Prevent using multi-irb and activating debugger at the same time

Multi-irb makes a few assumptions:

- IRB will manage all threads that host sub-irb sessions
- All IRB sessions will be run on the threads created by IRB itself

However, when using the debugger these assumptions are broken:

- `debug` will freeze ALL threads when it suspends the session (e.g. when
  hitting a breakpoint, or performing step-debugging).
- Since the irb-debug integration runs IRB as the debugger's interface,
  it will be run on the debugger's thread, which is not managed by IRB.

So we should prevent the 2 features from being used at the same time.
To do that, we check if the other feature is already activated when
executing the commands that would activate the other feature.

lib/irb/cmd/debug.rb

@@ -35,6 +35,11 @@ def execute(pre_cmds: nil, do_cmds: nil)
             return
           end
 
+          if IRB.respond_to?(:JobManager)
+            warn "Can't start the debugger when IRB is running in a multi-IRB session."
+            return
+          end
+
           unless IRB::Debug.setup(irb_context.irb)
             puts <<~MSG
               You need to install the debug gem before using this command.

lib/irb/cmd/subirb.rb

@@ -11,8 +11,7 @@ module IRB
 
   module ExtendCommand
     class MultiIRBCommand < Nop
-      def initialize(conf)
-        super
+      def execute(*args)
         extend_irb_context
       end
 
@@ -22,13 +21,23 @@ def extend_irb_context
         # this extension patches IRB context like IRB.CurrentContext
         require_relative "../ext/multi-irb"
       end
+
+      def print_debugger_warning
+        warn "Multi-IRB commands are not available when the debugger is enabled."
+      end
     end
 
     class IrbCommand < MultiIRBCommand
       category "IRB"
       description "Start a child IRB."
 
       def execute(*obj)
+        if irb_context.with_debugger
+          print_debugger_warning
+          return
+        end
+
+        super
         IRB.irb(nil, *obj)
       end
     end
@@ -38,6 +47,12 @@ class Jobs < MultiIRBCommand
       description "List of current sessions."
 
       def execute
+        if irb_context.with_debugger
+          print_debugger_warning
+          return
+        end
+
+        super
         IRB.JobManager
       end
     end
@@ -47,6 +62,12 @@ class Foreground < MultiIRBCommand
       description "Switches to the session of the given number."
 
       def execute(key = nil)
+        if irb_context.with_debugger
+          print_debugger_warning
+          return
+        end
+
+        super
         raise CommandArgumentError.new("Please specify the id of target IRB job (listed in the `jobs` command).") unless key
         IRB.JobManager.switch(key)
       end
@@ -57,6 +78,12 @@ class Kill < MultiIRBCommand
       description "Kills the session with the given number."
 
       def execute(*keys)
+        if irb_context.with_debugger
+          print_debugger_warning
+          return
+        end
+
+        super
         IRB.JobManager.kill(*keys)
       end
     end

test/irb/test_debug_cmd.rb

@@ -365,6 +365,38 @@ def bar
       assert_include(output, "show_source    Show the source code of a given method or constant.")
     end
 
+    def test_debugger_cant_be_activated_while_multi_irb_is_active
+      write_ruby <<~'ruby'
+        binding.irb
+        a = 1
+      ruby
+
+      output = run_ruby_file do
+        type "jobs"
+        type "next"
+        type "exit"
+      end
+
+      assert_match(/irb\(main\):001:0> jobs/, output)
+      assert_include(output, "Can't start the debugger when IRB is running in a multi-IRB session.")
+    end
+
+    def test_multi_irb_commands_are_not_available_after_activating_the_debugger
+      write_ruby <<~'ruby'
+        binding.irb
+        a = 1
+      ruby
+
+      output = run_ruby_file do
+        type "next"
+        type "jobs"
+        type "continue"
+      end
+
+      assert_match(/irb\(main\):001:0> next/, output)
+      assert_include(output, "Multi-IRB commands are not available when the debugger is enabled.")
+    end
+
     private
 
     TIMEOUT_SEC = 3