This page describes the support in Skara for both creating and reviewing backports.

Terminology

A backport commit (often abbreviated to just backport) is a replica of an existing commit applied to a different repository. In most cases the backport commit will be applied to a repository representing an older release, for example creating a backport commit of a commit in the jdk repository for the jdk11u-dev repository.

A backport commit is considered clean if the changes in the original commit are identical to the changes in the backport commit. Note that only the changes has to be identical, not the changed lines.

Commit Message

All backport commits features an additional commit message trailer (a line in the last paragraph of the commit message) - Backport-of. The Backport-of trailer describes which commit the backport commit was created from. The following commit message shows an example:

8123456: This is a bug

This commit fixes a tricky bug.

Reviewed-by: ehelin
Backport-of: 5a526c1c5716f6d9a7fc94741bcdb2f424d342df

In the example above it can be seen that the backport commit fixes the bug "JDK-8123456", has the summary "This commit fixes a tricky bug" and that the backport commit was reviewed by "ehelin". The Backport-of trailer shows that the backport commit is a replica of the commit with hash 5a526c1c5716f6d9a7fc94741bcdb2f424d342df.

Note that author metadata recorded in the backport commit is the author of the backport commit, not the author of the original commit. Likewise the reviewers recorded in the commit message are the reviewers of the backport commit, not the reviewers of the original commit. If the author of the backport isn't Author in the project that the backport commit is created for, then the backport commit can be sponsored. In the case of a backport being sponsored, then the author will recorded as the author of the commit and the sponsor as the committer of the commit (just as for regular commits).

Backport Pull Requests

Skara features a way to declare a pull request to be a "backport pull request" in order to ease the creation of backport commits. A "backport pull request" is a pull request which title is of the form "Backport [0-9a-z]{40}", for example "Backport 5a526c1c5716f6d9a7fc94741bcdb2f424d342df". The hash represents the hash of the original commit that is being backported. When the Skara bots encounter a "backport pull request" then they will:

Continuing with the example from the Commit Message section, a "backport pull request" with the title "Backport 5a526c1c5716f6d9a7fc94741bcdb2f424d342df" would result in bots marking the pull request as solving issue "JDK-8123456" and the final commit message to have the summary "This commit fixes a tricky bug". When the pull request is integrated the bots would also append "Backport-of: 5a526c1c5716f6d9a7fc94741bcdb2f424d342df" to the final commit message.

The bots will automatically detect if the commit in a backport pull request is a clean backport. If it is then bots will add the label "clean" to the pull requests. Backport pull requests which commit is a clean backport can be integrated without reviewers (configurable per repository). This means that not all commit messages for backport commits will have a "Reviewed-by" line. 

The Skara CLI tools features the tool git pr create for creating pull requests. The option --backport to git pr create makes the title of pull request consist of "Backport" and the given hash, for example git pr create --backport=5a526c1c5716f6d9a7fc94741bcdb2f424d342df creates a pull request with the title "Backport 5a526c1c5716f6d9a7fc94741bcdb2f424d342df". The value of --backport does not have to be a full hash, it can be an abbreviated hash or a reference.

Backport Commit Command

The /backport commit command can be used to quickly create a backport pull request for a given commit. Just navigate to the comment in source code hosting provider's web UI and add a comment consisting of "/backport <repo>". If the commit does not apply clean on the target repository then a message will be shown for the files with conflicts.

Workflow

Creating backports

Web UI

Navigate the to commit indented to be backported in the source code hosting provider's web UI and issue the /backport commit command. If the commit applied cleanly onto the target repository then go to the pull request linked in the reply and issue the /sponsor pull request command.

If the commit did not apply cleanly then the commit must be backport manually and backport pull request must be created manually. See the CLI section for an example of how to do this using the command-line.

CLI

Use the Skara CLI tool git-backport to try to automatically create a backport pull request for the given commit, for example git backport https://github.com/openjdk/jdk 5a526c1c5716f6d9a7fc94741bcdb2f424d342df. If the commit applied cleanly onto the target repository then sponsor the resulting backport pull request with git-pr sponsor, for example git pr sponsor 17.

If the commit could not be applied cleanly, then the conflicts must be manually resolved. After the conflicts have been resolved then a pull request must be created with the title "Backport <hash>". An example of how to do this is shown below:

$ git checkout -b backport-5a526c1c
$ git fetch https://github.com/openjdk/jdk 5a526c1c5716f6d9a7fc94741bcdb2f424d342df
$ git cherry-pick --no-commit FETCH_HEAD
$ # resolve conflicts
$ git commit -m 'Backport 5a526c1c5716f6d9a7fc94741bcdb2f424d342df'
$ git push -u origin backport-5a526c1c

The output from the final git push will return a link that can be used to create the pull request.

Reviewing backports