gh-152433: Windows: Use GetFileSizeEx instead of GetFileSize

GetFileSizeEx is available in all supported Windows versions and has the
advantage is also present in UWP, then allows build also in UWP with
the same common code.

It also simplifies error checking and makes handling files larger
than 4GB easier.

Author: thexai

Misc/NEWS.d/next/Windows/2026-06-28-13-03-57.gh-issue-152433.jUXFwC.rst

@@ -0,0 +1 @@
+Use the Windows API ``GetFileSizeEx()`` for memory mapped files, rather than the older ``GetFileSize()``.

Modules/mmapmodule.c

@@ -806,20 +806,12 @@ mmap_mmap_size_impl(mmap_object *self)
 
 #ifdef MS_WINDOWS
     if (self->file_handle != INVALID_HANDLE_VALUE) {
-        DWORD low,high;
-        long long size;
-        low = GetFileSize(self->file_handle, &high);
-        if (low == INVALID_FILE_SIZE) {
-            /* It might be that the function appears to have failed,
-               when indeed its size equals INVALID_FILE_SIZE */
-            DWORD error = GetLastError();
-            if (error != NO_ERROR)
-                return PyErr_SetFromWindowsErr(error);
+        LARGE_INTEGER size;
+        if (!GetFileSizeEx(self->file_handle, &size)) {
+          DWORD error = GetLastError();
+          return PyErr_SetFromWindowsErr(error);
         }
-        if (!high && low < LONG_MAX)
-            return PyLong_FromLong((long)low);
-        size = (((long long)high)<<32) + low;
-        return PyLong_FromLongLong(size);
+        return PyLong_FromLongLong(size.QuadPart);
     }
 #endif /* MS_WINDOWS */
 
@@ -2127,18 +2119,14 @@ new_mmap_object(PyTypeObject *type, PyObject *args, PyObject *kwdict)
             m_obj->file_handle = fh;
         }
         if (!map_size) {
-            DWORD low,high;
-            low = GetFileSize(fh, &high);
-            /* low might just happen to have the value INVALID_FILE_SIZE;
-               so we need to check the last error also. */
-            if (low == INVALID_FILE_SIZE &&
-                (dwErr = GetLastError()) != NO_ERROR)
+            LARGE_INTEGER li;
+            if (!GetFileSizeEx(fh, &li))
             {
+                dwErr = GetLastError();
                 Py_DECREF(m_obj);
                 return PyErr_SetFromWindowsErr(dwErr);
             }
-
-            size = (((long long) high) << 32) + low;
+            size = li.QuadPart;
             if (size == 0) {
                 PyErr_SetString(PyExc_ValueError,
                                 "cannot mmap an empty file");