Checking if a Git branch has been merged into master when SQUASHED?

Question

I'd like to automate cleaning of remote branches. I want to be able to check to see if a branch has been merged into master already. Originally, my plan was to use git merge-base to see the last common commit.

However, it turns out that we squash all of our branches as we merge them into master. As this creates a new commit hash, I'm unable to find the common commits between master and my branches.

How might I go about determining if a branch has been merged if we've squashed them all when merging?

Thanks in advance!

Answer 1

Here's one approach:

1. Use the GitHub API to get the HEAD SHA for all merged pull requests.
2. Match these SHAs against local branches.

If a local branch's HEAD has corresponds to that of a PR that's been merged, then you can safely delete it. This will work regardless of how the PR was merged (full merge, fast foward, squash and merge).

I implemented this approach in a delete-squashed-branches script. Here's what usage looks like:

(master) $ git branch
  delete-merged-prs
  fixup
  unmerged-branch
* master

(master) $ delete-squashed-branches
Finding local branches which were merged onto origin/master via GitHub...
warning: deleting branch 'delete-merged-prs' that has been merged to
     'refs/remotes/origin/delete-merged-prs', but not yet merged to HEAD.
Deleted branch delete-merged-prs (was 325a42b).
warning: deleting branch 'fixup' that has been merged to
     'refs/remotes/origin/fixup', but not yet merged to HEAD.
Deleted branch fixup (was e54f54b).

(master) $ git branch
  unmerged-branch
* master

(The warnings can be ignored since the branches were merged onto master via Squash & Merge.)

Caveats:

• You'll need to pip install pygithub to use the script.
• It will look for local branches that were merged onto origin/(current branch). So you'll want to run it on master, develop or whatever branch you work off of.
• If you have multiple long-lived branches, this approach won't work very well.

Answer 2

You could do an experimental merge and check whether it introduces any changes, like so:

git checkout -b foo-merge-test-branch master
git merge --squash foo
if [ -n "$(git status --porcelain)" ]; then 
  echo "foo has not been merged";
else 
  echo "foo is already merged";
fi
git reset --hard && git checkout master && git branch -d foo-merge-test-branch

This will only work if none of the files changed in the original git merge --squash foo have seen further conflicting changes on master since then.