Code review in Phabricator: Difference between revisions

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We use the [[Differential]] application of [[Phabricator]] to perform [[code review|code reviews]] in the context of [[Software Heritage]].
#REDIRECT [[swhdocs:devel/contributing/phabricator.html#patch-submission]]
 
* 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 ==
 
=== Arcanist configuration ===
 
When using git, [[Arcanist]] by default mess with the local history, rewriting commits at the time of first submission.<br />
To avoid that we use so called [https://secure.phabricator.com/book/phabricator/article/arcanist_new_project/#history-mutability-git history immutability].
 
To that end, you shall configure your <tt>arc</tt> accordingly:
<pre>
arc set-config history.immutable true
</pre>
 
Note that this does ''not'' mean that you are forbidden to rewrite your local branches (e.g., with <tt>git rebase</tt>).
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.
 
=== Enabling <tt>git push</tt> to our forge ===
 
The way we've configured our review setup for continuous integration needs you to configure git to allow pushes to our forge. There's two ways you can do this : setting a ssh key to push over ssh, or setting a specific password for git pushes over https.
 
==== SSH key for pushes ====
 
In your forge User settings page (On the top right, click on your avatar, then click ''Settings''), you have access to a ''Authentication'' > ''SSH Public Keys'' section (Direct link: <tt>hxxps://forge.softwareheritage.org/settings/user/'''<your username>'''/page/ssh/</tt>). You then have the option to upload a SSH public key, which will authenticate your pushes.
 
You then need to configure ssh/git to use that key pair, for instance by editing the <tt>~/.ssh/config</tt> file.
 
Finally, you should configure git to push over ssh when pushing to https://forge.softwareheritage.org, by running the following command:
git config --global url.git@forge.softwareheritage.org:.pushInsteadOf https://forge.softwareheritage.org
 
This lets git know that it should use <tt>git@forge.softwareheritage.org:</tt> as a base url when pushing repositories cloned from forge.softwareheritage.org over https.
 
==== VCS password for pushes ====
 
If you're not comfortable setting up SSH to upload your changes, you have the option of setting a VCS password. This password, ''separate from your account password'', allows Phabricator to authenticate your uploads over HTTPS.
 
In your forge User settings page (On the top right, click on your avatar, then click ''Settings''), you need to use the ''Authentication'' > ''VCS Password'' section to set your VCS password (Direct link: <tt>hxxps://forge.softwareheritage.org/settings/user/'''<your username>'''/page/vcspassword/</tt>).
 
If you still get a 403 error on push, this means you need a forge administrator to enable HTTPS pushes for the repository (which wasn't done by default in historical repositories). Please drop by on IRC and let us know!
 
== Workflow ==
 
* work in a feature branch: <tt>git checkout -b my-feat</tt>
* initial review request: hack/commit/hack/commit ; <tt>arc diff origin/master</tt>
* react to change requests: hack/commit/hack/commit ; <tt>arc diff --update Dxx origin/master</tt>
* landing change: <tt>git checkout master ; git merge my-feat ; git push</tt>
 
=== Starting a new feature and submit it for review ===
 
Use a '''one branch per feature''' workflow, with well-separated ''logical commits'' ([https://wiki.softwareheritage.org/wiki/Git_style_guide following those conventions]).
Please open one diff per logical commit to keep the diff size to a minimum.
 
<pre>
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 ...
</pre>
 
Please, follow the
To '''submit your code for review''' the first time:
<pre>
arc diff origin/master
</pre>
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:
<pre>
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
</pre>
 
After completing the message arc will submit the review request and tell you its number and URL:
<pre>
[...]
Created a new Differential revision:
        Revision URI: https://forge.softwareheritage.org/Dxx
</pre>
 
=== 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)
* pulling on master and rebasing your branch against it if meanwhile someone landed commits on master:
 
<pre>
git checkout master
git pull
git checkout my-shiny-feature
git rebase master
</pre>
 
 
When you're ready to '''update your review request''':
<pre>
arc diff --update Dxx HEAD~
</pre>
 
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.
 
=== Dependencies between diffs ===
 
Note that you can manage diff dependencies within the same module with the following keyword in the diff description:
 
<pre>
Depends on Dxx
</pre>
 
That allows to keep a logical view in your diff.
It's not strictly necessary (because the tooling now deals with it properly) but it might help reviewers or yourself to do so.
 
=== 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.
Update the diff [https://wiki.softwareheritage.org/wiki/Code_review_in_Phabricator#Updating_your_branch_to_reflect_requested_changes following the prior section].
(It'd be good to let the ci build finish to make sure everything is still green).
 
Once you're happy you can '''push to origin/master''' directly, e.g.:
<pre>
git checkout master
git merge --ff-only my-shiny-feature
git push
</pre>
 
<code>--ff-only</code> is optional, and makes sure you don't unintentionally create a merge commit.
 
Optionally you can then delete your local feature branch:
<pre>
git branch -d my-shiny-feature
</pre>
 
=== Reviewing locally / landing someone else's changes ===
 
You can do local reviews of code with arc patch:
 
<pre>
arc patch Dxyz
</pre>
 
This will create a branch '''arcpatch-Dxyz''' containing the changes on your local checkout.
 
You can then merge those changes upstream with
 
<pre>
git checkout master
git merge --ff arcpatch-Dxyz
git push origin master
</pre>
 
or, alternatively:
 
<pre>
arc land --squash
</pre>
 
== See also ==
 
* [[Code review]] for guidelines on how code is reviewed when developing for Software Heritage




[[Category:Software development]]
[[Category:Software development]]

Latest revision as of 12:44, 31 March 2021

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