Re: git pull/rebase bug: when "onto" branch has rebasing branch's commits in reflog

Am 03.01.21 um 23:02 schrieb Andrew Oates:

I've poked at the source code but haven't found exactly what causes
the issue --- but if you do a 'git pull --rebase' or 'git rebase' onto
a tracking branch that has previously pointed to a commit that the
rebasing branch includes, the rebase will be a noop.

In practice I've hit this a few times lately when splitting a topic
branch into two branches after the fact.

Here is a short repro:

git init
touch file1
git add file1
git commit -a -m "first commit"
touch file2
git add file2
git commit -a -m "second commit"
touch file3
git add file3
git commit -a -m "third commit"
git checkout -b branch
git branch --set-upstream-to=master
git checkout master
git reset --hard 'HEAD^1'

touch file2.5
git add file2.5
git commit -a -m "second-and-a-half commit"
git --no-pager log --oneline --all --graph

#rm .git/logs/refs/heads/master

git checkout branch
git pull -v --rebase
git --no-pager log --oneline --all --graph

This outputs,
* 58432a7 (branch) third commit
| * 0e4f775 (HEAD -> master) second-and-a-half commit
|/
* 37b2e3f second commit
* 5e9f0b7 first commit
...
Successfully rebased and updated refs/heads/branch.
* 0e4f775 (HEAD -> branch, master) second-and-a-half commit
* 37b2e3f second commit
* 5e9f0b7 first commit

showing that "third commit" is lost.  If you execute the "rm ..."
line, then the sequence works as expected, and the final state is,

Successfully rebased and updated refs/heads/branch.
* b636309 (HEAD -> branch) third commit
* 410a5dc (master) second-and-a-half commit
* 41981d0 second commit
* 286398d first commit

My best guess is that there's something odd happening in get_fork_point().

To me, the outcome looks reasonable:

By the time when the branch was created, it had no commits on top of
master. Then master was rewound and grew in a different direction, while
the branch didn't do anything. When you rebase it to its upstream, you
should expect that no commits are rebased.

-- Hannes