socket.if_nametoindex('no-such-iface') raises OSError() without setting errno

[socketpair]

try:
    if_nametoindex('qweqweq')
except Exception as err:
    print(err.errno)

None

I think it's a bug because I can not distinguish from any other OS errors precisely (like EPERM vs ENOENT or so)

[sobolevn]

There's a comment in the source code: /* if_nametoindex() doesn't set errno */

[socketpair]

Then it should not be OSError or it has to set some errno, like ENOENT or so.

static PyObject *
socket_if_nametoindex(PyObject *self, PyObject *args)
{
    PyObject *oname;
#ifdef MS_WINDOWS
    NET_IFINDEX index;
#else
    unsigned long index;
#endif
    if (!PyArg_ParseTuple(args, "O&:if_nametoindex",
                          PyUnicode_FSConverter, &oname))
        return NULL;

    index = if_nametoindex(PyBytes_AS_STRING(oname));
    Py_DECREF(oname);
    if (index == 0) {
        /* if_nametoindex() doesn't set errno */
        PyErr_SetString(PyExc_OSError, "no interface with this name");
        return NULL;
    }

    return PyLong_FromUnsignedLong(index);
}

Another example from the same file:

    if (h->h_addrtype != af) {
        /* Let's get real error message to return */
        errno = EAFNOSUPPORT;
        PyErr_SetFromErrno(PyExc_OSError);
        return NULL;
    }

I propose the same change. i.e. errno= ENOENT + PyErr_SetFromErrno(PyExc_OSError);

[socketpair]

man if_nametoindex:

On success, if_nametoindex() returns the index number of the network interface;
on error, 0 is returned and errno is set to indicate the error.

It MUST set errno on error! I think there was some incorrect libc implementation when the code was written. So, a crutch like the one I described before is not ever required. As a solution, we can do the following :

errno = ENOENT; // for buggy if_nametoindex implementations, where they don't set errno.
index = if_nametoindex(PyBytes_AS_STRING(oname));
Py_DECREF(oname);
if (index == 0) {
    PyErr_SetFromErrno(PyExc_OSError);
    return NULL;
}

[socketpair]

@sobolevn

[sobolevn]

Sorry, I am not qualified enought in this module to make decisions :)
I've just highlighted that this is a known thing. Probably, you can go to the original comment PR and try to figure out what were the alternatives.

[arhadthedev]

cc @gpshead as a network expert.