[PATCH 03/16] checkout: improve die_if_checked_out() robustness

[Eric Sunshine <sunshine@sunshineco.com>]

die_if_checked_out() is intended to check if the branch about to be
checked out is already checked out either in the main worktree or in a
linked worktree. However, if .git/worktrees directory does not exist,
then it never bothers checking the main worktree, even though the
specified branch might indeed be checked out there, which is fragile
behavior.

This hasn't been a problem in practice since the current implementation
of "git worktree add" (and, earlier, "git checkout --to") always creates
.git/worktrees before die_if_checked_out() is called by the child "git
checkout" invocation which populates the new worktree.

However, git-worktree will eventually be retrofitted to populate the new
worktree via "git reset --hard" rather than "git checkout", and it will
want to use die_if_checked_out() to perform this check as well. But,
the reliance upon order of operations (creating .git/worktrees before
checking if a branch is already checked out) is fragile, and, as a
general function, callers should not be expected to abide by this
undocumented and unwarranted restriction. Therefore, make
die_if_checked_out() more robust by checking the main worktree whether
.git/worktrees exists or not.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
---
 builtin/checkout.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/builtin/checkout.c b/builtin/checkout.c
index e75fb5e..fc8bd79 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -916,12 +916,6 @@ static void die_if_checked_out(struct branch_info *new)
 	DIR *dir;
 	struct dirent *d;
 
-	strbuf_addf(&path, "%s/worktrees", get_git_common_dir());
-	if ((dir = opendir(path.buf)) == NULL) {
-		strbuf_release(&path);
-		return;
-	}
-
 	/*
 	 * $GIT_COMMON_DIR/HEAD is practically outside
 	 * $GIT_DIR so resolve_ref_unsafe() won't work (it
@@ -929,6 +923,12 @@ static void die_if_checked_out(struct branch_info *new)
 	 */
 	check_linked_checkout(new, NULL);
 
+	strbuf_addf(&path, "%s/worktrees", get_git_common_dir());
+	if ((dir = opendir(path.buf)) == NULL) {
+		strbuf_release(&path);
+		return;
+	}
+
 	while ((d = readdir(dir)) != NULL) {
 		if (!strcmp(d->d_name, ".") || !strcmp(d->d_name, ".."))
 			continue;
-- 
2.5.0.rc1.201.ga12d9f8