Add support for creating symbolic links on Windows

Replace all calls to os.symlink with platform_utils.symlink.

The Windows implementation calls into the CreateSymbolicLinkW Win32
API, as os.symlink is not supported.

Separate the Win32 API definitions into a separate module
platform_utils_win32 for clarity.

Change-Id: I0714c598664c2df93383734e609d948692c17ec5

platform_utils_win32.py

@@ -0,0 +1,63 @@
+#
+# Copyright (C) 2016 The Android Open Source Project
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import errno
+
+from ctypes import WinDLL, get_last_error, FormatError, WinError
+from ctypes.wintypes import BOOL, LPCWSTR, DWORD
+
+kernel32 = WinDLL('kernel32', use_last_error=True)
+
+# Win32 error codes
+ERROR_SUCCESS = 0
+ERROR_PRIVILEGE_NOT_HELD = 1314
+
+# Win32 API entry points
+CreateSymbolicLinkW = kernel32.CreateSymbolicLinkW
+CreateSymbolicLinkW.restype = BOOL
+CreateSymbolicLinkW.argtypes = (LPCWSTR,  # lpSymlinkFileName In
+                                LPCWSTR,  # lpTargetFileName In
+                                DWORD)    # dwFlags In
+
+# Symbolic link creation flags
+SYMBOLIC_LINK_FLAG_FILE = 0x00
+SYMBOLIC_LINK_FLAG_DIRECTORY = 0x01
+
+
+def create_filesymlink(source, link_name):
+  """Creates a Windows file symbolic link source pointing to link_name."""
+  _create_symlink(source, link_name, SYMBOLIC_LINK_FLAG_FILE)
+
+
+def create_dirsymlink(source, link_name):
+  """Creates a Windows directory symbolic link source pointing to link_name.
+  """
+  _create_symlink(source, link_name, SYMBOLIC_LINK_FLAG_DIRECTORY)
+
+
+def _create_symlink(source, link_name, dwFlags):
+  # Note: Win32 documentation for CreateSymbolicLink is incorrect.
+  # On success, the function returns "1".
+  # On error, the function returns some random value (e.g. 1280).
+  # The best bet seems to use "GetLastError" and check for error/success.
+  CreateSymbolicLinkW(link_name, source, dwFlags)
+  code = get_last_error()
+  if code != ERROR_SUCCESS:
+    error_desc = FormatError(code).strip()
+    if code == ERROR_PRIVILEGE_NOT_HELD:
+      raise OSError(errno.EPERM, error_desc, link_name)
+    error_desc = 'Error creating symbolic link %s: %s'.format(
+        link_name, error_desc)
+    raise WinError(code, error_desc)