Gerrit/Commit message guidelines

The commit message of your change plays an important role. It is first thing other people will see about your change.

Subject
The first line of the commit message is known as the subject. The subject should be less than 80 characters long (aim for 50-70).
 * Summarize your change in the subject line. Keep in mind that this will be in the repository forever.
 * Optionally, prefix the subject with the relevant component. A component is the general area that your commit will change.
 * Use the imperative mood in your subject line. The imperative mood sounds like you are giving instructions to someone, and might start with words like "Change", "Fix", "Add", "Remove", "Document", "Refactor", etc.
 * Your subject line should be short, clearly state what your commit is changing. It should not be possible to use the same subject line for two commits that do different things.  People will read your subject line out of context, as it passes by in change feeds, code review emails, git-blame logs, release notes, deployment changelog, etc.  A good subject line helps decide quickly whether this commit among many will be relevant to a given interest or concern.
 * Do not end the subject line with a period/full stop/dot.

Body
When writing the body text, think about the following questions:
 * Why should this change be made? What is wrong with the current code?
 * Why should it be changed in this way? Are there other ways?
 * Did you consider other approaches? If so, describe why they were not as good.
 * How can a reviewer test or verify that your code is working correctly?

Recommended:


 * Separate the body from the subject with one empty line.
 * Wrap the message so that lines are a maximum of 100 characters long. Many editors/tools can do this automatically; 72 characters is a common width to wrap at. However, do not break URLs to make them 'fit', as this will make them un-clickable; keep them, even if they are longer.
 * Refer to other commits by using a (short) Gerrit Change-Id like or Git commit hash like  .  If the related change is not yet merged, always use the Gerrit Change-Id as the Git commit hash will change once it is merged, which would lead then become a dead-end.

Not recommended:


 * Don't refer to other commits with a URL or change number.
 * Instead, use the Gerrit Change-Id like or Git commit hash like  .  This kind of hash is automatically converted to a link when viewing the change in Gerrit, Gitiles, and other repository browsers.  It also allows for easy navigation in the Git repository during development such as via  or inside text editors.
 * On the other hand, a URL can only be resolved in a web browser and goes to a fixed location, which breaks code review workflows and departs from local context. For example, inside a Git browser the hash allows you to quickly go from one commit to a related one in the same tool, instead of being sent to Gerrit.  The hashes can also be searched for in Gerrit to automatically find commits that refer to it.
 * Another issue is that change numbers can ambiguous or become automatically linked to a different commit than you intend. This is because commit hashes are sometimes numbers only, e.g. commit 665661 is different than change 665661.
 * Don't use only a URL as the explanation for a change.
 * If the change is justified by a discussion elsewhere or some external documentation, try to summarise the key point(s) in your commit message and refer to the URL as well.

Footer and meta-data
The most important information of the footer is the (mandatory) and.

Format " " and "  " meta-data exactly like in the examples below, and place them together at the end of the body, after one empty line.

Find more information on individual meta-data fields below.

Good example
jquery.badge: Add ability to display the number zero

Cupcake ipsum dolor sit. Amet tart cheesecake tiramisu chocolate cake topping. Icing ice cream sweet roll. Biscuit dragée toffee wypas.

Does not yet address T44834 or T176. Follow-up to Id5e7cbb1.

Bug: T42 Change-Id: I88c5f819c42d9fe1468be6b2cf74413d7d6d6907

Bad example
Improved the code by fixing a bug.

Changed the files a.php and b.php

Bug: T42 Change-Id: I88c5f819c42d9fe1468be6b2cf74413d7d6d6907

Subject
Most programs we use that display Git commit, render the subject line as plain text. This means URLs do not work, and selecting/copying of text often is not possible. Therefore, do not mention Phabricator tasks, Git commits, or urls inside the subject line. Instead, mention those in the body text, or footer meta-data. That way, they can be universally selected, copied, or clicked.


 * Gerrit uses the subject in: email notifications, IRC notifications, search results.
 * GitHub uses the subject in: [ https://github.com/wikimedia/mediawiki-core/commits/ commit history], [ https://github.com/wikimedia/mediawiki-core/commit/a943d0da14f1ff4435409171a32f03cb958025ad commit subject].
 * The Git CLI uses the subject in:,  ,  ,  , etc.
 * and much more!
 * and much more!

Component
You may start the subject line with a component, which will indicate what area of the project is changed by your commit.

It should be one of the following:
 * A directory of PHP classes under or , such as " installer ", " jobqueue ", " objectcache ", " resourceloader ", " rdbms ", etc.
 * A PHP class name, such as " Title ", " User ", " OutputPage ", etc.; typically for classes without subdirectory in.
 * ResourceLoader module name (like " mediawiki.Title ", " mediawiki.util ", etc.).
 * Generic keyword affecting multiple areas relating to the type of change, such as:
 * "build" - for changes to files relating to the development workflow, such as updates to,  , etc.
 * "tests", "qunit", "phpunit" - for changes that only affect unit or integration test suites, or the test suite runners.

Phabricator
To reference a bug or task, in the commit message mention it inline using the Txxx notation (e.g. " That was caused by T169. ")

To express that a commit resolves (even partially) or is specially relevant to a bug, add in the footer at the end of the commit message.

(If you're amending a commit message, insert it immediately above the line, without an empty line between them. Remember to follow the overall structure rules and separate the body from the subject with one empty line.)

Bug: T169

A bot will automatically leave a comment on the Phabricator task about any significant events (being merged, abandoned, etc.). If a patch resolves two or more bugs, put each reference on its own line at the bottom.

Bug: T299087 Bug: T299088

Cross-references
Whenever you refer to another commit, use the SHA-1 git hash of the merged commit. If the commit in still pending review, use the Gerrit Change-Id hash instead of the git hash because the hash relates to an individual patch set (which changes when rebased, thus creating a dead-end).

Change-Id
's tool will automatically append the "  " keyword to new commits.

Dependencies
If you have cross-repo dependencies (your commit depends on another commit in a different repository), declare them by adding to the last paragraph. (" Ixxx "... is the of the other commit.) This will instruct Zuul to test the commit together with that one.

To provide additional guidance to developers, you can indicate the inverse relationship using  in last paragraph of the commit message in the other reposiory. (" Iyyy "... is the of your commit.) Note that Zuul does not react to this, it is just for the benefit of human readers. Also, Gerrit will automatically add backlinks based on the presence of, regardless of any.

Crediting others
Add this line before the  to credit other developers working on the change. You can add more than one separated by a line break.