitting a branch with fixes impossible.
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