python - 无法实现 readlink() 函数

Question

我一直在尝试找出一种方法来获得某种能力，以便能够在 Python 2.7 下返回 Windows 中符号链接(symbolic link)的真实绝对路径。 (我无法升级到 3.x，因为 Maya/3ds max 等大多数 DCC 不使用该版本的 Python)

我查看了 sid0 ntfs utils(其 islink() 函数有效，但 readlink() 函数出于某种原因总是为我返回一个空的 unicode 字符串)和 juntalis ntfs 库(不幸的是，我不能' t 开始工作)，以及有人发布的有用脚本:

import os, ctypes, struct
from ctypes import windll, wintypes

FSCTL_GET_REPARSE_POINT = 0x900a8

FILE_ATTRIBUTE_READONLY      = 0x0001
FILE_ATTRIBUTE_HIDDEN        = 0x0002
FILE_ATTRIBUTE_DIRECTORY     = 0x0010
FILE_ATTRIBUTE_NORMAL        = 0x0080
FILE_ATTRIBUTE_REPARSE_POINT = 0x0400


GENERIC_READ  = 0x80000000
GENERIC_WRITE = 0x40000000
OPEN_EXISTING = 3
FILE_READ_ATTRIBUTES = 0x80
FILE_FLAG_OPEN_REPARSE_POINT = 0x00200000
INVALID_HANDLE_VALUE = wintypes.HANDLE(-1).value

INVALID_FILE_ATTRIBUTES = 0xFFFFFFFF

FILE_FLAG_OPEN_REPARSE_POINT = 2097152
FILE_FLAG_BACKUP_SEMANTICS = 33554432
# FILE_FLAG_OPEN_REPARSE_POINT | FILE_FLAG_BACKUP_SEMANTI
FILE_FLAG_REPARSE_BACKUP = 35651584


GetFileAttributes = windll.kernel32.GetFileAttributesW
_CreateFileW = windll.kernel32.CreateFileW
_DevIoCtl = windll.kernel32.DeviceIoControl
_DevIoCtl.argtypes = [
    wintypes.HANDLE, #HANDLE hDevice
    wintypes.DWORD, #DWORD dwIoControlCode
    wintypes.LPVOID, #LPVOID lpInBuffer
    wintypes.DWORD, #DWORD nInBufferSize
    wintypes.LPVOID, #LPVOID lpOutBuffer
    wintypes.DWORD, #DWORD nOutBufferSize
    ctypes.POINTER(wintypes.DWORD), #LPDWORD lpBytesReturned
    wintypes.LPVOID] #LPOVERLAPPED lpOverlapped
_DevIoCtl.restype = wintypes.BOOL


def islink(path):
    # assert os.path.isdir(path), path
    if GetFileAttributes(path) & FILE_ATTRIBUTE_REPARSE_POINT:
        return True
    else:
        return False


def DeviceIoControl(hDevice, ioControlCode, input, output):
    # DeviceIoControl Function
    # http://msdn.microsoft.com/en-us/library/aa363216(v=vs.85).aspx
    if input:
        input_size = len(input)
    else:
        input_size = 0
    if isinstance(output, int):
        output = ctypes.create_string_buffer(output)
    output_size = len(output)
    assert isinstance(output, ctypes.Array)
    bytesReturned = wintypes.DWORD()
    status = _DevIoCtl(hDevice, ioControlCode, input,
                       input_size, output, output_size, bytesReturned, None)
    print "status(%d)" % status
    if status != 0:
        return output[:bytesReturned.value]
    else:
        return None


def CreateFile(path, access, sharemode, creation, flags):
    return _CreateFileW(path, access, sharemode, None, creation, flags, None)


SymbolicLinkReparseFormat = "LHHHHHHL"
SymbolicLinkReparseSize = struct.calcsize(SymbolicLinkReparseFormat);

def readlink(path):
    """ Windows readlink implementation. """
    # This wouldn't return true if the file didn't exist, as far as I know.
    assert islink(path)
    # assert type(path) == unicode

    # Open the file correctly depending on the string type.
    hfile = CreateFile(path, GENERIC_READ, 0, OPEN_EXISTING,
                       FILE_FLAG_REPARSE_BACKUP)
    # MAXIMUM_REPARSE_DATA_BUFFER_SIZE = 16384 = (16*1024)
    buffer = DeviceIoControl(hfile, FSCTL_GET_REPARSE_POINT, None, 16384)
    windll.CloseHandle(hfile)

    # Minimum possible length (assuming length of the target is bigger than 0)
    if not buffer or len(buffer) < 9:
        return None

    # Only handle SymbolicLinkReparseBuffer
    (tag, dataLength, reserver, SubstituteNameOffset, SubstituteNameLength,
     PrintNameOffset, PrintNameLength,
     Flags) = struct.unpack(SymbolicLinkReparseFormat,
                            buffer[:SymbolicLinkReparseSize])
    print tag, dataLength, reserver, SubstituteNameOffset, SubstituteNameLength
    start = SubstituteNameOffset + SymbolicLinkReparseSize
    actualPath = buffer[start : start + SubstituteNameLength].decode("utf-16")
    # This utf-16 string is null terminated
    index = actualPath.find(u"\0")
    assert index > 0
    if index > 0:
        actualPath = actualPath[:index]
    if actualPath.startswith(u"?\\"):
        return actualPath[2:]
    else:
        return actualPath

然而，大多数时候我尝试过的各种解决方案最终都会给我:

[错误126]找不到指定的模块

即使我能够导入 ctypes 并执行诸如导入 cdll 之类的操作:

libc = cdll.msvcrt
libc.printf
<_FuncPtr object at 0x0000000002A9F388>

我对 Python 的这一部分和一般的 ctypes 还很陌生，因此非常感谢任何有关处理符号链接(symbolic link)的指示!

Answer 1

ERROR_MOD_NOT_FOUND (126) 可能是由于 windll.CloseHandle(hfile) 尝试加载“closehandle.dll”。它缺少 kernel32。

这是处理联结和符号链接(symbolic link)的替代实现。

ctypes 定义

import ctypes
from ctypes import wintypes

kernel32 = ctypes.WinDLL('kernel32', use_last_error=True)

FILE_READ_ATTRIBUTES = 0x0080
OPEN_EXISTING = 3
FILE_FLAG_OPEN_REPARSE_POINT = 0x00200000
FILE_FLAG_BACKUP_SEMANTICS   = 0x02000000
FILE_ATTRIBUTE_REPARSE_POINT = 0x0400

IO_REPARSE_TAG_MOUNT_POINT = 0xA0000003
IO_REPARSE_TAG_SYMLINK     = 0xA000000C
FSCTL_GET_REPARSE_POINT    = 0x000900A8
MAXIMUM_REPARSE_DATA_BUFFER_SIZE = 0x4000

LPDWORD = ctypes.POINTER(wintypes.DWORD)
LPWIN32_FIND_DATA = ctypes.POINTER(wintypes.WIN32_FIND_DATAW)
INVALID_HANDLE_VALUE = wintypes.HANDLE(-1).value

def IsReparseTagNameSurrogate(tag):
    return bool(tag & 0x20000000)

def _check_invalid_handle(result, func, args):
    if result == INVALID_HANDLE_VALUE:
        raise ctypes.WinError(ctypes.get_last_error())
    return args

def _check_bool(result, func, args):
    if not result:
        raise ctypes.WinError(ctypes.get_last_error())
    return args

kernel32.FindFirstFileW.errcheck = _check_invalid_handle
kernel32.FindFirstFileW.restype = wintypes.HANDLE
kernel32.FindFirstFileW.argtypes = (
    wintypes.LPCWSTR,  # _In_  lpFileName
    LPWIN32_FIND_DATA) # _Out_ lpFindFileData

kernel32.FindClose.argtypes = (
    wintypes.HANDLE,) # _Inout_ hFindFile

kernel32.CreateFileW.errcheck = _check_invalid_handle
kernel32.CreateFileW.restype = wintypes.HANDLE
kernel32.CreateFileW.argtypes = (
    wintypes.LPCWSTR, # _In_     lpFileName
    wintypes.DWORD,   # _In_     dwDesiredAccess
    wintypes.DWORD,   # _In_     dwShareMode
    wintypes.LPVOID,  # _In_opt_ lpSecurityAttributes
    wintypes.DWORD,   # _In_     dwCreationDisposition
    wintypes.DWORD,   # _In_     dwFlagsAndAttributes
    wintypes.HANDLE)  # _In_opt_ hTemplateFile 

kernel32.CloseHandle.argtypes = (
    wintypes.HANDLE,) # _In_ hObject

kernel32.DeviceIoControl.errcheck = _check_bool
kernel32.DeviceIoControl.argtypes = (
    wintypes.HANDLE,  # _In_        hDevice
    wintypes.DWORD,   # _In_        dwIoControlCode
    wintypes.LPVOID,  # _In_opt_    lpInBuffer
    wintypes.DWORD,   # _In_        nInBufferSize
    wintypes.LPVOID,  # _Out_opt_   lpOutBuffer
    wintypes.DWORD,   # _In_        nOutBufferSize
    LPDWORD,          # _Out_opt_   lpBytesReturned
    wintypes.LPVOID)  # _Inout_opt_ lpOverlapped 

class REPARSE_DATA_BUFFER(ctypes.Structure):
    class ReparseData(ctypes.Union):
        class LinkData(ctypes.Structure):
            _fields_ = (('SubstituteNameOffset', wintypes.USHORT),
                        ('SubstituteNameLength', wintypes.USHORT),
                        ('PrintNameOffset',      wintypes.USHORT),
                        ('PrintNameLength',      wintypes.USHORT))
            @property
            def PrintName(self):
                dt = wintypes.WCHAR * (self.PrintNameLength //
                                       ctypes.sizeof(wintypes.WCHAR))
                name = dt.from_address(ctypes.addressof(self.PathBuffer) +
                                       self.PrintNameOffset).value
                if name.startswith(r'\??'):
                    name = r'\\?' + name[3:] # NT => Windows
                return name
        class SymbolicLinkData(LinkData):
            _fields_ = (('Flags',      wintypes.ULONG),
                        ('PathBuffer', wintypes.BYTE * 0))
        class MountPointData(LinkData):
            _fields_ = (('PathBuffer', wintypes.BYTE * 0),)
        class GenericData(ctypes.Structure):
            _fields_ = (('DataBuffer', wintypes.BYTE * 0),)
        _fields_ = (('SymbolicLinkReparseBuffer', SymbolicLinkData),
                    ('MountPointReparseBuffer',   MountPointData),
                    ('GenericReparseBuffer',      GenericData))
    _fields_ = (('ReparseTag',        wintypes.ULONG),
                ('ReparseDataLength', wintypes.USHORT),
                ('Reserved',          wintypes.USHORT),
                ('ReparseData',       ReparseData))
    _anonymous_ = ('ReparseData',)

函数

def islink(path):
    data = wintypes.WIN32_FIND_DATAW()
    kernel32.FindClose(kernel32.FindFirstFileW(path, ctypes.byref(data)))
    if not data.dwFileAttributes & FILE_ATTRIBUTE_REPARSE_POINT:
        return False
    return IsReparseTagNameSurrogate(data.dwReserved0)

def readlink(path):
    n = wintypes.DWORD()
    buf = (wintypes.BYTE * MAXIMUM_REPARSE_DATA_BUFFER_SIZE)()
    flags = FILE_FLAG_OPEN_REPARSE_POINT | FILE_FLAG_BACKUP_SEMANTICS
    handle = kernel32.CreateFileW(path, FILE_READ_ATTRIBUTES, 0, None,
                OPEN_EXISTING, flags, None)
    try:
        kernel32.DeviceIoControl(handle, FSCTL_GET_REPARSE_POINT, None, 0,
            buf, ctypes.sizeof(buf), ctypes.byref(n), None)
    finally:
        kernel32.CloseHandle(handle)
    rb = REPARSE_DATA_BUFFER.from_buffer(buf)
    tag = rb.ReparseTag
    if tag == IO_REPARSE_TAG_SYMLINK:
        return rb.SymbolicLinkReparseBuffer.PrintName
    if tag == IO_REPARSE_TAG_MOUNT_POINT:
        return rb.MountPointReparseBuffer.PrintName
    if not IsReparseTagNameSurrogate(tag):
        raise ValueError("not a link")
    raise ValueError("unsupported reparse tag: %d" % tag)

示例

>>> sys.version
'2.7.13 (v2.7.13:a06454b1afa1, Dec 17 2016, 20:53:40) 
[MSC v.1500 64 bit (AMD64)]'
>>> os.system(r'mklink /d spam C:\Windows')
symbolic link created for spam <<===>> C:\Windows
0
>>> islink('spam')
True
>>> readlink('spam')
u'C:\\Windows'
>>> islink('C:/Documents and Settings') # junction
True
>>> readlink('C:/Documents and Settings')
u'C:\\Users'
>>> islink('C:/Users/All Users') # symlinkd
True
>>> readlink('C:/Users/All Users')
u'C:\\ProgramData'