CMSSW on GitHub

Manual Forward Ports

When some pull request is committed in an old release, this does not get propagated in the subsequent one, apart from specific cases (e.g. THREADED builds).

In order to understand what is missing one can use the git log command, where the most recent release is the beginning of the log and the previous one is the end of the log. E.g.:

git fetch official-cmssw
git log --reverse --first-parent --topo-order official-cmssw/CMSSW_7_3_X..official-cmssw/CMSSW_7_2_X

will show the merged pull requests which are part of CMSSW_7_2_X but not of CMSSW_7_3_X. In order to merge a few of them, the best strategy is:

  1. Move to the most recent branch e.g. git fetch official-cmssw ; git reset --hard official-cmssw/CMSSW_7_3_X.
  2. Decide from the log which one is the most recent PR we want to merge.
  3. Use git merge <commit> to merge that pull request and all those preceding it.
  4. Push the branch to your repository and do a pull request. E.g. git push my-cmssw HEAD:forward-port-CMSSW_7_3_X

Variations for this include using a different merge strategy (e.g. -X ours) when merging so that in case of conflict ours (i.e. the newest branch) side of the conflict is preferred (you can also use theirs). If you wish to ignore one or more PR, you can use:

git merge -s ours <commit>

which will discard all the pull request up to <commit>.

Notice also that while you can do the merge incrementally, you should always do it in order.

Find below a web based view of what is missing to be forward ported for the various releases.

Automatic Forward Ports

The script auto-update-git-branches automatically forward ports the changes between some cmssw and cmsdist branches. You can see the script to know which branches are currenly being forward ported automatially. It is run by update-github-pages

Preventing a Pull Request for Being Forward Ported

Sometimes a pull request is merged in a branch and should not be merged into others. For example, #8480 was merged in CMSSW_7_5_X, but these changes should not be merged to CMSSW_7_5_ROOT5_X. The next time update-github-pages runs, it will merge those unwanted changes.

In order to prevent a pull request for being forward ported into another branch you can do the following:

  1. clone the official cmssw-repository. Add your clone and the repository from which the pr originally came from
git clone git@github.com:cms-sw/cmssw.git
git remote add my_clone \<url\>
git remote add pr_repo \<url\>
  1. checkout the branch that you don’t want to receive the changes. Create a new branch from it
git checkout -b avoid-merge origin/unwanted_receiving_branch
  1. merge the unanted branch with -s ours from the repository from wich the pr came from
git merge -s ours pr_repo/pr_branch_name
  1. push the new branch to your repository in order to create a new PR like #8487 from it.
git push my_clone HEAD:avoid-merge

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