import json
from invirt.common import *
from os.path import getmtime

default_src_path   = '/etc/invirt/master.yaml'
default_cache_path = '/var/lib/invirt/cache.json'
lock_file          = '/var/lib/invirt/cache.lock'

def load(src_path = default_src_path,
         cache_path = default_cache_path,
         force_refresh = False):
    """
    Try loading the configuration from the faster-to-load JSON cache at
    cache_path.  If it doesn't exist or is outdated, load the configuration
    instead from the original YAML file at src_path and regenerate the cache.
    I assume I have the permissions to write to the cache directory.
    """
    # Namespace container for various state variables, so that they can be
    # updated by closures.
    ns = struct()

    if force_refresh:
        ns.do_refresh = True
    else:
        src_mtime = getmtime(src_path)
        try:            cache_mtime   = getmtime(cache_path)
        except OSError: ns.do_refresh = True
        else:           ns.do_refresh = src_mtime > cache_mtime

    if not ns.do_refresh:
        # Try reading from the cache first.  This must be transactionally
        # isolated from concurrent writes to prevent reading an incomplete
        # (changing) version of the data (but the transaction can share the
        # lock with other concurrent reads).
        @with_lock_file(lock_file, False)
        def read_cache():
            try: ns.cfg = with_closing(file(cache_path))(lambda f: json.read(f.read()))
            except: ns.do_refresh = True

    if ns.do_refresh:
        # Atomically reload the source and regenerate the cache.  The read and
        # write must be a single transaction, or a stale version may be
        # written.
        @with_lock_file(lock_file)
        def refresh_cache():
            import yaml
            try:    default_loader = yaml.CSafeLoader
            except: default_loader = yaml.SafeLoader
            ns.cfg = with_closing(file(src_path))(lambda f: yaml.load(f, default_loader))
            try: with_closing(file(cache_path, 'w'))(lambda f: f.write(json.write(ns.cfg)))
            except: pass # silent failure
    return ns.cfg

dicts = load()
structs = dicts2struct(dicts)

# vim:et:sw=4:ts=4