repository: add a compatibility hash algorithm

We currently have support for using a full stage 4 SHA-256
implementation.  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 <[email protected]>
Signed-off-by: Eric W. Biederman <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: ebiederm

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 */

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);

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 =