Fixed #19373 -- Ported Windows file locking from PyWin32 to ctypes

There wasn't any file locking under Windows unless PyWin32 was
installed. This removes that (undocumented) dependency by using ctypes
instead.

Thanks to Anatoly Techtonik for writing the ctypes port upon which this
is based.

django/core/files/locks.py

@@ -1,10 +1,13 @@
 """
 Portable file locking utilities.
 
-Based partially on example by Jonathan Feignberg <jdf@pobox.com> in the Python
-Cookbook, licensed under the Python Software License.
+Based partially on an example by Jonathan Feignberg in the Python
+Cookbook [1] (licensed under the Python Software License) and a ctypes port by
+Anatoly Techtonik for Roundup [2] (license [3]).
 
-    http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/65203
+[1] http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/65203
+[2] http://sourceforge.net/p/roundup/code/ci/default/tree/roundup/backends/portalocker.py
+[3] http://sourceforge.net/p/roundup/code/ci/default/tree/COPYING.txt
 
 Example Usage::
 
@@ -13,58 +16,98 @@ Example Usage::
     ...     locks.lock(f, locks.LOCK_EX)
     ...     f.write('Django')
 """
+import os
 
 __all__ = ('LOCK_EX', 'LOCK_SH', 'LOCK_NB', 'lock', 'unlock')
 
-system_type = None
 
-try:
-    import win32con
-    import win32file
-    import pywintypes
-    LOCK_EX = win32con.LOCKFILE_EXCLUSIVE_LOCK
-    LOCK_SH = 0
-    LOCK_NB = win32con.LOCKFILE_FAIL_IMMEDIATELY
-    __overlapped = pywintypes.OVERLAPPED()
-    system_type = 'nt'
-except (ImportError, AttributeError):
-    pass
-
-try:
-    import fcntl
-    LOCK_EX = fcntl.LOCK_EX
-    LOCK_SH = fcntl.LOCK_SH
-    LOCK_NB = fcntl.LOCK_NB
-    system_type = 'posix'
-except (ImportError, AttributeError):
-    pass
-
-
-def fd(f):
+def _fd(f):
     """Get a filedescriptor from something which could be a file or an fd."""
     return f.fileno() if hasattr(f, 'fileno') else f
 
-if system_type == 'nt':
-    def lock(file, flags):
-        hfile = win32file._get_osfhandle(fd(file))
-        win32file.LockFileEx(hfile, flags, 0, -0x10000, __overlapped)
 
-    def unlock(file):
-        hfile = win32file._get_osfhandle(fd(file))
-        win32file.UnlockFileEx(hfile, 0, -0x10000, __overlapped)
-elif system_type == 'posix':
-    def lock(file, flags):
-        fcntl.lockf(fd(file), flags)
+if os.name == 'nt':
+    import msvcrt
+    from ctypes import (sizeof, c_ulong, c_void_p, c_int64,
+                        Structure, Union, POINTER, windll, byref)
+    from ctypes.wintypes import BOOL, DWORD, HANDLE
 
-    def unlock(file):
-        fcntl.lockf(fd(file), fcntl.LOCK_UN)
-else:
-    # File locking is not supported.
-    LOCK_EX = LOCK_SH = LOCK_NB = None
+    LOCK_SH = 0  # the default
+    LOCK_NB = 0x1  # LOCKFILE_FAIL_IMMEDIATELY
+    LOCK_EX = 0x2  # LOCKFILE_EXCLUSIVE_LOCK
+
+    # --- Adapted from the pyserial project ---
+    # detect size of ULONG_PTR
+    if sizeof(c_ulong) != sizeof(c_void_p):
+        ULONG_PTR = c_int64
+    else:
+        ULONG_PTR = c_ulong
+    PVOID = c_void_p
+
+    # --- Union inside Structure by stackoverflow:3480240 ---
+    class _OFFSET(Structure):
+        _fields_ = [
+            ('Offset', DWORD),
+            ('OffsetHigh', DWORD)]
+
+    class _OFFSET_UNION(Union):
+        _anonymous_ = ['_offset']
+        _fields_ = [
+            ('_offset', _OFFSET),
+            ('Pointer', PVOID)]
+
+    class OVERLAPPED(Structure):
+        _anonymous_ = ['_offset_union']
+        _fields_ = [
+            ('Internal', ULONG_PTR),
+            ('InternalHigh', ULONG_PTR),
+            ('_offset_union', _OFFSET_UNION),
+            ('hEvent', HANDLE)]
+
+    LPOVERLAPPED = POINTER(OVERLAPPED)
+
+    # --- Define function prototypes for extra safety ---
+    LockFileEx = windll.kernel32.LockFileEx
+    LockFileEx.restype = BOOL
+    LockFileEx.argtypes = [HANDLE, DWORD, DWORD, DWORD, DWORD, LPOVERLAPPED]
+    UnlockFileEx = windll.kernel32.UnlockFileEx
+    UnlockFileEx.restype = BOOL
+    UnlockFileEx.argtypes = [HANDLE, DWORD, DWORD, DWORD, LPOVERLAPPED]
+
+    def lock(f, flags):
+        hfile = msvcrt.get_osfhandle(_fd(f))
+        overlapped = OVERLAPPED()
+        ret = LockFileEx(hfile, flags, 0, 0, 0xFFFF0000, byref(overlapped))
+        return bool(ret)
+
+    def unlock(f):
+        hfile = msvcrt.get_osfhandle(_fd(f))
+        overlapped = OVERLAPPED()
+        ret = UnlockFileEx(hfile, 0, 0, 0xFFFF0000, byref(overlapped))
+        return bool(ret)
+
+elif os.name == 'posix':
+    import fcntl
+    LOCK_SH = fcntl.LOCK_SH  # shared lock
+    LOCK_NB = fcntl.LOCK_NB  # non-blocking
+    LOCK_EX = fcntl.LOCK_EX
+
+    def lock(f, flags):
+        ret = fcntl.flock(_fd(f), flags)
+        return (ret == 0)
+
+    def unlock(f):
+        ret = fcntl.flock(_fd(f), fcntl.LOCK_UN)
+        return (ret == 0)
+
+else:  # File locking is not supported.
+    LOCK_EX = LOCK_SH = LOCK_NB = 0
 
     # Dummy functions that don't do anything.
-    def lock(file, flags):
-        pass
+    def lock(f, flags):
+        # File is not locked
+        return False
 
-    def unlock(file):
-        pass
+    def unlock(f):
+        # File is unlocked
+        return True

docs/releases/1.7.txt

@@ -415,6 +415,14 @@ Email
 * The SMTP :class:`~django.core.mail.backends.smtp.EmailBackend` now accepts a
   :attr:`~django.core.mail.backends.smtp.EmailBackend.timeout` parameter.
 
+File Storage
+^^^^^^^^^^^^
+
+* File locking on Windows previously depended on the PyWin32 package; if it
+  wasn't installed, file locking failed silently. That dependency has been
+  removed, and file locking is now implemented natively on both Windows
+  and Unix.
+
 File Uploads
 ^^^^^^^^^^^^