source: trunk/packages/sipb-xen-base/files/usr/share/python-support/sipb-xen-base/invirt/config.py @ 806

Last change on this file since 806 was 806, checked in by y_z, 17 years ago

just read the master configuration if the cache refresh fails

File size: 2.7 KB
RevLine 
[784]1import json
[778]2from invirt.common import *
[781]3from os.path import getmtime
[726]4
[771]5default_src_path   = '/etc/invirt/master.yaml'
[781]6default_cache_path = '/var/lib/invirt/cache.json'
[793]7lock_file          = '/var/lib/invirt/cache.lock'
[726]8
[771]9def load(src_path = default_src_path,
10         cache_path = default_cache_path,
11         force_refresh = False):
12    """
13    Try loading the configuration from the faster-to-load JSON cache at
14    cache_path.  If it doesn't exist or is outdated, load the configuration
15    instead from the original YAML file at src_path and regenerate the cache.
16    I assume I have the permissions to write to the cache directory.
17    """
[806]18
[793]19    # Namespace container for various state variables, so that they can be
20    # updated by closures.
21    ns = struct()
22
[771]23    if force_refresh:
[806]24        do_refresh = True
[771]25    else:
26        src_mtime = getmtime(src_path)
[793]27        try:            cache_mtime   = getmtime(cache_path)
[806]28        except OSError: do_refresh = True
29        else:           do_refresh = src_mtime > cache_mtime
[771]30
[806]31    if not do_refresh:
[793]32        # Try reading from the cache first.  This must be transactionally
33        # isolated from concurrent writes to prevent reading an incomplete
34        # (changing) version of the data (but the transaction can share the
[806]35        # lock with other concurrent reads).  This isolation is accomplished
36        # using an atomic filesystem rename in the refreshing stage.
37        try: ns.cfg = with_closing(file(cache_path)) (
[796]38                lambda f: json.read(f.read()))
[806]39        except: do_refresh = True
[778]40
[806]41    if do_refresh:
[781]42        # Atomically reload the source and regenerate the cache.  The read and
43        # write must be a single transaction, or a stale version may be
[806]44        # written (if another read/write of a more recent configuration
45        # is interleaved).  The final atomic rename is to keep this
46        # transactionally isolated from the above cache read.  If we fail to
47        # acquire the lock, just try to load the master configuration.
48        import yaml
49        try:    loader = yaml.CSafeLoader
50        except: loader = yaml.SafeLoader
51        try:
52            @with_lock_file(lock_file)
53            def refresh_cache():
54                ns.cfg = with_closing(file(src_path)) (
55                        lambda f: yaml.load(f, loader))
56                try: with_closing(file(cache_path + '.tmp', 'w')) (
57                        lambda f: f.write(json.write(ns.cfg)))
58                except: pass # silent failure
59                else: os.rename(cache_path + '.tmp', cache_path)
60        except IOError:
[796]61            ns.cfg = with_closing(file(src_path)) (
[806]62                    lambda f: yaml.load(f, loader))
[793]63    return ns.cfg
[771]64
[778]65dicts = load()
66structs = dicts2struct(dicts)
67
[726]68# vim:et:sw=4:ts=4
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