Rebase Flow

This document outlines the technical details about the rebase flow as implemented in GitHub Desktop. It is intended to be a simple subset of what is technically possible to do with the git rebase command line interface, and various decisions have been made along the way to support this flow.

Testing the rebase

When the user is choosing which branch to rebase the current branch on, the application is expected to test and identify whether there will be problems with the rebase:

In the "happy path" case, there will be no conflicts when rebasing this branch on top of the chosen branch, and the number of commits that will be rebased is shown
If conflicts are expected that the user is required to resolve to rebase the current branch on top of the chosen branch, the application should provide a warning. Ideally the application would be able to identify the absolute number of conflicts, but because of the cascading nature of rebases (after the user resolves conflicts in one commit, the following commits may introduce conflicts that were not previously known) this is not able to be quantified definitively.

To work out this information, we need to emulate a Git rebase and test out the changes without making changes to the working directory or the Git index. To do this, we need to:

identify the commits that Git would apply from the target branch
generate a patch that represents the changes the rebase would apply to the base branch
test this patch and see if it applies cleanly to the base branch

Steps 2 and 3 are not currently implemented, but are being tracked in #6960.

The next section walks through these steps in more detail, referencing the underlying Git implementation.

Identifying the commits to rebase

The rebase flow that Desktop currently uses is the simple git rebase <upstream> <branch>. In the rest of this document I will refer to <upstream> as the base branch and <branch> as the target branch, to differentiate from the upstream remote.

When git invokes git rebase <upstream> <branch> it performs these steps as setup:

confirm <upstream> and <branch> are valid references in the repository, and resolve the oid of each ref
find the range of commits from <upstream_oid>..<branch_oid>
generate a patch series of the contents, ready to rebase

Detecting Conflicts

TODO: As part of detecting conflicts in #6960 there will be some words here eventually to help explain how the solution works.

Warning about remote commits

If the target branch is tracking a remote branch and the user has enabled the Show confirmation dialog before force pushing setting, Desktop will check if any commits exist in the range <upstream_oid>..<remote_branch_oid> before starting the rebase, where <remote_branch_oid> is the tip of the tracking branch.

This check is important to identify any remote commits that will be caught in this rebase, because their existence would require rewriting the history on the remote, which the user may not have permission to do based on branch protections or may not want to do because of the impact it can have on other collaborators.

If commits are found, a dialog is shown to ask the user to confirm they wish to proceed with the rebase.

Reporting Progress

When the user launches the rebase, the application will run git rebase <upstream> <branch> behind the scenes, which progress information to stdout as it works through the process of applying commits to the base branch to create the new history.

Desktop is able to parse this output and use in the application, but because this is all working locally it is typically very fast on machines with good I/O, the application will make the rebase appear slower than it actually is so that it can transition smoothly between "progress" and the end states of the rebase without confusing the user.

Encountering conflicts

Once the git rebase <upstream> <branch> has completed Desktop inspects the output returned by git. The rebase will end up in one of two states:

the commits from the target branch were applied cleanly, and the reference associated with the target branch now points to the rewritten history on top of the base branch.
the rebase could not complete because one of the commits had conflicts when it was applied to the base branch, and the user needs to resolve these conflicts if the user wishes to resolve them

This section will focus on the second scenario, and what Desktop does to surface this context to the user to help them figure out how to resolve conflicts.

When a rebase is in a conflicted state, the .git directory has a bunch of helpful context to help the user orient themselves and to figure out what is next.

Is a rebase in progress?

The .git/REBASE_HEAD file exists when the rebase is stopped because of conflicts, and contains the commit ID of the current patch that cannot be applied cleanly to the working directory

Details about the rebase

The information about what changes are being rebased can be identified found from a few files:

.git/rebase-merge/head-name - the name of the target branch associated with the current rebase - may be prefixed with refs/heads/

TODO: can we confirm when this happens or doesn't happen?

.git/rebase-merge/onto - the commit ID of the base branch, which is used as a starting point to apply the commits from the target branch
.git/rebase-merge/orig-head - the original commit ID of the target branch

This information is especially helpful if Desktop encounters a repository which had the rebase started outside Desktop but encountered conflicts - with this information the application can identify the current situation and continue with resolving conflicts without needing to restart the operation.

Details about the rebase progress

If a rebase was not started outside Desktop, or the rebase flow is paused, the details about the progress state can be built up from these fields.

When a rebase encounters conflicts, the patches themselves are stored in files in .git/rebase-merge/ in numeric order of which they will be applied, e.g. 0001, 0002, 0003 and so on.

TODO: can we confirm whether 9999 is the upper limit here for the rebase?

.git/rebase-merge/msgnum - this contains a number representing the current patch being applied as part of the rebase.
.git/rebase-merge/end - this number contains the total number of patches that will be applied as part of this rebase
.git/rebase-merge/orig-head - the original commit ID of the target branch
.git/rebase-merge/onto - the commit ID of the base branch, which is used as a starting point to apply the commits from the target branch

The last two files, orig-head and onto can be used to generate a range of commits which are part of the rebase, without needing to read all the patch files in the directory.

Completing the rebase

After detecting that the user has resolved all conflicts in the working directory - both in files with conflict markers and files requiring a manual resolution ("ours" or "theirs") - the rebase flow will determine what the next rebase action should be.

After staging all the tracked files, if there are no changes in the index this means the current commit is a no-op and the changes it contains are already available in the base branch. This means the commit can be omitted by the rebase by running git rebase --skip.

If there are changes in the index after staging all the changes, the rebase flow will run git rebase --continue to indicate the current commit should be updated and used in the rebased branch.

And like before, the rebase flow will monitor the output from this new rebase command and report progress, until it either completes or encounters conflicts again.

Aborting the rebase

If the rebase encounters conflicts, the user has the opportunity to abort the rebase if they feel uncomfortable with proceeding.

If the user has resolved conflicts before trying to abort, the rebase flow will ask the user to confirm they wish to abort the rebase, as those changes will not be available if they proceed.

Force Push

When the rebase is completed, the application will update its list of rebased branches to indicate that the current branch was updated by the user and is eligible to be "force pushed" to the remote repository.

This has potential downsides, so there are additional checks as part of this work:

only a branch that completed the rebase flow in Desktop will be eligible for a "force push" operation - branches rebased outside Desktop will be ignored
other branches which are ahead and behind will need to have these commits resolved before any commits can be pushed. Depending on your Git configuration, this could be a pull with rebase or a pull with merge commit
if the user has enabled "Show Confirmation Dialog before Force Pushing" (enabled by default), the user will see a prompt that explains the downstream impact of rewriting the branch for other contributors
when Desktop invokes git push it will also pass the --force-with-lease flag that guards against the tracking branch being updated without the user knowing, to avoid overwriting newer commits since the rebase was completed