[RFC/PATCH v3 1/3] tree.c: teach read_tree_recursive how to traverse gitlink entries

[Lars Hjemli <hjemli@gmail.com>]

When the callback function invoked from read_tree_recursive() returns
`READ_TREE_RECURSIVE` for a gitlink entry, the traversal will now
continue into the tree connected to the gitlinked commit. It is the
responsibility of the callback function to somehow make the gitlinked
commit (and corresponding tree/blob) objects available, possibly by
inserting the submodule object database as an alternate odb.

Also, all existing callback function has been updated to only return
READ_TREE_RECURSIVE for directory entries, so this patch should not
introduce any changes to current behavior.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
---
 archive.c         |    2 +-
 builtin-ls-tree.c |    9 ++-------
 merge-recursive.c |    2 +-
 tree.c            |   28 ++++++++++++++++++++++++++++
 4 files changed, 32 insertions(+), 9 deletions(-)

diff --git a/archive.c b/archive.c
index 9ac455d..e6de039 100644
--- a/archive.c
+++ b/archive.c
@@ -132,7 +132,7 @@ static int write_archive_entry(const unsigned char *sha1, const char *base,
 		err = write_entry(args, sha1, path.buf, path.len, mode, NULL, 0);
 		if (err)
 			return err;
-		return READ_TREE_RECURSIVE;
+		return (S_ISDIR(mode) ? READ_TREE_RECURSIVE : 0);
 	}
 
 	buffer = sha1_file_to_archive(path_without_prefix, sha1, mode,
diff --git a/builtin-ls-tree.c b/builtin-ls-tree.c
index 5b63e6e..fca4631 100644
--- a/builtin-ls-tree.c
+++ b/builtin-ls-tree.c
@@ -68,13 +68,8 @@ static int show_tree(const unsigned char *sha1, const char *base, int baselen,
 		 *
 		 * Something similar to this incomplete example:
 		 *
-		if (show_subprojects(base, baselen, pathname)) {
-			struct child_process ls_tree;
-
-			ls_tree.dir = base;
-			ls_tree.argv = ls-tree;
-			start_command(&ls_tree);
-		}
+		if (show_subprojects(base, baselen, pathname))
+			retval = READ_TREE_RECURSIVE;
 		 *
 		 */
 		type = commit_type;
diff --git a/merge-recursive.c b/merge-recursive.c
index b97026b..ee853b9 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -237,7 +237,7 @@ static int save_files_dirs(const unsigned char *sha1,
 		string_list_insert(newpath, &o->current_file_set);
 	free(newpath);
 
-	return READ_TREE_RECURSIVE;
+	return (S_ISDIR(mode) ? READ_TREE_RECURSIVE : 0);
 }
 
 static int get_files_dirs(struct merge_options *o, struct tree *tree)
diff --git a/tree.c b/tree.c
index 03e782a..dfe4d5f 100644
--- a/tree.c
+++ b/tree.c
@@ -131,6 +131,34 @@ int read_tree_recursive(struct tree *tree,
 			if (retval)
 				return -1;
 			continue;
+		} else if (S_ISGITLINK(entry.mode)) {
+			int retval;
+			struct strbuf path;
+			unsigned int entrylen;
+			struct commit *commit;
+
+			entrylen = tree_entry_len(entry.path, entry.sha1);
+			strbuf_init(&path, baselen + entrylen + 1);
+			strbuf_add(&path, base, baselen);
+			strbuf_add(&path, entry.path, entrylen);
+			strbuf_addch(&path, '/');
+
+			commit = lookup_commit(entry.sha1);
+			if (!commit)
+				die("Commit %s in submodule path %s not found",
+				    sha1_to_hex(entry.sha1), path.buf);
+
+			if (parse_commit(commit))
+				die("Invalid commit %s in submodule path %s",
+				    sha1_to_hex(entry.sha1), path.buf);
+
+			retval = read_tree_recursive(commit->tree,
+						     path.buf, path.len,
+						     stage, match, fn, context);
+			strbuf_release(&path);
+			if (retval)
+				return -1;
+			continue;
 		}
 	}
 	return 0;
-- 
1.6.1.150.g5e733b