From dc42feef5695727a219dd5b4fb4a6d9dca9f14cc Mon Sep 17 00:00:00 2001
From: Yang Zhang <y_z@mit.edu>
Date: Fri, 1 Aug 2008 01:11:58 -0400
Subject: [PATCH] just read the master configuration if the cache refresh
 fails

svn path=/trunk/packages/sipb-xen-base/; revision=806
---
 .../python-support/sipb-xen-base/invirt/config.py  |   46 ++++++++++++--------
 1 file changed, 28 insertions(+), 18 deletions(-)

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 4a27cea..6d0ab36 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
@@ -15,41 +15,51 @@ 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:
-        ns.do_refresh = True
+        do_refresh = True
     else:
         src_mtime = getmtime(src_path)
         try:            cache_mtime   = getmtime(cache_path)
-        except OSError: ns.do_refresh = True
-        else:           ns.do_refresh = src_mtime > cache_mtime
+        except OSError: do_refresh = True
+        else:           do_refresh = src_mtime > cache_mtime
 
-    if not ns.do_refresh:
+    if not 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)) (
+        # 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()))
-            except: ns.do_refresh = True
+        except: do_refresh = True
 
-    if ns.do_refresh:
+    if 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(lock_file)
-        def refresh_cache():
-            import yaml
+        # written (if another read/write of a more recent configuration
+        # is interleaved).  The final atomic rename is to keep this
+        # transactionally isolated from the above cache read.  If we fail to
+        # acquire the lock, just try to load the master configuration.
+        import yaml
+        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)))
+                except: pass # silent failure
+                else: os.rename(cache_path + '.tmp', cache_path)
+        except IOError:
             ns.cfg = with_closing(file(src_path)) (
-                lambda f: yaml.load(f, yaml.CSafeLoader))
-            try: with_closing(file(cache_path, 'w')) (
-                lambda f: f.write(json.write(ns.cfg)))
-            except: pass # silent failure
+                    lambda f: yaml.load(f, loader))
     return ns.cfg
 
 dicts = load()
-- 
1.7.9.5