Re: [PATCH v2 04/30] repository: add a compatibility hash algorithm

> However, we'd like to support interoperability with
> SHA-1 repositories as well.  The transition plan anticipates a
> compatibility hash algorithm configuration option that we can use to
> implement support for this.  Let's add an element to the repository
> structure that indicates the compatibility hash algorithm so we can use
> it when we need to consider interoperability between algorithms.
> 
> Add a helper function repo_set_compat_hash_algo that takes a
> compatibility hash algorithm and sets "repo->compat_hash_algo".  If
> GIT_HASH_UNKNOWN is passed as the compatibility hash algorithm
> "repo->compat_hash_algo" is set to NULL.
> 
> For now, the code results in "repo->compat_hash_algo" always being set
> to NULL, but that will change once a configuration option is added.
> 
> Inspired-by: brian m. carlson <sandals@crustytoothpaste.net>
> Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
> ---
>  repository.c | 8 ++++++++
>  repository.h | 4 ++++
>  setup.c      | 3 +++
>  3 files changed, 15 insertions(+)
> 
> diff --git a/repository.c b/repository.c
> index a7679ceeaa45..80252b79e93e 100644
> --- a/repository.c
> +++ b/repository.c
> @@ -104,6 +104,13 @@ void repo_set_hash_algo(struct repository *repo, int hash_algo)
>  	repo->hash_algo = &hash_algos[hash_algo];
>  }
>  
> +void repo_set_compat_hash_algo(struct repository *repo, int algo)
> +{
> +	if (hash_algo_by_ptr(repo->hash_algo) == algo)
> +		BUG("hash_algo and compat_hash_algo match");
> +	repo->compat_hash_algo = algo ? &hash_algos[algo] : NULL;
> +}
> +
>  /*
>   * Attempt to resolve and set the provided 'gitdir' for repository 'repo'.
>   * Return 0 upon success and a non-zero value upon failure.
> @@ -184,6 +191,7 @@ int repo_init(struct repository *repo,
>  		goto error;
>  
>  	repo_set_hash_algo(repo, format.hash_algo);
> +	repo_set_compat_hash_algo(repo, GIT_HASH_UNKNOWN);
>  	repo->repository_format_worktree_config = format.worktree_config;
>  
>  	/* take ownership of format.partial_clone */
> diff --git a/repository.h b/repository.h
> index 5f18486f6465..bf3fc601cc53 100644
> --- a/repository.h
> +++ b/repository.h
> @@ -160,6 +160,9 @@ struct repository {
>  	/* Repository's current hash algorithm, as serialized on disk. */
>  	const struct git_hash_algo *hash_algo;
>  
> +	/* Repository's compatibility hash algorithm. */
> +	const struct git_hash_algo *compat_hash_algo;
> +
>  	/* A unique-id for tracing purposes. */
>  	int trace2_repo_id;
>  
> @@ -199,6 +202,7 @@ void repo_set_gitdir(struct repository *repo, const char *root,
>  		     const struct set_gitdir_args *extra_args);
>  void repo_set_worktree(struct repository *repo, const char *path);
>  void repo_set_hash_algo(struct repository *repo, int algo);
> +void repo_set_compat_hash_algo(struct repository *repo, int compat_algo);
>  void initialize_the_repository(void);
>  RESULT_MUST_BE_USED
>  int repo_init(struct repository *r, const char *gitdir, const char *worktree);
> diff --git a/setup.c b/setup.c
> index 18927a847b86..aa8bf5da5226 100644
> --- a/setup.c
> +++ b/setup.c
> @@ -1564,6 +1564,8 @@ const char *setup_git_directory_gently(int *nongit_ok)
>  		}
>  		if (startup_info->have_repository) {
>  			repo_set_hash_algo(the_repository, repo_fmt.hash_algo);
> +			repo_set_compat_hash_algo(the_repository,
> +						  GIT_HASH_UNKNOWN);
>  			the_repository->repository_format_worktree_config =
>  				repo_fmt.worktree_config;
>  			/* take ownership of repo_fmt.partial_clone */
> @@ -1657,6 +1659,7 @@ void check_repository_format(struct repository_format *fmt)
>  	check_repository_format_gently(get_git_dir(), fmt, NULL);
>  	startup_info->have_repository = 1;
>  	repo_set_hash_algo(the_repository, fmt->hash_algo);
> +	repo_set_compat_hash_algo(the_repository, GIT_HASH_UNKNOWN);
>  	the_repository->repository_format_worktree_config =
>  		fmt->worktree_config;
>  	the_repository->repository_format_partial_clone =