ethtool/ioctl: Use byref instead of cpointer when possible

[JeThWifirst]

According to the ctypes documentation 1, calling cpointer creates a new instance, and it seems to lead to memory leaks: "Note that ctypes does not have OOR (original object return), it constructs a new, equivalent object each time you retrieve an attribute".

The cpointer documentation 2 indicates that byref should rather be used when passing "a pointer to an object to a foreign function call".

Reproducer

The following reproducer shows the memory leak:

#!/usr/bin/python3
import psutil
from pyroute2.ethtool import Ethtool


def main():
    process = psutil.Process()
    memory = []

    with Ethtool() as ethtool:
        for idx in range(10001):
            ethtool._with_ioctl.change_ifname("enp11s0")
            ethtool._with_ioctl.get_statistics()
            if idx % 1000 == 0:
                memory.append(round(process.memory_info().rss / 1024 / 1024, 2))

    print(memory)


if __name__ == "__main__":
    main()

Without the fix, on my computer:

$ ./reproducer.py
[39.05, 50.42, 62.05, 73.92, 85.42, 96.92, 109.17, 120.67, 132.17, 143.55, 155.05]

With the fix, on my computer:

$ ./reproducer.py
[38.92, 39.05, 39.05, 39.05, 39.05, 39.05, 39.05, 39.05, 39.05, 39.05, 39.05]

[svinota]

Oh, thanks! Merging tonight.