1 files changed, 60 insertions, 7 deletions
diff --git a/nova/utils.py b/nova/utils.py
index 499af2039..8936614cc 100644
--- a/nova/utils.py
+++ b/nova/utils.py
@@ -41,6 +41,7 @@ from xml.sax import saxutils
 
 from eventlet import event
 from eventlet import greenthread
+from eventlet import semaphore
 from eventlet.green import subprocess
 None
 from nova import exception
@@ -531,17 +532,69 @@ def loads(s):
     return json.loads(s)
 
 
-def synchronized(name):
+_semaphores_semaphore = semaphore.Semaphore()
+_semaphores = {}
+
+
+class _NoopContextManager(object):
+    def __enter__(self):
+        pass
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        pass
+
+
+def synchronized(name, external=False):
+    """Synchronization decorator
+
+    Decorating a method like so:
+    @synchronized('mylock')
+    def foo(self, *args):
+       ...
+
+    ensures that only one thread will execute the bar method at a time.
+
+    Different methods can share the same lock:
+    @synchronized('mylock')
+    def foo(self, *args):
+       ...
+
+    @synchronized('mylock')
+    def bar(self, *args):
+       ...
+
+    This way only one of either foo or bar can be executing at a time.
+
+    The external keyword argument denotes whether this lock should work across
+    multiple processes. This means that if two different workers both run a
+    a method decorated with @synchronized('mylock', external=True), only one
+    of them will execute at a time.
+    """
+
     def wrap(f):
         @functools.wraps(f)
         def inner(*args, **kwargs):
-            LOG.debug(_("Attempting to grab %(lock)s for method "
-                        "%(method)s..." % {"lock": name,
+            with _semaphores_semaphore:
+                if name not in _semaphores:
+                    _semaphores[name] = semaphore.Semaphore()
+                sem = _semaphores[name]
+            LOG.debug(_('Attempting to grab semaphore "%(lock)s" for method '
+                      '"%(method)s"...' % {"lock": name,
                                            "method": f.__name__}))
-            lock = lockfile.FileLock(os.path.join(FLAGS.lock_path,
-                                                  'nova-%s.lock' % name))
-            with lock:
-                return f(*args, **kwargs)
+            with sem:
+                if external:
+                    LOG.debug(_('Attempting to grab file lock "%(lock)s" for '
+                                'method "%(method)s"...' %
+                                {"lock": name, "method": f.__name__}))
+                    lock_file_path = os.path.join(FLAGS.lock_path,
+                                                  'nova-%s.lock' % name)
+                    lock = lockfile.FileLock(lock_file_path)
+                else:
+                    lock = _NoopContextManager()
+
+                with lock:
+                    return f(*args, **kwargs)
+
         return inner
     return wrap