X-Git-Url: http://xvm.mit.edu/gitweb/invirt/packages/invirt-base.git/blobdiff_plain/dc42feef5695727a219dd5b4fb4a6d9dca9f14cc..cb5ceb33ff4e87e35e30cffb07c2dec47fa5937e:/files/usr/share/python-support/sipb-xen-base/invirt/config.py

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 6d0ab36..69cb899 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
@@ -1,10 +1,14 @@
+from __future__ import with_statement
+
 import json
 from invirt.common import *
+from os import rename
 from os.path import getmtime
+from contextlib import closing
 
 default_src_path   = '/etc/invirt/master.yaml'
 default_cache_path = '/var/lib/invirt/cache.json'
-lock_file          = '/var/lib/invirt/cache.lock'
+lock_path          = '/var/lib/invirt/cache.lock'
 
 def load(src_path = default_src_path,
          cache_path = default_cache_path,
@@ -16,17 +20,35 @@ def load(src_path = default_src_path,
     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.
+    # Namespace container for state variables, so that they can be updated by
+    # closures.
     ns = struct()
 
     if force_refresh:
         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: do_refresh  = True
+        else:           do_refresh  = src_mtime + 1 >= cache_mtime
+
+        # We chose not to simply say
+        #
+        #   do_refresh = src_mtime >= cache_time
+        #
+        # because between the getmtime(src_path) and the time the cache is
+        # rewritten, the master configuration may have been updated, so future
+        # checks here would find a cache with a newer mtime than the master
+        # (and thus treat the cache as containing the latest version of the
+        # master).  The +1 means that for at least a full second following the
+        # update to the master, this function will refresh the cache, giving us
+        # 1 second to write the cache.  Note that if it takes longer than 1
+        # second to write the cache, then this situation could still arise.
+        #
+        # The getmtime calls should logically be part of the same transaction
+        # as the rest of this function (cache read + conditional cache
+        # refresh), but to wrap everything in an flock would cause the
+        # following cache read to be less streamlined.
 
     if not do_refresh:
         # Try reading from the cache first.  This must be transactionally
@@ -34,8 +56,9 @@ def load(src_path = default_src_path,
         # (changing) version of the data (but the transaction can share the
         # lock with other concurrent reads).  This isolation is accomplished
         # using an atomic filesystem rename in the refreshing stage.
-        try: ns.cfg = with_closing(file(cache_path)) (
-                lambda f: json.read(f.read()))
+        try: 
+            with closing(file(cache_path)) as f:
+                ns.cfg = json.read(f.read())
         except: do_refresh = True
 
     if do_refresh:
@@ -49,17 +72,17 @@ def load(src_path = default_src_path,
         try:    loader = yaml.CSafeLoader
         except: loader = yaml.SafeLoader
         try:
-            @with_lock_file(lock_file)
-            def refresh_cache():
-                ns.cfg = with_closing(file(src_path)) (
-                        lambda f: yaml.load(f, loader))
-                try: with_closing(file(cache_path + '.tmp', 'w')) (
-                        lambda f: f.write(json.write(ns.cfg)))
+            with lock_file(lock_path):
+                with closing(file(src_path)) as f:
+                    ns.cfg = yaml.load(f, loader)
+                try: 
+                    with closing(file(cache_path + '.tmp', 'w')) as f:
+                        f.write(json.write(ns.cfg))
                 except: pass # silent failure
-                else: os.rename(cache_path + '.tmp', cache_path)
+                else: rename(cache_path + '.tmp', cache_path)
         except IOError:
-            ns.cfg = with_closing(file(src_path)) (
-                    lambda f: yaml.load(f, loader))
+            with closing(file(src_path)) as f:
+                ns.cfg = yaml.load(f, loader)
     return ns.cfg
 
 dicts = load()