vfs: add walk
To eliminate "path prefix" (= "the root of vfs") part from "dirpath"
yielded by "os.walk()" correctly, "path prefix" should have "os.sep"
at the end of own string, but it isn't easy to ensure it, because:
- examination by "path.endswith(os.sep)" isn't portable
Some problematic encodings use 0x5c (= "os.sep" on Windows) as the
tail byte of some multi-byte characters.
- "os.path.join(path, '')" isn't portable
With Python 2.7.9, this invocation doesn't add "os.sep" at the end
of UNC path (see issue4557 for detail).
Python 2.7.9 changed also behavior of "os.path.normpath()" (see *) and
"os.path.splitdrive()" for UNC path.
vfs root normpath splitdrive os.sep required
=============== ============== =================== ============
z:\ z:\ z: + \ no
z:\foo z:\foo z: + \foo yes
z:\foo\ z:\foo z: + \foo yes
[before Python 2.7.9]
\\foo\bar \\foo\bar '' + \\foo\bar yes
\\foo\bar\ \\foo\bar (*) '' + \\foo\bar yes
\\foo\bar\baz \\foo\bar\baz '' + \\foo\bar\baz yes
\\foo\bar\baz\ \\foo\bar\baz '' + \\foo\bar\baz yes
[Python 2.7.9]
\\foo\bar \\foo\bar \\foo\bar + '' yes
\\foo\bar\ \\foo\bar\ (*) \\foo\bar + \ no
\\foo\bar\baz \\foo\bar\baz \\foo\bar + \baz yes
\\foo\bar\baz\ \\foo\bar\baz \\foo\bar + \baz yes
If it is ensured that "normpath()"-ed vfs root is passed to
"splitdrive()", adding "os.sep" is required only when "path" part of
"splitdrive()" result isn't "os.sep" itself. This is just what
"pathutil.nameasprefix()" examines.
This patch applies "os.path.normpath()" on "self.join(None)"
explicitly, because it isn't ensured that vfs root is already
normalized: vfs itself is constructed with "realpath=False" (= avoid
normalizing in "vfs.__init__()") in many code paths.
This normalization should be much cheaper than subsequent file I/O for
directory traversal.
