[BUG] Amending a shallow clone -> orphan branch

Consider the following steps :

git clone --depth 1 --single-branch --branch master 
https://github.com/git/git
cd git
 >1.txt
git add 1.txt
git commit --amend

This results in an orphaned branch, where a single commit contains 
entire tree.

I understand that this is a bug, because certainly git knew shallow 
commit's parents and could reuse that when amending?

Hi Alexandr, welcome to Git mailing list!

On 06/08/21 07.04, Alexandr Miloslavskiy wrote:

Consider the following steps :

git clone --depth 1 --single-branch --branch master 
https://github.com/git/git
cd git
 >1.txt
git add 1.txt
git commit --amend

This results in an orphaned branch, where a single commit contains 
entire tree.

I understand that this is a bug, because certainly git knew shallow 
commit's parents and could reuse that when amending?

I tested that, and below is `git status` after the test (using Git 2.32.0):

On branch master
Your branch and 'origin/master' have diverged,
and have 1 and 1 different commits each, respectively.
  (use "git pull" to merge the remote branch into yours)

nothing to commit, working tree clean

So you're amending merge commit by adding dummy file, so that your 
`master` branch becomes divergent against origin. That dummy file isn't 
contained in any commits that are included with that merge commit.

-- 
An old man doll... just what I always wanted! - Clara

Bagas,

Thanks for testing.

 > I tested that, and below is `git status` after the test (using Git 
2.32.0):
 >
 >> On branch master
 >> Your branch and 'origin/master' have diverged,
 >> and have 1 and 1 different commits each, respectively.
 >>   (use "git pull" to merge the remote branch into yours)
 >>
 >> nothing to commit, working tree clean
 >
 > So you're amending merge commit by adding dummy file, so that your
 > `master` branch becomes divergent against origin. That dummy file isn't
 > contained in any commits that are included with that merge commit.

What you probably didn't notice is that the new commit has no parents, 
that is, it formed a new orphaned branch. The expected behavior is 
amending the top commit and keeping it connected to the previous branch.

A bit more details:

$ git clone --depth 1 --single-branch --branch master 
https://github.com/git/git
$ cd git

Here, a shallow clone is made with just 1 commit.

$ git show -s --pretty=%P HEAD

Outputs nothing; that's already a small bug.
Surely the commit has parents!

$ git cat-file -p HEAD | grep parent

OK, that's better, finally the true parents are shown.

$ >1.txt
$ git add 1.txt
$ git commit --amend

Here, the top (and only in our clone) commit is amended.

$ git cat-file -p HEAD | grep parent

Nothing! This is the bug I'm reporting: amending the commit orphaned it.

To give a bit more background: I have a heavy-weight repo which I was 
testing on different machines. Since it's heavy, I decided to shallow 
clone just 1 commit. Upon testing on some machine, I found a small bug 
and amended a fix. Then I force-pushed. I expected the branch to be amended.

Instead, what I got was an orphaned branch, disconnected from all repo's 
branches, containing all files at once! Clearly not the best thing.

On 06.08.2021 15:21, Bagas Sanjaya wrote:

So you're amending merge commit by adding dummy file, so that your 
`master` branch becomes divergent against origin. That dummy file isn't 
contained in any commits that are included with that merge commit.

Note, the problem isn't about merge commits, it happens for regular 
commits as well. Maybe I should have picked a different repo for 
example. Still, the problem with losing all parents is the same 
regardless of merge/regular commit.