warn if saving zero-length file

lib/mongo/gridfs/grid_io.rb

@@ -78,6 +78,7 @@ module Mongo
     # @return [String]
     #   the data in the file
     def read(length=nil)
+      return '' if @file_length.zero?
       if length == 0
         return ''
       elsif length.nil? && @file_position.zero?
@@ -165,6 +166,9 @@ module Mongo
     # @return [True]
     def close
       if @mode[0] == ?w
+        if @current_chunk['n'].zero? && @chunk_position.zero?
+          warn "Warning: Storing a file with zero length."
+        end
         @upload_date = Time.now.utc
         @files.insert(to_mongo_object)
       end

test/grid_test.rb

@@ -45,6 +45,18 @@ class GridTest < Test::Unit::TestCase
       end
     end
 
+    context "Storing data with a length of zero" do
+      setup do
+        @grid = Grid.new(@db, 'test-fs')
+        @id   = @grid.put('', 'sample', :metadata => {'app' => 'photos'})
+      end
+
+      should "return the zero length" do
+        data = @grid.get(@id)
+        assert_equal 0, data.read.length
+      end
+    end
+
     context "Streaming: " do || {}
       setup do
         def read_and_write_stream(filename, read_length, opts={})