Enable "step back <n>" while replay

lib/debug/session.rb

@@ -1061,12 +1061,14 @@ def step_command type, arg
         else
           leave_subsession [:step, type, arg&.to_i]
         end
-      when /\Aback\z/, /\Areset\z/
+      when /\A(back)\z/, /\A(back)\s+(\d+)\z/, /\A(reset)\z/
         if type != :in
           @ui.puts "only `step #{arg}` is supported."
           :retry
         else
-          request_tc [:step, arg.to_sym]
+          type = $1.to_sym
+          iter = $2&.to_i
+          request_tc [:step, type, iter]
         end
       else
         @ui.puts "Unknown option: #{arg}"

lib/debug/thread_client.rb

@@ -890,11 +890,12 @@ def wait_next_action_
             break
 
           when :back
+            iter = iter || 1
             if @recorder&.can_step_back?
               unless @recorder.backup_frames
                 @recorder.backup_frames = @target_frames
               end
-              @recorder.step_back
+              @recorder.step_back iter
               raise SuspendReplay
             else
               puts "Can not step back more."
@@ -1201,8 +1202,11 @@ def enabled?
         @tp_recorder.enabled?
       end
 
-      def step_back
-        @index += 1
+      def step_back iter
+        @index += iter
+        if @index > @log.size
+          @index = @log.size
+        end
       end
 
       def step_forward

test/console/record_test.rb

@@ -139,6 +139,74 @@ def test_1629263892
     end
   end
 
+  class StepBackWithNumWhileReplayTest < ConsoleTestCase
+    def program
+      <<~RUBY
+         1| def a
+         2|   return b()
+         3| end
+         4| 
+         5| def b
+         6|   return 1
+         7| end
+         8| 
+         9| a()
+        10| a()
+        11| a()
+        12| a()
+        13| a()
+        14| a()
+      RUBY
+    end
+
+    def test_1663648816
+      debug_code(program) do
+        type 'b 11'
+        type 'record on'
+        type 'c'
+        assert_line_num 11
+        type 'step back   2' # multiple whitespaces
+        assert_line_num 6
+        type 'step back 2'
+        assert_line_num 10
+        type 'q!'
+      end
+    end
+  end
+
+  class StepBackWhenNumberIsLargetThanLogIndex < ConsoleTestCase
+    def program
+      <<~RUBY
+         1| def a
+         2|   return b()
+         3| end
+         4| 
+         5| def b
+         6|   return 1
+         7| end
+         8| 
+         9| a()
+        10| a()
+        11| a()
+        12| a()
+        13| a()
+        14| a()
+      RUBY
+    end
+
+    def test_1663648816
+      debug_code(program) do
+        type 'b 11'
+        type 'record on'
+        type 'c'
+        assert_line_num 11
+        type 'step back 100'
+        assert_line_num 5
+        type 'q!'
+      end
+    end
+  end
+
   class RecordOnAfterStoppingOnceTest < ConsoleTestCase
     def program
       <<~RUBY