Improve BufferedIO performance

`BufferedIO` is a bit inefficient for reading large responses because
it use the classic `buffer.slice!` technique which cause a lot of
unnecessary string copying.

This is particularly visible on line based protocol when reading
line by line.

Instead of repeatedly shifting the string, we can keep track of
which offset we're at, to know how many bytes are left in the buffer.

This change also open the door to further optimization by increasing
the buffer size, as previously `slice!` would get slower the larger
the buffer is.

Benchmark results:

```
=== 1k ===
Warming up --------------------------------------
                  1k     1.234k i/100ms
              1k opt     1.283k i/100ms
Calculating -------------------------------------
                  1k     12.615k (± 0.9%) i/s -     64.168k in   5.086995s
              1k opt     12.856k (± 0.9%) i/s -     65.433k in   5.090051s

Comparison:
                  1k:    12615.2 i/s
              1k opt:    12856.0 i/s - 1.02x  (± 0.00) faster

=== 10k ===
Warming up --------------------------------------
                 10k     1.165k i/100ms
             10k opt     1.269k i/100ms
Calculating -------------------------------------
                 10k     11.550k (± 2.4%) i/s -     58.250k in   5.046378s
             10k opt     12.736k (± 1.0%) i/s -     64.719k in   5.081969s

Comparison:
                 10k:    11550.3 i/s
             10k opt:    12736.3 i/s - 1.10x  (± 0.00) faster

=== 100k ===
Warming up --------------------------------------
                100k   809.000  i/100ms
            100k opt   926.000  i/100ms
Calculating -------------------------------------
                100k      8.054k (± 3.0%) i/s -     40.450k in   5.028299s
            100k opt      9.286k (± 2.2%) i/s -     47.226k in   5.088841s

Comparison:
                100k:     8053.6 i/s
            100k opt:     9285.5 i/s - 1.15x  (± 0.00) faster

=== 1M ===
Warming up --------------------------------------
                  1M   249.000  i/100ms
              1M opt   315.000  i/100ms
Calculating -------------------------------------
                  1M      2.448k (± 2.5%) i/s -     12.450k in   5.089744s
              1M opt      3.119k (± 2.6%) i/s -     15.750k in   5.053772s

Comparison:
                  1M:     2447.8 i/s
              1M opt:     3118.8 i/s - 1.27x  (± 0.00) faster
```

Profiling before (1MB responses):

```
==================================
  Mode: wall(1000)
  Samples: 5276 (0.00% miss rate)
  GC: 394 (7.47%)
==================================
     TOTAL    (pct)     SAMPLES    (pct)     FRAME
      1622  (30.7%)        1622  (30.7%)     IO#wait_readable
       777  (14.7%)         777  (14.7%)     IO#read_nonblock
       365   (6.9%)         365   (6.9%)     (sweeping)
      2705  (51.3%)         364   (6.9%)     Net::BufferedIO#rbuf_fill
       264   (5.0%)         264   (5.0%)     String#index
       223   (4.2%)         223   (4.2%)     String#sub
       221   (4.2%)         221   (4.2%)     String#slice!
       185   (3.5%)         185   (3.5%)     String#split
       108   (2.0%)         108   (2.0%)     IO#write_nonblock
       101   (1.9%)         101   (1.9%)     String#downcase
        66   (1.3%)          66   (1.3%)     Net::BufferedIO#LOG
        57   (1.1%)          57   (1.1%)     String#count
        51   (1.0%)          51   (1.0%)     String#to_s
       391   (7.4%)          50   (0.9%)     Net::HTTPGenericRequest#write_header
        50   (0.9%)          50   (0.9%)     String#capitalize
        49   (0.9%)          49   (0.9%)     Array#join
        47   (0.9%)          47   (0.9%)     String#b
       106   (2.0%)          36   (0.7%)     Net::HTTPHeader#set_field
        34   (0.6%)          34   (0.6%)     Module#===
        33   (0.6%)          33   (0.6%)     String#[]
       140   (2.7%)          29   (0.5%)     Net::BufferedIO#write0
        29   (0.5%)          29   (0.5%)     (marking)
       281   (5.3%)          27   (0.5%)     Net::BufferedIO#rbuf_consume
      1195  (22.6%)          25   (0.5%)     Net::HTTPResponse#read_body
      1024  (19.4%)          25   (0.5%)     Net::HTTPResponse.each_response_header
        86   (1.6%)          24   (0.5%)     Net::HTTPHeader#set_field
        23   (0.4%)          23   (0.4%)     Net::HTTP#proxy_uri
        51   (1.0%)          23   (0.4%)     Net::HTTPHeader#initialize_http_header
      2225  (42.2%)          22   (0.4%)     Net::BufferedIO#readuntil
        20   (0.4%)          20   (0.4%)     Regexp#===
```

Profiling after (1MB responses):

```
==================================
  Mode: wall(1000)
  Samples: 15180 (0.00% miss rate)
  GC: 1688 (11.12%)
==================================
     TOTAL    (pct)     SAMPLES    (pct)     FRAME
      4534  (29.9%)        4534  (29.9%)     IO#read_nonblock
     10650  (70.2%)        3944  (26.0%)     Net::HTTPOpt::BufferedIOOpt#rbuf_fill
      2101  (13.8%)        2101  (13.8%)     IO#wait_readable
      1442   (9.5%)        1442   (9.5%)     (sweeping)
       360   (2.4%)         360   (2.4%)     String#sub
       312   (2.1%)         312   (2.1%)     String#split
       265   (1.7%)         265   (1.7%)     String#bytesize
       246   (1.6%)         246   (1.6%)     (marking)
       151   (1.0%)         151   (1.0%)     IO#write_nonblock
       125   (0.8%)         125   (0.8%)     String#downcase
       116   (0.8%)         116   (0.8%)     String#index
       113   (0.7%)         113   (0.7%)     Module#===
       162   (1.1%)          89   (0.6%)     Net::HTTPOpt::BufferedIOOpt#rbuf_consume_all_shareable!
       158   (1.0%)          65   (0.4%)     Net::HTTPHeader#set_field
        63   (0.4%)          63   (0.4%)     String#capitalize
        63   (0.4%)          63   (0.4%)     BasicObject#equal?
        58   (0.4%)          58   (0.4%)     Regexp#match
        58   (0.4%)          58   (0.4%)     String#[]
       449   (3.0%)          56   (0.4%)     Net::HTTPGenericRequest#write_header
        53   (0.3%)          53   (0.3%)     String#to_s
        52   (0.3%)          52   (0.3%)     Net::HTTPOpt::BufferedIOOpt#LOG
        52   (0.3%)          52   (0.3%)     String#count
        44   (0.3%)          44   (0.3%)     String#byteslice
        44   (0.3%)          44   (0.3%)     Array#join
      1096   (7.2%)          42   (0.3%)     Net::HTTPResponse.each_response_header
      2617  (17.2%)          40   (0.3%)     Net::HTTPOpt::BufferedIOOpt#readuntil
       132   (0.9%)          30   (0.2%)     Net::HTTPOpt::BufferedIOOpt#rbuf_consume
        28   (0.2%)          28   (0.2%)     Regexp#===
        27   (0.2%)          27   (0.2%)     Net::HTTP#proxy_uri
      8862  (58.4%)          27   (0.2%)     Net::HTTPResponse#read_body
````

Benchmark code:

```ruby

require "fileutils"
DIR = "/tmp/www"
FileUtils.mkdir_p(DIR)
HOST = "127.0.0.1"
PORT = 8080
CONF = <<~EOS
daemon            off;
worker_processes  2;

events {
    worker_connections  128;
}

http {
    server_tokens off;
    charset       utf-8;

    server {
        server_name   localhost;
        listen        #{HOST}:#{PORT};

        keepalive_requests 10000000;
        keepalive_timeout 3600s;

        error_page    500 502 503 504  /50x.html;

        location      / {
            root      #{DIR};
        }

    }

}
EOS

File.write(File.join(DIR, "1k.txt"), 'a' * 1024)
File.write(File.join(DIR, "10k.txt"), 'a' * 1024 * 10)
File.write(File.join(DIR, "100k.txt"), 'a' * 1024 * 100)
File.write(File.join(DIR, "1M.txt"), 'a' * 1024 * 1024)

File.write(File.join(DIR, "nginx.conf"), CONF)

require "benchmark/ips"
require "net/http"

nginx_pid = Process.spawn('nginx', '-c', File.join(DIR, "nginx.conf"))

module Net
  class HTTPOpt < HTTP

    class BufferedIOOpt < ::Net::BufferedIO  #:nodoc: internal use only
      def initialize(io, read_timeout: 60, write_timeout: 60, continue_timeout: nil, debug_output: nil)
        @io = io
        @read_timeout = read_timeout
        @write_timeout = write_timeout
        @continue_timeout = continue_timeout
        @debug_output = debug_output
        @rbuf = ''.b
        @rbuf_offset = 0
      end

      attr_reader :io
      attr_accessor :read_timeout
      attr_accessor :write_timeout
      attr_accessor :continue_timeout
      attr_accessor :debug_output

      def inspect
        "#<#{self.class} io=#{@io}>"
      end

      def eof?
        @io.eof?
      end

      def closed?
        @io.closed?
      end

      def close
        @io.close
      end

      #
      # Read
      #

      public

      def read(len, dest = ''.b, ignore_eof = false)
        LOG "reading #{len} bytes..."
        read_bytes = 0
        begin
          while read_bytes + rbuf_size < len
            if s = rbuf_consume_all_shareable!
              read_bytes += s.bytesize
              dest << s
            end
            rbuf_fill
          end
          s = rbuf_consume(len - read_bytes)
          read_bytes += s.bytesize
          dest << s
        rescue EOFError
          raise unless ignore_eof
        end
        LOG "read #{read_bytes} bytes"
        dest
      end

      def read_all(dest = ''.b)
        LOG 'reading all...'
        read_bytes = 0
        begin
          while true
            if s = rbuf_consume_all_shareable!
              read_bytes += s.bytesize
              dest << s
            end
            rbuf_fill
          end
        rescue EOFError
          ;
        end
        LOG "read #{read_bytes} bytes"
        dest
      end

      def readuntil(terminator, ignore_eof = false)
        offset = @rbuf_offset
        begin
          until idx = @rbuf.index(terminator, offset)
            offset = @rbuf.bytesize
            rbuf_fill
          end
          return rbuf_consume(idx + terminator.bytesize - @rbuf_offset)
        rescue EOFError
          raise unless ignore_eof
          return rbuf_consume
        end
      end

      def readline
        readuntil("\n").chop
      end

      private

      BUFSIZE = 1024 * 16

      def rbuf_fill
        tmp = @rbuf_empty ? @rbuf : nil
        case rv = @io.read_nonblock(BUFSIZE, tmp, exception: false)
        when String
          @rbuf_empty = false
          if rv.equal?(tmp)
            @rbuf_offset = 0
          else
            @rbuf << rv
            rv.clear
          end
          return
        when :wait_readable
          (io = @io.to_io).wait_readable(@read_timeout) or raise Net::ReadTimeout.new(io)
          # continue looping
        when :wait_writable
          # OpenSSL::Buffering#read_nonblock may fail with IO::WaitWritable.
          # http://www.openssl.org/support/faq.html#PROG10
          (io = @io.to_io).wait_writable(@read_timeout) or raise Net::ReadTimeout.new(io)
          # continue looping
        when nil
          raise EOFError, 'end of file reached'
        end while true
      end

      def rbuf_flush
        if @rbuf_empty
          @rbuf.clear
          @rbuf_offset = 0
        end
        nil
      end

      def rbuf_size
        @rbuf.bytesize - @rbuf_offset
      end

      # Warning: this method may share the buffer to avoid
      # copying. The caller must no longer use the returned
      # string once rbuf_fill has been called again
      def rbuf_consume_all_shareable!
        @rbuf_empty = true
        buf = if @rbuf_offset == 0
          @rbuf
        else
          @rbuf.byteslice(@rbuf_offset..-1)
        end
        @rbuf_offset = @rbuf.bytesize
        buf
      end

      def rbuf_consume(len = nil)
        if @rbuf_offset == 0 && (len.nil? || len == @rbuf.bytesize)
          s = @rbuf
          @rbuf = ''.b
          @rbuf_offset = 0
          @rbuf_empty = true
        elsif len.nil?
          s = @rbuf.byteslice(@rbuf_offset..-1)
          @rbuf = ''.b
          @rbuf_offset = 0
          @rbuf_empty = true
        else
          s = @rbuf.byteslice(@rbuf_offset, len)
          @rbuf_offset += len
          @rbuf_empty = @rbuf_offset == @rbuf.bytesize
          rbuf_flush
        end

        @debug_output << %Q[-> #{s.dump}\n] if @debug_output
        s
      end

      #
      # Write
      #

      public

      def write(*strs)
        writing {
          write0(*strs)
        }
      end

      alias << write

      def writeline(str)
        writing {
          write0 str + "\r\n"
        }
      end

      private

      def writing
        @written_bytes = 0
        @debug_output << '<- ' if @debug_output
        yield
        @debug_output << "\n" if @debug_output
        bytes = @written_bytes
        @written_bytes = nil
        bytes
      end

      def write0(*strs)
        @debug_output << strs.map(&:dump).join if @debug_output
        orig_written_bytes = @written_bytes
        strs.each_with_index do |str, i|
          need_retry = true
          case len = @io.write_nonblock(str, exception: false)
          when Integer
            @written_bytes += len
            len -= str.bytesize
            if len == 0
              if strs.size == i+1
                return @written_bytes - orig_written_bytes
              else
                need_retry = false
                # next string
              end
            elsif len < 0
              str = str.byteslice(len, -len)
            else # len > 0
              need_retry = false
              # next string
            end
            # continue looping
          when :wait_writable
            (io = @io.to_io).wait_writable(@write_timeout) or raise Net::WriteTimeout.new(io)
            # continue looping
          end while need_retry
        end
      end

      #
      # Logging
      #

      private

      def LOG_off
        @save_debug_out = @debug_output
        @debug_output = nil
      end

      def LOG_on
        @debug_output = @save_debug_out
      end

      def LOG(msg)
        return unless @debug_output
        @debug_output << msg + "\n"
      end
    end
    BufferedIO = BufferedIOOpt

    # Unchanged from ruby 3.1.1, only allow to lookup the mofidied BufferedIO
    def connect
      if use_ssl?
        # reference early to load OpenSSL before connecting,
        # as OpenSSL may take time to load.
        @ssl_context = OpenSSL::SSL::SSLContext.new
      end

      if proxy? then
        conn_addr = proxy_address
        conn_port = proxy_port
      else
        conn_addr = conn_address
        conn_port = port
      end

      D "opening connection to #{conn_addr}:#{conn_port}..."
      begin
        s = Socket.tcp conn_addr, conn_port, @local_host, @local_port, connect_timeout: @open_timeout
      rescue => e
        e = Net::OpenTimeout.new(e) if e.is_a?(Errno::ETIMEDOUT) #for compatibility with previous versions
        raise e, "Failed to open TCP connection to " +
          "#{conn_addr}:#{conn_port} (#{e.message})"
      end
      s.setsockopt(Socket::IPPROTO_TCP, Socket::TCP_NODELAY, 1)
      D "opened"
      if use_ssl?
        if proxy?
          plain_sock = BufferedIO.new(s, read_timeout: @read_timeout,
                                      write_timeout: @write_timeout,
                                      continue_timeout: @continue_timeout,
                                      debug_output: @debug_output)
          buf = "CONNECT #{conn_address}:#{@PORT} HTTP/#{HTTPVersion}\r\n"
          buf << "Host: #{@address}:#{@PORT}\r\n"
          if proxy_user
            credential = ["#{proxy_user}:#{proxy_pass}"].pack('m0')
            buf << "Proxy-Authorization: Basic #{credential}\r\n"
          end
          buf << "\r\n"
          plain_sock.write(buf)
          HTTPResponse.read_new(plain_sock).value
          # assuming nothing left in buffers after successful CONNECT response
        end

        ssl_parameters = Hash.new
        iv_list = instance_variables
        SSL_IVNAMES.each_with_index do |ivname, i|
          if iv_list.include?(ivname)
            value = instance_variable_get(ivname)
            unless value.nil?
              ssl_parameters[SSL_ATTRIBUTES[i]] = value
            end
          end
        end
        @ssl_context.set_params(ssl_parameters)
        @ssl_context.session_cache_mode =
          OpenSSL::SSL::SSLContext::SESSION_CACHE_CLIENT |
          OpenSSL::SSL::SSLContext::SESSION_CACHE_NO_INTERNAL_STORE
        @ssl_context.session_new_cb = proc {|sock, sess| @ssl_session = sess }
        D "starting SSL for #{conn_addr}:#{conn_port}..."
        s = OpenSSL::SSL::SSLSocket.new(s, @ssl_context)
        s.sync_close = true
        # Server Name Indication (SNI) RFC 3546
        s.hostname = @address if s.respond_to? :hostname=
        if @ssl_session and
           Process.clock_gettime(Process::CLOCK_REALTIME) < @ssl_session.time.to_f + @ssl_session.timeout
          s.session = @ssl_session
        end
        ssl_socket_connect(s, @open_timeout)
        if (@ssl_context.verify_mode != OpenSSL::SSL::VERIFY_NONE) && @ssl_context.verify_hostname
          s.post_connection_check(@address)
        end
        D "SSL established, protocol: #{s.ssl_version}, cipher: #{s.cipher[0]}"
      end
      @socket = BufferedIO.new(s, read_timeout: @read_timeout,
                               write_timeout: @write_timeout,
                               continue_timeout: @continue_timeout,
                               debug_output: @debug_output)
      @last_communicated = nil
      on_connect
    rescue => exception
      if s
        D "Conn close because of connect error #{exception}"
        s.close
      end
      raise
    end
    private :connect
  end
end

begin
  sleep 0.2

  connection = Net::HTTP.start(HOST, PORT)
  connection.keep_alive_timeout = 3600
  connection_opt = Net::HTTPOpt.start(HOST, PORT)
  connection_opt.keep_alive_timeout = 3600

  unless connection.request_get("/100k.txt").body == connection_opt.request_get("/100k.txt").body
    abort("bug?")
  end

  if ARGV.first == "profile"
    require 'stackprof'
    require 'json'

    StackProf.run(mode: :wall, out: "/tmp/stackprof-net-http.dump", raw: true) do
      40_000.times do
        connection.request_get("/1M.txt").body
      end
    end
    File.write("/tmp/stackprof-net-http.json", JSON.dump(Marshal.load(File.binread("/tmp/stackprof-net-http.dump"))))
    system("stackprof", "/tmp/stackprof-net-http.rb")

    StackProf.run(mode: :wall, out: "/tmp/stackprof-net-http-opt.dump", raw: true) do
      40_000.times do
        connection_opt.request_get("/1M.txt").body
      end
    end
    File.write("/tmp/stackprof-net-http-opt.json", JSON.dump(Marshal.load(File.binread("/tmp/stackprof-net-http-opt.dump"))))
    system("stackprof", "/tmp/stackprof-net-http-opt.dump")

  else
    %w(1k 10k 100k 1M).each do |size|
      puts "=== #{size} ==="
      Benchmark.ips do |x|
        path = "/#{size}.txt"
        x.report("#{size}") { connection.request_get(path).body }
        x.report("#{size} opt") { connection_opt.request_get(path).body }
        x.compare!(order: :baseline)
      end
      puts
    end
  end
ensure
  Process.kill('TERM', nginx_pid)
  Process.wait(nginx_pid)
end

```

Author: byroot

lib/net/protocol.rb

@@ -120,6 +120,7 @@ def initialize(io, read_timeout: 60, write_timeout: 60, continue_timeout: nil, d
       @continue_timeout = continue_timeout
       @debug_output = debug_output
       @rbuf = ''.b
+      @rbuf_offset = 0
     end
 
     attr_reader :io
@@ -154,14 +155,15 @@ def read(len, dest = ''.b, ignore_eof = false)
       LOG "reading #{len} bytes..."
       read_bytes = 0
       begin
-        while read_bytes + @rbuf.size < len
-          s = rbuf_consume(@rbuf.size)
-          read_bytes += s.size
-          dest << s
+        while read_bytes + rbuf_size < len
+          if s = rbuf_consume_all_shareable!
+            read_bytes += s.bytesize
+            dest << s
+          end
           rbuf_fill
         end
         s = rbuf_consume(len - read_bytes)
-        read_bytes += s.size
+        read_bytes += s.bytesize
         dest << s
       rescue EOFError
         raise unless ignore_eof
@@ -175,9 +177,10 @@ def read_all(dest = ''.b)
       read_bytes = 0
       begin
         while true
-          s = rbuf_consume(@rbuf.size)
-          read_bytes += s.size
-          dest << s
+          if s = rbuf_consume_all_shareable!
+            read_bytes += s.bytesize
+            dest << s
+          end
           rbuf_fill
         end
       rescue EOFError
@@ -188,14 +191,16 @@ def read_all(dest = ''.b)
     end
 
     def readuntil(terminator, ignore_eof = false)
+      offset = @rbuf_offset
       begin
-        until idx = @rbuf.index(terminator)
+        until idx = @rbuf.index(terminator, offset)
+          offset = @rbuf.bytesize
           rbuf_fill
         end
-        return rbuf_consume(idx + terminator.size)
+        return rbuf_consume(idx + terminator.bytesize - @rbuf_offset)
       rescue EOFError
         raise unless ignore_eof
-        return rbuf_consume(@rbuf.size)
+        return rbuf_consume
       end
     end
 
@@ -208,12 +213,16 @@ def readline
     BUFSIZE = 1024 * 16
 
     def rbuf_fill
-      tmp = @rbuf.empty? ? @rbuf : nil
+      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
+        @rbuf_empty = false
+        if rv.equal?(tmp)
+          @rbuf_offset = 0
+        else
+          @rbuf << rv
+          rv.clear
+        end
         return
       when :wait_readable
         (io = @io.to_io).wait_readable(@read_timeout) or raise Net::ReadTimeout.new(io)
@@ -228,13 +237,50 @@ def rbuf_fill
       end while true
     end
 
-    def rbuf_consume(len)
-      if len == @rbuf.size
+    def rbuf_flush
+      if @rbuf_empty
+        @rbuf.clear
+        @rbuf_offset = 0
+      end
+      nil
+    end
+
+    def rbuf_size
+      @rbuf.bytesize - @rbuf_offset
+    end
+
+    # Warning: this method may share the buffer to avoid
+    # copying. The caller must no longer use the returned
+    # string once rbuf_fill has been called again
+    def rbuf_consume_all_shareable!
+      @rbuf_empty = true
+      buf = if @rbuf_offset == 0
+        @rbuf
+      else
+        @rbuf.byteslice(@rbuf_offset..-1)
+      end
+      @rbuf_offset = @rbuf.bytesize
+      buf
+    end
+
+    def rbuf_consume(len = nil)
+      if @rbuf_offset == 0 && (len.nil? || len == @rbuf.bytesize)
         s = @rbuf
         @rbuf = ''.b
+        @rbuf_offset = 0
+        @rbuf_empty = true
+      elsif len.nil?
+        s = @rbuf.byteslice(@rbuf_offset..-1)
+        @rbuf = ''.b
+        @rbuf_offset = 0
+        @rbuf_empty = true
       else
-        s = @rbuf.slice!(0, len)
+        s = @rbuf.byteslice(@rbuf_offset, len)
+        @rbuf_offset += len
+        @rbuf_empty = @rbuf_offset == @rbuf.bytesize
+        rbuf_flush
       end
+
       @debug_output << %Q[-> #{s.dump}\n] if @debug_output
       s
     end