from __future__ import with_statement import json from invirt.common import * from os import rename from os.path import getmtime from contextlib import closing default_src_path = '/etc/invirt/master.yaml' default_cache_path = '/var/lib/invirt/cache.json' lock_path = '/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 If it doesn't exist or is outdated, load the configuration instead from the original YAML file at src_path and regenerate the cache.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 state variables, so that they can be updated by # closures. ns = struct() if force_refresh: do_refresh = True else: src_mtime = getmtime(src_path) try: cache_mtime = getmtime(cache_path) except OSError: do_refresh = True else: do_refresh = src_mtime + 1 >= cache_mtime # We chose not to simply say # # do_refresh = src_mtime >= cache_time # # because between the getmtime(src_path) and the time the cache is # rewritten, the master configuration may have been updated, so future # checks here would find a cache with a newer mtime than the master # (and thus treat the cache as containing the latest version of the # master). The +1 means that for at least a full second following the # update to the master, this function will refresh the cache, giving us # 1 second to write the cache. Note that if it takes longer than 1 # second to write the cache, then this situation could still arise. # # The getmtime calls should logically be part of the same transaction # as the rest of this function (cache read + conditional cache # refresh), but to wrap everything in an flock would cause the # following cache read to be less streamlined. if not 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). This isolation is accomplished # using an atomic filesystem rename in the refreshing stage. try: with closing(file(cache_path)) as f: ns.cfg = json.read(f.read()) except: do_refresh = True if 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 (if another read/write of a more recent configuration # is interleaved). The final atomic rename is to keep this # transactionally isolated from the above cache read. If we fail to # acquire the lock, just try to load the master configuration. import yaml try: loader = yaml.CSafeLoader except: loader = yaml.SafeLoader try: with lock_file(lock_path): with closing(file(src_path)) as f: ns.cfg = yaml.load(f, loader) try: with closing(file(cache_path + '.tmp', 'w')) as f: f.write(json.write(ns.cfg)) except: pass # silent failure else: rename(cache_path + '.tmp', cache_path) except IOError: with closing(file(src_path)) as f: ns.cfg = yaml.load(f, loader) return ns.cfg dicts = load() structs = dicts2struct(dicts) # vim:et:sw=4:ts=4