another benchmark for IO.copy_stream socket writing

For testing Linux socket-only workaround for
https://bugs.ruby-lang.org/issues/13085

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57364 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

benchmark/bm_io_copy_stream_write_socket.rb

@@ -0,0 +1,35 @@
+# 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
+  require 'socket'
+  src = Zero.new
+  rd, wr = UNIXSocket.pair
+  pid = fork do
+    wr.close
+    buf = String.new
+    while rd.read(16384, buf)
+    end
+  end
+  rd.close
+  IO.copy_stream(src, wr)
+rescue Errno::ENOENT, NotImplementedError, NameError
+  # not *nix: missing /dev/zero, fork, or UNIXSocket
+rescue LoadError # no socket?
+ensure
+  wr.close if wr
+  Process.waitpid(pid) if pid
+end if IO.respond_to?(:copy_stream)