#!/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
import os

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

    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

    currentallocation = int(open('/sys/devices/system/xen_memory/xen_memory0/info/current_kb', 'r').read())
# this information seems to have vanished in recent linux - we _want_ min_target_kb, but it doesn't seem to be
# exposed anymore.
#    minimumtarget = int(open('/sys/devices/system/xen_memory/xen_memory0/target_kb', 'r').read())
    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 kibibytes
    dom0minmem = int(xminfoc['dom0-min-mem']) * 1024

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

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