# Copyright 2014 Gentoo Foundation
# Distributed under the terms of the GNU General Public License v2

import errno

from portage import os

def first_existing(path):
	"""
	Returns the first existing path element, traversing from the given
	path to the root directory. A path is considered to exist if lstat
	either succeeds or raises an error other than ENOENT or ESTALE.

	This can be particularly useful to check if there is permission to
	create a particular file or directory, without actually creating
	anything.

	@param path: a filesystem path
	@type path: str
	@rtype: str
	@return: the element that exists
	"""
	existing = False
	for path in iter_parents(path):
		try:
			os.lstat(path)
			existing = True
		except OSError as e:
			if e.errno not in (errno.ENOENT, errno.ESTALE):
				existing = True

		if existing:
			return path

	return os.sep

def iter_parents(path):
	"""
	@param path: a filesystem path
	@type path: str
	@rtype: iterator
	@return: an iterator which yields path and all parents of path,
		ending with the root directory
	"""
	yield path
	while path != os.sep:
		path = os.path.dirname(path)
		yield path