util: make symlink() work on older Windows versions

tools/util.py

@@ -53,21 +53,38 @@ def remove_and_symlink(target, name, target_is_dir=False):
 
 def symlink(target, name, target_is_dir=False):
  if os.name == "nt":
- import ctypes
- CreateSymbolicLinkW = ctypes.windll.kernel32.CreateSymbolicLinkW
- CreateSymbolicLinkW.restype = ctypes.c_ubyte
- CreateSymbolicLinkW.argtypes = (ctypes.c_wchar_p, ctypes.c_wchar_p,
- ctypes.c_uint32)
-
- # Replace forward slashes by backward slashes.
- # Strangely it seems that this is only necessary for symlinks to files.
- # Forward slashes don't cause any issues when the target is a directory.
- target = target.replace("/", "\\")
- flags = 0x02 # SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE
- if (target_is_dir):
- flags |= 0x01 # SYMBOLIC_LINK_FLAG_DIRECTORY
- if not CreateSymbolicLinkW(name, target, flags):
- raise ctypes.WinError()
+ from ctypes import WinDLL, WinError, GetLastError
+ from ctypes.wintypes import BOOLEAN, DWORD, LPCWSTR
+
+ kernel32 = WinDLL('kernel32', use_last_error=False)
+ CreateSymbolicLinkW = kernel32.CreateSymbolicLinkW
+ CreateSymbolicLinkW.restype = BOOLEAN
+ CreateSymbolicLinkW.argtypes = (LPCWSTR, LPCWSTR, DWORD)
+
+ # File-type symlinks can only use backslashes as separators.
+ target = os.path.normpath(target)
+
+ # If the symlink points at a directory, it needs to have the appropriate
+ # flag set, otherwise the link will be created but it won't work.
+ if target_is_dir:
+ type_flag = 0x01 # SYMBOLIC_LINK_FLAG_DIRECTORY
+ else:
+ type_flag = 0
+
+ # Before Windows 10, creating symlinks requires admin privileges.
+ # As of Win 10, there is a flag that allows anyone to create them.
+ # Initially, try to use this flag.
+ unpriv_flag = 0x02 # SYMBOLIC_LINK_FLAG_ALLOW_UNPRIVILEGED_CREATE
+ r = CreateSymbolicLinkW(name, target, type_flag | unpriv_flag)
+
+ # If it failed with ERROR_INVALID_PARAMETER, try again without the
+ # 'allow unprivileged create' flag.
+ if not r and GetLastError() == 87: # ERROR_INVALID_PARAMETER
+ r = CreateSymbolicLinkW(name, target, type_flag)
+
+ # Throw if unsuccessful even after the second attempt.
+ if not r:
+ raise WinError()
  else:
  os.symlink(target, name)