Re: [PATCH v4 2/8] builtin/refs: get worktrees without reading head information

[Karthik Nayak <karthik.188@gmail.com>]

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shejialuo <shejialuo@gmail.com> writes:

> In "packed-backend.c", there are some functions such as "create_snapshot"
> and "next_record" which would check the correctness of the content of
> the "packed-ref" file. When anything is bad, the program will die.
>
> It may seem that we have nothing relevant to above feature, because we
> are going to read and parse the raw "packed-ref" file without creating
> the snapshot and using the ref iterator to check the consistency.
>
> However, when using "get_worktrees" in "builtin/refs", we would parse
> the "HEAD" information. If the referent of the "HEAD" is inside the
> "packed-ref", we will call "create_snapshot" function to parse the
> "packed-ref" to get the information. No matter whether the entry of
> "HEAD" in "packed-ref" is correct, "create_snapshot" would call
> "verify_buffer_safe" to check whether there is a newline in the last
> line of the file. If not, the program will die.
>

Nit: while the second paragraph above makes sense in the context of what
we're trying to achieve in this patch series. It doesn't make much sense
for this patch in isolation. Perhaps we want to give some more context
around what we're trying to solve for in the upcoming patches and hence
how it hinders that.

> Although this behavior has no harm for the program, it will
> short-circuit the program. When the users execute "git refs verify" or
> "git fsck", we should avoid reading the head information, which may
> execute the read operation in packed backend with stricter checks to die
> the program. Instead, we should continue to check other parts of the
> "packed-refs" file completely.
>
> Fortunately, in 465a22b338 (worktree: skip reading HEAD when repairing
> worktrees, 2023-12-29), we have introduced a function
> "get_worktrees_internal" which allows us to get worktrees without
> reading head information.
>
> Create a new exposed function "get_worktrees_without_reading_head", then
> replace the "get_worktrees" in "builtin/refs" with the new created
> function.
>
> Mentored-by: Patrick Steinhardt <ps@pks.im>
> Mentored-by: Karthik Nayak <karthik.188@gmail.com>
> Signed-off-by: shejialuo <shejialuo@gmail.com>
> ---
>  builtin/refs.c | 2 +-
>  worktree.c     | 5 +++++
>  worktree.h     | 6 ++++++
>  3 files changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/builtin/refs.c b/builtin/refs.c
> index a29f195834..55ff5dae11 100644
> --- a/builtin/refs.c
> +++ b/builtin/refs.c
> @@ -88,7 +88,7 @@ static int cmd_refs_verify(int argc, const char **argv, const char *prefix,
>  	git_config(git_fsck_config, &fsck_refs_options);
>  	prepare_repo_settings(the_repository);
>
> -	worktrees = get_worktrees();
> +	worktrees = get_worktrees_without_reading_head();
>  	for (size_t i = 0; worktrees[i]; i++)
>  		ret |= refs_fsck(get_worktree_ref_store(worktrees[i]),
>  				 &fsck_refs_options, worktrees[i]);
> diff --git a/worktree.c b/worktree.c
> index 248bbb39d4..89b7d86cef 100644
> --- a/worktree.c
> +++ b/worktree.c
> @@ -175,6 +175,11 @@ struct worktree **get_worktrees(void)
>  	return get_worktrees_internal(0);
>  }
>
> +struct worktree **get_worktrees_without_reading_head(void)
> +{
> +	return get_worktrees_internal(1);
> +}
> +
>  const char *get_worktree_git_dir(const struct worktree *wt)
>  {
>  	if (!wt)
> diff --git a/worktree.h b/worktree.h
> index 38145df80f..1ba4a161a0 100644
> --- a/worktree.h
> +++ b/worktree.h
> @@ -30,6 +30,12 @@ struct worktree {
>   */
>  struct worktree **get_worktrees(void);
>
> +/*
> + * Like `get_worktrees`, but does not read HEAD. This is useful when checking
> + * the consistency, as reading HEAD may not be necessary.

Checking what consistency? We should be a bit more verbose here. You can
mention that skipping HEAD allows to get the worktree without worrying
about failures pertaining to parsing the HEAD ref.

> + */
> +struct worktree **get_worktrees_without_reading_head(void);
> +
>  /*
>   * Returns 1 if linked worktrees exist, 0 otherwise.
>   */
> --
> 2.48.1

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