Re: [PATCH] rebase --update-refs: avoid unintended ref deletion

[Phillip Wood <phillip.wood123@gmail.com>]

Hi Victoria

On 04/11/2022 16:57, Victoria Dye wrote:
> In b3b1a21d1a5 (sequencer: rewrite update-refs as user edits todo list,
> 2022-07-19), the 'todo_list_filter_update_refs()' step was added to handle
> the removal of 'update-ref' lines from a 'rebase-todo'. Specifically, it
> removes potential ref updates from the "update refs state" if a ref does not
> have a corresponding 'update-ref' line.
> 
> However, because 'write_update_refs_state()' will not update the state if
> the 'refs_to_oids' list was empty, removing *all* 'update-ref' lines will
> result in the state remaining unchanged from how it was initialized (with
> all refs' "after" OID being null). Then, when the ref update is applied, all
> refs will be updated to null and consequently deleted.

Thanks for taking the time to track down the cause of this bug and fix it.

> To fix this, add a 'force_if_empty' flag to allow writing the update refs
> state even if 'refs_to_oids' is empty. The three usages of
> 'write_update_refs_state()' are updated as follows:
> 
> - in 'todo_list_filter_update_refs()': force_if_empty is 1 because update
>    ref entries are removed here. This setting fixes the ref deletion issue.
> - in 'do_update_ref()': force_if_empty is 0, since this method only modifies
>    (does not add or delete) ref update entries.
> - in 'todo_list_add_update_ref_commands()': force_if_empty is 0, since this
>    method strictly adds ref update entries.

I think not writing the list if it is empty is just an optimization to 
avoid creating an empty file. I wonder if it would be simpler to 
unlink() any existing file if write_update_refs_state() is called with 
an empty list rather than adding the force flag.

> Additionally, add a test covering the "all update-ref lines removed" case.

That's great

Best Wishes

Phillip

> Reported-by: herr.kaste <herr.kaste@gmail.com>
> Signed-off-by: Victoria Dye <vdye@github.com>
> ---
> This fixes the issue reported in [1]. I initially misinterpreted the root
> cause (thought that 'todo_list_filter_update_refs()' was only applied in the
> case of '--edit-todo'). After looking into it a bit more, it appears that
> the actual failure case is much narrower, occurring only when *all*
> 'update-ref' lines were deleted from the 'rebase-todo'.
> 
> Thanks!
> - Victoria
> 
> [1] https://lore.kernel.org/git/CAFzd1+5F4zqQ1CNeY2xaaf0r__JmE4ECiBt5h5OdiJHbaE78VA@mail.gmail.com/
> 
>   sequencer.c                   | 15 ++++++++++-----
>   t/t3404-rebase-interactive.sh | 24 ++++++++++++++++++++++++
>   2 files changed, 34 insertions(+), 5 deletions(-)
> 
> diff --git a/sequencer.c b/sequencer.c
> index e658df7e8ff..4d99a4fd6ca 100644
> --- a/sequencer.c
> +++ b/sequencer.c
> @@ -4122,7 +4122,7 @@ static int do_merge(struct repository *r,
>   	return ret;
>   }
> 
> -static int write_update_refs_state(struct string_list *refs_to_oids)
> +static int write_update_refs_state(struct string_list *refs_to_oids, int force_if_empty)
>   {
>   	int result = 0;
>   	struct lock_file lock = LOCK_INIT;
> @@ -4130,7 +4130,12 @@ static int write_update_refs_state(struct string_list *refs_to_oids)
>   	struct string_list_item *item;
>   	char *path;
> 
> -	if (!refs_to_oids->nr)
> +	/*
> +	 * If 'force' is specified, we want to write the updated refs even if
> +	 * the list is empty. This is only needed for callers that may have
> +	 * deleted items from 'refs_to_oids'.
> +	 */
> +	if (!refs_to_oids->nr && !force_if_empty)
>   		return 0;
> 
>   	path = rebase_path_update_refs(the_repository->gitdir);
> @@ -4260,7 +4265,7 @@ void todo_list_filter_update_refs(struct repository *r,
>   	}
> 
>   	if (updated)
> -		write_update_refs_state(&update_refs);
> +		write_update_refs_state(&update_refs, 1);
>   	string_list_clear(&update_refs, 1);
>   }
> 
> @@ -4281,7 +4286,7 @@ static int do_update_ref(struct repository *r, const char *refname)
>   		}
>   	}
> 
> -	write_update_refs_state(&list);
> +	write_update_refs_state(&list, 0);
>   	string_list_clear(&list, 1);
>   	return 0;
>   }
> @@ -6015,7 +6020,7 @@ static int todo_list_add_update_ref_commands(struct todo_list *todo_list)
>   		}
>   	}
> 
> -	res = write_update_refs_state(&ctx.refs_to_oids);
> +	res = write_update_refs_state(&ctx.refs_to_oids, 0);
> 
>   	string_list_clear(&ctx.refs_to_oids, 1);
> 
> diff --git a/t/t3404-rebase-interactive.sh b/t/t3404-rebase-interactive.sh
> index 4f5abb5ad25..e7d3721ece8 100755
> --- a/t/t3404-rebase-interactive.sh
> +++ b/t/t3404-rebase-interactive.sh
> @@ -1964,6 +1964,30 @@ test_expect_success 'respect user edits to update-ref steps' '
>   	test_cmp_rev HEAD refs/heads/no-conflict-branch
>   '
> 
> +test_expect_success '--update-refs: do not delete refs if all update-ref are removed' '
> +	git checkout -b test-refs-not-removed no-conflict-branch &&
> +	git branch -f base HEAD~4 &&
> +	git branch -f first HEAD~3 &&
> +	git branch -f second HEAD~3 &&
> +	git branch -f third HEAD~1 &&
> +	git branch -f tip &&
> +	(
> +		set_cat_todo_editor &&
> +		test_must_fail git rebase -i --update-refs base >todo.raw &&
> +		sed -e "/^update-ref/d" <todo.raw >todo
> +	) &&
> +	(
> +		set_replace_editor todo &&
> +		git rebase -i --update-refs base
> +	) &&
> +
> +	test_cmp_rev HEAD~3 refs/heads/first &&
> +	test_cmp_rev HEAD~3 refs/heads/second &&
> +	test_cmp_rev HEAD~1 refs/heads/third &&
> +	test_cmp_rev HEAD refs/heads/tip &&
> +	test_cmp_rev HEAD refs/heads/no-conflict-branch
> +'
> +
>   test_expect_success '--update-refs: check failed ref update' '
>   	git checkout -B update-refs-error no-conflict-branch &&
>   	git branch -f base HEAD~4 &&
> --
> 2.38.0
>