Todd Wolfson

Software Engineer

August 06, 2015

Pull requests (PRs) should always be:

  • Focused on a single feature
  • Digestible (e.g. maximum of 150 lines changed)

The reasoning for these requisites are:

  • Allows us to identify/create better abstractions
  • Makes it easier for reviewing and re-reviewing changes in a timely manner

This article is a guide to managing branches on a non-trivial/high velocity project (e.g. a lot of changes/PRs occur frequently) which requires squashed commits.

Setup

As a consequence of "single feature" PRs, we sometimes need to write dependent features/PRs (e.g. abstract a utility function, use utility in new feature). For example, our git structure would look like:

        o feature-1b (bbbbbb)
       /
      o feature-1a (aaaaaa)
     /
o---o master (ffffff)

which when landed looks like:

o---o---o---o master, feature-1b (bbbbbb)

Problem

Typical workflows like git rebase, git commit --amend, and git merge --squash fall apart when an earlier PRs needs a change. For example, adding a comment for clarity on feature-1a bumps all commits:

        o feature-1b (b22222)
       /
      o feature-1a (a22222)
     /
o---o master (ffffff)

Most ways of doing this lead to hard to debug merge conflicts. Here is an amateur example:

# Navigate to our first PR's branch
git checkout feature-1a

# Edit our file
echo "# This is a clarifying comment" >> file.txt

# Update our PR commit
git add file.txt
git commit --amend --no-edit
# New commit is now a22222

# Push our updated PR
git push origin feature-1a --force

# Navigate to our second PR's branch
git checkout feature-1b

# Merge in our past work and sort out our merge conflicts
git merge feature-1a
git mergetool -y
# Need to update every file that was edited in `bbbbbb`
# Additionally, hard to know if we kept all of our intended changes
# For example, what if all files were indented?
#   How would we verify all 150 lines of our changes were retained?
#   What if we are 3 PR's deep and reviewing 450 lines of code?

# Commit our merge resolution
git commit --no-edit

# Squash our commits via `git rebase`
git rebase -i feature-1a
git rebase --continue
# New commit is now b22222

# Push our updated PR
git push origin feature-1b --force

Solution

To solve this problem, we are going to lean on git's ability to handle non-squashed merges well.

Historical and squashed branches

As foundation for our solution, we will use 2 branches per new feature:

  • A historical branch operates as its own master branch for the feature
    • Typically named {{feature}} (e.g. add-homepage)
  • A squashed branch has all work from our historical branch in 1 commit
    • Typically named {{feature}}.squashed (e.g. add-homepage.squashed)

Here is an example workflow:

# Work on and build our historical branch
git checkout -b feature
echo "hello" > file
git add file
git commit -m "Added hello file"
echo "world" > file2
git add file2
git commit -m "Added world file"

# Generate squashed branch for our PR
# DEV: We use -B to overwrite any past squashed branches
# DEV: `merge --squash` combines all changes from `feature` into 1 commit
git checkout -B feature.squashed master
git merge --squash feature
git commit -m "Added hello and world files"

# Push our squashed branch and open PR for `feature.squashed` to `master`
git push origin feature.squashed

Here's a visualization of our git history:

        +---o feature.squashed (aaaaaa)
       /
      /---o---o feature (a12345)
     /
o---o master (ffffff)

When we land this PR, it will look like:

o---o---o master, feature.squashed (aaaaaa)

Application

Now that we are established with the historical/squashed workflow, let's apply it to our first example:

        o feature-1b (bbbbbb)
       /
      o feature-1a (aaaaaa)
     /
o---o master (ffffff)

For the purpose of dependent PRs, we are going to add one more concept known as a base branch. This is where the historical branch forked from it's dependent branch. For our example, this will look like:

                +---o feature-1b.squashed (bbbbbb)
               /
              /---o---o feature-1b (b12345)
             /
        +---o feature-1a.squashed, feature-1b.base (aaaaaa)
       /
      /---o---o feature-1a (a12345)
     /
o---o master (ffffff)

To reiterate, the contents of a12345 and aaaaaa are the same (similarly with b12345 and bbbbbb).

Building the current git structure would look like:

# Build our feature-1a on a historical branch
git checkout -b feature-1a
echo "hello" > file
git add file
git commit -m "Added hello file"
# This is commit `a12345`

# Squash into our feature-1a squashed branch
git checkout -B feature-1a.squashed master
git merge --squash feature-1a
git commit -m "Added hello file"
# This is commit `aaaaaa`

# Open our first PR
git push origin feature-1a.squashed

# Mark our origin for feature-1b
git checkout -b feature-1b.base

# Build our feature-1b on a historical branch
git checkout -b feature-1b
echo "world" > file2
git add file2
git commit -m "Added world file"
# This is commit `b12345`

# Squash into our feature-1b squashed branch
#   but for this one, we base off of our feature-1b base
git checkout -B feature-1b.squashed feature-1b.base
git merge --squash feature-1b
git commit -m "Added world file"
# This is commit `bbbbbb`

# Open our second PR
git push origin feature-1b.squashed

When we land our stack of PR's, it happens the same as a normal PR:

# Land our first PR
git checkout master
git merge feature-1a.squashed
git push origin master

# Land our second PR
git merge feature-1b.squashed
git push origin master

We don't land feature-1b.squashed directly, as we should detect conflicting merges at each PR.

Solution in action

As with the initial problem, we will now update our first PR. The result will look like:

                +---o feature-1b.squashed (b22222)
               /
              /---o---o feature-1b (b23456)
             /
        +---o feature-1a.squashed, feature-1b.base (a22222)
       /
      /---o---o---o feature-1a (a23456)
     /
o---o master (ffffff)
# Checkout our first historical branch
git checkout feature-1a
echo "hello world" > file
git add file
git commit -m "Corrected file's content"
# This is commit `a23456`

# Update our squashed branch (still using -B to override the branch)
git checkout -B feature-1a.squashed master
git merge --squash feature-1a
git commit -m "Added hello file"
# This is commit `a22222`

# Force push our squashed branch which automatically updates the PR
git push origin feature-1a.squashed --force

# Navigate to our base for the second feature and merge in our changes
git checkout feature-1b.base
git merge feature-1a.squashed
# Pro-tip: Use `git merge -` to merge past branch

# Handle any merge conflicts in a resolution `merge` commit
git mergetool -y
git commit

# Verify there are no differences between our base and its origin
# DEV: This is a nice step between branches to verify there are no gotchas
git diff feature-1a.squashed

# Navigate to our second historical branch and merge its base changes
git checkout feature-1b
git merge feature-1b.base
# Pro-tip: Use `git merge -` to merge past branch
# This is commit `b23456`

This is a very important point in the workflow so let's explain in more detail what is happening.

Both feature-1b.base and feature-1b share a common commit (aaaaaa) outside of master. This is a known state where the 2 branches agree.

When we merged in the new feature-1a.squashed to feature-1b.base, we built a new merge commit (a22222*) that makes sure feature-1a changes take priority as they are newest .

When we merge this new merge commit (a22222*) into feature-1b (b12345), git respects all past changes between feature-1b and feature-1b.base. However, any changes in the new merge commit that conflict with the set of changes since feature-1b.base will be brought up as conflicts.

These conflicts will be a smaller (possibly empty) set than a typical rebase workflow.

Anyway, back to the code:

# Diff our base to make sure all the changes we expected exist
git diff feature-1b.base
# Pro-tip: We can open a GitHub window to skip over the diff and verify it looks consistent
# Pro-tip: If we feel something changed, then
#   we can take a diff of our squashed branch to its past commit
#   and compare that diff to the diff in `git diff feature-1b.base`
#   We are diffing our diffs to see any lines missed/added during merging
#   git diff feature-1b.base > new-diff
#   git diff feature-1b.squashed~1..feature-1b.squashed > old-diff
#   diff new-diff old-diff
#      I prefer copying to clipboard over writing to files (e.g. `| xclip -selection c`)
#      and using a diff tool in Sublime Text (e.g. FileDiffs)

# If there are any changes that we didn't want, then we can use
#   git checkout -p {branch} # Uses patch mode to select bits from {branch}
#   git checkout -p {branch} -- {filepath} # Same as last command, specific to 1 file
#   git reset -p # Uses patch mode to unstage specific parts of our staged changes
#   git stash -p # Uses patch mode to stash specific parts of our working directory
#   git add -p # Uses patch mode to stage specific parts of our working directory

# Overwrite the `.base` branch with the only squash commit
# DEV: This removes a `merge` commit from our PR (`a22222*` -> `a22222`)
git checkout -B feature-1b.base feature-1a.squashed

# Squash our branch for the second PR
git checkout -B feature-1b.squashed feature-1b.base
git merge --squash feature-1b
git commit -m "Added world file"

# Force push our squashed branch which automatically updates the PR
git push origin feature-1b.squashed --force
# This is commit `bbbbbb`

Cleanup

Once our PR is landed and deployed, we can clean up our branches via git-delete-branch from git-extras.

I prefer to be cautious and perform this over a few commands:

# Find which branches have recently been merged in
git checkout master
git branch --merged
#   feature-1a.squashed
#   feature-1b.squashed
# * master

# Generate a grep to find all relevant branches
git branch | grep -E 'feature-1a|feature-1b'
#   feature-1a
#   feature-1a.squashed
#   feature-1b
#   feature-1b.base
#   feature-1b.squashed

# Run each of our branches against `git-delete-branch`
git branch | grep -E 'feature-1a|feature-1b' | xargs git-delete-branch

Additional tooling

In order to make my life easier, I have written 2 tools that I use with this workflow:

  • git-sqwish, a git utility to automate .squashed branch generation
  • git shortref, a git utility that returns name of current branch
    • Define the following in your global .gitconfig under [alias]
      • shortref = symbolic-ref --short HEAD
    • This can be used like git checkout -B "$(git shortref).squashed"

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