Re: Git submodule update remote keeps using previous default branch

On Fri, Oct 15, 2021 at 1:35 AM Sjoerd Langkemper
[off-list ref] wrote:

I am having trouble with git 2.33.1 checking out the wrong branch for
submodules for which the default branch has changed. `git submodule
update --remote` seems to remember the branch name to retrieve, while
I expect it to use the remote HEAD every time. This causes unexpected
behaviour when the remote HEAD starts pointing to another branch.

I create a new git project and add a submodule, with `git submodule
add git@host:foo/testproject.git`.

This checks out the default branch, `master` in this case. `git remote
show origin` also shows that `master` is the HEAD branch. Running `git
submodule update --remote` updates the submodule to the latest master.

Now I change the default branch on the remote (using Gitlab's web
interface) to `newmaster`.

`git remote show origin` now correctly shows `newmaster` as the
remote's HEAD branch. However, running `git submodule update --remote`
still updates the submodule to the latest `master` branch, while I
expect it to update to the lastest `newmaster` branch.

I suspect this is going to be the same answer I gave on the last
thread about submodules[1]. Submodules in Git store the _commit ID_
they point to in the tree, but they do not store a _ref name_, nor do
they have some indicator for "It was the default branch". They choose
a ref by (essentially) looping over the refs from the remote (which
are always sent in alphabetical order) and choosing the first one with
a matching SHA. If your "newmaster" is alphabetically after "master",
and "master" still exists, and both refs point to the same commit (or
only "master" points to it), I suspect your submodule is going to
choose that.

[1] https://lore.kernel.org/git/CAGyf7-FAHJOb6iQYqYNt0WSk+zUHUJ_FjrU1xis1bBQd9Z6KPQ@mail.gmail.com/ (local)

There's no branch specified in .gitmodules or .git/config. I am not
sure how git remembers the branch. When switching `testproject` to
`newmaster` manually and then running `git submodule update --remote`,
it is reset to `master` again.

Is this a bug? Can I change the branch somehow?

If you updated the submodule to reference a commit that was on
"newmaster" and not present on "master", I think it'd change.

Regards,

Sjoerd Langkemper