RUBY-679 raise error if exhaust used with mongos

Author: TylerBrock

lib/mongo/cursor.rb

@@ -386,8 +386,12 @@ def query_opts
     def add_option(opt)
       check_modifiable
 
-      if exhaust?(opt) && (@limit != 0)
-        raise MongoArgumentError, "Exhaust option is incompatible with limit."
+      if exhaust?(opt)
+        if @limit != 0
+          raise MongoArgumentError, "Exhaust is incompatible with limit."
+        elsif @connection.mongos?
+          raise MongoArgumentError, "Exhaust is incompatible with mongos."
+        end
       end
 
       @options |= opt

test/functional/cursor_test.rb

@@ -110,6 +110,15 @@ def test_limit_after_exhaust_error
     end
   end
 
+  def test_exhaust_with_mongos
+    @@connection.expects(:mongos?).returns(:true)
+    c = Cursor.new(@@coll)
+
+    assert_raise MongoArgumentError do
+      c.add_option(OP_QUERY_EXHAUST)
+    end
+  end
+
   def test_inspect
     selector = {:a => 1}
     cursor = @@coll.find(selector)