MTU detection

[gronke]

When no MTU was specified explicitly, the MTU of the host bridge (defined in interfaces) is detected.

python-iocage has implemented this feature in https://github.com/iocage/iocage/pull/722, but looks up the first bridge members MTU instead of the bridges value.

[gronke]

A quick test on performance:

Popen ifconfig

import iocage.decorators

@iocage.decorators.timeit
def test_popen():
    import subprocess
    subprocess.Popen(
        ["ifconfig", "em0"],
        stdout=subprocess.PIPE,
        stderr=subprocess.PIPE
    ).communicate()

@iocage.decorators.timeit
def test_ioctl_helper():
    import iocage.helpers_ioctl
    iocage.helpers_ioctl.get_interface_ip4_address("em0")

def run(iterations):
    print("----- subprocess.Popen -----")
    [test_popen() for x in range(iterations)]
    print("--- iocage.helpers_ioctl ---")
    [test_ioctl_helper() for x in range(iterations)]

run(iterations=5)

$ python3.6 speedtest.py
----- subprocess.Popen -----
function [test_popen] finished in 24.06597137451172 ms
function [test_popen] finished in 25.435209274291992 ms
function [test_popen] finished in 27.526378631591797 ms
function [test_popen] finished in 24.75738525390625 ms
function [test_popen] finished in 24.570941925048828 ms
--- iocage.helpers_ioctl ---
function [test_ioctl_helper] finished in 5.7430267333984375 ms
function [test_ioctl_helper] finished in 0.04482269287109375 ms
function [test_ioctl_helper] finished in 0.032901763916015625 ms
function [test_ioctl_helper] finished in 0.019550323486328125 ms
function [test_ioctl_helper] finished in 0.028848648071289062 ms