RUBY-716 Add parallel_collection_scan helper

Author: estolfo

lib/mongo/collection.rb

@@ -824,6 +824,24 @@ def group(opts, condition={}, initial={}, reduce=nil, finalize=nil)
       end
     end
 
+
+    def parallel_scan(num_cursors, opts={})
+      cmd                          = BSON::OrderedHash.new
+      cmd[:parallelCollectionScan] = self.name
+      cmd[:numCursors]             = num_cursors
+      result                       = @db.command(cmd, command_options(opts))
+
+      result['cursors'].collect do |cursor_info|
+        seed = {
+          :cursor_id   => cursor_info['cursor']['id'],
+          :first_batch => cursor_info['cursor']['firstBatch'],
+          :pool        => @connection.pinned_pool
+        }
+        Cursor.new(self, seed.merge!(opts))
+      end
+
+    end
+
     private
 
     def new_group(opts={})

test/functional/collection_test.rb

@@ -1180,6 +1180,23 @@ def test_map_reduce_with_collection_output_to_other_db
     end
   end
 
+  if @@version >= '2.5.5'
+    def test_parallel_scan
+      8000.times { |i| @@test.insert({ :_id => i }) }
+
+      n_docs = {}
+      threads = []
+      cursors = @@test.parallel_scan(3)
+      cursors.each_with_index do |cursor, i|
+        threads << Thread.new do
+          n_docs[i] = cursor.to_a.size
+        end
+      end
+      threads.each(&:join)
+      assert_equal @@test.count, n_docs.values.inject(0) { |sum, n| sum + n }
+    end
+  end
+
   if @@version > "1.3.0"
     def test_find_and_modify
       @@test << { :a => 1, :processed => false }