added shared/exclusive locking; added shared locking of initial JSON cache read

[invirt/packages/invirt-base.git] / 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 37fafb4..29f7ce5 100644 (file)
--- a/files/usr/share/python-support/sipb-xen-base/invirt/config.py
+++ b/files/usr/share/python-support/sipb-xen-base/invirt/config.py
@@ -1,13 +1,57 @@
-from __future__ import with_statement
-import yaml
+import json
+from invirt.common import *
+from os.path import getmtime
 
-default_path = '/etc/invirt/master.yaml'
+default_src_path   = '/etc/invirt/master.yaml'
+default_cache_path = '/var/lib/invirt/cache.json'
+lock_file          = '/var/lib/invirt/cache.lock'
 
-try:    default_loader = yaml.CSafeLoader
-except: default_loader = yaml.SafeLoader
+def load(src_path = default_src_path,
+         cache_path = default_cache_path,
+         force_refresh = False):
+    """
+    Try loading the configuration from the faster-to-load JSON cache at
+    cache_path.  If it doesn't exist or is outdated, load the configuration
+    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()
 
-def load( path = default_path ):
-    with file( path ) as f:
-        return yaml.load( f, default_loader )
+    if force_refresh:
+        ns.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
+
+    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 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(lock_file)
+        def refresh_cache():
+            import yaml
+            try:    default_loader = yaml.CSafeLoader
+            except: default_loader = yaml.SafeLoader
+            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 ns.cfg
+
+dicts = load()
+structs = dicts2struct(dicts)
 
 # vim:et:sw=4:ts=4