[PATCH 06/32] repository: Implement core.compatMap

@@ -736,3 +736,9 @@ core.abbrev::
 	If set to "no", no abbreviation is made and the object names
 	are shown in their full length.
 	The minimum length is 4.
+
+core.compatMap::
+	Enables the use of a compat map to recored the hash in the
+	other object format.  This allows repositories in different
+	objects formats to interoperate.  It allows looking up old oids
+	in a repository that has been converted from sha1 to sha256.

@@ -104,6 +104,16 @@ void repo_set_hash_algo(struct repository *repo, int hash_algo)
 	repo->hash_algo = &hash_algos[hash_algo];
 }
 
+void repo_enable_compat_map(struct repository *repo, int enable_compat)
+{
+	const struct git_hash_algo *other_algo =
+		&hash_algos[(hash_algo_by_ptr(repo->hash_algo) == GIT_HASH_SHA1) ?
+			GIT_HASH_SHA256 :
+			GIT_HASH_SHA1];
+
+	repo->compat_hash_algo = enable_compat ? other_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 +194,7 @@ int repo_init(struct repository *repo,
 		goto error;
 
 	repo_set_hash_algo(repo, format.hash_algo);
+	repo_enable_compat_map(repo, format.use_compat_map);
 	repo->repository_format_worktree_config = format.worktree_config;
 
 	/* take ownership of format.partial_clone */

@@ -202,6 +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_enable_compat_map(struct repository *repo, int enable_compat);
 void initialize_the_repository(void);
 RESULT_MUST_BE_USED
 int repo_init(struct repository *r, const char *gitdir, const char *worktree);

@@ -623,6 +623,8 @@ static int check_repo_format(const char *var, const char *value,
 			return 0;
 		}
 	}
+	else if (strcmp(var, "core.compatmap") == 0)
+		data->use_compat_map = git_config_bool(var, value);
 
 	return read_worktree_config(var, value, ctx, vdata);
 }

@@ -1564,8 +1566,10 @@ 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_enable_compat_map(the_repository, repo_fmt.use_compat_map);
 			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;

@@ -1657,6 +1661,7 @@ 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_enable_compat_map(the_repository, fmt->use_compat_map);
 	the_repository->repository_format_worktree_config =
 		fmt->worktree_config;
 	the_repository->repository_format_partial_clone =

@@ -86,6 +86,7 @@ struct repository_format {
 	int worktree_config;
 	int is_bare;
 	int hash_algo;
+	int use_compat_map;
 	int sparse_index;
 	char *work_tree;
 	struct string_list unknown_extensions;