in_forward: Suppress non-freezed strings ocurrences on hot-path

Signed-off-by: Hiroshi Hatake <hiroshi@chronosphere.io>

Author: cosmo0920

lib/fluent/plugin/in_forward.rb

@@ -68,7 +68,12 @@ def each(unpacker: nil, &block)
     helpers :server
 
     LISTEN_PORT = 24224
+    OPTION_ACK = 'ack'.freeze
+    OPTION_CHUNK = 'chunk'.freeze
+    OPTION_COMPRESSED = 'compressed'.freeze
     OPTION_FLUENT_SIGNAL = 'fluent_signal'.freeze
+    OPTION_SIZE = 'size'.freeze
+    OPTION_TEXT = 'text'.freeze
 
     desc 'The port to listen to.'
     config_param :port, :integer, default: LISTEN_PORT
@@ -313,8 +318,8 @@ def read_messages(conn, &block)
     end
 
     def response(option)
-      if option && option['chunk']
-        return { 'ack' => option['chunk'] }
+      if option && option[OPTION_CHUNK]
+        return { OPTION_ACK => option[OPTION_CHUNK] }
       end
       nil
     end
@@ -348,10 +353,11 @@ def on_message(msg, chunk_size, conn)
       when String
         # PackedForward
         option = msg[2] || {}
-        size = option['size'] || 0
+        size = option[OPTION_SIZE] || 0
+        compressed = option[OPTION_COMPRESSED]
 
-        if option['compressed'] && option['compressed'] != 'text'
-          es = Fluent::CompressedMessagePackEventStream.new(entries, nil, size.to_i, compress: option['compressed'].to_sym, decompression_size_limit: @decompression_size_limit)
+        if compressed && compressed != OPTION_TEXT
+          es = Fluent::CompressedMessagePackEventStream.new(entries, nil, size.to_i, compress: compressed.to_sym, decompression_size_limit: @decompression_size_limit)
         else
           es = Fluent::MessagePackEventStream.new(entries, nil, size.to_i)
         end