Code review in Phabricator

We use the Differential application of Phabricator to do code reviews in the context of Software Heritage.

• we use Git and history.immutable=true (but beware as that is partly a Phabricator misnomer, read on)
• when code reviews are required, developers will be allowed to push directly to master once an accepted Differential diff exists

Configuration

When using git, Arcanist by default mess with the local history, rewriting commits at the time of first submission.
To avoid that we use so called history immutability.

To that end, you shall configure your arc accordingly:

arc set-config history.immutable true

Note that this does not mean that you are forbidden to rewrite your local branches (e.g., with git rebase). Quite the contrary: you are encouraged to locally rewrite branches before pushing to ensure that commits are logically separated and your commit history easy to bisect. The above setting just means that arc will not rewrite commit history under your nose.

Workflow

TL;DR:

• work in a feature branch: git checkout -b my-feat
• initial review request: hack/commit/hack/commit ; arc diff origin/master
• react to change requests: hack/commit/hack/commit ; arc diff --update Dxx origin/master
• landing change: git checkout master ; git merge my-feat ; git push

Starting a new feature and submit it for review

Use a one branch per feature workflow, with well-separated logical commits:

git checkout -b my-shiny-feature
... hack hack hack ...
git commit -m 'architecture skeleton for my-shiny-feature'
... hack hack hack ...
git commit -m 'my-shiny-feature: implement module foo'
... etc ...

To submit your code for review the first time:

arc diff origin/master

arc will prompt for a code review message. Provide the following information:

• first line: short description of the overall work (i.e., the feature you're working on). This will become the title of the review
• Summary field (optional): long description of the overall work; the field can continue in subsequent lines, up to the next field. This will become the "Summary" section of the review
• Test Plan field (optional): write here if something special is needed to test your change
• Reviewers field (optional): the (Phabricator) name(s) of desired reviewers. If you don't specify one (recommended) the default reviewers will be chosen
• Subscribers field (optional): the (Phabricator) name(s) of people that will be notified about changes to this review request. In most cases it should be left empty

For example:

mercurial loader

Summary: first stab at a mercurial loader (T329)

The implementation follows the plan detailed in F2F discussion with @foo.

Performances seem decent enough for a first trial (XXX seconds for YYY repository
that contains ZZZ patches).

Test plan: 

Reviewers: 

Subscribers: foo

After completing the message arc will submit the review request and tell you its number and URL:

[...]
Created a new Differential revision:
        Revision URI: https://forge.softwareheritage.org/Dxx

Updating your branch to reflect requested changes

Your feature might get accepted as is, YAY! Or, reviewers might request changes; no big deal!

Use the Differential web UI to follow-up to received comments, if needed.

To implement requested changes in the code, hack on your branch as usual by:

• adding new commits, and/or
• rewriting old commits with git rebase (to preserve a nice, easy to bisect history)

When you're ready to update your review request:

arc diff --update Dxx origin/master

Arc will prompt you for a message: describe what you've changed w.r.t. the previous review request, free form. Your message will become the changelog entry in Differential for this new version of the diff.

Differential only care about the code diff, and not about the commits or their order. Therefore each "update" can be a completely different series of commits, possibly rewritten from the previous submission.

Landing your change onto master

Once your change has been approved in Differential, you will be able to land it onto the master branch.

Before doing so, you're encouraged to clean up your git commit history, reordering/splitting/merging commits as needed to have separate logical commits and an easy to bisect history. The correspondence between the accepted review and pushed code is checked by looking only at the code diff, so commit fiddling will not impact your ability to push to master.

Once you're happy you can push to origin/master directly, e.g.:

git checkout master
git merge my-shiny-feature
git push

Optionally you can then delete your local feature branch:

git branch -d my-shiny-feature

Reviewing locally / landing someone else's changes

You can do local reviews of code with arc patch:

arc patch Dxyz

This will create a branch arcpatch-Dxyz containing the changes on your local checkout.

You can then merge those changes upstream with

git checkout master
git merge --ff arcpatch-Dxyz
git push origin master