I really don't get the point of stacked PRs.
Just using git, you'd send a set of patches, which can be reviewed, tested and applied individually.
The PR workflow makes a patch series an undivisible set of changes, which must be reviewed, tested and applied in unison.
And stacked PRs tries to work around this issue, but the issue is how PRs are implemented in the first place.
What you really want is the ability to review individual commits/patches again, rather than work on entire bundles at once. Stacked PRs seems like a second layer of abstraction to work around issues with the first layer of abstractions.
The teams that I have worked with still apply the philosophy you’re describing, but they consider PRs to be the “commit”, i.e. the smallest thing that is sane to apoly individually.
Then the commits in the PR are not held to the standard of being acceptable to apply, and they are squashed together when the PR is merged.
This allows for a work flow in which up until the PR is merged the “history of developing the PR” is preserved but once it is merged, the entire PR is applied as one change to the main branch.
This workflow combined with stacked PRs allows developers to think in terms of the “smallest reviewable and applicable change” without needing to ensure that during development their intermediate states are safe to apply to main.
Doesn’t this mean that a first review might request that a specific change be reverted, and then a later reviewer reviews that reversion? That’s essentially reviewing a noop, but understanding the it’s a noop requires carefully checking all previous now-invalidated changes.
No, each PR is based on the previous one, so the reviewer only needs to consider the ideas that are new in each PR one at a time.
Squashing is fine if you’re just making a mess of temporary commits as you work and you don’t want to keep any of those changes separate in master, but that’s not a useful review workflow. A lot of times I’ve built a feature in a way that decomposed naturally into e.g. two commits: one to do a preparatory refactor (which might have a lot of noisy and repetitive changes, like changing a function signature) and another to actually change the behavior. You want those changes to be separate because it makes the changes easier to review; the reviewer quickly skims the first commit, observes that it’s a mechanical refactor, and the change in behavior has its own, smaller commit without all the noise.
“What if there’s feedback and you need to make changes after the code review?” Then I do the same thing I did before I posted the code review: make separate “fixup” commits and do an interactive rebase to squash them into my commits. (And yes, I do validate that the intermediate commits build cleanly.)
There’s nothing you get from stacked PR’s that you don’t also get from saying “please review my feature branch commit by commit”.
Yes what you’re describing is literally the thing GitHub has built but instead of having to make a bunch of compromises, there is dedicated UI and product metaphor for it.
Some examples of compromises:
You can’t merge partially merge a large “review commit by commit” PR so you are forced to wait until it is all ready to merge.
Exactly! A stack of PRs is really the same beast as a branch of commits.
The traditional tools (mailing-lists, git branches, Phabricator) represented each change as a difference between an old version of the code and the proposed new version. I believe Phabricator literally stored the diff. They were called “diffs” and you could make a new one by copying and pasting into a <textarea> before pressing save*.
The new fangled stuff (GitHub and its clones) recorded your change as being between branches A and B, showed you the difference on the fly, and let you modify branch B. After fifteen years of this we are now seeing the option for branch A to be something other than main, or at least for this to be a well supported workflow.
In traditional git land, having your change as a first class object — an email or printout or ph/D1234 with the patch included — was the default workflow!
*Or some other verb meaning save.
It’s useful for large PRs in large repos with many contributors. It reduces the burden for reviewers.
Still not sure this is the right solution. My problem is if one of your first stages gets rejected in review or requires significant changes, it invalidates so much work that comes after it. I've always when possible preferred to get small stuff merged in to production as it happens rather than build an entire feature and put it up for review.
> it invalidates so much work that comes after it.
No, not necessarily.
I work on a large repo and new features often involve changes to 3 different services: 2 from the backend, and the frontend UI. Sending a single PR with changes to all 3 services is really not ideal: the total diff size in a feature I added recently was maybe 600+ lines, and the reviewers for frontend and backend changes are different people. The changes in the 2 backend services can be thought of as business logic on one side and interactions with external platforms on the other. The business logic can't work without integrating calls to external APIs, and the UI can't work without the business logic.
These days I open 3 separate PRs and the software only works once all 3 are merged and built. It would be great to have all of them as a single package that's still testable and reviewable as 3 distinct parts. The UI reviewer can check out the whole stacked PR and see it running locally with a functional backend, something that's not possible without a lot of manual work when we have 3 PRs.
The LLVM community used this model for years with Phabricator before it was EOL'd and moving to GH and PRs was forced. It's a proven model and works very well in complex code bases, multiple components and dependencies that can have very different reviewer groups. E.g: 1) A foundational change to the IR is the baseline commit 2) Then some tweaks on top to lay the groundwork for uses of that change 3) Some implementation of a new feature that uses the new IR change 4) A final change that flips the feature flag on to enable by default.
Each of these changes are dependent on the last. Without stacked PRs you have o only one PR and reviewing this is huge. Maybe thousands of lines of complex code. Worse, some reviewers only need to see some parts of it and not the rest.
Stacked diffs were a godsend and the LLVM community's number one complaint about moving to GitHub was losing this feature.
this works much better in Phabricator because commits to diffs are a 1:1 relationship, diffs are updated by amending the commit, etc., the Github implementation does seem a bit like gluing on an additional feature.
Right, a PR is "just" a set of commits (all must be in the same branch) that are intended to land atomically.
Stacked PRs are not breaking up a set of commits into divisible units. Like you said, you can already do that yourself. They let you continue to work off of a PR as your new base. This lets you continue to iterate asynchronously to a review of the earlier PRs, and build on top of them.
You often, very often, need to stage your work into reviewer-consumable units. Those units are the stack.