File locking fails if argtypes redefined

[Simon Sawicki]

django.core.files.locks uses ctypes.windll.kernel32 which is a global instance. If other code redefines windll.kernel32.LockFileEx.argtypes the function fails with an error.
I am not sure how to write a test since its global

import ctypes
import ctypes.wintypes

from django.core.files import locks


class OVERLAPPED(ctypes.Structure):
    _fields_ = [
        ("Internal", ctypes.wintypes.LPVOID),
        ("InternalHigh", ctypes.wintypes.LPVOID),
        ("Offset", ctypes.wintypes.DWORD),
        ("OffsetHigh", ctypes.wintypes.DWORD),
        ("hEvent", ctypes.wintypes.HANDLE),
    ]


ctypes.windll.kernel32.LockFileEx.argtypes = [
    ctypes.wintypes.HANDLE,
    ctypes.wintypes.DWORD,
    ctypes.wintypes.DWORD,
    ctypes.wintypes.DWORD,
    ctypes.wintypes.DWORD,
    ctypes.wintypes.DWORD,
]

with open("./file", "w") as f:
    locks.lock(f, locks.LOCK_EX)

Possible patch:

django/core/files/locks.py

@@ -32 +32 @@
         POINTER,
         Structure,
         Union,
+        WinDLL,
         byref,
         c_int64,
         c_ulong,
         c_void_p,
         sizeof,
-        windll,
     )
     from ctypes.wintypes import BOOL, DWORD, HANDLE
 
@@ -73 +73 @@
     LPOVERLAPPED = POINTER(OVERLAPPED)
 
     # --- Define function prototypes for extra safety ---
-    LockFileEx = windll.kernel32.LockFileEx
+    kernel32 = WinDLL("kernel32")
+    LockFileEx = kernel32.LockFileEx
     LockFileEx.restype = BOOL
     LockFileEx.argtypes = [HANDLE, DWORD, DWORD, DWORD, DWORD, LPOVERLAPPED]
-    UnlockFileEx = windll.kernel32.UnlockFileEx
+    UnlockFileEx = kernel32.UnlockFileEx
     UnlockFileEx.restype = BOOL
     UnlockFileEx.argtypes = [HANDLE, DWORD, DWORD, DWORD, LPOVERLAPPED]
 

tests/files/tests.py

@@ -200 +200 @@
             self.assertIs(locks.unlock(f1), True)
             self.assertIs(locks.unlock(f2), True)
 
+    @unittest.skipIf(os.name != "nt", "File locking usees WinDLL on windows only")      
+    def test_local_windll(self):
+        import ctypes
+        import ctypes.wintypes
+
+        old_argtypes = ctypes.windll.kernel32.LockFileEx.argtypes
+        try:
+            ctypes.windll.kernel32.LockFileEx.argtypes = [
+                ctypes.wintypes.HANDLE,
+                ctypes.wintypes.DWORD,
+                ctypes.wintypes.DWORD,
+                ctypes.wintypes.DWORD,
+                ctypes.wintypes.DWORD,
+                ctypes.wintypes.DWORD,
+            ]
+            file_path = Path(__file__).parent / "test.png"
+            with open(file_path) as file:
+                try:
+                    locks.lock(file, locks.LOCK_EX)
+                except Exception:
+                    self.fail("Locking raised exception with changed argtypes")
+                else:
+                    locks.unlock(file)
+        finally:
+            ctypes.windll.kernel32.LockFileEx.argtypes = old_argtypes
+
 
 class NoNameFileTestCase(unittest.TestCase):
     """