Extend IDomainObjectModification

Currently the notify signal is sent before the database commit has occurred. It
is not possible to listen for modification events and know that the changes
have made it to the database. This can lead to a race condition if you try to
get a record from the database.

For an example see:

ckan/ckanext-archiver#6

This extension of the interface would allow notification after the database
commit has occurred.

ckan/model/modification.py

@@ -32,7 +32,12 @@ def notify_observers(self, func):
 
 
     def before_commit(self, session):
+        self.notify_observers(session, self.notify)
 
+    def after_commit(self, session):
+        self.notify_observers(session, self.notify_after_commit)
+
+    def notify_observers(self, session, method):
         session.flush()
         if not hasattr(session, '_object_cache'):
             return
@@ -44,13 +49,13 @@ def before_commit(self, session):
 
         for obj in set(new):
             if isinstance(obj, (_package.Package, resource.Resource)):
-                self.notify(obj, domain_object.DomainObjectOperation.new)
+                method(obj, domain_object.DomainObjectOperation.new)
         for obj in set(deleted):
             if isinstance(obj, (_package.Package, resource.Resource)):
-                self.notify(obj, domain_object.DomainObjectOperation.deleted)
+                method(obj, domain_object.DomainObjectOperation.deleted)
         for obj in set(changed):
             if isinstance(obj, resource.Resource):
-                self.notify(obj, domain_object.DomainObjectOperation.changed)
+                method(obj, domain_object.DomainObjectOperation.changed)
             if getattr(obj, 'url_changed', False):
                 for item in plugins.PluginImplementations(plugins.IResourceUrlChange):
                     item.notify(obj)
@@ -69,7 +74,7 @@ def before_commit(self, session):
                     if package and package not in deleted | new:
                         changed_pkgs.add(package)
         for obj in changed_pkgs:
-            self.notify(obj, domain_object.DomainObjectOperation.changed)
+            method(obj, domain_object.DomainObjectOperation.changed)
 
 
     def notify(self, entity, operation):
@@ -82,3 +87,14 @@ def notify(self, entity, operation):
                 # We reraise all exceptions so they are obvious there
                 # is something wrong
                 raise
+
+    def notify_after_commit(self, entity, operation):
+        for observer in plugins.PluginImplementations(
+                plugins.IDomainObjectModification):
+            try:
+                observer.notify_after_commit(entity, operation)
+            except Exception, ex:
+                log.exception(ex)
+                # We reraise all exceptions so they are obvious there
+                # is something wrong
+                raise

ckan/plugins/interfaces.py

@@ -185,6 +185,9 @@ class IDomainObjectModification(Interface):
     def notify(self, entity, operation):
         pass
 
+    def notify_after_commit(self, entity, operation):
+        pass
+
 
 class IResourceUrlChange(Interface):
     """