Subject: Re: [PATCH] setup: copy repository_format using helper

From: Victoria Dye <[email-masked]>
Date: Mon, 12 Jun 2023 17:03:17 -0700

Glen Choo wrote:
> In several parts of the setup machinery, we set up a repository_format
> and then use it to set up the_repository in nearly the exact same way,
> suggesting that we might be able to use a helper function to standardize
> the behavior and make future modifications easier. Create this helper
> function, setup_repository_from_format(), thus standardizing this
> behavior.
> 
> To determine what the 'standardized behavior' should be, we can compare
> the candidate call sites in repo_init(), setup_git_directory_gently(),
> check_repository_format() and discover_git_directory(),
> 
> - All of them copy .worktree_config.
> 
> - All of them 'copy' .partial_clone. Most perform a shallow copy of the
>   pointer, then set the .partial_clone = NULL so that it doesn't get
>   cleared by clear_repository_format(). However,
>   check_repository_format() copies the string deeply because the
>   repository_format is sometimes read back (it is an "out" parameter).
>   To accomodate both shallow copying and deep copying, toggle this
>   behavior using the "modify_fmt_ok" parameter.

Do you have a specific example of this happening? I see two uses of
'check_repository_format()' in the codebase:

1. in 'enter_repo()' ('path.c')
2. in 'init_db()' ('init-db.c')

The first one calls 'check_repository_format()' with 'NULL', which causes
the function to create a temporary 'struct repository_format' that is then
discarded at the end of the function - no need to worry about the value
being cleared there.

The second one does call 'check_repository_format()' with a 'struct
repository_format' instance, but the 'partial_clone' field field is not
accessed again after that. The only subsequent usages of the 'repo_fmt'
variable in 'init_db()' are:

- in 'validate_hash_algorithm()', where only the 'version' and 'hash_algo'
  fields are accessed.
- in 'create_default_files()', where only 'hash_algo' is accessed.

So, shouldn't it be safe to shallow-copy-and-NULL? But as I noted earlier
[1], if you do that it'll make the name 'check_repository_format()' a bit
misleading (since it's actually modifying its arg in place). So, if you
update to always shallow copy, 'check_repository_format()' should be renamed
to reflect its side effects.

[1] https://lore.kernel.org/git/[email-masked]/

> 
> - Most of them set up repository.hash_algo, except
>   discover_git_directory(). Our helper function unconditionally sets up
>   .hash_algo because it turns out that discover_git_directory() probably
>   doesn't need to set up "struct repository" at all!

If that's the case, shouldn't the 'repository_format' assignments in
'discover_git_directory()' be removed altogether? 

>   discover_git_directory() isn't actually used in the setup process - its
>   only caller in the Git binary is read_early_config(). As explained by
>   16ac8b8db6 (setup: introduce the discover_git_directory() function,
>   2017-03-13), it is supposed to be an entrypoint into setup.c machinery
>   that allows the Git directory to be discovered without side effects,
>   in other words, we shouldn't have introduced side effects in
>   ebaf3bcf1ae (repository: move global r_f_p_c to repo struct,
>   2021-06-17). Fortunately, we didn't start to rely on this unintended
>   behavior between then and now, so we can just drop it.
> 
> Signed-off-by: Glen Choo <[email-masked]>
> ---
> Here's the helper function I had in mind. I was initially mistaken and
> it turns out that we need to support deep copying, but fortunately,
> t0001 is extremely thorough and will catch virtually any mistake in the
> setup process. CI seems to pass, though it appears to be a little flaky
> today and sometimes cancels jobs
> (https://github.com/chooglen/git/actions/runs/5249029150).
> 
> If you're comfortable with it, I would prefer for you to squash this
> into your patches so that we don't just end up changing the same few
> lines. If not, I'll Reviewed-by your patches (if I don't find any other
> concerns on a re-read) and send this as a 1-patch on top.

Reading through the commit message & patch, I'm still not convinced this
refactor is a good idea - to me, it doesn't leave the code in a clearly
better state. If you feel strongly that it does, though, I'm happy to leave
it to others to review/decide but I would prefer that you keep it a separate
patch submission on top.

Thanks!

> diff --git a/repository.c b/repository.c
> index 104960f8f5..50f0b26a6c 100644
> --- a/repository.c
> +++ b/repository.c
> @@ -181,12 +181,7 @@ int repo_init(struct repository *repo,
>  	if (read_and_verify_repository_format(&format, repo->commondir))
>  		goto error;
>  
> -	repo_set_hash_algo(repo, format.hash_algo);
> -	repo->repository_format_worktree_config = format.worktree_config;
> -
> -	/* take ownership of format.partial_clone */
> -	repo->repository_format_partial_clone = format.partial_clone;
> -	format.partial_clone = NULL;
> +	setup_repository_from_format(repo, &format, 1);
>  
>  	if (worktree)
>  		repo_set_worktree(repo, worktree);
> diff --git a/setup.c b/setup.c
> index d866395435..33ce58676f 100644
> --- a/setup.c
> +++ b/setup.c
> @@ -1561,13 +1561,8 @@ const char *setup_git_directory_gently(int *nongit_ok)
>  			setup_git_env(gitdir);
>  		}
>  		if (startup_info->have_repository) {
> -			repo_set_hash_algo(the_repository, repo_fmt.hash_algo);
> -			the_repository->repository_format_worktree_config =
> -				repo_fmt.worktree_config;
> -			/* take ownership of repo_fmt.partial_clone */
> -			the_repository->repository_format_partial_clone =
> -				repo_fmt.partial_clone;
> -			repo_fmt.partial_clone = NULL;
> +			setup_repository_from_format(the_repository,
> +						     &repo_fmt, 1);
>  		}
>  	}
>  	/*
> @@ -1654,14 +1649,26 @@ void check_repository_format(struct repository_format *fmt)
>  		fmt = &repo_fmt;
>  	check_repository_format_gently(get_git_dir(), fmt, NULL);
>  	startup_info->have_repository = 1;
> -	repo_set_hash_algo(the_repository, fmt->hash_algo);
> -	the_repository->repository_format_worktree_config =
> -		fmt->worktree_config;
> -	the_repository->repository_format_partial_clone =
> -		xstrdup_or_null(fmt->partial_clone);
> +	setup_repository_from_format(the_repository, fmt, 0);
>  	clear_repository_format(&repo_fmt);
>  }
>  

I think you may be missing changes to 'discover_git_directory()'? Like I
mentioned above, though, if you don't think 'discover_git_directory()' needs
to set up 'the_repository', then those assignments should just be removed
(not replaced with 'setup_repository_from_format()').