Lock results file while doing merge.

lib/simplecov.rb

@@ -1,6 +1,8 @@
 #
 # Code coverage for ruby 1.9. Please check out README for a full introduction.
 #
+require 'lockfile'
+
 module SimpleCov
   # Indicates invalid coverage data
   class CoverageDataError < StandardError; end;
@@ -52,8 +54,12 @@ def result
       # If we're using merging of results, store the current result
       # first, then merge the results and return those
       if use_merging
-        SimpleCov::ResultMerger.store_result(@result) if @result
-        return SimpleCov::ResultMerger.merged_result
+        lockfile = ::Lockfile.new ResultMerger.resultset_path + '.lockfile'
+
+        lockfile.lock do
+          SimpleCov::ResultMerger.store_result(@result) if @result
+          return SimpleCov::ResultMerger.merged_result
+        end
       else
         return @result if defined? @result
       end

simplecov.gemspec

@@ -13,6 +13,7 @@ Gem::Specification.new do |gem|
   gem.summary     = gem.description
 
   gem.add_dependency 'multi_json', '~> 1.0'
+  gem.add_dependency 'lockfile'
   gem.add_dependency 'simplecov-html', '~> 0.7.1'
 
   gem.add_development_dependency 'aruba'