Merge branch 'jc/reopen-lock-file'

There are cases where you lock and open to write a file, close it to
show the updated contents to external processes, and then have to
update the file again while still holding the lock, but the lockfile
API lacked support for such an access pattern.

* jc/reopen-lock-file:
  lockfile: allow reopening a closed but still locked file

cache.h

@@ -585,6 +585,7 @@ extern NORETURN void unable_to_lock_index_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 int commit_lock_file(struct lock_file *);
+extern int reopen_lock_file(struct lock_file *);
 extern void update_index_if_able(struct index_state *, struct lock_file *);
 
 extern int hold_locked_index(struct lock_file *, int);

lockfile.c

@@ -237,6 +237,16 @@ int close_lock_file(struct lock_file *lk)
 	return close(fd);
 }
 
+int reopen_lock_file(struct lock_file *lk)
+{
+	if (0 <= lk->fd)
+		die(_("BUG: reopen a lockfile that is still open"));
+	if (!lk->filename[0])
+		die(_("BUG: reopen a lockfile that has been committed"));
+	lk->fd = open(lk->filename, O_WRONLY);
+	return lk->fd;
+}
+
 int commit_lock_file(struct lock_file *lk)
 {
 	char result_file[PATH_MAX];