[RFC] branch: delete gone-upstream branches only when proven merged into a named ref

[DonQuinleone <don@donquinleone.com>]

Hi all,

This is my first time posting to the list, so good to meet everyone,
and thanks in advance for bearing with me.

I've just noticed that "git branch --delete-merged" is now available,
and that it never deletes a topic in the workflow that hosting
services (e.g. GitHub) encourage. Before writing a patch, I wanted to
ask how the list might consider closing that gap.

In that workflow, the topic tracks and pushes to a branch of its own
name; when the pull request is merged with a merge commit, the server
deletes that branch. With master as of c51408889:

  $ git push -u origin topic
       ... merged on the server, and origin's topic deleted ...

  $ git fetch origin
  $ git branch --dry-run --delete-merged 'origin/*'
       ... spared, as pushing would update the upstream ...

  $ git fetch --prune origin
  $ git branch -vv
  * master 61e9886 [origin/master: behind 2] base
    topic  87d0c48 [origin/topic: gone] topic work
  $ git branch --dry-run --delete-merged 'origin/*'
       ... skipped, because the upstream is gone ...
  $ git merge-base --is-ancestor topic origin/master && echo merged
  merged

So the branch is provably merged, but at no point in this workflow
will --delete-merged remove it. A script reproducing this is at the
bottom.

From reading back through the archives, this is deliberate. When v7
of the series fell back to the remote's default branch for a gone
upstream, Junio wrote [1]:

> If the upstream itself is gone, removing anything based on some other
> criteria cannot by definition a "safety check".

I agree that a vanished upstream proves nothing, so I would like to
propose separating selection from proof:

"Upstream gone" may select a candidate, but it is never proof. The
only proof is that the branch tip is reachable from a ref the user
names explicitly, so nothing is inferred: neither HEAD, nor the
remote's default branch.

I can see two possible shapes.

(A) A modifier on --delete-merged:

  $ git branch --delete-merged 'origin/*' --into origin/master

Branches whose upstream matches the pattern but no longer exists would
become candidates, and would be deleted only if their tip is reachable
from the named commit. check_branch_commit() would be handed that
commit in place of HEAD, as suggested on v7 [1]. Branches whose
upstream still exists would be judged exactly as today, and the
worktree, stacked-branch and deleteMerged=false exclusions would still
apply.

(B) Composable filters on -d:

  $ git branch --upstream-gone --merged=origin/master      # preview
  $ git branch -d --upstream-gone --merged=origin/master   # delete

--upstream-gone would be useful on its own for listing (today the
usual approach is to grep "git branch -vv" for ": gone]"). But it
turns -d into a second bulk-delete mode beside --delete-merged, and
--merged defaults to HEAD when it comes last, so a forgotten argument
would quietly bring back HEAD as the proxy.

I lean towards (A) because it is small and keeps every existing
protection, but I'd rather settle the shape here than assume anything.

So:

 1. Is "selection is not proof" an acceptable basis for touching
    gone-upstream branches?

 2. (A), (B), or something else? If (A), is there a better name than
    --into (--merged-into?), and should it stay limited to branches
    whose upstream is gone?

I would leave squashing and rebasing out of scope, in keeping with
--delete-merged staying "one stable and strict" operation [2]. I've
Cc'd Harald, as this relates closely to his in-flight --delete-merged
topics [3][4], and Phillip, who reviewed the series.

Thanks in advance. Look forward to hearing what people think.

~ Don

[1] https://lore.kernel.org/git/xmqq5x4sn393.fsf@gitster.g/
[2] https://lore.kernel.org/git/xmqqy0c21gq1.fsf@gitster.g/
[3] https://lore.kernel.org/git/pull.2428.v2.git.git.1791102886740.gitgitgadget@gmail.com/
[4] https://lore.kernel.org/git/94a6d23f8b96d4a07fb58c71f4a2f7dc4d3055bc.1791711665.git.gitgitgadget@gmail.com/

-- >8 --
#!/bin/sh
set -e
export GIT_CONFIG_GLOBAL=/dev/null GIT_CONFIG_NOSYSTEM=1
export GIT_AUTHOR_NAME=A GIT_AUTHOR_EMAIL=a@example.com
export GIT_COMMITTER_NAME=C GIT_COMMITTER_EMAIL=c@example.com
cd "$(mktemp -d)"
git init -q --bare -b master origin.git
git clone -q origin.git work 2>/dev/null && cd work
git commit -q --allow-empty -m base && git push -q origin HEAD:master
git switch -q -c topic && git commit -q --allow-empty -m "topic work"
git push -q -u origin topic 2>/dev/null && git switch -q master
git clone -q ../origin.git ../server && (cd ../server &&
	git merge -q --no-ff -m "Merge topic" origin/topic &&
	git push -q origin master :topic)
echo "after fetch:";         git fetch -q
git branch --dry-run --delete-merged 'origin/*'
echo "after fetch --prune:"; git fetch -q --prune
git branch --dry-run --delete-merged 'origin/*'
git merge-base --is-ancestor topic origin/master && echo merged