repository: enable initialization of submodules

Introduce 'repo_submodule_init()' which performs initialization of a
'struct repository' as a submodule of another 'struct repository'.

The resulting submodule 'struct repository' can be in one of three states:

  1. The submodule is initialized and has a worktree.

  2. The submodule is initialized but does not have a worktree.  This
     would occur when the submodule's gitdir is present in the
     superproject's 'gitdir/modules/' directory yet the submodule has not
     been checked out in superproject's worktree.

  3. The submodule remains uninitialized due to an error in the
     initialization process or there is no matching submodule at the
     provided path in the superproject.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

repository.c

@@ -144,6 +144,58 @@ error:
 	return -1;
 }
 
+/*
+ * Initialize 'submodule' as the submodule given by 'path' in parent repository
+ * 'superproject'.
+ * Return 0 upon success and a non-zero value upon failure.
+ */
+int repo_submodule_init(struct repository *submodule,
+			struct repository *superproject,
+			const char *path)
+{
+	const struct submodule *sub;
+	struct strbuf gitdir = STRBUF_INIT;
+	struct strbuf worktree = STRBUF_INIT;
+	int ret = 0;
+
+	sub = submodule_from_cache(superproject, null_sha1, path);
+	if (!sub) {
+		ret = -1;
+		goto out;
+	}
+
+	strbuf_repo_worktree_path(&gitdir, superproject, "%s/.git", path);
+	strbuf_repo_worktree_path(&worktree, superproject, "%s", path);
+
+	if (repo_init(submodule, gitdir.buf, worktree.buf)) {
+		/*
+		 * If initilization fails then it may be due to the submodule
+		 * not being populated in the superproject's worktree.  Instead
+		 * we can try to initilize the submodule by finding it's gitdir
+		 * in the superproject's 'modules' directory.  In this case the
+		 * submodule would not have a worktree.
+		 */
+		strbuf_reset(&gitdir);
+		strbuf_repo_git_path(&gitdir, superproject,
+				     "modules/%s", sub->name);
+
+		if (repo_init(submodule, gitdir.buf, NULL)) {
+			ret = -1;
+			goto out;
+		}
+	}
+
+	submodule->submodule_prefix = xstrfmt("%s%s/",
+					      superproject->submodule_prefix ?
+					      superproject->submodule_prefix :
+					      "", path);
+
+out:
+	strbuf_release(&gitdir);
+	strbuf_release(&worktree);
+	return ret;
+}
+
 void repo_clear(struct repository *repo)
 {
 	free(repo->gitdir);
@@ -158,6 +210,8 @@ void repo_clear(struct repository *repo)
 	repo->index_file = NULL;
 	free(repo->worktree);
 	repo->worktree = NULL;
+	free(repo->submodule_prefix);
+	repo->submodule_prefix = NULL;
 
 	if (repo->config) {
 		git_configset_clear(repo->config);

repository.h

@@ -43,6 +43,13 @@ struct repository {
 	 */
 	char *worktree;
 
+	/*
+	 * Path from the root of the top-level superproject down to this
+	 * repository.  This is only non-NULL if the repository is initialized
+	 * as a submodule of another repository.
+	 */
+	char *submodule_prefix;
+
 	/* Subsystems */
 	/*
 	 * Repository's config which contains key-value pairs from the usual
@@ -80,6 +87,9 @@ extern struct repository *the_repository;
 extern void repo_set_gitdir(struct repository *repo, const char *path);
 extern void repo_set_worktree(struct repository *repo, const char *path);
 extern int repo_init(struct repository *repo, const char *gitdir, const char *worktree);
+extern int repo_submodule_init(struct repository *submodule,
+			       struct repository *superproject,
+			       const char *path);
 extern void repo_clear(struct repository *repo);
 
 extern int repo_read_index(struct repository *repo);