New submission from Tey <[email protected]>:
Making PyString_FromFormatV() build an empty string on 2.7 will fail and raise
the following error:
SystemError: Objects/stringobject.c:3903: bad argument to internal function
It does not matter if format is empty or not (e.g., both
PyString_FromFormat("") and PyString_FromFormat("%s", "")).
The exception is raised from _PyString_Resize() (called at the end of
PyString_FromFormatV()), because the string given to that method has a ref
count different than 1. This is expected for empty strings, because
PyString_FromStringAndSize() returns a reference to <nullstring> when <size> is
0.
A possible fix would be to prevent the call to _PyString_Resize() from
PyString_FromFormatV() if <string> is equal to <nullstring>, or if there is no
need to resize the string.
Python 3 versions before 3.6 might be impacted as well through
PyBytes_FromFormatV() (cannot check).
The following code can be used to trigger the bug:
static int dobug(const char *fmt, ...) {
va_list args;
va_start(args, fmt);
PyObject *str = PyString_FromFormatV(fmt, args);
va_end(args);
if(str == NULL) {
fprintf(stderr, "Error: PyString_FromFormatV(%s) returned NULL\n",
fmt);
return -1;
}
Py_DECREF(str);
return 0;
}
static PyObject* bug(PyObject *self) {
fprintf(stderr, "dobug(\"\") => %d\n", dobug(""));
fprintf(stderr, "dobug(\"%%s\", \"\") => %d\n", dobug("%s", ""));
if(PyErr_Occurred()) return NULL;
Py_RETURN_NONE;
}
----------
components: Interpreter Core
messages: 319180
nosy: Tey
priority: normal
severity: normal
status: open
title: PyString_FromFormatV() fails to build empty strings
type: behavior
versions: Python 2.7, Python 3.4, Python 3.5
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Python tracker <[email protected]>
<https://bugs.python.org/issue33817>
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