From c4e45141f8612ae5977869c4ea6babb777cf6bd6 Mon Sep 17 00:00:00 2001 From: Yang Zhang Date: Wed, 30 Jul 2008 21:50:42 -0400 Subject: [PATCH] added shared/exclusive locking; added shared locking of initial JSON cache read svn path=/trunk/packages/sipb-xen-base/; revision=793 --- .../python-support/sipb-xen-base/invirt/common.py | 6 ++- .../python-support/sipb-xen-base/invirt/config.py | 39 ++++++++++++-------- 2 files changed, 28 insertions(+), 17 deletions(-) diff --git a/files/usr/share/python-support/sipb-xen-base/invirt/common.py b/files/usr/share/python-support/sipb-xen-base/invirt/common.py index 5f669ba..3dc67a4 100644 --- a/files/usr/share/python-support/sipb-xen-base/invirt/common.py +++ b/files/usr/share/python-support/sipb-xen-base/invirt/common.py @@ -39,7 +39,7 @@ def with_closing(rsrc): finally: rsrc.close() return wrapper -def with_lock_file(path): +def with_lock_file(path, exclusive = True): """ Context manager for lock files. Example: @@ -53,7 +53,9 @@ def with_lock_file(path): def wrapper(func): @with_closing(file(path, 'w')) def g(f): - flock(f, LOCK_EX) + if exclusive: locktype = LOCK_EX + else: locktype = LOCK_SH + flock(f, locktype) try: return func() finally: flock(f, LOCK_UN) return g diff --git a/files/usr/share/python-support/sipb-xen-base/invirt/config.py b/files/usr/share/python-support/sipb-xen-base/invirt/config.py index c27ba6b..29f7ce5 100644 --- a/files/usr/share/python-support/sipb-xen-base/invirt/config.py +++ b/files/usr/share/python-support/sipb-xen-base/invirt/config.py @@ -4,6 +4,7 @@ from os.path import getmtime default_src_path = '/etc/invirt/master.yaml' 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, @@ -14,33 +15,41 @@ def load(src_path = default_src_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 not do_refresh: - # try reading from the cache first - try: cfg = with_closing(file(cache_path))(lambda f: json.read(f.read())) - except: do_refresh = True + 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 do_refresh: + 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('/var/lib/invirt/cache.lock') - def cfg(): + @with_lock_file(lock_file) + def refresh_cache(): import yaml try: default_loader = yaml.CSafeLoader except: default_loader = yaml.SafeLoader - cfg = with_closing(file(src_path))(lambda f: yaml.load(f, default_loader)) - try: with_closing(file(cache_path, 'w'))(lambda f: f.write(json.write(cfg))) + ns.cfg = with_closing(file(src_path))(lambda f: yaml.load(f, default_loader)) + try: with_closing(file(cache_path, 'w'))(lambda f: f.write(json.write(ns.cfg))) except: pass # silent failure - return cfg - return cfg + return ns.cfg dicts = load() structs = dicts2struct(dicts) -- 1.7.9.5