= Gerrit Code Review - Crafting Changes

Here are some hints as to what approvers may be looking for
before approving or submitting changes to the Gerrit project.
Let's start with the simple nit picky stuff.  You are likely
excited that your code works; help us share your excitement
by not distracting us with the simple stuff.  Thanks to Gerrit,
problems are often highlighted and we find it hard to look
beyond simple spacing issues.  Blame it on our short attention
spans, we really do want your code.


[[branch]]
== Branch

Gerrit provides support for more than one version, which naturally
raises the question of which branch you should start your contribution
on. There are no hard and fast rules, but below we try to outline some
guidelines:

* Genuinely new and/or disruptive features, should generally start on
  `master`. Also consider submitting a
  link:dev-design-docs.html[design doc] beforehand to allow discussion
  by the ESC and the community.
* Improvements of existing features should also generally go into
  `master`. But we understand that if you cannot run `master`, it
  might take a while until you could benefit from it. In that case,
  start on the newest `stable-*` branch that you can run.
* Bug-fixes should generally at least cover the oldest affected and
  still supported version. If you're affected and run an even older
  version, you're welcome to upload to that older version, even if
  it is no longer officially supported, bearing in mind that
  verification and release may happen only once merged upstream.

Regardless of the above, changes might get moved to a different branch
before being submitted or might get cherry-picked/re-merged to a
different branch even after they've landed.

For each of the above items, you'll find ad-hoc exceptions. The point
is: We'd much rather see your code and fixes than not see them.


[[commit-message]]
== Commit Message

It is essential to have a good commit message if you want your
change to be reviewed.

  * Keep lines no longer than 72 chars
  * Start with a short one line summary
  * Followed by a blank line
  * Followed by one or more explanatory paragraphs
  * Use the present tense (fix instead of fixed)
  * Use the past tense when describing the status before this commit
  * Include a `Bug: Issue <#>` line if fixing a Gerrit issue, or a
    `Feature: Issue <#>` line if implementing a feature request.
  * Include a `Change-Id` line

[[vim-setup]]
=== Setting up Vim for Git commit message

Git uses Vim as the default commit message editor. Put this into your
`$HOME/.vimrc` file to configure Vim for Git commit message formatting
and writing:

====
  " Enable spell checking, which is not on by default for commit messages.
  au FileType gitcommit setlocal spell

  " Reset textwidth if you've previously overridden it.
  au FileType gitcommit setlocal textwidth=72
====


[[git-commit-settings]]
=== A sample good Gerrit commit message:
====
  Add sample commit message to guidelines doc

  The original patch set for the contributing guidelines doc did not
  include a sample commit message, this new patchset does.  Hopefully this
  makes things a bit clearer since examples can sometimes help when
  explanations don't.

  Note that the body of this commit message can be several paragraphs, and
  that I word wrap it at 72 characters.  Also note that I keep the summary
  line under 50 characters since it is often truncated by tools which
  display just the git summary.

  Bug: Issue 98765605
  Change-Id: Ic4a7c07eeb98cdeaf44e9d231a65a51f3fceae52
====

The `Change-Id` line is, as usual, created by a local git hook.  To install it,
simply copy it from the checkout and make it executable:

====
  cp ./gerrit-server/src/main/resources/com/google/gerrit/server/tools/root/hooks/commit-msg .git/hooks/
  chmod +x .git/hooks/commit-msg
====

If you are working on core plugins, you will also need to install the
same hook in the submodules:

====
  export hook=$(pwd)/.git/hooks/commit-msg
  git submodule foreach 'cp -p "$hook" "$(git rev-parse --git-dir)/hooks/"'
====


To set up git's remote for easy pushing, run the following:

====
  git remote add gerrit https://gerrit.googlesource.com/gerrit
====

The HTTPS access requires proper username and password; this can be obtained
by clicking the 'Obtain Password' link on the
link:https://gerrit-review.googlesource.com/#/settings/http-password[HTTP
Password tab of the user settings page].

Alternately, you may use the
link:https://pypi.org/project/git-review/[git-review] tool to submit changes
to Gerrit. If you do, it will set up the Change-Id hook and `gerrit` remote
for you. You will still need to do the HTTP access step.

[[style]]
== Style

This project has a policy of Eclipse's warning free code. Eclipse
configuration is added to git and we expect the changes to be
warnings free.

We do not ask you to use Eclipse for editing, obviously.  We do ask you
to provide Eclipse's warning free patches only. If for some reasons, you
are not able to set up Eclipse and verify, that your patch hasn't
introduced any new Eclipse warnings, mention this in a comment to your
change, so that reviewers will do it for you. Yes, the way to go is to
extend gerrit CI to take care of this, but it's not yet implemented.

Gerrit follows the
link:https://google.github.io/styleguide/javaguide.html[Google Java Style
Guide].

To format Java source code, Gerrit uses the
link:https://github.com/google/google-java-format[`google-java-format`]
tool (version 1.7), and to format Bazel BUILD, WORKSPACE and .bzl files the
link:https://github.com/bazelbuild/buildtools/tree/master/buildifier[`buildifier`]
tool (version 4.0.0). Unused dependencies are found and removed using the
link:https://github.com/bazelbuild/buildtools/tree/master/unused_deps[`unused_deps`]
build tool, a sibling of `buildifier`.

These tools automatically apply format according to the style guides; this
streamlines code review by reducing the need for time-consuming, tedious,
and contentious discussions about trivial issues like whitespace.

You may download and run `google-java-format` on your own, or you may
run `./tools/setup_gjf.sh` to download a local copy and set up a
wrapper script. If you run your own copy, please use the same version,
as there may be slight differences between versions.

[[code-rules]]
== Code Rules
=== Final
When to use `final` modifier and when not (in new code):

Always:

  * final fields: marking fields as final forces them to be
  initialized in the constructor or at declaration
  * final static fields: clearly communicates the intent
  * to use final variables in inner anonymous classes

Optional:

  * final classes: use when appropriate, e.g. API restriction
  * final methods: similar to final classes

Never:

  * local variables: it clutters the code, and makes the code less
  readable. When copying old code to new location, finals should
  be removed
  * method parameters: similar to local variables

=== Optional / Nullable
Recommended:

  * Optionals in arguments are discouraged (use @Nullable instead)
  * Return types should be objects or Optionals of objects, but not null/nullable

[[code-organization]]
== Code Organization

Do your best to organize classes and methods in a logical way.
Here are some guidelines that Gerrit uses:

  * Ensure a standard copyright header is included at the top
    of any new files (copy it from another file, update the year).
  * Always place loggers first in your class!
  * Define any static interfaces next in your class.
  * Define non static interfaces after static interfaces in your
    class.
  * Next you should define static types, static members, and
    static methods, in decreasing order of visibility (public to private).
  * Finally instance types, instance members, then constructors,
    and then instance methods.
  * Some common exceptions are private helper static methods, which
    might appear near the instance methods which they help (but may
    also appear at the top).
  * Getters and setters for the same instance field should usually
    be near each other barring a good reason not to.
  * If you are using assisted injection, the factory for your class
    should be before the instance members.
  * Annotations should go before language keywords (`final`, `private`, etc) +
    Example: `@Assisted @Nullable final type varName`
  * Prefer to open multiple AutoCloseable resources in the same
    try-with-resources block instead of nesting the try-with-resources
    blocks and increasing the indentation level more than necessary.

Wow that's a lot!  But don't worry, you'll get the habit and most
of the code is organized this way already; so if you pay attention
to the class you are editing you will likely pick up on it.
Naturally new classes are a little harder; you may want to come
back and consult this section when creating them.

[[design]]
== Design

Here are some design level objectives that you should keep in mind
when coding:

  * Most client pages should perform only one RPC to load so as to
    keep latencies down.  Exceptions would apply to RPCs which need
    to load large data sets if splitting them out will help the
    page load faster.  Generally page loads are expected to complete
    in under 100ms.  This will be the case for most operations,
    unless the data being fetched is not using Gerrit's caching
    infrastructure.  In these slower cases, it is worth considering
    mitigating this longer load by using a second RPC to fill in
    this data after the page is displayed (or alternatively it might
    be worth proposing caching this data).
  * `@Inject` should be used on constructors, not on fields.  The
    current exceptions are the ssh commands, these were implemented
    earlier in Gerrit's development.  To stay consistent, new ssh
    commands should follow this older pattern; but eventually these
    should get converted to eliminate this exception.
  * Don't leave repository objects (git or schema) open.  Use a
    try-with-resources statement to ensure that repository objects get
    closed after use.
  * Don't leave UI components, which can cause new actions to occur,
    enabled during RPCs which update Git repositories, including NoteDb.
    This is to prevent people from submitting actions more than once
    when operating on slow links.  If the action buttons are disabled,
    they cannot be resubmitted and the user can see that Gerrit is still
    busy.

[[tests]]
== Tests

  * Tests for new code will greatly help your change get approved.

[[change-size]]
== Change Size/Number of Files Touched

And finally, I probably cannot say enough about change sizes.
Generally, smaller is better, hopefully within reason.  Do try to
keep things which will be confusing on their own together,
especially if changing one without the other will break something!

  * If a new feature is implemented and it is a larger one, try to
    identify if it can be split into smaller logical features; when
    in doubt, err on the smaller side.
  * Separate bug fixes from feature improvements.  The bug fix may
    be an easy candidate for approval and should not need to wait
    for new features to be approved.  Also, combining the two makes
    reviewing harder since then there is no clear line between the
    fix and the feature.
  * Separate supporting refactoring from feature changes.  If your
    new feature requires some refactoring, it helps to make the
    refactoring a separate change which your feature change
    depends on.  This way, reviewers can easily review the refactor
    change as a something that should not alter the current
    functionality, and feel more confident they can more easily
    spot errors this way.  Of course, it also makes it easier to
    test and locate later on if an unfortunate error does slip in.
    Lastly, by not having to see refactoring changes at the same
    time, it helps reviewers understand how your feature changes
    the current functionality.
  * Separate logical features into separate changes.  This
    is often the hardest part.  Here is an example:  when adding a
    new ability, make separate changes for the UI and the ssh
    commands if possible.
  * Do only what the commit message describes.  In other words, things which
    are not strictly related to the commit message shouldn't be part of
    a change, even trivial things like externalizing a string somewhere
    or fixing a typo.  This helps keep `git blame` more useful in the future
    and it also makes `git revert` more useful.
  * Use topics to link your separate changes together.

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