#!/usr/bin/env python
"""
Gathers availability data for the VM host it's running on.
"""

from subprocess import PIPE, Popen, call
import sys
import yaml

# return the amount of memory in kilobytes represented by s
def parseUnits(s):
    num, unit = s.split(' ')
    return int(num) * {'kb':1, 'mb':1024}[unit.lower()]

def main(argv):
    """
    Calculate the amount of memory available for new VMs
    The numbers returned by xm info and xm info -c are in MB
    The numbers in /proc/xen/balloon have nice units
    All math is done in kilobytes for consistency
    Output is in MB

    Bail if /etc/invirt/nocreate exists
    """
    try:
        os.stat('/etc/invirt/nocreate')
        print 0
        return 0
    except OSError:
        pass

    p = Popen(['/usr/sbin/xm', 'info'], stdout=PIPE)
    output = p.communicate()[0]
    if p.returncode != 0:
        raise RuntimeError("Command '%s' returned non-zero exit status %d"
                           % ('/usr/sbin/xm info', p.returncode)) 
    xminfo = yaml.load(output, yaml.CSafeLoader)

    free_memory = int(xminfo['free_memory']) * 1024

    ballooninfo = yaml.load(open('/proc/xen/balloon', 'r').read())
    currentallocation = parseUnits(ballooninfo['Current allocation'])
    minimumtarget = parseUnits(ballooninfo['Minimum target'])

    p = Popen(['/usr/sbin/xm', 'info', '-c'], stdout=PIPE)
    output = p.communicate()[0]
    if p.returncode != 0:
        raise RuntimeError("Command '%s' returned non-zero exit status %d"
                           % ('/usr/sbin/xm info -c', p.returncode)) 
    xminfoc = yaml.load(output, yaml.CSafeLoader)

    # In kilobytes
    dom0minmem = int(xminfoc['dom0-min-mem']) * 1024

    dom0_spare_memory = currentallocation - max(minimumtarget, dom0minmem)
    print int((free_memory + dom0_spare_memory)/1024)
    return 0

if __name__ == '__main__':
    sys.exit(main(sys.argv))