commit-msg - Edit commit messages to insert a Change-Id tag.
A Git hook automatically invoked by git commit, and most other commit creation tools such as git citool or git gui. The Gerrit Code Review supplied implementation of this hook is a short shell script which automatically inserts a globally unique Change-Id tag in the footer of a commit message. When present, Gerrit uses this tag to track commits across cherry-picks and rebases.
After the hook has been installed in the user’s local Git repository for a project, the hook will modify a commit message such as:
Improve foo widget by attaching a bar. We want a bar, because it improves the foo by providing more wizbangery to the dowhatimeanery. Signed-off-by: A. U. Thor <firstname.lastname@example.org>
by inserting a new `Change-Id: ` line in the footer:
Improve foo widget by attaching a bar. We want a bar, because it improves the foo by providing more wizbangery to the dowhatimeanery. Change-Id: Ic8aaa0728a43936cd4c6e1ed590e01ba8f0fbf5b Signed-off-by: A. U. Thor <email@example.com>
The hook implementation is reasonably intelligent at inserting the Change-Id line before any Signed-off-by or Acked-by lines placed at the end of the commit message by the author, but if no such lines are present then it will just insert a blank line, and add the Change-Id at the bottom of the message.
If a Change-Id line is already present in the message footer, the script will do nothing, leaving the existing Change-Id unmodified. This permits amending an existing commit, or allows the user to insert the Change-Id manually after copying it from an existing change viewed on the web.
To obtain the commit-msg script use scp, wget or curl to copy it to your local system:
$ scp -p -P 29418 firstname.lastname@example.org:hooks/commit-msg .git/hooks/
$ curl http://review.example.com/tools/hooks/commit-msg
The hook generates unique Change-Id lines by creating a virtual commit object within the local Git repository, and obtaining the SHA-1 hash from it. Like any other Git commit, the following properties are included in the computation:
SHA-1 of the tree being committed
SHA-1 of the parent commit
Name, email address, timestamp of the author
Name, email address, timestamp of the committer
Proposed commit message (before Change-Id was inserted)
Because the names of the tree and parent commit, as well as the committer timestamp are included in the hash computation, the output Change-Id is sufficiently unique.
Part of Gerrit Code Review