gh-86179: Avoid making case-only changes when calculating realpath() during initialization (GH-113077)

Author: zooba

Modules/getpath.c

@@ -506,12 +506,17 @@ getpath_realpath(PyObject *Py_UNUSED(self) , PyObject *args)
     HANDLE hFile;
     wchar_t resolved[MAXPATHLEN+1];
     int len = 0, err;
+    Py_ssize_t pathlen;
     PyObject *result;
 
-    wchar_t *path = PyUnicode_AsWideCharString(pathobj, NULL);
+    wchar_t *path = PyUnicode_AsWideCharString(pathobj, &pathlen);
     if (!path) {
         return NULL;
     }
+    if (wcslen(path) != pathlen) {
+        PyErr_SetString(PyExc_ValueError, "path contains embedded nulls");
+        return NULL;
+    }
 
     Py_BEGIN_ALLOW_THREADS
     hFile = CreateFileW(path, 0, 0, NULL, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS, NULL);
@@ -535,7 +540,11 @@ getpath_realpath(PyObject *Py_UNUSED(self) , PyObject *args)
                 len -= 4;
             }
         }
-        result = PyUnicode_FromWideChar(p, len);
+        if (CompareStringOrdinal(path, (int)pathlen, p, len, TRUE) == CSTR_EQUAL) {
+            result = Py_NewRef(pathobj);
+        } else {
+            result = PyUnicode_FromWideChar(p, len);
+        }
     } else {
         result = Py_NewRef(pathobj);
     }