Re: Bug in git submodule update --remote

I am also reposting your message, adding hard line breaks so that
it displays more readably in the list archive, and also so that
anyone else can choose to quote you from this.

On 14-May-2021, at 23:41, Ben Avison [off-list ref] wrote:

A recent-ish change in git 2.27.1, introduced in commit f0a96e8d, has
also got me wondering about whether some related functionality is
correct. I'm not sure what the best way to fix things is, so can I
invite opinions?

The scenario: I have a repository with a submodule. The submodule's
upstream repository uses a fork workflow, so the submodule has two
remotes, one for pulling in other people's changes, and one for
uploading my own pull requests.

$ mkdir myproject
$ cd myproject
$ git init
$ git submodule add https://github.com/acme/library
$ cd library
$ git remote add -f myfork https://github.com/user/library
$ cd ..
$ git add .
$ git commit -m "Initial commit"
$ git remote add origin https://github.com/user/myproject
$ git push -u origin master

Furthermore, assume I forked https://github.com/acme/example some time
ago, so that the master branches between it and my fork have diverged.

Time passes. People hack away on both projects. I want to fix a bug or
implement a new feature in the library, so I start by ensuring both are
up-to-date:

$ git pull $ git submodule update --remote
$ cd library
$ git checkout -b new-feature
$ # hack away
$ git add .
$ git commit -m "New feature"
$ git push -u myfork new-feature
$ cd ..

Some more time passes, and I want to work on it again. Again, I start by
ensuring I'm up-to-date. Before git 2.27.1, I could do:

$ git submodule update --remote

Now, I get:

fatal: Needed a single revision Unable to find current myfork/HEAD
revision in submodule path 'library'

What's going on is that within "git submodule update --remote", the sha1
used is formed by looking up the submodule's ref "<remote>/<branch>".
The change in git 2.27.1 is that if no remote tracking branch is stated
in the superproject's .gitmodules file, <branch> defaults to "HEAD"
rather than "master" as previously. That's fine if <remote> is "origin",
but in practice, it depends on how the submodule is currently checked
out:

* if it's in detached HEAD state, <remote> is set to "origin"
* if its branch is not a tracking branch, "origin" is also used
* but if it's a tracking branch (as I used in my workflow above - not
  uncommon in pull request workflows because you might grant other
  people access to the branch during the review process) then it looks
  up the name of the remote which is being tracked

First observation: a ref called "myfork/HEAD" presumably *could* have
been created by the "git remote add" command, reflecting that remote's
default branch. Should it?

In practice, that's not actually what I'd want, though. If I do "git
submodule update --remote", I personally normally do so as a shortcut
compared to cloning everything again. I don't expect the behaviour to
change depending on whether I happen to have left one of the submodules
checked out on a tracking branch or not: myfork/master is potentially
quite out of date compared to origin/master.

It also makes very little sense to me that issuing "git submodule update
--remote" twice in quick succession would leave us in a different state,
because the first call puts all the submodules into detached HEAD state,
meaning that the second call always looks up the remote tracking
branches from the submodule's "origin" remote.

One way this could be fixed is that if <branch> turns out to be "HEAD",
we could force <remote> to be "origin". However, this doesn't address
the equivalent problem that arises if the remote tracking branch *is*
named in .gitmodules.

I'd therefore like to propose that "git submodule update --remote"
*always* uses the remote named "origin" to form the ref used to check
out the submodule. However, I'm not sure whether everyone would agree
that this constitutes a bugfix, or whether I'd need to introduce a new
switch to enforce this behaviour.

So, what do you think?

Ben