Restore MAIN_CONTEXT correctly (#937)

lib/irb.rb

@@ -992,6 +992,7 @@ def debug_readline(binding)
     def run(conf = IRB.conf)
       in_nested_session = !!conf[:MAIN_CONTEXT]
       conf[:IRB_RC].call(context) if conf[:IRB_RC]
+      prev_context = conf[:MAIN_CONTEXT]
       conf[:MAIN_CONTEXT] = context
 
       save_history = !in_nested_session && conf[:SAVE_HISTORY] && context.io.support_history_saving?
@@ -1014,6 +1015,9 @@ def run(conf = IRB.conf)
           eval_input
         end
       ensure
+        # Do not restore to nil. It will cause IRB crash when used with threads.
+        IRB.conf[:MAIN_CONTEXT] = prev_context if prev_context
+
         RubyVM.keep_script_lines = keep_script_lines_backup if defined?(RubyVM.keep_script_lines)
         trap("SIGINT", prev_trap)
         conf[:AT_EXIT].each{|hook| hook.call}

test/irb/test_irb.rb

@@ -125,6 +125,26 @@ def test_empty_input_echoing_behaviour
     end
   end
 
+  class NestedBindingIrbTest < IntegrationTestCase
+    def test_current_context_restore
+      write_ruby <<~'RUBY'
+        binding.irb
+      RUBY
+
+      output = run_ruby_file do
+        type '$ctx = IRB.CurrentContext'
+        type 'binding.irb'
+        type 'p context_changed: IRB.CurrentContext != $ctx'
+        type 'exit'
+        type 'p context_restored: IRB.CurrentContext == $ctx'
+        type 'exit'
+      end
+
+      assert_include output, '{:context_changed=>true}'
+      assert_include output, '{:context_restored=>true}'
+    end
+  end
+
   class IrbIOConfigurationTest < TestCase
     Row = Struct.new(:content, :current_line_spaces, :new_line_spaces, :indent_level)