infra-manual/doc/source/developers.rst

:title: Developer's Guide

.. _developer_manual:

Developer's Guide
#################

The goal of this document is to walk you through the concepts and
specifics that should be understood while contributing to projects
hosted in the OpenDev infrastructure.

Accounts creation
=================

The steps necessary to create a Gerrit account are described in
:ref:`getting_started`. Only extra recommended steps will be described here.

IRC Account
-----------

While development on OpenDev only requires an account on the OpenDev
Gerrit Code Review System, **effective** development in OpenDev hosted
projects often requires interacting with other developers in IRC
channels on OFTC. It is recommended to start by getting set up on
IRC so that one can ask questions if one encounters issues with other
phases of account setup.

If you do not know how to connect to OFTC, the `Connecting to OFTC`_
document will help. If you're going to be interacting with others
through OFTC on a regular basis, it's also recommended you `Register
your IRC Nick`_ since it's how they'll know you're you. This will
allow you to reclaim your name later if someone starts using it
while you're not connected, or even (with some additional settings)
preventing anyone else from using it at all.

You can find the OpenDev community in the ``#opendev`` IRC channel on OFTC.

For further information about the use of IRC in OpenStack, see
:ref:`irc-guide`.

.. _Connecting to OFTC: https://www.oftc.net/
.. _Register your IRC Nick: https://www.oftc.net/Services/#register-your-account


Extra Gerrit account setup steps
--------------------------------

.. _openstack-individual-contributor-license-agreement:

OpenStack Individual Contributor License Agreement
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Projects that are part of OpenStack project require signing the
Individual Contributor License Agreement, see `these detailed
instructions
<https://docs.openstack.org/contributors/common/setup-gerrit.html#individual-contributor-license-agreement>`_.

.. _gerrit-ssh-host-keys:

Gerrit SSH Host Keys
^^^^^^^^^^^^^^^^^^^^

Careful users may wish to verify the SSH Host key fingerprints for the
Gerrit service the first time they connect from a new system. Depending on
which key types your client is configured to negotiate, you may see some or
all of these listed::

   256 SHA256:/aPoKpg+804wdezs21L9djZ4bOsLudpGF7m7779XVuQ [review.opendev.org]:29418 (ECDSA)
   2048 SHA256:RXNl/GKyDaKiIQ93BoDvrNSKUPFvA1PNeAO9QiirYZU [review.opendev.org]:29418 (RSA)
   256 SHA256:lHsyuBxtcAiZeJM+viHllq52he9JNPqg8FFKv5+/BJ8 [review.opendev.org]:29418 (ED25519)

Accessing Gerrit over HTTPS
^^^^^^^^^^^^^^^^^^^^^^^^^^^

Git-review normally communicates with Gerrit using SSH over port 29418 with
no further configuration needed. However, if you suspect that ssh
over non-standards ports might be blocked (or you need to access the web
using https) then you can configure git-review to use an https endpoint
instead of ssh. Keep in mind that you will need to generate an
`HTTP password in Gerrit
<https://review.opendev.org/#/settings/http-password>`_ to use this
connection. You should run the following command before "git review -s"::

  git remote add gerrit https://<username>@review.opendev.org/<umbrella repository name>/<repository name>.git

In case you had already tried to setup git-review and it failed, it might
be necessary to remove the Gerrit remote from git::

  git remote rm gerrit

Development Workflow
====================

Working on Bugs
---------------

Bug reports for a project are generally tracked on Launchpad at
https://bugs.launchpad.net/<projectname>, or on StoryBoard (
https://storyboard.openstack.org). Contributors may review these
reports regularly when looking for work to complete.

There are 4 key tasks with regards to bugs that anyone can do:

#. Confirm new bugs: When a bug is filed, it is set to the "New" status.
   A "New" bug can be marked "Confirmed" once it has been reproduced
   and is thus confirmed as genuine.
#. Solve inconsistencies: Make sure bugs are Confirmed, and if assigned
   that they are marked "In Progress"
#. Review incomplete bugs: See if information that caused them to be marked
   "Incomplete" has been provided, determine if more information is required
   and provide reminders to the bug reporter if they haven't responded after
   2-4 weeks.
#. Review stale In Progress bugs: Work with assignee of bugs to determine
   if the bug is still being worked on, if not, unassign them and mark them
   back to Confirmed or Triaged.

Learn more about working with bugs for various projects at:

https://wiki.openstack.org/wiki/BugTriage

Bug statuses are documented here:

https://docs.openstack.org/project-team-guide/bugs.html

If you find a bug that you wish to work on, you may assign it to yourself.
When you upload a review, include the bug in the commit message for
automatic updates back to Launchpad or StoryBoard. The following options
are available for Launchpad::

  Closes-Bug: #######
  Partial-Bug: #######
  Related-Bug: #######

and for StoryBoard::

  Task: ######
  Story: ######

Mentioning the story will create a handy link to the story from gerrit,
and link to the gerrit patch in StoryBoard.
Mentioning the task will change the task status in StoryBoard to
'review' while the patch is in review, and then 'merged' once the
patch is merged. When all tasks in a story are marked 'merged',
the story will automatically change status from 'active' to 'merged'.
If the patch is abandoned, the task status will change back to 'todo'.
It's currently best to note both story and task so that the task status
will update and people will be able to find the related story.

Also see the `Including external references
<https://wiki.openstack.org/wiki/GitCommitMessages#Including_external_references>`_
section of the OpenStack Git Commit Good Practices wiki page.

Working on Specifications and Blueprints
----------------------------------------

Many OpenStack project teams have a <projectteam>-specs repository which
is used to hold approved design specifications for additions and changes to
the project team's code repositories.

The layout of the repository will typically be something like::

  specs/<release>/

It may also have subdirectories to make clear which specifications are approved
and which have already been implemented::

  specs/<release>/approved
  specs/<release>/implemented

You can typically find an example spec in ``specs/template.rst``.

Check the repository for the project team you're working on for specifics
about repository organization.

Specifications are proposed for a given release by adding them to the
``specs/<release>`` directory and posting it for review.  The implementation
status of a blueprint for a given release can be found by looking at the
blueprint in Launchpad.  Not all approved blueprints will get fully implemented.

Specifications have to be re-proposed for every release.  The review may be
quick, but even if something was previously approved, it should be re-reviewed
to make sure it still makes sense as written.

Historically, Launchpad blueprints were used to track the implementation of
these significant features and changes in OpenStack. For many project teams,
these Launchpad blueprints are still used for tracking the current
status of a specification. For more information, see `the Blueprints wiki page
<https://wiki.openstack.org/wiki/Blueprints>`_.

View all approved project team's specifications at
https://specs.openstack.org/.

Starting a change
-----------------

The :ref:`getting_started` page explains how to originally clone and prepare
a git repository. This only has to be done once, as you can reuse the cloned
repository for multiple changes.

Before creating your topic branch, just make sure you have the latest
upstream changes::

  git remote update
  git checkout master
  git pull --ff-only origin master


Git branch names
----------------

You may pick any name for your git branch names. By default, it will
be reused as the topic for your change in Gerrit::

  git checkout -b TOPIC-BRANCH

Best practices recommend, if you are working on a specific blueprint, to
name your topic branch ``bp/BLUEPRINT`` where BLUEPRINT is
the name of a blueprint in Launchpad (for example,
``bp/authentication``).  The general convention when working on bugs
is to name the branch ``bug/BUG-NUMBER`` (for example,
``bug/1234567``).

If you want to use a different gerrit topic name from the git branch name,
you can use the following command to submit your change::

  git review -t TOPIC


Committing changes
------------------

`Git commit messages
<https://wiki.openstack.org/wiki/GitCommitMessages>`_ should start
with a short 50 character or less summary in a single paragraph.  The
following paragraph(s) should explain the change in more detail.

If your changes addresses a blueprint or a bug, be sure to mention them in the commit message using the following syntax::

  Implements: blueprint BLUEPRINT
  Closes-Bug: ####### (Partial-Bug or Related-Bug are options)

For example::

  Adds keystone support

  ...Long multiline description of the change...

  Implements: blueprint authentication
  Closes-Bug: #123456
  Change-Id: I4946a16d27f712ae2adf8441ce78e6c0bb0bb657

Note that in most cases the Change-Id line should be automatically
added by a Gerrit commit hook installed by git-review.  If you already
made the commit and the Change-Id was not added, do the Gerrit setup
step and run: ``git commit --amend``. The commit hook will
automatically add the Change-Id when you finish amending the commit
message, even if you don't actually make any changes. Do not change
the Change-Id when amending a change as that will confuse Gerrit.

Make your changes, commit them, and submit them for review::

  git commit -a

.. Note:: Do not check in changes on your master branch.  Doing so will
    cause merge commits when you pull new upstream changes, and merge
    commits will not be accepted by Gerrit.

Using Signed-off-by
-------------------

Projects may require the use of a ``Signed-off-by``, and even if they
do not, you are welcome to include ``Signed-off-by`` in your commits.
By doing so, you are certifying that the following is true::

        Developer's Certificate of Origin 1.1

        By making a contribution to this project, I certify that:

        (a) The contribution was created in whole or in part by me and I
            have the right to submit it under the open source license
            indicated in the file; or

        (b) The contribution is based upon previous work that, to the best
            of my knowledge, is covered under an appropriate open source
            license and I have the right under that license to submit that
            work with modifications, whether created in whole or in part
            by me, under the same open source license (unless I am
            permitted to submit under a different license), as indicated
            in the file; or

        (c) The contribution was provided directly to me by some other
            person who certified (a), (b) or (c) and I have not modified
            it.

        (d) I understand and agree that this project and the contribution
            are public and that a record of the contribution (including all
            personal information I submit with it, including my sign-off) is
            maintained indefinitely and may be redistributed consistent with
            this project or the open source license(s) involved.

A ``Signed-off-by`` header takes the following form in a commit message::

    Signed-off-by: Full Name <email@example.com>

If you add the ``-s`` option to ``git commit``, this header will be added
automatically::

    git commit -s

Running Unit Tests
------------------

Before submitting your change, you should test it. To learn how to run
python based unit tests in OpenStack projects see
`Running Python Unit Tests`_

Previewing a Change
-------------------

Before submitting your change, you should make sure that your change
does not contain the files or lines you do not explicitly change::

  git show

Submitting a Change for Review
------------------------------

Once you have committed a change to your local repository, all you
need to do to send it to Gerrit for code review is run::

  git review

When that completes, automated tests will run on your change and other
developers will peer review it.

Updating a Change
-----------------
If the code review process suggests additional changes, make and amend
the changes to the existing commit. Leave the existing Change-Id:
footer in the commit message as-is. Gerrit knows that this is an
updated patchset for an existing change::

  git commit -a --amend
  git review

Understanding Changes and Patch Sets
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

It's important to understand how Gerrit handles changes and patch
sets. Gerrit combines the Change-Id in the commit message, the
project, and the target branch to uniquely identify a change.

A new patch set is determined by any modification in the commit
hash. When a change is initially pushed up it only has one patch
set. When an update is done for that change, ``git commit --amend``
will change the most current commit's hash because it is essentially a
new commit with the changes from the previous state combined with the
new changes added. Since it has a new commit hash, once a ``git
review`` is successfully processed, a new patch set appears in Gerrit.

Since a patch set is determined by a modification in the commit hash,
many git commands will cause new patch sets. Three common ones that do
this are:

* ``git commit --amend``
* ``git rebase``
* ``git cherry-pick``

As long as you leave the "Change-Id" line in the commit message alone
and continue to propose the change to the same target branch, Gerrit
will continue to associate the new commit with the already existing
change, so that reviewers are able to see how the change evolves in
response to comments.

Squashing Changes
-----------------
If you have made many small commits, you should squash them so that
they do not show up in the public repository. Remember: each commit
becomes a change in Gerrit, and must be approved separately. If you
are making one "change" to the project, squash your many "checkpoint"
commits into one commit for public consumption. Here's how::

  git checkout master
  git pull origin master
  git checkout TOPIC-BRANCH
  git rebase -i master

Use the editor to squash any commits that should not appear in the
public history. If you want one change to be submitted to Gerrit, you
should only have one "pick" line at the end of this process. After
completing this, you can prepare your public commit message(s) in your
editor. You start with the commit message from the commit that you
picked, and it should have a Change-Id line in the message. Be sure to
leave that Change-Id line in place when editing.

Once the commit history in your branch looks correct, run git review
to submit your changes to Gerrit.

Adding a Dependency
-------------------
When you want to start new work that is based on the commit under the
review, you can add the commit as a dependency.

Fetch change under review and check out branch based on that change::

  git review -d $PARENT_CHANGE_NUMBER
  git checkout -b $DEV_TOPIC_BRANCH

Edit files, add files to git::

  git commit -a
  git review

.. Note:: git review rebases the existing change (the dependency) and the
    new commit if there is a conflict against the branch they are being
    proposed to. Typically this is desired behavior as merging cannot
    happen until these conflicts are resolved. If you don't want to deal
    with new patchsets in the existing change immediately you can pass
    the ``-R`` option to git review in the last step above to prevent
    rebasing. This requires future rebasing to resolve conflicts.

If the commit your work depends on is updated, and you need to get the
latest patchset from the depended commit, you can do the following.

Fetch and checkout the parent change::

  git review -d $PARENT_CHANGE_NUMBER

Cherry-pick your commit on top of it::

  git review -x $CHILD_CHANGE_NUMBER

Submit rebased change for review::

  git review

The note for the previous example applies here as well. Typically you
want the rebase behavior in git review. If you would rather postpone
resolving merge conflicts you can use git review ``-R`` as the last step
above.

Rebasing a commit
-----------------

Sometimes the target branch you are working on has changed, which can create
a merge conflict with your patch. In this case, you need to rebase your
commit on top of the current state of the branch. This rebase needs to
be done manually:

#. Checkout and update master:

   .. code-block:: console

      $ git checkout master
      $ git remote update

#. Checkout the working branch and rebase on master:

   .. code-block:: console

      $ git review -d 180503
      $ git rebase origin/master

#. If git indicates there are merge conflicts, view the affected files:

   .. code-block:: console

      $ git status

#. Edit the listed files to fix conflicts, then add the modified files:

   .. code-block:: console

      $ git add <file1> <file2> <file3>

#. Confirm that all conflicts are resolved, then continue the rebase:

   .. code-block:: console

      $ git status
      $ git rebase --continue

Cross-Repository Dependencies
-----------------------------

If your change has a dependency on a change outside of that
repository, like a change for another repository or some manual
setup, you have to ensure that the change merge at the right time.

For a change depending on a manual setup, mark your change with the
"Work in Progress" label until the manual setup is done. A core
reviewer might also block an important change with a -2 so that it
does not get merged accidentally before the manual setup is done.

If your change has a dependency on a change in another repository,
you can use cross-repo dependencies (CRD) in Zuul:

* To use them, include "Depends-On: <gerrit-change-url>" in the footer
  of your commit message. Use the permalink of the change.  This is
  output by Gerrit when running git-review on the change, or you can
  find it in the top-left corner of the Gerrit web interface.  Where
  it says "Change ###### - Needs ..." the number is the link to the
  change; you can copy and paste that URL.  A patch can also depend on
  multiple changes as explained in :ref:`multiple_changes`.

* These are one-way dependencies only -- do not create a cycle.


Gate Pipeline
^^^^^^^^^^^^^

When Zuul sees CRD changes, it serializes them in the usual manner when
enqueuing them into a pipeline. This means that if change A depends on
B, then when they are added to the gate pipeline, B will appear first
and A will follow. If tests for B fail, both B and A will be removed
from the pipeline, and it will not be possible for A to merge until B
does.

Note that if changes with CRD do not share a change queue (such as the
"integrated gate"), then Zuul is unable to enqueue them together, and the
first will be required to merge before the second is enqueued.

Check Pipeline
^^^^^^^^^^^^^^

When changes are enqueued into the check pipeline, all of the related
dependencies (both normal git-dependencies that come from parent
commits as well as CRD changes) appear in a dependency graph, as in
the gate pipeline. This means that even in the check pipeline, your
change will be tested with its dependency. So changes that were
previously unable to be fully tested until a related change landed in
a different repo may now be tested together from the start.

All of the changes are still independent (so you will note that the
whole pipeline does not share a graph as in the gate pipeline), but
for each change tested, all of its dependencies are visually connected
to it, and they are used to construct the git references that Zuul
uses when testing.  When looking at this graph on the `Zuul
status page <https://zuul.opendev.org/>`__, you will note that
the dependencies show up as grey dots, while the actual change tested
shows up as red or green. This is to indicate that the grey changes
are only there to establish dependencies. Even if one of the
dependencies is also being tested, it will show up as a grey dot when
used as a dependency, but separately and additionally will appear as
its own red or green dot for its test.

.. _multiple_changes:

Multiple Changes
^^^^^^^^^^^^^^^^

A Gerrit URL refers to a single change on a single branch, so if your
change depends on multiple changes, or the same change on multiple
branches of a project, you will need to explicitly list each URL.
Simply add another "Depends-On:" line to the footer for each
additional change.

Cycles
^^^^^^

If a cycle is created by use of CRD, Zuul will abort its work very
early. There will be no message in Gerrit and no changes that are part
of the cycle will be enqueued into any pipeline. This is to protect
Zuul from infinite loops. The developers hope that they can improve
this to at least leave a message in Gerrit in the future. But in the
meantime, please be cognizant of this and do not create dependency
cycles with Depends-On lines.

Limitations and Caveats
^^^^^^^^^^^^^^^^^^^^^^^

Keep in mind that these dependencies are dependencies on changes in
other repositories. Thus, a Depends-on only enforces an ordering but
is not visible otherwise especially in these cases:

* Changes for the CI infrastructure like changes
  ``openstack/project-config`` are never tested in a production
  simulated environment. So, if one of the changes adjusts the job
  definitions or creates a new job, a Depends-On will not test the new
  definition, the CI infrastructure change needs to merge to master
  and be in production to be fully evaluated.
* If a test job installs packages from PyPI and not via source, be
  aware that the package from PyPI will always be used, a Depends-On
  will not cause a modified package to be used instead of installing
  from PyPI.

  As an example, if you are testing a change in python-novaclient that
  needs a change in python-keystoneclient, you add a Depends-On in the
  python-novaclient change. If a python-novaclient job installs
  python-keystoneclient from PyPI, the Depends-On will not have any
  effect since the PyPI version is used. If a python-novaclient job
  installs python-keystoneclient from source, the checked out source
  will have the change applied.

Do not add a Depends-On an abandoned change, your change will never
merge.

If you backport a change to another branch, it will recieve a new URL.
If you need to additionally depend on the backported change, you will
need to amend the commit message to add another Depends-On footer.

A change that is dependent on another can be approved before the
dependent change merges. If the repositories share the gate queue, it
will merge automatically after the dependent change merged. But if the
repositories do not share the gate queue, it will not merge
automatically when the dependent change has merged, even a ``recheck``
will not help. Zuul waits for a status change and does not see it. The
change needs another approval or a toggle of the approval, toggle
means removing the approval and readding it again.


Code Review
===========

Log in to https://review.opendev.org/ to see proposed changes, and
review them.

To provide a review for a proposed change in the Gerrit UI, click on
the ``Reply...`` button. In the code review, you
can add a message, as well as a vote (+1,0,-1).

It's also possible to add comments to specific lines in the file, for
giving context to the comment. For that look at the diff of changes
done in the file (click the file name), and click on the line number
for which you want to add the inline comment. After you add one or
more inline comments, you still have to send the Review message (see
above, with or without text and vote). Prior to sending the inline
comments in a review comment the inline comments are stored as Drafts
in your browser. Other reviewers can only see them after you have
submitted them as a comment on the patchset.

Any developer may propose or comment on a change (including
voting +1/0/-1 on it). A vote of +2 is
allowed from core reviewers, and should be used to indicate that
they are a core reviewer and are leaving a vote that should be
counted as such.

Some OpenDev hosted projects, like many OpenStack project teams, have
a policy requiring two positive reviews from core reviewers.

When a review has enough +2 reviews and one of the core team believes it
is ready to be merged, he or she should leave a +1 vote in the
"Workflow" category. You may do so by clicking the "Review" button
again, with or without changing your code review vote and optionally
leaving a comment. When a +1 Approved review is received, Zuul will
run tests on the change, and if they pass, it will be merged.

A green checkmark indicates that the review has met the requirement
for that category. Under "Code-Review", only one +2 gets the green
check.

For more details on reviews in Gerrit, check the
`Gerrit documentation
<https://review.opendev.org/Documentation/intro-quick.html#_reviewing_the_change>`_.

.. _automated-testing:

Automated Testing
-----------------

When a new patchset is uploaded to Gerrit, that project's "check"
tests are run on the patchset by Zuul. Once completed the test
results are reported to Gerrit by Zuul in the form of a Verified:
+/-1 vote. After code reviews have been completed and a change
receives an Approved: +1 vote that project's "gate" tests are run
on the change by Zuul. Zuul reports the results of these tests
back to Gerrit in the form of a Verified: +/-2 vote. Code merging
will only occur after the gate tests have passed successfully and
received a Verified: +2. You can view the state of tests currently
being run on the `Zuul Status page <https://zuul.opendev.org/>`__.

If a change fails tests in Zuul, please follow the steps below:

1. Zuul leaves a comment in the review with links to the log files
   for the test run. Follow those links and examine the output from
   the test. It will include a console log, and in the case of unit
   tests, HTML output from the test runner, or in the case of a
   devstack-gate test, it may contain quite a large number of system
   logs.
2. Examine the console log or other relevant log files to determine
   the cause of the error. If it is related to your change, you should
   fix the problem and upload a new patchset. Do not use "recheck".
3. It is possible that the CI infrastructure may be having some issues which
   are causing your tests to fail.  You can verify the status of the OpenDev
   infrastructure by doing the following:

   * https://wiki.openstack.org/wiki/Infrastructure_Status
   * `@OpenStackInfra <https://twitter.com/openstackinfra>`_ on Twitter.
   * the topic in your project's IRC channel (or ``#opendev``)

   .. note::

      If a job fails in the automated testing system with the status of
      ``POST_FAILURE`` rather than a normal ``FAILURE``, it could either be
      that your tests resulted with the system under test losing network
      connectivity or an issue with the automated testing system.

      If you are seeing repeated ``POST_FAILURE`` reports with no indication of
      problems in the CI system, make sure that your tests are not impacting
      the network of the system.

4. It may be the case that the problem is due to non-deterministic
   behavior unrelated to your change that has already merged. In this
   situation, you can help other developers and focus the attention of
   QA, CI, and developers working on a fix by performing the following
   steps:

   1. For OpenStack projects, check `elastic-recheck
      <https://docs.openstack.org/contributors/code-and-documentation/elastic-recheck.html>`_
      to see whether the bug is already identified and if not, add it.

      If your error is not there, then:

   2. Identify which project or projects are affected, and search for a
      related bug on Launchpad. You can search for bugs affecting all
      OpenStack Projects here: https://bugs.launchpad.net/openstack/ If
      you do not find an existing bug, file a new one (be sure to
      include the error message and a link to the logs for the
      failure). If the problem is due to an infrastructure problem
      (such as Zuul or Gerrit), file (or search for) the bug against
      the openstack-gate project.

5. It may also happen that the CI infrastructure somehow cannot finish
   a job and restarts it. If this happens several times, the job is
   marked as failed with a message of ``RETRY_LIMIT``. Usually this
   means that network connectivity for the job was lost and the change
   itself causes the job node to become unreachable consistently.

6. To re-run check or gate jobs, leave a comment on the review
   with the form "recheck".

A patchset has to be approved to run tests in the gate pipeline. If the
patchset has failed in the gate pipeline (it will have been approved to get
into the gate pipeline) a recheck will first run the check jobs and if those
pass, it will again run the gate jobs. There is no way to only run the gate
jobs, the check jobs will first be run again.

More information on debugging automated testing failures can be found in the
following recordings:

- `Tales From The Gate <https://www.youtube.com/watch?v=sa67J6yMYZ0>`_
- `Debugging Failures in the OpenStack Gate <https://www.youtube.com/watch?v=fowBDdLGBlU>`_

Post Processing
---------------

After patches land, jobs can be run in the post queue. Finding build logs for
these jobs works a bit differently to the results of the pre-merge check and
gate queues.

For jobs in the post queue, logs are found via the builds tab of
https://zuul.opendev.org/, for example to search for post jobs of the
openstack tenant, go to the `Zuul openstack build tab
<http://zuul.opendev.org/t/openstack/builds?pipeline=post>`_ .

Peer Review
-----------

Anyone can be a reviewer: participating in the review process is a
great way to learn about social norms and the development
processes.

General review flow:

1. Review is a conversation that works best when it flows back and
   forth. Submitters need to be responsive to questions asked in
   comments, even if the score is `+0` from the reviewer. Likewise,
   reviewers should not use a negative score to elicit a response if
   they are not sure the patch should be changed before merging.

   For example, if there is a patch submitted which a reviewer cannot
   fully understand because there are changes that aren't documented
   in the commit message or code documentation, this is a good time
   to issue a negative score. Patches need to be clear in their commit
   message and documentation.

   As a counter-example, a patch which is making use of a new library,
   which the reviewer has never used before, should not elicit a
   negative score from the reviewer with a question like "Is this
   library using standard python sockets for communication?" That is
   a question the reviewer can answer themselves, and which should
   not hold up the review process while the submitter explains
   things. Either the author or a reviewer should try to add a review
   comment answering such questions, unless they indicate a need to
   better extend the commit message, code comments, docstrings or
   accompanying documentation files.

2. In almost all cases, a negative review should be accompanied by
   clear instructions for the submitter how they might fix the patch.

There may be more specific items to be aware of inside the
projects' documentation for contributors.

Contributors may notice a review that has several +1's from other
reviewers, passes the functional tests, etc. but the code still has
not been merged. As only core contributors can approve code for
merging, you can help things along by getting a core reviewer's
attention in IRC (never on the mailing lists) and letting them know
there is a changeset with lots of positive reviews and needs final
approval.

Work in Progress
----------------

To get early feedback on a change which is not fully finished yet, you
can submit a change to Gerrit and mark it as "Work in Progress" (WIP).

.. note::
   The OpenDev Gerrit system does not support drafts, use
   "Work in Progress" instead. Draft changes have been disabled because
   people assume they are private when they are not. They also create
   confusion if child changes are not drafts. Additionally it is
   difficult to run CI on them. It is better to assume changes are public
   and mark the not yet ready state.

To do so, after submitting a change to Gerrit in usual way (``git review``),
You should go to Gerrit, and do `Code Review`_ of your own change while
setting "Workflow" vote to "-1", which marks the change as WIP.

This allows others to review the change, while at the same time
blocking it from being merged, as you already plan to continue working on it.

.. note:: After uploading a new patchset, this -1 (WIP) vote disappears.
    So if you still plan to do additional changes, do not forget to
    set Workflow to -1 on the new patchset.

Merging
=======

Once a change has been approved and passed the gate jobs, Gerrit
automatically merges the latest patchset.

Each patchset gets merged to the head of the branch before testing it. If
Gerrit cannot merge a patchset, it will give a -1 review and add a
comment notifying of merge failure.

Each time a change merges, the "merge-check" pipeline verifies that
all open changes on the same project are still mergeable. If any
job is not mergeable, Zuul will give a -1 review and add a
comment notifying of merge failure.

After a change is merged, project-specific post jobs are run.
Most often the post jobs publish documentation, run coverage, or
send strings to the translation server.

Project Gating
--------------

Project gating refers to the process of running regression tests
before a developer's patchset is merged. The intent of running
regression tests is to validate that new changes submitted
against the source code repository will not introduce new
bugs. Gating prevents regressions by ensuring that a series
of tests pass successfully before allowing a patchset to
be merged into the mainline of development.

The system used for gating is Zuul, which listens to the Gerrit
event stream and is configured with YAML files to define a series
of tests to be run in response to an event.

The jobs in the gate queue are executed once a core reviewer approves
a change (using a +1 Workflow vote) and a verified +1 vote
exist. When approving, at least one +2 Code-Review vote needs to exist
(can be given by core reviewer when approving). The convention is that
two +2 Code-Reviews are needed for approving.

Once all of the jobs report success on an approved patchset in the
configured gate pipeline, then Gerrit will merge the code into trunk.

Besides running the gate tests, the gate pipeline determines the order
of changes to merge across multiple projects. The changes are tested
and merged in this order, so that for each change the state of all
other repositories can be identified.

Additional information about project gating and Zuul can be found in
the Zuul documentation, located at:
https://zuul-ci.org/docs/zuul/user/gating.html

.. _`Running Python Unit Tests`: https://docs.openstack.org/project-team-guide/project-setup/python.html#running-python-unit-tests