Lock example_status_persistence_file to prevent race conditions

This introduces file locking to ExampleStatusPersister#persist to
avoid a race condition where parallel or unrelated spec runs race to
update the same file:

- read 1
- read 2
- write 1
- write 2 - write 1 is lost

Because we need to read and write the file under the same lock,
I inline #write into #persist so that dumped_statuses is called
after the file is opened/locked.

Author: Empact

lib/rspec/core/example_status_persister.rb

@@ -21,16 +21,19 @@ def initialize(examples, file_name)
       end
 
       def persist
-        write dumped_statuses
+        RSpec::Support::DirectoryMaker.mkdir_p(File.dirname(@file_name))
+        # lock the file while reading / persisting to avoid a race
+        # condition where parallel or unrelated spec runs race to
+        # update the same file
+        File.open(@file_name, File::RDWR | File::CREAT | File::LOCK_EX) { |f|
+          @previous_runs = File.read(f)
+          f.truncate(0)
+          f.write(dumped_statuses)
+        }
       end
 
     private
 
-      def write(statuses)
-        RSpec::Support::DirectoryMaker.mkdir_p(File.dirname(@file_name))
-        File.open(@file_name, "w") { |f| f.write(statuses) }
-      end
-
       def dumped_statuses
         ExampleStatusDumper.dump(merged_statuses)
       end
@@ -52,7 +55,7 @@ def statuses_from_this_run
       end
 
       def statuses_from_previous_runs
-        self.class.load_from(@file_name)
+        ExampleStatusParser.parse(@previous_runs)
       end
     end