get_worktrees() must return main worktree as first item even on error

This is required by git-worktree.txt, stating that the main worktree is
the first line (especially in --porcelain mode when we can't just change
behavior at will).

There's only one case when get_worktrees() may skip main worktree, when
parse_ref() fails. Update the code so that we keep first item as main
worktree and return something sensible in this case:

 - In user-friendly mode, since we're not constraint by anything,
   returning "(error)" should do the job (we already show "(detached
   HEAD)" which is not machine-friendly). Actually errors should be
   printed on stderr by parse_ref() (*)

 - In plumbing mode, we do not show neither 'bare', 'detached' or
   'branch ...', which is possible by the format description if I read
   it right.

Careful readers may realize that when the local variable "head_ref" in
get_main_worktree() is emptied, add_head_info() will do nothing to
wt->head_sha1. But that's ok because head_sha1 is zero-ized in the
previous patch.

(*) Well, it does not. But it's supposed to be a stop gap implementation
    until we can reuse refs code to parse "ref: " stuff in HEAD, from
    resolve_refs_unsafe(). Now may be the time since refs refactoring is
    mostly done.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/worktree.c

@@ -388,7 +388,7 @@ static void show_worktree_porcelain(struct worktree *wt)
 		printf("HEAD %s\n", sha1_to_hex(wt->head_sha1));
 		if (wt->is_detached)
 			printf("detached\n");
-		else
+		else if (wt->head_ref)
 			printf("branch %s\n", wt->head_ref);
 	}
 	printf("\n");
@@ -408,8 +408,10 @@ static void show_worktree(struct worktree *wt, int path_maxlen, int abbrev_len)
 				find_unique_abbrev(wt->head_sha1, DEFAULT_ABBREV));
 		if (wt->is_detached)
 			strbuf_addstr(&sb, "(detached HEAD)");
-		else
+		else if (wt->head_ref)
 			strbuf_addf(&sb, "[%s]", shorten_unambiguous_ref(wt->head_ref, 0));
+		else
+			strbuf_addstr(&sb, "(error)");
 	}
 	printf("%s\n", sb.buf);
 

t/t2027-worktree-list.sh

@@ -96,4 +96,25 @@ test_expect_success 'bare repo cleanup' '
 	rm -rf bare1
 '
 
+test_expect_success 'broken main worktree still at the top' '
+	git init broken-main &&
+	(
+		cd broken-main &&
+		test_commit new &&
+		git worktree add linked &&
+		cat >expected <<-EOF &&
+		worktree $(pwd)
+		HEAD $_z40
+
+		EOF
+		cd linked &&
+		echo "worktree $(pwd)" >expected &&
+		echo "ref: .broken" >../.git/HEAD &&
+		git worktree list --porcelain | head -n 3 >actual &&
+		test_cmp ../expected actual &&
+		git worktree list | head -n 1 >actual.2 &&
+		grep -F "(error)" actual.2
+	)
+'
+
 test_done

worktree.c

@@ -88,16 +88,13 @@ static struct worktree *get_main_worktree(void)
 
 	strbuf_addf(&path, "%s/HEAD", get_git_common_dir());
 
-	if (parse_ref(path.buf, &head_ref, &is_detached) < 0)
-		goto done;
-
 	worktree = xcalloc(1, sizeof(*worktree));
 	worktree->path = strbuf_detach(&worktree_path, NULL);
 	worktree->is_bare = is_bare;
 	worktree->is_detached = is_detached;
-	add_head_info(&head_ref, worktree);
+	if (!parse_ref(path.buf, &head_ref, &is_detached))
+		add_head_info(&head_ref, worktree);
 
-done:
 	strbuf_release(&path);
 	strbuf_release(&worktree_path);
 	strbuf_release(&head_ref);
@@ -173,8 +170,7 @@ struct worktree **get_worktrees(void)
 
 	list = xmalloc(alloc * sizeof(struct worktree *));
 
-	if ((list[counter] = get_main_worktree()))
+	list[counter++] = get_main_worktree();
-		counter++;
 
 	strbuf_addf(&path, "%s/worktrees", get_git_common_dir());
 	dir = opendir(path.buf);