[RFC][PATCH 1/2] worktree: represent the primary worktree with '/' instead of NULL

[Shreyansh Paliwal <shreyanshpaliwalcmsmn@gmail.com>]

The worktree API uses NULL to represent the primary worktree. As a result,
many callers pass NULL to implicitly refer to it, which in turn requires
additional checks to ensure that `wt` is defined before use.

Represent the main worktree explicitly by setting `wt->id` to "/" in
`get_main_worktree()` and update `is_main_worktree()` accordingly. Replace
checks for wt to be defined in worktree.c functions with calls to
`is_main_worktree()` (or strcmp(wt->id, "/")).

Signed-off-by: Shreyansh Paliwal <shreyanshpaliwalcmsmn@gmail.com>
---
 worktree.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/worktree.c b/worktree.c
index 9308389cb6..b29934407f 100644
--- a/worktree.c
+++ b/worktree.c
@@ -101,6 +101,7 @@ static struct worktree *get_main_worktree(int skip_reading_head)
 
 	CALLOC_ARRAY(worktree, 1);
 	worktree->repo = the_repository;
+	worktree->id = xstrdup("/");
 	worktree->path = strbuf_detach(&worktree_path, NULL);
 	worktree->is_current = is_current_worktree(worktree);
 	worktree->is_bare = (is_bare_repository_cfg == 1) ||
@@ -127,6 +128,8 @@ struct worktree *get_linked_worktree(const char *id,
 
 	if (!id)
 		die("Missing linked worktree name");
+	if (!strcmp(id, "/"))
+		die("'/' is reserved for primary worktree");
 
 	repo_common_path_append(the_repository, &path, "worktrees/%s/gitdir", id);
 	if (strbuf_read_file(&worktree_path, path.buf, 0) <= 0)
@@ -206,9 +209,7 @@ struct worktree **get_worktrees_without_reading_head(void)
 
 char *get_worktree_git_dir(const struct worktree *wt)
 {
-	if (!wt)
-		return xstrdup(repo_get_git_dir(the_repository));
-	else if (!wt->id)
+	if (is_main_worktree(wt))
 		return xstrdup(repo_get_common_dir(the_repository));
 	else
 		return repo_common_path(the_repository, "worktrees/%s", wt->id);
@@ -277,7 +278,7 @@ struct worktree *find_worktree_by_path(struct worktree **list, const char *p)
 
 int is_main_worktree(const struct worktree *wt)
 {
-	return !wt->id;
+	return !strcmp(wt->id, "/");
 }
 
 const char *worktree_lock_reason(struct worktree *wt)
@@ -566,7 +567,7 @@ void strbuf_worktree_ref(const struct worktree *wt,
 {
 	if (parse_worktree_ref(refname, NULL, NULL, NULL) ==
 		    REF_WORKTREE_CURRENT &&
-	    wt && !wt->is_current) {
+	    !wt->is_current) {
 		if (is_main_worktree(wt))
 			strbuf_addstr(sb, "main-worktree/");
 		else
@@ -629,6 +630,9 @@ static void repair_gitfile(struct worktree *wt,
 	char *path = NULL;
 	int err;
 
+	if (is_main_worktree(wt))
+		goto done;
+
 	/* missing worktree can't be repaired */
 	if (!file_exists(wt->path))
 		goto done;
-- 
2.53.0