test for IO.copy_stream CPU usage (r58534)

I'm likely to make similar mistakes in the future when working
on Fiber auto-scheduling.  Start adding assertions for existing
code, first.

* test/ruby/test_io.rb (test_copy_stream_no_busy_wait): added
* test/lib/test/unit/assertions.rb (assert_cpu_usage_low): added

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58924 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/lib/test/unit/assertions.rb

@@ -715,6 +715,13 @@ eom
         skip
       end
 
+      def assert_cpu_usage_low(msg = nil, pct: 0.005)
+        require 'benchmark'
+        tms = Benchmark.measure(msg || '') { yield }
+        max = pct * tms.real
+        assert_operator tms.total, :<=, max, msg
+      end
+
       def assert_is_minus_zero(f)
         assert(1.0/f == -Float::INFINITY, "#{f} is not -0.0")
       end

test/ruby/test_io.rb

@@ -532,6 +532,18 @@ class TestIO < Test::Unit::TestCase
   end
 
   if have_nonblock?
+    def test_copy_stream_no_busy_wait
+      IO.pipe do |r,w|
+        r.nonblock = true
+        assert_cpu_usage_low('r58534 [ruby-core:80969] [Backport #13533]') do
+          th = Thread.new { IO.copy_stream(r, IO::NULL) }
+          sleep 0.1
+          w.close
+          th.join
+        end
+      end
+    end
+
     def test_copy_stream_pipe_nonblock
       mkcdtmpdir {
         with_read_pipe("abc") {|r1|