Added locking around update and delete methods on main API objects. This should remove the 500 errors which come from concurrent overlapping updates. Experimental - might be too slow for production.

app/models/node.rb

@@ -130,6 +130,7 @@ class Node < ActiveRecord::Base
     # provide repeatable reads for the used-by checks. this means it
     # shouldn't be possible to get race conditions.
     Node.transaction do
+      self.lock!
       check_consistency(self, new_node, user)
       way = WayNode.find(:first, :joins => :way, 
                          :conditions => [ "current_ways.visible = ? AND current_way_nodes.node_id = ?", true, self.id ])
@@ -150,25 +151,28 @@ class Node < ActiveRecord::Base
   end
 
   def update_from(new_node, user)
-    check_consistency(self, new_node, user)
+    Node.transaction do
+      self.lock!
+      check_consistency(self, new_node, user)
       
-    # update changeset first
-    self.changeset_id = new_node.changeset_id
-    self.changeset = new_node.changeset
+      # update changeset first
+      self.changeset_id = new_node.changeset_id
+      self.changeset = new_node.changeset
       
-    # update changeset bbox with *old* position first
-    changeset.update_bbox!(bbox);
+      # update changeset bbox with *old* position first
+      changeset.update_bbox!(bbox);
       
-    # FIXME logic needs to be double checked
-    self.latitude = new_node.latitude 
-    self.longitude = new_node.longitude
-    self.tags = new_node.tags
-    self.visible = true
+      # FIXME logic needs to be double checked
+      self.latitude = new_node.latitude 
+      self.longitude = new_node.longitude
+      self.tags = new_node.tags
+      self.visible = true
       
-    # update changeset bbox with *new* position
-    changeset.update_bbox!(bbox);
+      # update changeset bbox with *new* position
+      changeset.update_bbox!(bbox);
       
-    save_with_history!
+      save_with_history!
+    end
   end
   
   def create_with_history(user)

app/models/relation.rb

@@ -240,6 +240,7 @@ class Relation < ActiveRecord::Base
     # provide repeatable reads for the used-by checks. this means it
     # shouldn't be possible to get race conditions.
     Relation.transaction do
+      self.lock!
       check_consistency(self, new_relation, user)
       # This will check to see if this relation is used by another relation
       rel = RelationMember.find(:first, :joins => :relation, 
@@ -255,16 +256,19 @@ class Relation < ActiveRecord::Base
   end
 
   def update_from(new_relation, user)
-    check_consistency(self, new_relation, user)
-    unless new_relation.preconditions_ok?(self.members)
-      raise OSM::APIPreconditionFailedError.new("Cannot update relation #{self.id}: data or member data is invalid.")
+    Relation.transaction do
+      self.lock!
+      check_consistency(self, new_relation, user)
+      unless new_relation.preconditions_ok?(self.members)
+        raise OSM::APIPreconditionFailedError.new("Cannot update relation #{self.id}: data or member data is invalid.")
+      end
+      self.changeset_id = new_relation.changeset_id
+      self.changeset = new_relation.changeset
+      self.tags = new_relation.tags
+      self.members = new_relation.members
+      self.visible = true
+      save_with_history!
     end
-    self.changeset_id = new_relation.changeset_id
-    self.changeset = new_relation.changeset
-    self.tags = new_relation.tags
-    self.members = new_relation.members
-    self.visible = true
-    save_with_history!
   end
   
   def create_with_history(user)

app/models/way.rb

@@ -204,17 +204,20 @@ class Way < ActiveRecord::Base
   end
 
   def update_from(new_way, user)
-    check_consistency(self, new_way, user)
-    unless new_way.preconditions_ok?(self.nds)
-      raise OSM::APIPreconditionFailedError.new("Cannot update way #{self.id}: data is invalid.")
-    end
+    Way.transaction do
+      self.lock!
+      check_consistency(self, new_way, user)
+      unless new_way.preconditions_ok?(self.nds)
+        raise OSM::APIPreconditionFailedError.new("Cannot update way #{self.id}: data is invalid.")
+      end
       
-    self.changeset_id = new_way.changeset_id
-    self.changeset = new_way.changeset
-    self.tags = new_way.tags
-    self.nds = new_way.nds
-    self.visible = true
-    save_with_history!
+      self.changeset_id = new_way.changeset_id
+      self.changeset = new_way.changeset
+      self.tags = new_way.tags
+      self.nds = new_way.nds
+      self.visible = true
+      save_with_history!
+    end
   end
 
   def create_with_history(user)
@@ -258,6 +261,7 @@ class Way < ActiveRecord::Base
     # provide repeatable reads for the used-by checks. this means it
     # shouldn't be possible to get race conditions.
     Way.transaction do
+      self.lock!
       check_consistency(self, new_way, user)
       rel = RelationMember.find(:first, :joins => :relation,
                              :conditions => [ "visible = ? AND member_type='Way' and member_id=? ", true, self.id])