Maintainer’s handbook

This document describes the mechanics of maintaining Scylla - how to do things, rather than what do do (e.g. accept or reject a patch).

General git tips

Enable reusing recorded resolutions

The command

git config --global rerere.enabled true

will record merge conflict resolutions and replay them when git encounters the same conflict. This is helpful when managing multiple branches that see similar conflicts.

Set merge.conflictstyle to diff3

The command

git config --global diff.conflictstyle = diff3

will set the conflict markers to three-way diff style. This records not only “ours” and “theirs”, but also the common ancestor. This allows the maintainer to see the intent of each change and aids in resolution.

git submodule sync

Avoid using git submodule sync as it sets internal state that is easy to forget, and which can wreak havoc if you do forget it.

Applying patches and patch series

Patches can arrive via mailing lists and github pull requests. Either way, they should be applied to the next branch, not master.

Sometimes, patches and patch series have dependencies. It’s important to verify that those dependencies are satisfied. In the case of patch series, make sure the series base contains the dependency, otherwise bisectability is compromised.

A series base can be found with the command

git merge-base remote/series_branch_or_tag origin/master

Applying patches and patch series from the mailing list

Before you begin, check out the next branch and pull from scylla.git to be up to date. If the pull isn’t clean, abort the merge and use git pull --rebase. Examine the result to see if you forgot to push a previously applied patch.

Applying single patches from the mailing list

Save the patch(es) to some directory, and use the command

git am -im3 /path/to/patches/*.eml

to apply the patches. -i makes the process interactive and lets you edit the commit message, -m sets the Message-Id tag (which is used by Commit Bot to set the Reply-To header, so that the commit acknowledgement appears as a response to the patch), and -3 enables 3-way merging which is rumored to reduce conflicts.

Use git push to publish the patches.

Applying patch series from the mailing list

Indentify the git url and branch/tag identifier, and issue the command

git pull --log --no-ff --no-rebase <url> <branch/tag>

The --log flag generates a list of patches in the commit log, while --no-ff and --no-rebase ensure a merge commit is created.

Copy the cover letter subject and body to the merge commit’s subject and body, respectively. Make sure the merge commit supplies enough information to understand what the series is doing without having to read individual commits.

Use git push to publish the patches.

Applying patches and patch series from github pull requests

A common contributor mistake is to base patches on next rather than on master. This results in random commits appearing in the pull request, if next is edited for some reason.

Ensure that the target branch in the pull request page is set to scylladb:next (click Edit and change it if that’s not the case).

When merging, verify that github didn’t mangle the commit log, check both your and the contributor’s email address are correct (no or similar address, or home vs. work addresses).

Applying single patches from github pull requests

Select “Squash and merge” and follow through.

Applying patch series from github pull requests

Select “Create a merge commit” and do NOT follow through - github will attribute itself as the commiter. Instead, click “view command line options”, select the “git pull” line, paste it to a terminal and add --no-ff --log and execute.

Dequeuing bad patches

Sometimes, a patch fails promotion by Jenkins, or needs to be dequeued for some other reason. This section explains how.

  1. Synchronize with origin by checking out next and issuing a git pull

  2. Issue git rebase -i --rebase-merges origin/master

  3. Identify the final section that contains the pick/merge command that will contain the result. Ignore any intermediate sections that describe branches.

  4. Delete pick/merge commands that correspond to bad commits

  5. Save the file and let git rebase do the work

  6. Publish your changes with git push --force-with-lease

Note: git contains a bug where branch descriptions with the characters ['":\.] confuse it. Best to search-and-replace those characters with nothing.

Updating submodule references

Submodules are maintained in separate repositories. For example, Seastar is developed upstream independently of Scylla. We want to periodically (and upon contributor request) refresh scylla.git to include the latest submodules.

  1. Check out the next branch and synchronize it using git pull

  2. Run the scripts/ script, which will open a git commit log editor for every submodule to show that commits are being updated.

  3. Edit the submodule update commits with any necessary additional imformation. For example, amend the message with Fixes tags.

  4. Use git push to publish your work.

Backporting patches

To backport a patch, check out the next branch of the relevant release branch (e.g. next-3.2), syncrhonize it with scylla.git, and use the cherry-pick command:

git cherry-pick -x <commit hash>

for individual commits, and

git cherry-pick -x -m 1 <commit hash>

for merge commits. -x leaves a reference to the original commit hash, and -m 1 indicates which is the “mainline” parent of the merge commit.

If conflicts cannot be resolved with reasonable effort, ask the contributor for help.

Backporting Seastar commits

The first time a release branch needs a Seastar backport requires creating a Seastar branch. This is done in a separate repository:

  1. Check out the next branch for your release branch (e.g. next-3.2) and synchronize using git pull.

  2. Use git submodule update to syncrhonize the submodule

  3. Use cd seastar to enter the submodule

  4. Create a new branch (e.g. git checkout -b branch-3.2) corresponding to the release series you are backporting to. Note, the regular branch name is used, not the next branch.

  5. Use `git push -u scylla-seastar branch-3.2’ to publish the branch. Note, scylla-seastar here is a git remote that refers to, a repository used for holding seastar backports for scylla.git.

  6. Use cd .. to return to scylla.git.

  7. Edit .gitmodules to change ../seastar to ../scylla-seastar. This points the seastar submodule at the backports repository.

  8. Commit with a descriptive message and push to the relevant next branch.

After this is done, backporting seastar patches can proceed:

  1. Check out the next branch for your release branch (e.g. next-3.2) and synchronize using git pull.

  2. Use git submodule update to syncrhonize the submodule

  3. Use cd seastar to enter the submodule

  4. Check out the relevant branch (branch-3.2 in our example)

  5. Use git cherry-pick -x <hash> (or git cherry-pick -x -m 1 <hash>) to backport patches.

  6. Use git push to publish the scylla-seastar.git patches.

  7. Use cd .. to return to scylla.git.

  8. Use git submodule summary seastar to create a change log.

  9. Commit using git commit seastar, populate change log from step 8.

  10. Publish using git push.