Phabricator: developing feature on top of other feature

NOTES: This post assumes that you're familiar with git rebase and phabricator terms.

We use phabricator as our code review tool. A typical workflow for submitting & merging your feature:

1. Create a new feature branch based on master branch, develop and git commit locally from there.
2. Submit code review arc diff when we're done.
3. Run arc land to merge our diff to remote master branch.

arc land will squash all of the commits into 1 commit on the feature branch, it uses diff title and description as the commit message. We can also simulate arc land manually, maybe not 100% exact though:

1. git checkout master to make sure local master branch is up to date.
2. git checkout <feature-branch>
3. git rebase master -i and squash all of the <feature-branch> commits into 1 commit on top of master branch.
4. Merge the rebased & squashed feature branch to remote master.
5. Checkout to master and pull origin master.

This typical flow works if we work on separate features at the same time, but it could fail if we try to develop feature on top of other feature that's still in code review (submitted to phabricator). There are many cases this failure could happen, one of the frequent one is when we develop feature-b on top of feature-a where feature-a is still in code review.

# Simulation

We'll be using this repo as example.

Assumptions:

M = master
FA = feature a
FB = feature b

mx<n> = commit on master branch
ax<n> = commit on feature-a branch
bx<n> = commit on feature-b branch

The condition will look like below:

• master branch: 3 commits mx1, mx2, and mx3.
• feature-a branch: 2 commits ax1 and ax2.
• feature-b branch: 3 commits bx1, bx2 and bx3, it's built on top of feature-a.

mx1---mx2---mx3---> [M]
               \
                ax1---ax2---> [FA]
                         \
                          bx1---bx2---bx3---> [FB]

# Arc land feature-a branch

Let's say that your feature-a is accepted via code review process and you do arc land, it'll squash commits ax1 and ax2 into 1 commit.

# Process view
# S = squash
# --------------------
mx1---mx2---mx3---> [M]
               \         /---bx1---bx2---bx3---> [FB]
                ax1---ax2---> [FA]
                 |     |
                 S     S
# End result
# There's a new commit "ax" that represents commit ax1 and ax2.
# --------------------
mx1---mx2---mx3---ax---> [M]
               \
                ax1---ax2---bx1---bx2---bx3---> [FB]

feature-b doesn't need ax1 and ax2 because it's already represented by ax. It will fail if you've submitted feature-b for code review and do arc land, the reason is because it could not merge "cleanly". The fix is actually pretty easy, you need to point commit bx1 to ax. We will do git rebase to rewrite the history, PLEASE BACKUP feature-b first before you do this, you can do it by checking-out out from feature-b to another branch.

git checkout feature-b

# Backup for safety
git checkout -b feature-b-backup

# Rewriting history

First, make sure that we're on feature-b and then we will run rebase interactive to drop commits ax1 and ax2. It should be safe because both commits are already in master (represented by ax commit).

# Do interactive rebase onto master
# --------------------
git rebase -i master

There will be an editor prompt that should look like

pick 233a9b8 ax1
pick 5f5e3ce ax2
pick e145774 bx1
pick 1834f7d bx2
pick 95d155a bx3

You have 2 options remove ax1 and ax2:

1. Delete the lines
2. Replace pick with drop

You can do either one but I'll go with drop to make it more explicit in this post

drop 233a9b8 ax1
drop 5f5e3ce ax2
pick e145774 bx1
pick 1834f7d bx2
pick 95d155a bx3

Save your changes and quit editor, the rebase process will continue by itself. Now your feature-b should point to the latest commit on local master branch, you can verify this via git log or use a GUI tool.

git log --oneline --decorate --color --graph

# End result
# --------------------
mx1---mx2---mx3---ax---> [M]
                    \
                     bx1---bx2---bx3---> [FB]

That's it, now you can do arc land cleanly. I hope it helps, wish there's an easier way.

# Post Updates

Most recent updates:

• Got feedback from Daniel Albert regarding the rebase process, we could use git rebase -i master to reduce confusion. See the diff here.