[pathfinder] Use LOAD_WITH_ALTERED_SEARCH_PATH for system DLL search on Windows

cuda_pathfinder/cuda/pathfinder/_dynamic_libs/load_dl_windows.py

@@ -13,6 +13,7 @@
 )
 
 # Mirrors WinBase.h (unfortunately not defined already elsewhere)
+WINBASE_LOAD_WITH_ALTERED_SEARCH_PATH = 0x00000008
 WINBASE_LOAD_LIBRARY_SEARCH_DLL_LOAD_DIR = 0x00000100
 WINBASE_LOAD_LIBRARY_SEARCH_DEFAULT_DIRS = 0x00001000
 
@@ -45,6 +46,17 @@
 kernel32.AddDllDirectory.argtypes = [ctypes.wintypes.LPCWSTR]
 kernel32.AddDllDirectory.restype = ctypes.c_void_p  # DLL_DIRECTORY_COOKIE
 
+# SearchPathW - find a file in the system search path
+kernel32.SearchPathW.argtypes = [
+    ctypes.wintypes.LPCWSTR,  # lpPath (NULL to use standard search)
+    ctypes.wintypes.LPCWSTR,  # lpFileName
+    ctypes.wintypes.LPCWSTR,  # lpExtension
+    ctypes.wintypes.DWORD,  # nBufferLength
+    ctypes.wintypes.LPWSTR,  # lpBuffer
+    ctypes.POINTER(ctypes.wintypes.LPWSTR),  # lpFilePart
+]
+kernel32.SearchPathW.restype = ctypes.wintypes.DWORD
+
 
 def ctypes_handle_to_unsigned_int(handle: ctypes.wintypes.HMODULE) -> int:
     """Convert ctypes HMODULE to unsigned int."""
@@ -113,6 +125,31 @@ def check_if_already_loaded_from_elsewhere(libname: str, have_abs_path: bool) ->
     return None
 
 
+def _search_path_for_dll(dll_name: str) -> str | None:
+    """Search for a DLL using Windows SearchPathW.
+
+    Args:
+        dll_name: The name of the DLL to find
+
+    Returns:
+        The absolute path to the DLL if found, None otherwise
+    """
+    buffer = ctypes.create_unicode_buffer(260)  # MAX_PATH
+    length = kernel32.SearchPathW(None, dll_name, None, len(buffer), buffer, None)
+
+    if length == 0:
+        return None
+
+    # If buffer was too small, try with larger buffer
+    if length > len(buffer):
+        buffer = ctypes.create_unicode_buffer(length)
+        length = kernel32.SearchPathW(None, dll_name, None, len(buffer), buffer, None)
+        if length == 0:
+            return None
+
+    return buffer.value
+
+
 def load_with_system_search(libname: str) -> LoadedDL | None:
     """Try to load a DLL using system search paths.
 
@@ -124,10 +161,15 @@ def load_with_system_search(libname: str) -> LoadedDL | None:
     """
     # Reverse tabulated names to achieve new → old search order.
     for dll_name in reversed(SUPPORTED_WINDOWS_DLLS.get(libname, ())):
-        handle = kernel32.LoadLibraryExW(dll_name, None, 0)
-        if handle:
-            abs_path = abs_path_for_dynamic_library(libname, handle)
-            return LoadedDL(abs_path, False, ctypes_handle_to_unsigned_int(handle), "system-search")
+        # First, find the DLL's full path using SearchPathW
+        found_path = _search_path_for_dll(dll_name)
+        if found_path:
+            # Load with LOAD_WITH_ALTERED_SEARCH_PATH so Windows searches for
+            # dependencies from the DLL's directory (required for CUDA DLLs
+            # whose dependencies are co-located)
+            handle = kernel32.LoadLibraryExW(found_path, None, WINBASE_LOAD_WITH_ALTERED_SEARCH_PATH)
+            if handle:
+                return LoadedDL(found_path, False, ctypes_handle_to_unsigned_int(handle), "system-search")
 
     return None