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Setting up Git mergetool

Merge conflict resolution tooling

For Apache Pulsar core developers, handling git merge conflict resolution is necessary. To efficiently resolve merge conflicts, setting up tools that assist in visualizing these conflicts and resolving them is essential.

For developers starting to use automated tools to resolve merge conflicts during cherry-picking, IntelliJ is a recommended option. It offers excellent tooling, but its integration with a command-line workflow is not seamless. It performs well when you initiate the cherry-picking process in IntelliJ and handle the merge conflict resolution within the same environment. However, resolving a merge conflict often involves multiple steps, including reverting and amending changes until a satisfactory resolution is achieved. In many cases, using a combination of tools may be more effective than relying solely on IntelliJ for all required operations.

For more advanced users who use git on the command line, setting up the git mergetool is recommended. Here's an example of how to set up kdiff3 as a mergetool.

kdiff3 configuration on MacOS

Install kdiff3's cask version with brew:

# important! install the cask version of kdiff3
brew install --cask kdiff3

Configure kdiff3 as the mergetool and difftool of git

git config --global mergetool.kdiff3.path /Applications/
git config --global mergetool.kdiff3.args '$base $local $other -o $output'
git config --global mergetool.kdiff3.trustExitCode false
git config --global merge.tool kdiff3
git config --global difftool.kdiff3.path /Applications/
git config --global difftool.kdiff3.args '$base $local $other -o $output'
git config --global difftool.kdiff3.trustExitCode false
git config --global diff.guitool kdiff3

kdiff3 configuration on Linux

Install kdiff3 from your package manager. For example, on Ubuntu:

sudo apt install kdiff3

Configure kdiff3 as the mergetool and difftool of git

git config --global mergetool.kdiff3.path /usr/bin/kdiff3
git config --global merge.tool kdiff3
git config --global difftool.kdiff3.path /usr/bin/kdiff3
git config --global diff.guitool kdiff3

Using the mergetool kdiff3

If any merge conflicts arise after a cherry-pick, merge, or rebase, you should run git mergetool to resolve them. You can run git mergetool anytime, as the command will return when there are no conflicts to resolve.

The kdiff3 tool isn't the most user-friendly tool, and it takes time to get accustomed to its workings. There's commentary on mergetools in this blog post that could help you understand what the tools do and how merges are visualized in different tools. One of the advantages of kdiff3 is that it contains a custom merge algorithm which can resolve some conflicts without requiring a choice. In some cases, there may be chances for mistakes, but these are rare and could also occur when manually choosing the resolution. The resolution will need to be verified in any case.

Tips for Using kdiff3

  • When the merge conflict resolution process begins, a view with three panes and a split pane at the bottom of the window will appear.
    • The left pane displays the diff from the common version of the file. This can be confusing and is often not very useful. You can hide it by deselecting "Window -> Show Window A".
    • The middle pane shows the local version.
    • The right pane shows the remote version.
    • The bottom pane is the output, which is the result of the merge. You can also make manual edits in this pane to resolve conflicts manually.
  • It's beneficial to learn how to navigate to the next and previous merge conflict and how to choose the resolution using keyboard shortcuts.
    • On MacOS, you may need to remap some of the keyboard shortcuts to improve usability. This is especially necessary when using an external keyboard.

Git revert and commit amending tooling

Resolving merge conflicts can sometimes be more complex with merge tools than simply reverting some changes and modifying the original source code in an IDE. This process may involve multiple steps, including reverting and amending changes to the merge commit. The merge commit should also incorporate the necessary changes for backporting. In many cases, it is also necessary to fix the import statements in an IDE and amending those changes to the merge commit.

For this purpose, the git gui tool is excellent. It allows for partial reverts to previous commits and makes it easy to amend additional changes to the latest commit, all with clear visualization.

Installing the git gui tool:

# on MacOS
brew install git-gui
# on Linux install "git-gui" from your package manager, example of Ubuntu
sudo apt install git-gui

There are many tools available for this purpose, but git gui is one of the simplest and most effective.

Using IntelliJ for cherry-picking and merge conflict resolution.