Re: Submodules: two confusing situations

From: Stefan Beller <sbeller@google.com>
Date: Nov 1, 2016, 22:56 UTC
Message-ID: <CAGZ79kbqVphQpDSEKFs=_6E1XNsn6rz=1ifRx9iEHFhRd9ZPOA@mail.gmail.com>
In-Reply-To: <6533af94ebd74952b75e51c5609d8c20@exmbdft7.ad.twosigma.com>

On Tue, Nov 1, 2016 at 3:13 PM, David Turner <David.Turner@twosigma.com> wrote:

Show 17 quoted lines

> Consider the following two cases:
>
> We have commit X and commit Y.  X is an ancestor of Y.
>
> We're at X and doing get checkout Y -- or we're at Y and we're doing
> git checkout X.
>
> Case 1: Between X and Y, we have deleted a submodule.
> In order to move from X to Y, git removes the submodule
> from the working tree.
>
> Case 2: Between X and Y, we have instead added a submodule.  In order
> to move from Y to X (that is, the opposite direction), git *does not*
> remove the submodule from the tree; instead, it gives a warning and
> leaves the submodule behind.
>
> I don't understand why these two cases are not symmetric.

Because you are using git-submodule.sh that is only an approximation of what is supposed to happen. ("git checkout [X | Y] && git submodule update" I'd guess).

"git submodule update" only *updates* the submodules and doesn't *delete* them at all. I think this originated from the historic behavior of having the submodules .git directory inside the submodule and not in the superprojects .git/module. When having the .git dir inside the submodule, you cannot delete the submodule as you'd potentially loose information (local commits) from the submodule.

I am currently working on implementing a flag --recurse-submodules for git-checkout that would be symmetrical from X -> Y and back, but that feature hasn't seen the mailing list yet as I discovered yet another bug locally. I think I found a reviewer though. ;)

The problem with doing a propoer checkout, i.e. update and deletion of
a submodule,
you need to be sure the submodule in its state can go away:
* no untracked files that are lost (except for gitignored files)
* clean index in the submodule and
* the submodule points at the recorded sha1 (i.e. there are no
  commits that would be dangling)

Show 10 quoted lines

>
> Perhaps relatedly, consider the attached shell-script, which I have not yet made into a full git test because I'm not sure I understand the issues fully.
>
> It creates three commits:
>
> Commit 1 adds a submodule
> Commit 2 removes that submodule and re-adds it into a subdirectory (sub1 to sub1/sub1).
> Commit 3 adds an unrelated file.
>
> Then it checks out commit 1 (first deinitializing the submodule to avoid case 2 above), and attempts to cherry-pick commit 3.  This seems like it ought to work, based on my understanding of cherry-pick.  But in fact it gives a conflict on the stuff from commit 2 (which is unrelated to commit 3).

That sounds like a bug indeed.

>
> This is confusing to me, and looks like a bug.  What am I missing?
>