Handling Github PRs

how to create a PR (for contributers)

Read [1].

Pull requests are made to apache/mahout repository on Github.

merging a PR and closing it (for committers).

Remember that pull requests are equivalent to a remote branch with potentially a multitude of commits. In this case it is recommended to squash remote commit history to have one commit per issue, rather than merging in a multitude of contributer’s commits. In order to do that, as well as close the PR at the same time, it is recommended to use squash commits.

Read [2] (merging locally). Merging pull requests are equivalent to merging contributor’s branch:

git checkout master      # switch to local master branch
git pull apache master   # fast-forward to current remote HEAD
git pull --squash https://github.com/cuser/mahout cbranch  # merge to master 

In this example we assume that contributor Github handle is “cuser” and the PR branch name is “cbranch” there. We also assume that apache remote is configured as

apache  https://git-wip-us.apache.org/repos/asf/mahout.git (fetch)
apache  https://git-wip-us.apache.org/repos/asf/mahout.git (push)

Squash pull ensures all PR history is squashed into single commit. Also, it is not yet committed, even if fast forward is possible, so you get chance to change things before committing.

At this point resolve conflicts, if any, or ask contributor to rebase on top of master, if PR went out of sync.

Also run regular patch checks and change CHANGELOG.

Suppose everything is fine, you now can commit the squashed request

git commit -a

edit message to contain “MAHOUT-YYYY description closes #ZZ”, where ZZ is the pull request number. Including “closes #ZZ” will close PR automatically. More information [3].

push apache master

(this will require credentials).

Note on squashing: Since squash discards remote branch history, repeated PRs from the same remote branch are difficult for merging. The workflow implies that every new PR starts with a new rebased branch. This is more important for contributors to know, rather than for committers, because if new PR is not mergeable, github would warn to begin with. Anyway, watch for dupe PRs (based on same source branches). This is a bad practice.

Closing a PR without committing

When we want to reject a PR (close without committing), just do the following commit on master’s HEAD without merging the PR:

git commit --allow-empty -m "closes #ZZ *Won't fix*"
git push apache master

that should close PR without merging and any code modifications in the master repository.

Apache/github integration features

Read [4]. Issue handles mentioned in comments and PR name should post to mailing lists and Jira.