[PATCH v2 3/5] worktree.c: add is_worktree_locked()

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

This provides an API for checking if a worktree is locked. We need to
check this to avoid double locking a worktree, or try to unlock one when
it's not even locked.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 worktree.c | 21 +++++++++++++++++++++
 worktree.h |  6 ++++++
 2 files changed, 27 insertions(+)

diff --git a/worktree.c b/worktree.c
index 6b3015b..cb30af3 100644
--- a/worktree.c
+++ b/worktree.c
@@ -231,6 +231,27 @@ int is_main_worktree(const struct worktree *wt)
 	return !wt->id;
 }
 
+const char *is_worktree_locked(const struct worktree *wt)
+{
+	static struct strbuf sb = STRBUF_INIT;
+	struct strbuf path = STRBUF_INIT;
+
+	strbuf_git_common_path(&path, "worktrees/%s/locked", wt->id);
+
+	if (!file_exists(path.buf)) {
+		strbuf_release(&path);
+		return NULL;
+	}
+
+	strbuf_reset(&sb);
+	if (strbuf_read_file(&sb, path.buf, 0) < 0)
+		die_errno(_("failed to read '%s'"), path.buf);
+	strbuf_release(&path);
+
+	strbuf_trim(&sb);
+	return sb.buf;
+}
+
 int is_worktree_being_rebased(const struct worktree *wt,
 			      const char *target)
 {
diff --git a/worktree.h b/worktree.h
index 39f1aa2..3a780c2 100644
--- a/worktree.h
+++ b/worktree.h
@@ -40,6 +40,12 @@ extern struct worktree *find_worktree_by_path(struct worktree **list,
  */
 extern int is_main_worktree(const struct worktree *wt);
 
+/*
+ * Return the reason string if the given worktree is locked. Return
+ * NULL otherwise. Return value is only valid until the next invocation.
+ */
+extern const char *is_worktree_locked(const struct worktree *wt);
+
 /*
  * Free up the memory for worktree(s)
  */
-- 
2.8.2.524.g6ff3d78