Race condition in ActionController::Caching::Pages

Using JRuby 1.6.0, we saw a corrupt file created by Rails' page caching. It appeared that multiple threads were able to write simultaneously to a cache file.

We were able to duplicate the behavior we saw with the following contrived example:

f1 = File.open("out", "wb+")
f2 = File.open("out", "wb+")

10000.times do
  f1.write "a"
  f2.write "b"
end

f2.close
f1.close

This results with a file with interleaved "a" and "b" characters.

I'm not sure if this behavior occurs across processes as well, but it's likely to be much more prevalent in a multi-threaded program.