Gerrit/Commit message guidelines

The commit message plays an important role in revision control systems. They are the first thing other people will see of your commit.

Subject
The first line of the commit message is known as the subject.
 * It describes the change briefly.
 * It should be no more than 50 characters (must be less than 62).
 * Optionally prefix it with the changed component.

Second line
Keep the second line empty.
 * This separates the subject from the body.
 * Without this the following lines will be interpreted as part of the subject.

Body
Use the body of the commit message to describe your change in detail.
 * Give an overview of why you’re committing this change.
 * What does the commit change.
 * Any new design choices made.
 * Areas to focus on for recommendations or to verify correct implementation.
 * Any research you might have done.
 * Bytes are cheap, so just write!
 * Wrap the body of the message between 70 and 100 characters.

Patch set comments
It can be tempting to add patch set specific comments to the commit message body, don’t do it; add them as gerrit patch set comments on the specific code lines or in the general patch set comment area.

Subject
The first line of the commit message (the "subject") is especially important. It helps reviewers to see at a glance what the commit is about.

Phrase your subject in imperative mood. For example "make foo do bar" instead of "[This patch] makes foo do bar" or "[I am] changing foo to do bar".

If you are unable to summarise your change in a short line, perhaps it is too early to push it to Gerrit, or maybe it contains too many different things that should be broken up into different commits.

Many interfaces use the subject to identify a commit, such as:
 * Gerrit: E-mail notifications, IRC notifications, Search results
 * Gitblit: Commit subject
 * GitHub: Commit history, Commit subject
 * ,  etc.
 * Release notes of Wikimedia deployment branches of MediaWiki
 * and much more

In all these cases the subject is rendered as plain text. Avoid using bugzilla or git references in the subject as they have no meaning in plain text. Instead mention them in the body text and/or as key-value pairs in the footer. When someone views the change in Gerrit this data is parsed (bug references, git hashes, urls all become linkified).

As being a title (not a sentence in a paragraph) it should not end in a full stop. Though one can argue about the semantics of it being a sentence or a title, consistency is important and we don't end in a full stop.

Body
Try to make your commit message understandable without external resources. Instead of just giving a URL to a mailing list archive, summarize the relevant points of the discussion.

Whenever you refer to another commit, use either the SHA-1 of the merged commit, or the Gerrit Change-Id of the changeset in Gerrit (avoid using Gerrit or Gitweb urls). If the commit in question has not been merged yet, use the Gerrit Change-Id instead of the SHA1 because the SHA1 changes every patch set (the SH1 reference would potentially be a dead end).

Referencing bugs
To reference a bug (from our Bugzilla) in the commit message mention it inline using the  keyword (lowercase) followed by one space and only the bug number (e.g. " That fixes bug 44441. ")

To express that a commit addresses a bug, add a line like the following above the  line in the footer (see example): Bug: bugnumber Make sure that no text follows the bugnumber. Both these and regular mentions of bugs in the commit message get automatically turned into links to bugzilla within Gerrit.

Gerrit is capable of automatically notifying Bugzilla about patch comments, merging, abandoning etc., Additionally, the bug's status gets set to  if appropriate. This behaviour is only triggered when a change has the bug linked in the footer.

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 bug 44441. Follows-up Id5e7cbb1.

Bug: 12345 Change-Id: I88c5f819c42d9fe1468be6b2cf74413d7d6d6907