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 [off-list ref]
Signed-off-by: Eric W. Biederman <redacted>
---
 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 =