[Bug report] Git gets stuck in rebase mode

[Bug report] Git gets stuck in rebase mode

[LeSeulArtichaut]

What did you do before the bug happened? (Steps to reproduce your issue)

I tried to rebase a branch, say `master`, onto a commit that does not exist in my local repository, say `37b52f3b750b11b9ef60db118f737a66af3bf242`:
`git rebase master 37b52f3b750b11b9ef60db118f737a66af3bf242`

- The branch has to be valid (e.g. `git rebase foo 37b52f3b750b11b9ef60db118f737a66af3bf242` doesn’t reproduce the bug)
- The commit SHA has to be a full SHA (e.g. `git rebase master 37b52f3b750b11b9ef60db118f737a66af3bf2 #42` doesn’t reproduce the bug)

Repro "from scratch":
```zsh
mkdir git-bug
cd git-bug
git init
touch foo.txt
git add foo.txt
git commit -m 'Initial commit'
git rebase master 37b52f3b750b11b9ef60db118f737a66af3bf242
```

What did you expect to happen? (Expected behavior)
Git either successfully applies the rebase or gives an error.

What happened instead? (Actual behavior)
```
error: Could not read 37b52f3b750b11b9ef60db118f737a66af3bf242
fatal: Invalid symmetric difference expression b91ebd8a08983ed5c27a3946821be02a39f99b3c…37b52f3b750b11b9ef60db118f737a66af3bf242
```

Git then instead gets « stuck » in rebase mode (i.e. a `.git/rebase-merge` directory exists), with the following visible effects:
 - `git status` kind of gets confused:
```bash
$ git status
On branch master
git-rebase-todo is missing.
No commands done.
No commands remaining.
You are currently editing a commit during a rebase.
  (use "git commit --amend" to amend the current commit)
  (use "git rebase --continue" once you are satisfied with your changes)

nothing to commit, working tree clean
```
 - powerlevel10k displays a `rebase-i` state
 - `git rebase --continue` and `git rebase --abort` fail:
```bash
$ git rebase --continue
error: could not open '.git/rebase-merge/git-rebase-todo': No such file or directory
$ git rebase --abort
fatal: unable to read tree 37b52f3b750b11b9ef60db118f737a66af3bf242
```

This disappears by deleting the `.git/rebase-merge` directory.

What's different between what you expected and what actually happened?
I didn’t expect Git to stay stuck in rebase. After the failure in the `rebase` command, the `.git/rebase-merge` directory should be deleted.


[System Info]
git version:
git version 2.30.0
cpu: x86_64
no commit associated with this build
sizeof-long: 8
sizeof-size_t: 8
shell-path: /bin/sh
uname: Darwin 19.6.0 Darwin Kernel Version 19.6.0: Tue Nov 10 00:10:30 PST 2020; root:xnu-6153.141.10~1/RELEASE_X86_64 x86_64
compiler info: clang: 12.0.0 (clang-1200.0.32.28)
libc info: no libc information available
$SHELL (typically, interactive shell): /bin/zsh


[Enabled Hooks]

[PATCH] rebase: verify commit parameter

[René Scharfe]

If the user specifies a base commit to switch to, check if it actually
references a commit right away to avoid getting confused later on when
it turns out to be an invalid object.

Reported-by: LeSeulArtichaut <leseulartichaut@gmail.com>
Signed-off-by: René Scharfe <l.s.r@web.de>
---
git-legacy-rebase.sh also accepted non-commits, if I read the code
correctly.

 builtin/rebase.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/builtin/rebase.c b/builtin/rebase.c
index 19c7b377aa..840dbd7eb7 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -1917,7 +1917,9 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 			die_if_checked_out(buf.buf, 1);
 			options.head_name = xstrdup(buf.buf);
 		/* If not is it a valid ref (branch or commit)? */
-		} else if (!get_oid(branch_name, &options.orig_head))
+		} else if (!get_oid(branch_name, &options.orig_head) &&
+			   lookup_commit_reference(the_repository,
+						   &options.orig_head))
 			options.head_name = NULL;
 		else
 			die(_("fatal: no such branch/commit '%s'"),
--
2.30.0