git bare repositories, worktrees and tracking branches

Question

I'm working with a code base where I need to be working on several branches at once for different purposes. So I clone to a bare repository and then set up some worktrees:

git clone --bare ssh://[email protected]/project/repo repo.git
cd repo.git
git worktree add ../branch-1 branch-1
git worktree add ../branch-2 branch-2
... someone else creates branch-3 and pushes is ...
git fetch origin +refs/heads/*:refs/heads/* --prune
git worktree add ../branch-3 branch-3

Now the branch-3 worktree isn't set to track the remote tree and trying to make it do so, I get into a horrible mess.

$ cd ../branch-3
$ git branch -u origin/branch-3
error: the requested upstream branch 'origin/refs/heads/feature/SW-5884-move-database-container-to-alpine-base-2' does not exist
hint: ...<snip>
$ git fetch +refs/heads/*:refs/remotes/origin/* --prune
$ git branch -u origin/branch-3
fatal: Cannot setup tracking information; starting point 'origin/feature/SW-5884-move-database-container-to-alpine-base-2' is not a branch.

What's the right magic to get this to work?

Answer 1

First, a side note: if you are going to use git worktree add for nontrival periods (more than two weeks at a time), be sure your Git is at least version 2.15.¹

For your particular purpose I'd recommend not using git clone --bare. Instead, use a regular clone, followed by the git worktree adds that you intend to do. You note in a comment that:

... you end up having to create a dummy branch to sit the repository itself on, because it's not possible to have both the repository and a worktree on the same branch at the same time.

There are several simple workarounds for this:

• Pick one of the N work-trees that you'll add, and use that as the branch in the main work-tree:

  git checkout -b branch-1 ssh://[email protected]/project/repo branch-1
  

  The drawback is that you now have a special distinguished "main" branch that you cannot just remove at any time—all the other branches depend on it.

• Or, after the clone, use git checkout --detach in the work-tree, to get a detached HEAD on the default branch:

  git clone ssh://[email protected]/project/repo repo.git
  cd repo.git
  git checkout --detach
  

• The only drawback to this second approach is that the work-tree is full of files, probably a waste of space. There's a solution to that as well: create a blank commit using the empty tree, and check that out:

  git clone ssh://[email protected]/project/repo repo.git
  cd repo.git
  git checkout $(git commit-tree $(git hash-object -t tree /dev/null) < /dev/null)
  

OK, the last one is not exactly obvious. It really is pretty simple though. The git hash-object -t tree /dev/null produces the hash ID of the empty tree that already exists in every repository. The git commit-tree makes a commit to wrap that empty tree—there isn't one, so we must make one to check it out—and prints out the hash ID of this new commit, and the git checkout checks that out as a detached HEAD. The effect is to empty out our index and work-tree, so that the only thing in the repository work-tree is the .git directory. The empty commit we made is on no branch and has no parent commit (it's a lone root commit) that we will never push anywhere.

---

¹The reason for this is that Git 2.5, where git worktree first appeared, has a bug I consider very bad: git gc never scans the added work-trees' HEAD files, nor their index files. If the added work-trees are always on some branch and never have any git added but uncommitted work, this never causes any problems. If uncommitted work does not sit around for at least 14 days, the default prune protection time suffices to keep it from being destroyed. But if you git add some work or commit on a detached HEAD, go on holiday for a month or otherwise leave this added work-tree undisturbed, and then come back to it, and a git gc --auto ran after that two-week grace period ran out, the files you saved have been destroyed!

This bug was fixed in Git 2.15.

---

Why --bare goes wrong

The root of the problem here is that git clone --bare does two things:

• it makes a bare repository (core.bare set to true) that has no work-tree and does no initial git checkout; and
• it changes the default fetch refspec from +refs/heads/*:refs/remotes/origin/* to +refs/heads/*:refs/heads/*.

This second item means that there are no refs/remotes/origin/ names, as you discovered. That's not easily repaired because of the (mostly hidden / internal) concept of refmaps, which show up very briefly in the git fetch documentation (see link).

Worse, it means that refs/heads/* will be updated on every git fetch. There is a reason git worktree add refuses to create a second work-tree that refers to the same branch that is checked out in any existing work-tree, and that is that Git fundamentally assumes that no one will mess with the refs/heads/name reference that HEAD is attached-to in this work-tree. So even if you did work around the refmap issue, when you run git fetch and it updates—or even removes, due to --prune and an upstream removal of the same name—the refs/heads/name name, the added work-tree's HEAD breaks, and the work-tree itself becomes problematic. (See Why does Git allow pushing to a checked-out branch in an added worktree? How shall I recover?)

There's one other thing you could try, which I have not tested at all, and that is: do the bare clone as you are doing, but then change the refspec and re-fetch, and delete all the existing branch names since they have no upstreams set (or, equivalently, set their upstreams):

git clone --bare ssh://[email protected]/project/repo repo.git
cd repo.git
git config remote.origin.fetch '+refs/heads/*:refs/remotes/origin/*'
git fetch
git for-each-ref --format='%(refname:short)' refs/heads | xargs git branch -d

(or replace the xargs with xargs -n1 -I{} git branch --set-upstream-to=origin/{} {}).

Answer 2

I thing there is a very simplest way;

1. clone the repo without checkout (no waste space)

git clone --no-checkout ssh://[email protected]/project/repo repo.git

2. go to the repo

cd repo.git

3. create a dummy branch to have the possibility to create a worktree on each existing one

git switch -c dummy

4. Now create your worktree as you want

git worktree add branch-1

or

git worktree add pathtobr1 branch-1

That's all. It is clean, easy without wasting space

Hope this helps ;-)