Eryk Sun added the comment:
I overlooked some aspects of the problem:
* A short relative path may end up exceeding MAX_PATH when normalized as a
fully qualified path.
* The working directory may be a UNC path or may already have the \\?\ prefix.
It's not thread-safe to check this beforehand.
* A path that contains a reserved DOS device name results in a \\.\ path.
Thus on second thought I think it's safer to call GetFullPathNameW for all
paths that lack the \\?\ prefix, and then copy the result if it needs to be
prefixed by \\?\ or \\?\UNC. The final path, if it's a filesystem path, should
always use the \\?\ namespace to ensure that functions such as shutil.rmtree
won't fail.
For example:
_DOS_DEVICES = "\\\\.\\"
_NT_DOS_DEVICES = "\\\\?\\"
_NT_UNC_DEVICE = "\\\\?\\UNC"
def winapi_path(path):
path = os.fsdecode(path) or '.'
if path.startswith(_NT_DOS_DEVICES):
return path
temp = os.path._getfullpathname(path)
if temp.startswith((_NT_DOS_DEVICES, _DOS_DEVICES)):
return path if temp == path else temp
if temp.startswith('\\\\'):
return _NT_UNC_DEVICE + temp[1:]
return _NT_DOS_DEVICES + temp
For reference, here's a typical call pattern when Windows 10.0.10586 converts a
DOS path to an NT path:
RtlInitUnicodeStringEx
RtlDosPathNameToRelativeNtPathName_U_WithStatus
RtlInitUnicodeStringEx
RtlDosPathNameToRelativeNtPathName
RtlGetFullPathName_Ustr
RtlDetermineDosPathNameType_Ustr
RtlAllocateHeap
memcpy
RtlGetFullPathName_Ustr is called with a buffer that's sizeof(WCHAR) * MAX_PATH
bytes. GetFullPathNameW also calls RtlGetFullPathName_Ustr, but with a
caller-supplied buffer that can be up to sizeof(WCHAR) * 32768 bytes.
Here's the call pattern for a \\?\ path:
RtlInitUnicodeStringEx
RtlDosPathNameToRelativeNtPathName_U_WithStatus
RtlInitUnicodeStringEx
RtlDosPathNameToRelativeNtPathName
RtlpWin32NtNameToNtPathName
RtlAllocateHeap
RtlAppendUnicodeStringToString
RtlAppendUnicodeStringToString
RtlpWin32NtNameToNtPathName copies the path, replacing \\? with the object
manager's \?? virtual DOS devices directory.
Here's some background information for those who don't already know the basics
of how Windows implements DOS devices in NT's object namespace, which you can
explore using Microsoft's free WinObj tool.
In Windows NT 3 and 4 (before Terminal Services) there was a single \DosDevices
directory, which is where the system created DOS device links to the actual NT
devices in \Device, such as C: => \Device\HarddiskVolume2. Windows 2000 changed
this in ways that were problematic, mostly due to using a per-session directory
instead of a per-logon directory. (Tokens for multiple logon sessions can be
used in a single Windows session, and almost always are since UAC split tokens
arrived in Vista.)
The design was changed again in Windows XP. \DosDevices is now just a link to
the virtual \?? directory. The system creates DOS devices in a local (per
logon) directory, except for system threads and LocalSystem logons (typically
services), which use the \GLOBAL?? directory. The per-logon directories are
located at \Sessions\0\DosDevices\[LogonAuthenticationId]. The object manager
parses \?? by first checking the local DOS devices and then the global DOS
devices. Each local DOS devices directory also has a Global link back to
\GLOBAL??. It's accessible as \\?\Global\[Device Name], which is useful when a
local device has the same name as a global one. The root directory of the
object namespace is accessible to administrators using the \GLOBAL??\GLOBALROOT
link, which from the Windows API is \\?\GLOBALROOT.
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<http://bugs.python.org/issue27730>
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