Add prepare_value parameter

Author: repeatedly

lib/fluent/plugin/filter_record_modifier.rb

@@ -4,6 +4,11 @@ module Fluent
   class Plugin::RecordModifierFilter < Plugin::Filter
     Fluent::Plugin.register_filter('record_modifier', self)
 
+    config_param :prepare_value, :string, default: nil,
+                 desc: <<-DESC
+Prepare values for filtering in configure phase. Prepared values can be used in <record>.
+You can write any ruby code.
+DESC
     config_param :char_encoding, :string, default: nil,
                  desc: <<-DESC
 Fluentd including some plugins treats the logs as a BINARY by default to forward.
@@ -26,7 +31,7 @@ class Plugin::RecordModifierFilter < Plugin::Filter
 This option is exclusive with `remove_keys`.
 DESC
 
-    BUILTIN_CONFIGURATIONS = %W(type @type log_level @log_level id @id char_encoding remove_keys whitelist_keys)
+    BUILTIN_CONFIGURATIONS = %W(type @type log_level @log_level id @id char_encoding remove_keys whitelist_keys prepare_value)
 
     def configure(conf)
       super
@@ -68,7 +73,7 @@ def configure(conf)
           check_config_placeholders(k, v)
           element.has_key?(k) # to suppress unread configuration warning
           @has_tag_parts = true if v.include?('tag_parts')
-          @map[k] = DynamicExpander.new(k, v)
+          @map[k] = DynamicExpander.new(k, v, @prepare_value)
         end
       end
 
@@ -153,7 +158,7 @@ def check_config_placeholders(k, v)
     end
 
     class DynamicExpander
-      def initialize(param_key, param_value)
+      def initialize(param_key, param_value, prepare_value)
         if param_value.include?('${')
           __str_eval_code__ = parse_parameter(param_value)
 
@@ -169,6 +174,12 @@ def expand(tag, time, record, tag_parts)
           @param_value = param_value
         end
 
+        begin
+          eval prepare_value if prepare_value
+        rescue SyntaxError
+          raise ConfigError, "Pass invalid syntax parameter : key = prepare_value, value = #{prepare_value}"
+        end
+
         begin
           # check eval genarates wrong code or not
           expand(nil, nil, nil, nil)