[ruby/irb] Fix history-saving feature

(ruby/irb#642)

* Define RelineInputMethod::HISTORY

The HistorySavingAbility module doesn't do anything if the input method
class doesn't define HISTORY.

- https://github.com/ruby/irb/blob/3ac96be660bf052902fb4e532c7a46cf294b71eb/lib/irb/history.rb#L10
- https://github.com/ruby/irb/blob/3ac96be660bf052902fb4e532c7a46cf294b71eb/lib/irb/history.rb#L34

This patch defines RelineInputMethod::HISTORY to avoid this.

* Improve history-saving's ability check

Instead of checking the existence of `input_method_class::HISTORY`, we should
make every input method class declare if it supports history saving or not.

Since the default value is `false`, it shouldn't break any custom input method
that inherits from `IRB::InputMethod`.

ruby/irb@aec7a5b3f5

lib/irb/context.rb

@@ -154,8 +154,12 @@ def initialize(irb, workspace = nil, input_method = nil)
 
     def save_history=(val)
       IRB.conf[:SAVE_HISTORY] = val
+
       if val
-        (IRB.conf[:MAIN_CONTEXT] || self).init_save_history
+        context = (IRB.conf[:MAIN_CONTEXT] || self)
+        if context.io.support_history_saving? && !context.io.singleton_class.include?(HistorySavingAbility)
+          context.io.extend(HistorySavingAbility)
+        end
       end
     end
 
@@ -576,11 +580,5 @@ def transform_args?(command)
       command = command_aliases.fetch(command.to_sym, command)
       ExtendCommandBundle.load_command(command)&.respond_to?(:transform_args)
     end
-
-    def init_save_history# :nodoc:
-      unless (class<<@io;self;end).include?(HistorySavingAbility)
-        @io.extend(HistorySavingAbility)
-      end
-    end
   end
 end

lib/irb/history.rb

@@ -7,7 +7,6 @@ def HistorySavingAbility.extended(obj)
     end
 
     def load_history
-      return unless self.class.const_defined?(:HISTORY)
       history = self.class::HISTORY
       if history_file = IRB.conf[:HISTORY_FILE]
         history_file = File.expand_path(history_file)
@@ -31,7 +30,6 @@ def load_history
     end
 
     def save_history
-      return unless self.class.const_defined?(:HISTORY)
       history = self.class::HISTORY
       if num = IRB.conf[:SAVE_HISTORY] and (num = num.to_i) != 0
         if history_file = IRB.conf[:HISTORY_FILE]

lib/irb/input-method.rb

@@ -47,6 +47,10 @@ def readable_after_eof?
       false
     end
 
+    def support_history_saving?
+      false
+    end
+
     # For debug message
     def inspect
       'Abstract InputMethod'
@@ -230,6 +234,10 @@ def readable_after_eof?
         true
       end
 
+      def support_history_saving?
+        true
+      end
+
       # Returns the current line number for #io.
       #
       # #line counts the number of times #gets is called.
@@ -256,6 +264,7 @@ def inspect
   end
 
   class RelineInputMethod < InputMethod
+    HISTORY = Reline::HISTORY
     # Creates a new input method object using Reline
     def initialize
       IRB.__send__(:set_encoding, Reline.encoding_system_needs.name, override: false)
@@ -458,6 +467,10 @@ def inspect
       str += " and #{inputrc_path}" if File.exist?(inputrc_path)
       str
     end
+
+    def support_history_saving?
+      true
+    end
   end
 
   class ReidlineInputMethod < RelineInputMethod