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Re: [PATCH v4 3/3] rebase: set REF_HEAD_DETACH in checkout_up_to_date()

From
Junio C Hamano <gitster@pobox.com>
Date
Mar 17, 2022, 21:36 UTC
Message-ID
<xmqq8rt8dzxr.fsf@gitster.g>
In-Reply-To
<13c5955c317713bbc6a91b9f44081395880abb67.1647546828.git.gitgitgadget@gmail.com>
"John Cai via GitGitGadget" <gitgitgadget@gmail.com> writes:
> From: John Cai <johncai86@gmail.com>
>
> Fixes a bug whereby rebase updates the deferenced reference HEAD points
> to instead of HEAD directly.
Perhaps
    "git rebase A B", where B is not a commit, should behave as if
    the HEAD got detached at B and then the detached HEAD got
    rebased on top of A.  A bug however overwrites the current
    branch to point at B, when B is a descendant of A (i.e. the
    rebase ends up being a fast-forward).
> ... See [1] for
> the original bug report.
OK (URL is wrong; see below).

The explanation of how the bug occurs (elided) in the patch looked all reasonable. It read well.

> ...
> Also add a test to ensure the correct behavior.

Yup. _Add_ a test to ensure that. Not replace a misleading test that expected to see a wrong behaviour.

> 1. https://lore.kernel.org/git/xmqqsfrpbepd.fsf@gitster.g/

This is not the original bug report. It was an early hint for diagnosis.

[1] https://lore.kernel.org/git/YiokTm3GxIZQQUow@newk/
would be a more appropriate pointer.
Show 6 quoted lines
>  	ropts.oid = &options->orig_head;
>  	ropts.branch = options->head_name;
>  	ropts.flags = RESET_HEAD_RUN_POST_CHECKOUT_HOOK;
> +	if (!ropts.branch)
> +		ropts.flags |=  RESET_HEAD_DETACH;
>  	ropts.head_msg = buf.buf;

OK. If head_name is not set, we do not want to touch the branch the HEAD happens to be pointing at, so we want to detach.

Show 10 quoted lines
> +test_expect_success 'switch to non-branch detaches HEAD' '
>  	git checkout main &&
>  	old_main=$(git rev-parse HEAD) &&
>  	git rebase First Second^0 &&
> -	test_cmp_rev HEAD main &&
> -	test_cmp_rev main $(git rev-parse Second) &&
> -	git symbolic-ref HEAD
> +	test_cmp_rev HEAD Second &&
> +	test_cmp_rev main $old_main &&
> +	test_must_fail git symbolic-ref HEAD

As we want (1) HEAD (detached) is pointing at Second, (2) 'main' stayed at $old_main, and (3) HEAD is detched, these three conditions look sane.

Thanks.

For reference, I discarded [1/3], queued [2/3] and replaced this [3/3] with the following for now.

---- >8 ---- ---- >8 ---- ---- >8 ---- ---- >8 ---- ---- >8 ----
From: John Cai <johncai86@gmail.com>
Subject: [PATCH] rebase: set REF_HEAD_DETACH in checkout_up_to_date()

"git rebase A B" where B is not a commit should behave as if the HEAD got detached at B and then the detached HEAD got rebased on top of A. A bug however overwrites the current branch to point at B, when B is a descendant of A (i.e. the rebase ends up being a fast-forward). See [1] for the original bug report.

The callstack from checkout_up_to_date() is the following:
cmd_rebase()
-> checkout_up_to_date()
   -> reset_head()
      -> update_refs()
         -> update_ref()

When B is not a valid branch but an oid, rebase sets the head_name of rebase_options to NULL. This value gets passed down this call chain through the branch member of reset_head_opts also getting set to NULL all the way to update_refs().

Then update_refs() checks ropts.branch to decide whether or not to switch branches. If ropts.branch is NULL, it calls update_ref() to update HEAD. At this point however, from rebase's point of view, we want a detached HEAD. But, since checkout_up_to_date() does not set the RESET_HEAD_DETACH flag, the update_ref() call will deference HEAD and update the branch its pointing to. We want the HEAD detached at B instead.

Fix this bug by adding the RESET_HEAD_DETACH flag in checkout_up_to_date if B is not a valid branch, so that once reset_head() calls update_refs(), it calls update_ref() with REF_NO_DEREF which updates HEAD directly intead of deferencing it and updating the branch that HEAD points to.

Also add a test to ensure the correct behavior.
[1] https://lore.kernel.org/git/YiokTm3GxIZQQUow@newk/
Reported-by: Michael McClimon <michael@mcclimon.org>
Signed-off-by: John Cai <johncai86@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 builtin/rebase.c  | 2 ++
 t/t3400-rebase.sh | 9 +++++++++
 2 files changed, 11 insertions(+)
diff --git a/builtin/rebase.c b/builtin/rebase.c
index b29ad2b65e..27fde7bf28 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -829,6 +829,8 @@ static int checkout_up_to_date(struct rebase_options *options)
 	ropts.oid = &options->orig_head;
 	ropts.branch = options->head_name;
 	ropts.flags = RESET_HEAD_RUN_POST_CHECKOUT_HOOK;
+	if (!ropts.branch)
+		ropts.flags |=  RESET_HEAD_DETACH;
 	ropts.head_msg = buf.buf;
 	if (reset_head(the_repository, &ropts) < 0)
 		ret = error(_("could not switch to %s"), options->switch_to);
diff --git a/t/t3400-rebase.sh b/t/t3400-rebase.sh
index 6dc8df8be7..cf55b017ff 100755
--- a/t/t3400-rebase.sh
+++ b/t/t3400-rebase.sh
@@ -389,6 +389,15 @@ test_expect_success 'switch to branch not checked out' '
 	git rebase main other
 '
 
+test_expect_success 'switch to non-branch detaches HEAD' '
+	git checkout main &&
+	old_main=$(git rev-parse HEAD) &&
+	git rebase First Second^0 &&
+	test_cmp_rev HEAD Second &&
+	test_cmp_rev main $old_main &&
+	test_must_fail git symbolic-ref HEAD
+'
+
 test_expect_success 'refuse to switch to branch checked out elsewhere' '
 	git checkout main &&
 	git worktree add wt &&

-- 
2.35.1-757-g4266a5c05c
Previous: John Cai via GitGitGadgetNext: John Cai
Message 35 of 41 in “rebase: set REF_HEAD_DETACH in checkout_up_to_date()”
  1. rebase: set REF_HEAD_DETACH in checkout_up_to_date()John Cai via GitGitGadget, Mar 11, 2022
  2. Junio C HamanoMar 11, 2022
  3. Junio C HamanoMar 11, 2022
  4. John CaiMar 11, 2022
  5. Phillip WoodMar 11, 2022
  6. John CaiMar 11, 2022
  7. John CaiMar 11, 2022
  8. Phillip WoodMar 11, 2022
  9. 0/2 rebase: update HEAD when is an oidJohn Cai via GitGitGadget, Mar 11, 2022
  10. 1/2 rebase: use test_commit helper in setupJohn Cai via GitGitGadget, Mar 11, 2022
  11. Junio C HamanoMar 13, 2022
  12. Phillip WoodMar 14, 2022
  13. Junio C HamanoMar 14, 2022
  14. 2/2 rebase: set REF_HEAD_DETACH in checkout_up_to_date()John Cai via GitGitGadget, Mar 11, 2022
  15. Junio C HamanoMar 13, 2022
  16. Phillip WoodMar 14, 2022
  17. Phillip WoodMar 14, 2022
  18. John CaiMar 14, 2022
  19. Phillip WoodMar 14, 2022
  20. 0/2 rebase: update HEAD when is an oidJohn Cai via GitGitGadget, Mar 17, 2022
  21. 2/2 rebase: set REF_HEAD_DETACH in checkout_up_to_date()John Cai via GitGitGadget, Mar 17, 2022
  22. Ævar Arnfjörð BjarmasonMar 17, 2022
  23. Junio C HamanoMar 17, 2022
  24. 1/2 rebase: use test_commit helper in setupJohn Cai via GitGitGadget, Mar 17, 2022
  25. Ævar Arnfjörð BjarmasonMar 17, 2022
  26. 0/3 rebase: update HEAD when is an oidJohn Cai via GitGitGadget, Mar 17, 2022
  27. 1/3 rebase: test showing bug in rebase with non-branchJohn Cai via GitGitGadget, Mar 17, 2022
  28. Junio C HamanoMar 17, 2022
  29. Junio C HamanoMar 17, 2022
  30. John CaiMar 17, 2022
  31. 2/3 rebase: use test_commit helper in setupJohn Cai via GitGitGadget, Mar 17, 2022
  32. Phillip WoodMar 18, 2022
  33. John CaiMar 18, 2022
  34. 3/3 rebase: set REF_HEAD_DETACH in checkout_up_to_date()John Cai via GitGitGadget, Mar 17, 2022
  35. Junio C HamanoMar 17, 2022
  36. John CaiMar 18, 2022
  37. 0/2 rebase: update HEAD when is an oidJohn Cai via GitGitGadget, Mar 18, 2022
  38. 1/2 rebase: use test_commit helper in setupJohn Cai via GitGitGadget, Mar 18, 2022
  39. Junio C HamanoMar 18, 2022
  40. 2/2 rebase: set REF_HEAD_DETACH in checkout_up_to_date()John Cai via GitGitGadget, Mar 18, 2022
  41. Junio C HamanoMar 18, 2022

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