Below is a test case which shows a way that the git-annex branch can become corrupted and lose data, including location log records and uuid.log lines.

At the end, a commit on the git-annex branch removes one of the 2 lines from the uuid.log; which should never happen.

The actual problem occurs earlier, at the "push point". Here a repo is cloned from the main one, initialized (adding the last uuid.log line), and then pushed back to the main one. That push is a fast-forward, so is allowed to directly update the git-annex branch in the main repo:

b884fe5..c497739  git-annex -> git-annex

Now the git-annex branch has a change that is not reflected in .git/annex/index, so the next time a change is made, it's committed using the out of date index, which causes a reversion of the changes that were pushed to the branch.


Thoughts

This is essentially the same reason why git blocks pushes to the checked-out branch of a non-bare repository.

This problem only affects workflows that involve pushing. Pulling workflows do not directly update the local git-annex branch, so avoid the problem.

And while bare repos are pushed to, they rarely have changes made directly to their git-annex branches, so while I think the same problem could happen with pushing to a bare repo, it's unlikely.

None of which is to say this is not a bad bug that needs to be comprehensively fixed.

Probably git-annex needs to record which ref of the git-annex branch corresponds to its index, and if the branch is at a different ref, merge it into the index.

And now that's done. I managed to do it with very little slowdown.

A side benefit is that users can now safely check out the git-annex branch and commit changes to it, and git-annex will notice them. Before, it was documented to ignore such changes. --Joey


Workaround

Users who want to prevent this bug from occuring when pushing to their non-bare repositories can install this script as .git/hooks/update

#!/bin/sh
if [ "$1" = refs/heads/git-annex ]; then
    exit 1
fi

--Joey


Test Case

#!/bin/sh
mkdir annextest
cd annextest

git init dir1
cd dir1
git annex init
touch foo 
echo hi > bar
git annex add
git commit -m add

cd ..
git clone dir1 dir2
cd dir2
git annex init otherdir
git annex get
# push point
git push

cd ..
cd dir1
echo "before"
git show git-annex:uuid.log
git annex drop foo --force
echo "after"
git show git-annex:uuid.log
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