add benchmark for IO.copy_stream IO#write case

I will attempt to reduce garbage in proposed fix
for https://bugs.ruby-lang.org/issues/13085

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57237 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

benchmark/bm_io_copy_stream_write.rb

@@ -0,0 +1,24 @@
+# The goal of this is to use a synthetic (non-IO) reader
+# to trigger the read/write loop of IO.copy_stream,
+# bypassing in-kernel mechanisms like sendfile for zero copy,
+# so we wrap the /dev/zero IO object:
+
+class Zero
+  def initialize
+    @n = 100000
+    @in = File.open('/dev/zero', 'rb')
+  end
+
+  def read(len, buf)
+    return if (@n -= 1) == 0
+    @in.read(len, buf)
+  end
+end
+
+begin
+  src = Zero.new
+  dst = File.open(IO::NULL, 'wb')
+  n = IO.copy_stream(src, dst)
+rescue Errno::ENOENT
+  # not *nix
+end if IO.respond_to?(:copy_stream) && IO.const_defined?(:NULL)