Fix nested IRB sessions' history saving

1. Dynamically including `HistorySavingAbility` makes things unnecessarily
   complicated and should be avoided.
2. Because both `Reline` and `Readline` use a single `HISTORY` constant
   to store history data. When nesting IRB sessions, only the first IRB
   session should handle history loading and saving so we can avoid
   duplicating history.
3. History saving callback should NOT be stored in `IRB.conf` as it's
   recreated every time `IRB.setup` is called, which would happen when
   nesting IRB sessions.

lib/irb.rb

@@ -482,9 +482,16 @@ def debug_break
     end
 
     def run(conf = IRB.conf)
+      in_nested_session = !!conf[:MAIN_CONTEXT]
       conf[:IRB_RC].call(context) if conf[:IRB_RC]
       conf[:MAIN_CONTEXT] = context
 
+      save_history = !in_nested_session && conf[:SAVE_HISTORY] && context.io.support_history_saving?
+
+      if save_history
+        context.io.load_history
+      end
+
       prev_trap = trap("SIGINT") do
         signal_handle
       end
@@ -496,6 +503,7 @@ def run(conf = IRB.conf)
       ensure
         trap("SIGINT", prev_trap)
         conf[:AT_EXIT].each{|hook| hook.call}
+        context.io.save_history if save_history
       end
     end
 

lib/irb/context.rb

@@ -8,7 +8,6 @@
 require_relative "inspector"
 require_relative "input-method"
 require_relative "output-method"
-require_relative "history"
 
 module IRB
   # A class that wraps the current state of the irb session, including the
@@ -130,8 +129,6 @@ def initialize(irb, workspace = nil, input_method = nil)
       else
         @io = input_method
       end
-      self.save_history = IRB.conf[:SAVE_HISTORY] if IRB.conf[:SAVE_HISTORY]
-
       @extra_doc_dirs = IRB.conf[:EXTRA_DOC_DIRS]
 
       @echo = IRB.conf[:ECHO]
@@ -154,13 +151,6 @@ def initialize(irb, workspace = nil, input_method = nil)
 
     def save_history=(val)
       IRB.conf[:SAVE_HISTORY] = val
-
-      if val
-        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
 
     def save_history

lib/irb/history.rb

@@ -1,9 +1,7 @@
 module IRB
   module HistorySavingAbility # :nodoc:
-    def HistorySavingAbility.extended(obj)
-      IRB.conf[:AT_EXIT].push proc{obj.save_history}
-      obj.load_history
-      obj
+    def support_history_saving?
+      true
     end
 
     def load_history

lib/irb/input-method.rb

@@ -5,6 +5,7 @@
 #
 
 require_relative 'completion'
+require_relative "history"
 require 'io/console'
 require 'reline'
 
@@ -180,6 +181,8 @@ def self.initialize_readline
         include ::Readline
       end
 
+      include HistorySavingAbility
+
       # Creates a new input method object using Readline
       def initialize
         self.class.initialize_readline
@@ -233,10 +236,6 @@ 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.
@@ -264,6 +263,7 @@ def inspect
 
   class RelineInputMethod < InputMethod
     HISTORY = Reline::HISTORY
+    include HistorySavingAbility
     # Creates a new input method object using Reline
     def initialize
       IRB.__send__(:set_encoding, Reline.encoding_system_needs.name, override: false)
@@ -466,10 +466,6 @@ def inspect
       str += " and #{inputrc_path}" if File.exist?(inputrc_path)
       str
     end
-
-    def support_history_saving?
-      true
-    end
   end
 
   class ReidlineInputMethod < RelineInputMethod

test/irb/test_history.rb

@@ -1,9 +1,12 @@
 # frozen_string_literal: false
 require 'irb'
 require 'readline'
+require "tempfile"
 
 require_relative "helper"
 
+return if RUBY_PLATFORM.match?(/solaris|mswin|mingw/i)
+
 module TestIRB
   class HistoryTest < TestCase
     def setup
@@ -205,4 +208,57 @@ def with_temp_stdio
       end
     end
   end
-end if not RUBY_PLATFORM.match?(/solaris|mswin|mingw/i)
+
+  class NestedIRBHistoryTest < IntegrationTestCase
+    def test_history_saving_with_debug
+      write_history <<~HISTORY
+        old_history_1
+        old_history_2
+        old_history_3
+      HISTORY
+
+      write_ruby <<~'RUBY'
+        def foo
+          binding.irb
+        end
+
+        binding.irb
+      RUBY
+
+      run_ruby_file do
+        type "'outer session'"
+        type "foo"
+        type "'inner session'"
+        type "exit"
+        type "'outer session again'"
+        type "exit"
+      end
+
+      assert_equal <<~HISTORY, @history_file.open.read
+        old_history_1
+        old_history_2
+        old_history_3
+        'outer session'
+        foo
+        'inner session'
+        exit
+        'outer session again'
+        exit
+      HISTORY
+    end
+
+    private
+
+    def write_history(history)
+      @history_file = Tempfile.new('rdbg_history')
+      @history_file.write(history)
+      @history_file.close
+      @irbrc = Tempfile.new('irbrc')
+      @irbrc.write <<~RUBY
+        IRB.conf[:HISTORY_FILE] = "#{@history_file.path}"
+      RUBY
+      @irbrc.close
+      @envs['IRBRC'] = @irbrc.path
+    end
+  end
+end