source: trunk/packages/invirt-base/python/invirt/config.py @ 1619

Last change on this file since 1619 was 1423, checked in by price, 16 years ago

use run-parts semantics in conf.d directory

File size: 5.0 KB
RevLine 
[1197]1from __future__ import with_statement
2
[784]3import json
[778]4from invirt.common import *
[1422]5import os
[816]6from os import rename
[781]7from os.path import getmtime
[1197]8from contextlib import closing
[1421]9import yaml
[1423]10import re
[726]11
[1421]12try:    loader = yaml.CSafeLoader
13except: loader = yaml.SafeLoader
14
[1422]15src_path    = '/etc/invirt/master.yaml'
16src_dirpath = '/etc/invirt/conf.d'
17cache_path  = '/var/lib/invirt/cache.json'
18lock_path   = '/var/lib/invirt/cache.lock'
[726]19
[1422]20def augment(d1, d2):
21    """Splice dict-tree d2 into d1.  Return d1.
22
23    Example:
24    >>> d = {'a': {'b': 1}, 'c': 2}
25    >>> augment(d, {'a': {'d': 3}})
26    {'a': {'b', 1, 'd': 3}, 'c': 2}
27    >>> d
28    {'a': {'b', 1, 'd': 3}, 'c': 2}
29    """
30    for k in d2:
31        if k in d1 and isinstance(d1[k], dict):
32            augment(d1[k], d2[k])
33        else:
34            d1[k] = d2[k]
35    return d1
36
[1423]37def run_parts_list(dirname):
38    """Reimplements Debian's run-parts --list.
39
40    One difference from run-parts's behavior: run-parts --list /foo/
41    will give output like /foo//bar, because Python code tends to expect this.
42
43    Matches documented behavior of run-parts in debianutils v2.28.2, dated 2007.
44    """
45    # From run-parts(8).
46    lanana_re   = re.compile('^[a-z0-9]+$')
47    lsb_re      = re.compile('^_?([a-z0-9_.]+-)+[a-z0-9]+$')
48    deb_cron_re = re.compile('^[a-z0-9][a-z0-9-]*$')
49    for name in os.listdir(dirname):
50        if lanana_re.match(name) or lsb_re.match(name) or deb_cron_re.match(name):
51            yield os.path.join(dirname, name)
52
[1422]53def list_files():
54    yield src_path
[1423]55    for name in run_parts_list(src_dirpath):
56        yield name
[1422]57
[1421]58def load_master():
[1422]59    config = dict()
60    for filename in list_files():
61        with closing(file(filename)) as f:
62            augment(config, yaml.load(f, loader))
63    return config
[1421]64
65def get_src_mtime():
[1422]66    return max(max(getmtime(filename) for filename in list_files()),
67               getmtime(src_dirpath))
[1421]68
[1420]69def load(force_refresh = False):
[771]70    """
71    Try loading the configuration from the faster-to-load JSON cache at
72    cache_path.  If it doesn't exist or is outdated, load the configuration
73    instead from the original YAML file at src_path and regenerate the cache.
74    I assume I have the permissions to write to the cache directory.
75    """
[806]76
[807]77    # Namespace container for state variables, so that they can be updated by
78    # closures.
[793]79    ns = struct()
80
[771]81    if force_refresh:
[806]82        do_refresh = True
[771]83    else:
[1421]84        src_mtime = get_src_mtime()
[807]85        try:            cache_mtime = getmtime(cache_path)
86        except OSError: do_refresh  = True
87        else:           do_refresh  = src_mtime + 1 >= cache_mtime
[771]88
[807]89        # We chose not to simply say
90        #
91        #   do_refresh = src_mtime >= cache_time
92        #
93        # because between the getmtime(src_path) and the time the cache is
94        # rewritten, the master configuration may have been updated, so future
95        # checks here would find a cache with a newer mtime than the master
96        # (and thus treat the cache as containing the latest version of the
97        # master).  The +1 means that for at least a full second following the
98        # update to the master, this function will refresh the cache, giving us
99        # 1 second to write the cache.  Note that if it takes longer than 1
100        # second to write the cache, then this situation could still arise.
101        #
102        # The getmtime calls should logically be part of the same transaction
103        # as the rest of this function (cache read + conditional cache
104        # refresh), but to wrap everything in an flock would cause the
105        # following cache read to be less streamlined.
106
[806]107    if not do_refresh:
[793]108        # Try reading from the cache first.  This must be transactionally
109        # isolated from concurrent writes to prevent reading an incomplete
110        # (changing) version of the data (but the transaction can share the
[806]111        # lock with other concurrent reads).  This isolation is accomplished
112        # using an atomic filesystem rename in the refreshing stage.
[1197]113        try: 
114            with closing(file(cache_path)) as f:
115                ns.cfg = json.read(f.read())
[806]116        except: do_refresh = True
[778]117
[806]118    if do_refresh:
[781]119        # Atomically reload the source and regenerate the cache.  The read and
120        # write must be a single transaction, or a stale version may be
[806]121        # written (if another read/write of a more recent configuration
122        # is interleaved).  The final atomic rename is to keep this
123        # transactionally isolated from the above cache read.  If we fail to
124        # acquire the lock, just try to load the master configuration.
125        try:
[1197]126            with lock_file(lock_path):
[1421]127                ns.cfg = load_master()
[1197]128                try: 
129                    with closing(file(cache_path + '.tmp', 'w')) as f:
130                        f.write(json.write(ns.cfg))
[806]131                except: pass # silent failure
[816]132                else: rename(cache_path + '.tmp', cache_path)
[806]133        except IOError:
[1421]134            ns.cfg = load_master()
[793]135    return ns.cfg
[771]136
[778]137dicts = load()
138structs = dicts2struct(dicts)
139
[726]140# vim:et:sw=4:ts=4
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