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Comments on What's the correct way to merge a branch and its dependent branch back to master?

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What's the correct way to merge a branch and its dependent branch back to master?

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In git I branched feature-A from master. To reduce eventual merge conflicts later, I branched feature-B, which heavily overlaps and depends on A, from feature-A. A build of the feature-B branch shows both sets of changes, as expected. (There have been no further commits on feature-A after this branch.)

What is the best way to land both features back on master? If I just merge feature-B, will it bring along its feature-A baseline, or just the feature-B-specific changes? I think a branch is a set of commits and a commit is a set of deltas, so my concern is that merging feature-B to master would reflect only the work that is unique to feature-B, and without feature-A it would break. But my mental model of git might not be correct.

Alternatively, I could merge feature-A first and then feature-B. That seems safe, but I don't know if it's the best way.

A third approach would be to merge feature-B back into feature-A and then merge feature-A, which came from master originally, back to master. I'm reluctant to do that because that breaks the isolation between feature-A and feature-B; if in the future I need to backport feature-A but not feature-B, I'm not sure how I would do that.

Goals:

  • feature-A and feature-B are both on master and arrive in that order (or together)
  • it remains possible to backport feature-A without bringing along feature-B in the future (I do not have to support the reverse)

In order to provide more details, that's what my repository looks like:

$ git log --graph --format="%ad %h [%p] %d"

Fri Jul 23 10:42:47 2021 -0400 725d9d1c11 [ca99f826dc] (HEAD -> feature/ct-comparisons-VER-75425, origin/feature/ct-comparisons-VER-75425)
Thu Jul 22 16:27:48 2021 -0400 ca99f826dc [6498847a48]
Thu Jul 22 11:05:17 2021 -0400 6498847a48 [9b2ddff48d] (origin/feature/null-rows-VER-74021-VER-75002, feature/null-rows-VER-74021-VER-75002)
Thu Jul 22 11:02:59 2021 -0400 9b2ddff48d [37329f2ec7]
...

feature/null-rows-VER-74021-VER-75002 is branch A and feature/ct-comparisons-VER-75425 is branch B, and I want to merge both to master.

git git-branch branching-strategy
posted over 3 years ago
3y ago by hkotsubo‭
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Monica Cellio‭ staff
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1 comment thread

If I understood correctly, just merging B is enough, as B was branched from A (thus, B "contains" al... (10 comments)
If I understood correctly, just merging B is enough, as B was branched from A (thus, B "contains" al...
hkotsubo‭ wrote over 3 years ago
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If I understood correctly, just merging B is enough, as B was branched from A (thus, B "contains" all A commits, as there were no further commits in A). Can you add the output of git log --graph --format="%ad %h [%p] %d", just to confirm how your repo history is?

Monica Cellio‭ wrote over 3 years ago · edited over 3 years ago
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$ git log --graph --format="%ad %h [%p] %d"

  • Fri Jul 23 10:42:47 2021 -0400 725d9d1c11 [ca99f826dc] (HEAD -> feature/ct-comparisons-VER-75425, origin/feature/ct-comparisons-VER-75425)

  • Thu Jul 22 16:27:48 2021 -0400 ca99f826dc [6498847a48]

  • Thu Jul 22 11:05:17 2021 -0400 6498847a48 [9b2ddff48d] (origin/feature/null-rows-VER-74021-VER-75002, feature/null-rows-VER-74021-VER-75002)

  • Thu Jul 22 11:02:59 2021 -0400 9b2ddff48d [37329f2ec7]

...

Monica Cellio‭ wrote over 3 years ago · edited over 3 years ago
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Not sure what I'm looking at; does this help? (Lots of older stuff from master, as expected.)

Monica Cellio‭ wrote over 3 years ago
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The latest two commits there are on feature-B; the previous two on feature-A; previous ones from master.

hkotsubo‭ wrote over 3 years ago
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I'm guessing that feature/null-rows-VER-74021-VER-75002 is branch A and feature/ct-comparisons-VER-75425 is branch B, and you want to merge both to master, right?

Monica Cellio‭ wrote over 3 years ago
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Correct. I was going to massage that output to match the feature names in the question, but I didn't want to accidentally remove something important.

hkotsubo‭ wrote over 3 years ago
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I've answered the first part (merge both A and B). Regarding the second part ("backport feature-A without bringing along feature-B in the future"), I didn't understand what you want to do...

Monica Cellio‭ wrote over 3 years ago
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Thank you! That explanation is very helpful, and I think you've answered my other question. In the future I might need to merge feature-A, without feature-B, into a previous release's branch, so I was looking for a solution that wouldn't pollute that. I think in that case I'd just merge feature-A into release-whatever, same as if feature-B never existed. I just need to isolate A from B; if B gets backported A would have to too.

hkotsubo‭ wrote over 3 years ago
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Yes, in this case, if you merge only branch A, it won't include B. Another alternative is to apply specific commits, using cherry-pick: https://git-scm.com/docs/git-cherry-pick

Monica Cellio‭ wrote over 3 years ago
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Thanks. We usually do backports by cherry-picking the commit to master (so, merge feature-x to master, then cherry-pick that commit to release-foo). That wouldn't work in this case because feature-A isn't coming to master in its own commit, but we could cherry-pick the last commit on that branch instead. Thanks again for the help and for straightening out my mental model!