# 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 Identify the git url and branch/tag identifier, and issue the command git pull --log --no-ff --no-rebase 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 @noreply.users.github.com or similar address, or home vs. work addresses). ### Applying pull requests with a helper script For maintainers' convenience, a script can be used to properly prepare a pull request for merging. The script works correctly both on single- and multi-commit series. In order to apply a pull request with given number, change the directory to Scylla's root source directory and follow the example below. 1. Fetch and checkout the `next` branch 2. `./scripts/pull_github_pr.sh ${PUT_PULL_REQUEST_NUMBER_HERE}` 3. Verify that the merge or cherry-pick was performed correctly 4. Push the next branch to the remote repository The script can also be used for backports. ### 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 committer. 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/refresh-submodules.sh` 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. By default refresh-submodules.sh will refresh all submodules from their master branches. It's possible to specify submodules and branches as command line arguments. Each is treated by the script as `name[:branch]`, so for example the `refresh-submodules.sh seastar` will only refresh the seastar submodule and `refresh-submodules.sh seastar:foo` will do it fetching from the remote `foo` branch. ## Backporting patches To backport a patch, check out the next branch of the relevant release branch (e.g. next-3.2), synchronize it with scylla.git, and use the cherry-pick command: git cherry-pick -x for individual commits, and git cherry-pick -x -m 1 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 synchronize 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 https://github.com/scylladb/scylla-seastar.git, 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 synchronize 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 ` (or `git cherry-pick -x -m 1 `) 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`. Alternatively * In the `scylla-seastar` repository 1. Check out the relevant branch (the `branch-3.2` from the example above) 2. Apply all the necessary patches (by cherry-picking them or doing any other relevant manipulations) 3. Push the udpated branch. * In the `scylla` repository 1. Check out the relevant next branch (e.g. `next-3.2` one) 2. Use the refreshing script specifying the `seastar:` as its first argument (`refresh-submodules.sh seastar:branch-3.2`) 3. Publish with `git push`.