Subject: Re: [GSoC][PATCH] commit: warn the usage of reverse_commit_list() helper

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Tue, 07 Feb 2023 18:56:26 +0100

On Tue, Feb 07 2023, Kousik Sanagavarapu wrote:

> The helper function reverse_commit_list() has destructive behavior when
> used to reverse a list in-place. Warn about this behavior.
>
> Signed-off-by: Kousik Sanagavarapu <[email-masked]>
> ---
>
> This patch has been sent based on the confusion that can be caused while
> using the reverse_commit_list() helper function. One example of this is
> a recent patch that I submitted[1] where the use of this function broke
> try_merge_strategy() in merge.
>
> It is also based on the discussions[2] there that I send this patch.
>
> [1]: https://lore.kernel.org/git/[email-masked]/
> [2]: https://lore.kernel.org/git/[email-masked]/
>
>  commit.h | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/commit.h b/commit.h
> index fa39202fa6..9dba07748f 100644
> --- a/commit.h
> +++ b/commit.h
> @@ -198,7 +198,12 @@ void commit_list_sort_by_date(struct commit_list **list);
>  /* Shallow copy of the input list */
>  struct commit_list *copy_commit_list(struct commit_list *list);
>  
> -/* Modify list in-place to reverse it, returning new head; list will be tail */
> +/*
> + * Modify list in-place to reverse it, returning new head; list will be tail.
> + *
> + * NOTE! The reversed list is constructed using the elements of the original
> + * list, hence losing the original list.
> + */
>  struct commit_list *reverse_commit_list(struct commit_list *list);

Junio can clarify, but I understood from his original comment on this
suggesting a comment that he wasn't aware of the existing documentation.

I think it's better just to chuck this up to an understandable one-off
mistake, if we're going to update the docs here I don't really see
what's being added by this addition.

It seems to me that this is just rephrasing what's being said more
succinctly with "modifies in-place", it's understood that any function
which does that is going to schred the input data for its own purposes.

If that wording is thought to be too technical or obscure wouldn't we be
better off with replacing the existing wording with something using less
jargon, rather than keeping the jargon & adding a rephrasing of it?

Having said that, I think the existing version is fine, and we could
just ascribe the issue that prompted this to a one-off mistake :)

I think if you want to pursue this, a much better improvement here would
be to show what the user *should* do.

E.g. show one code example of using the API in-place, and then the
preferred pattern if one wants to produce a new reversed commit list,
while retaining the original (presumably just copy_commit_list()
followed by reverse_commit_list()).