net/protocol: read directly into rbuf if it's empty

There's no need to allocate a temporary string when @rbuf is
empty, we can use it as the read_nonblock destination buffer to
save both allocation overhead and avoid a later memcpy.

This results in a halving user CPU time and tiny memory
reduction with the script below:

             user     system      total        real
before   0.603333   0.539999   1.143332 (  1.143347)
       RssAnon:     5624 kB

after    0.283334   0.560000   0.843334 (  0.846072)
       RssAnon:     5592 kB

------
require 'net/http'
require 'benchmark'
s = TCPServer.new('127.0.0.1', 0)
len = 1024 * 1024 * 1024 * 2
pid = fork do
  c = s.accept
  c.readpartial(16384).clear
  c.send("HTTP/1.0 200 OK\r\nContent-Length: #{len}\r\n\r\n", Socket::MSG_MORE)
  IO.copy_stream('/dev/zero', c, len)
  c.close
end

addr = s.addr
Net::HTTP.start(addr[3], addr[1]) do |http|
  http.request_get('/') do |res|
    puts(Benchmark.measure { res.read_body(&:clear) })
  end
end
puts File.readlines("/proc/self/status").grep(/RssAnon/)[0]
Process.waitpid2(pid)
------

* lib/net/protocol.rb (rbuf_fill): avoid allocation if rbuf is empty
  [ruby-core:84678] [Feature #14326]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61663 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/net/protocol.rb

@@ -172,8 +172,10 @@ module Net # :nodoc:
     BUFSIZE = 1024 * 16
 
     def rbuf_fill
-      case rv = @io.read_nonblock(BUFSIZE, exception: false)
+      tmp = @rbuf.empty? ? @rbuf : nil
+      case rv = @io.read_nonblock(BUFSIZE, tmp, exception: false)
       when String
+        return if rv.equal?(tmp)
         @rbuf << rv
         rv.clear
         return