X-Git-Url: http://xvm.mit.edu/gitweb/invirt/packages/invirt-base.git/blobdiff_plain/dc42feef5695727a219dd5b4fb4a6d9dca9f14cc..bed209caf968dd992a7f849f8b982b4b46a0f876:/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..8648c70 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
@@ -1,5 +1,6 @@
 import json
 from invirt.common import *
+from os import rename
 from os.path import getmtime
 
 default_src_path   = '/etc/invirt/master.yaml'
@@ -16,17 +17,35 @@ 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
@@ -56,7 +75,7 @@ def load(src_path = default_src_path,
                 try: with_closing(file(cache_path + '.tmp', 'w')) (
                         lambda 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))