Eryk Sun added the comment:
Standard users have SeChangeNotifyPrivilege, which allows traversing a
directory that they can't access, so Python should only work with paths as
strings instead of trying to open a directory handle.
I think it's best to make Windows do as much of the normalization work as
possible. Why reinvent the wheel instead of relying on GetFullPathNameW [1]?
For example, you propose to trim leading and trailing spaces and trailing dots
from each component, but Windows itself doesn't go that far. Leading spaces are
never removed. Only the final path component has all trailing spaces and dots
trimmed. From the preceding components Windows will strip one and only one
trailing dot, and a trailing space is never removed. Some examples:
>>> os.path.exists(r'C:\Temp\test\dir1\dir2\file')
True
>>> os.path.exists(r'C:\Temp\test\dir1\dir2\file. . . . . .')
True
>>> os.path.exists(r'C:\Temp\test\dir1.\dir2.\file')
True
>>> os.path.exists(r'C:\Temp\test\dir1..\dir2\file')
False
>>> os.path.exists(r'C:\Temp\test\dir1 \dir2\file')
False
>>> os.path.exists(r'C:\Temp\test\ dir1\dir2\file')
False
Components that consist of only "." and ".." should also be normalized:
>>> os.path.abspath(r'C:\Temp\test\dir1\..\dir1\.\dir2\...\file')
'C:\\Temp\\test\\dir1\\dir2\\...\\file'
Paths with DOS devices also need to be translated beforehand, since the
existence of classic DOS devices in every directory is emulated by the NT
runtime library when it translates from DOS paths to native NT paths. For
example:
>>> os.path.abspath(r'C:\Temp\con')
'\\\\.\\con'
>>> os.path.abspath(r'C:\Temp\nul')
'\\\\.\\nul'
>>> os.path.abspath(r'C:\Temp\prn')
'\\\\.\\prn'
>>> os.path.abspath(r'C:\Temp\aux')
'\\\\.\\aux'
GetFullPathNameW handles all of these corner cases already, so I think a
simpler algorithm is to just rely on Windows to do most of the work:
* If len(path) < 260 or the path starts with L"\\\\?\\" or L"\\\\.\\", don't do
anything.
* Call GetFullPathNameW to calculate the required path length.
* If the path starts with L"\\\\", over-allocate by sizeof(WCHAR) * 6.
Otherwise over-allocate by sizeof(WCHAR) * 4.
* Call GetFullPatheNameW again, with the buffer pointer adjusted past the
overallocation.
* If the path is a UNC path, copy the L"\\\\?\\UNC" prefix to the start of the
buffer. Otherwise copy L"\\\\?\\".
Contrary to the documentation on MSDN, Windows doesn't need the \\?\ prefix to
use a long path with GetFullPathNameW. On NT systems it has always worked with
long paths. The implementation uses the RtlGetFullPathName_U* family of
functions, which immediately wrap the input buffer in a UNICODE_STRING, which
has a limit of 32,768 characters.
The only MAX_PATH limit here is one that can't be avoided. The process working
directory is limited to MAX_PATH, as are the per-drive working directories
(stored in hidden environment variables, e.g. "=C:"). At least that's the case
prior to the upcoming change in Windows 10. With the change you propose in
issue 27731, Windows 10 users should be able to set a working directory that
exceeds MAX_PATH.
For example, the following demonstrates (in Windows 10.0.10586) that the value
of "=Z:" is only used when its length is less than MAX_PATH and the target
directory exists.
Create a long test path:
>>> path = 'Z:' + r'\test' * 50
>>> os.makedirs('\\\\?\\' + path + r'\last\test')
A drive-relative path is resolved relative to the root directory if the current
directory on the drive doesn't exist or is inaccessible:
>>> kernel32.SetEnvironmentVariableW('=Z:', path + r'\test')
1
>>> os.path._getfullpathname('Z:file')
'Z:\\file'
It also uses the root directory if the current directory on the drive exceeds
MAX_PATH:
>>> kernel32.SetEnvironmentVariableW('=Z:', path + r'\last\test')
1
>>> os.path._getfullpathname('Z:file')
'Z:\\file'
It resolves correctly if the current directory can be opened and the path
length doesn't exceed MAX_PATH:
>>> kernel32.SetEnvironmentVariableW('=Z:', path + r'\last')
1
>>> os.path._getfullpathname('Z:file')
'Z:\\test\\test\\test\\test\\test\\test\\test\\test\\test\\test\\
test\\test\\test\\test\\test\\test\\test\\test\\test\\test\\test\\
test\\test\\test\\test\\test\\test\\test\\test\\test\\test\\test\\
test\\test\\test\\test\\test\\test\\test\\test\\test\\test\\test\\
test\\test\\test\\test\\test\\test\\test\\last\\file'
>>> shutil.rmtree(r'\\?\Z:\test')
[1]: https://msdn.microsoft.com/en-us/library/aa364963
----------
nosy: +eryksun
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<http://bugs.python.org/issue27730>
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