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The second parameter of the PyUnicode_AsWideCharString() function
 wchar_t* PyUnicode_AsWideCharString(PyObject *unicode, Py_ssize_t *size)
is a pointer to Py_ssize_t. The size of created wchar_t array is saved on this pointer if it is not NULL. If NULL is passed as the second argument, the only way to determine the size of the wchar_t string is using wcslen(). But if the string contains the null characters, it looks truncated for wcslen() and other C API functions.
Reliable code should always pass the non-NULL second argument and check that wcslen() is equal to the returned string size. See for example the code in Modules/_io/winconsoleio.c. Passing NULL as the second argument is unsafe. But most code doesn't do such check (see all other usages of PyUnicode_AsWideCharString(..., NULL)). And this check complicates the callers code.
I propose to make the check for null characters inside of PyUnicode_AsWideCharString() if NULL is passes as the second argument. This will fix all unsafe usages of PyUnicode_AsWideCharString() and allow to simplify the reliable code.
This issue fixes the part of issue13617.