IO.popen will call the close method which was overridden when given a…

… block.

Test Script:
{{{
require 'test/unit/assertions.rb'
include Test::Unit::Assertions

class IO
  alias :close_orig :close
  def close
    warn "*** custom close"
    $close = :closed
    close_orig
  end
end

IO.popen("ruby -v", "r") {}

assert_equal($close, :closed)

puts :ok
}}}

io.c

@@ -2437,7 +2437,7 @@ rb_io_s_popen(VALUE klass, SEL sel, int argc, VALUE *argv)
 
     VALUE io = io_pipe_open(klass, process_name, mode);
     if (rb_block_given_p()) {
-	return rb_ensure(rb_yield, io, rb_io_close, io);
+	return rb_ensure(rb_yield, io, io_close2, io);
     }
     return io;
 }