source: trunk/packages/sipb-xen-base/python/invirt/config.py @ 1206

Last change on this file since 1206 was 1206, checked in by broder, 15 years ago

Switch sipb-xen-base to build using a setup.py script

File size: 3.7 KB
Line 
1from __future__ import with_statement
2
3import json
4from invirt.common import *
5from os import rename
6from os.path import getmtime
7from contextlib import closing
8
9default_src_path   = '/etc/invirt/master.yaml'
10default_cache_path = '/var/lib/invirt/cache.json'
11lock_path          = '/var/lib/invirt/cache.lock'
12
13def load(src_path = default_src_path,
14         cache_path = default_cache_path,
15         force_refresh = False):
16    """
17    Try loading the configuration from the faster-to-load JSON cache at
18    cache_path.  If it doesn't exist or is outdated, load the configuration
19    instead from the original YAML file at src_path and regenerate the cache.
20    I assume I have the permissions to write to the cache directory.
21    """
22
23    # Namespace container for state variables, so that they can be updated by
24    # closures.
25    ns = struct()
26
27    if force_refresh:
28        do_refresh = True
29    else:
30        src_mtime = getmtime(src_path)
31        try:            cache_mtime = getmtime(cache_path)
32        except OSError: do_refresh  = True
33        else:           do_refresh  = src_mtime + 1 >= cache_mtime
34
35        # We chose not to simply say
36        #
37        #   do_refresh = src_mtime >= cache_time
38        #
39        # because between the getmtime(src_path) and the time the cache is
40        # rewritten, the master configuration may have been updated, so future
41        # checks here would find a cache with a newer mtime than the master
42        # (and thus treat the cache as containing the latest version of the
43        # master).  The +1 means that for at least a full second following the
44        # update to the master, this function will refresh the cache, giving us
45        # 1 second to write the cache.  Note that if it takes longer than 1
46        # second to write the cache, then this situation could still arise.
47        #
48        # The getmtime calls should logically be part of the same transaction
49        # as the rest of this function (cache read + conditional cache
50        # refresh), but to wrap everything in an flock would cause the
51        # following cache read to be less streamlined.
52
53    if not do_refresh:
54        # Try reading from the cache first.  This must be transactionally
55        # isolated from concurrent writes to prevent reading an incomplete
56        # (changing) version of the data (but the transaction can share the
57        # lock with other concurrent reads).  This isolation is accomplished
58        # using an atomic filesystem rename in the refreshing stage.
59        try: 
60            with closing(file(cache_path)) as f:
61                ns.cfg = json.read(f.read())
62        except: do_refresh = True
63
64    if do_refresh:
65        # Atomically reload the source and regenerate the cache.  The read and
66        # write must be a single transaction, or a stale version may be
67        # written (if another read/write of a more recent configuration
68        # is interleaved).  The final atomic rename is to keep this
69        # transactionally isolated from the above cache read.  If we fail to
70        # acquire the lock, just try to load the master configuration.
71        import yaml
72        try:    loader = yaml.CSafeLoader
73        except: loader = yaml.SafeLoader
74        try:
75            with lock_file(lock_path):
76                with closing(file(src_path)) as f:
77                    ns.cfg = yaml.load(f, loader)
78                try: 
79                    with closing(file(cache_path + '.tmp', 'w')) as f:
80                        f.write(json.write(ns.cfg))
81                except: pass # silent failure
82                else: rename(cache_path + '.tmp', cache_path)
83        except IOError:
84            with closing(file(src_path)) as f:
85                ns.cfg = yaml.load(f, loader)
86    return ns.cfg
87
88dicts = load()
89structs = dicts2struct(dicts)
90
91# vim:et:sw=4:ts=4
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