Git: Authoring a commit that depends on multiple other Gerrit changes?

Question

At my place of work, we use a Gerrit server for code review. Often times different people will author commits that are unrelated. I've run into an issue where I must author a commit which depends on two other, unrelated commits.

Let's say there are two commits on a gerrit server:

• Commit A: "Added function flipImage"
• Commit B: "Added function rotateImage"

These two are independent of each other. My job is to implement Commit C: "Added function flipRotateImage". This requires both commits A and B to be present.

I have two questions:

1. How to I 'pull in' both A and B, and then author C, in such a way that I'm not actually changing A or B? In other words I can't cherry-pick A or B because it would change them. My goal is to be able to upload C for review without needing to update A & B.
2. Let's say the author of A uploads a new patch set. I want to update my working branch such that the "A" in my chain is the latest "A", not an older patchset. Can this be done?

Answer 1

1. Unless A or B is rebased to depend on the other you can't have C depend on both A and B. One possibility would be to check out A and merge from B to create a merge commit D onto which you could put C, but then you'd have to upload this otherwise useless merge commit for review, but graph-wise it's the only way of making both A and B reachable from C without making A reachable from B or vice versa.
2. Yes, fetch the new A into your workspace with the URL provided in the Gerrit UI and put C on top of it using e.g. git rebase --onto. The exact procedure would depend slightly on how you choose to deal with the problem in the first question, but ignoring that and assuming you have checked out the topic branch for C then git rebase --onto newA oldA will do.

Answer 2

A change has been proposed in Gerrit to support this kind of dependency specification outside of the git DAG. The change is intended for cross-project dependencies, but it should also support this kind of intra-project commit dependency as well. Sorry it doesn't help you now, though.

https://gerrit-review.googlesource.com/#/c/55243