-import json, yaml
-from os import error, makedirs
-from os.path import dirname, getmtime
+import json
+from invirt.common import *
+from os.path import getmtime
default_src_path = '/etc/invirt/master.yaml'
-default_cache_path = '/var/lib/invirt/invirt.json'
-
-try: default_loader = yaml.CSafeLoader
-except: default_loader = yaml.SafeLoader
-
-def wrap(rsrc, func):
- "Utility to that emulates with Python 2.5's `with closing(rsrc)`."
- try: return func(rsrc)
- finally: rsrc.close()
+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,
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:
- do_refresh = True
+ ns.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 > cache_mtime
+ try: cache_mtime = getmtime(cache_path)
+ except OSError: ns.do_refresh = True
+ else: ns.do_refresh = src_mtime > cache_mtime
- if do_refresh:
- # reload the source and regenerate the cache
- cfg = wrap(file(src_path), lambda f: yaml.load(f, default_loader))
- wrap(file(cache_path, 'w'), lambda f: f.write(json.write(cfg)))
- else:
- cfg = wrap(file(cache_path), lambda f: json.read(f.read()))
- return cfg
+ 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
+ ns.cfg = with_closing(file(src_path)) (
+ lambda f: yaml.load(f, yaml.CSafeLoader))
+ 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