get_locked_file_path(): new function

Add a function to return the path of the file that is locked by a
lock_file object. This reduces the knowledge that callers have to have
about the lock_file layout.

Suggested-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-lockfile.txt

@@ -143,6 +143,11 @@ hold_lock_file_for_append::
 	the existing contents of the file (if any) to the lockfile and
 	position its write pointer at the end of the file.
 
+get_locked_file_path::
+
+	Return the path of the file that is locked by the specified
+	lock_file object. The caller must free the memory.
+
 commit_lock_file::
 
 	Take a pointer to the `struct lock_file` initialized with an

cache.h

@@ -590,6 +590,7 @@ extern void unable_to_lock_message(const char *path, int err,
 extern NORETURN void unable_to_lock_die(const char *path, int err);
 extern int hold_lock_file_for_update(struct lock_file *, const char *path, int);
 extern int hold_lock_file_for_append(struct lock_file *, const char *path, int);
+extern char *get_locked_file_path(struct lock_file *);
 extern int commit_lock_file_to(struct lock_file *, const char *path);
 extern int commit_lock_file(struct lock_file *);
 extern int reopen_lock_file(struct lock_file *);

lockfile.c

@@ -257,6 +257,15 @@ int hold_lock_file_for_append(struct lock_file *lk, const char *path, int flags)
 	return fd;
 }
 
+char *get_locked_file_path(struct lock_file *lk)
+{
+	if (!lk->active)
+		die("BUG: get_locked_file_path() called for unlocked object");
+	if (lk->filename.len <= LOCK_SUFFIX_LEN)
+		die("BUG: get_locked_file_path() called for malformed lock object");
+	return xmemdupz(lk->filename.buf, lk->filename.len - LOCK_SUFFIX_LEN);
+}
+
 int close_lock_file(struct lock_file *lk)
 {
 	int fd = lk->fd;

refs.c

@@ -2606,9 +2606,7 @@ static int delete_ref_loose(struct ref_lock *lock, int flag)
 		 * loose.  The loose file name is the same as the
 		 * lockfile name, minus ".lock":
 		 */
-		char *loose_filename = xmemdupz(
-				lock->lk->filename.buf,
-				lock->lk->filename.len - LOCK_SUFFIX_LEN);
+		char *loose_filename = get_locked_file_path(lock->lk);
 		int err = unlink_or_warn(loose_filename);
 		free(loose_filename);
 		if (err && errno != ENOENT)