feat(dap): capture debuggee stdio

lib/debug/server_dap.rb

@@ -298,6 +298,7 @@ def process
         process_request(req)
       end
     ensure
+      restore_debuggee_stdio
       send_event :terminated unless @sock.closed?
     end
 
@@ -314,6 +315,7 @@ def process_request req
         UI_DAP.local_fs_map_set req.dig('arguments', 'localfs') || req.dig('arguments', 'localfsMap') || true
         @nonstop = true
 
+        capture_debuggee_stdio
         load_extensions req
 
       when 'attach'
@@ -326,6 +328,7 @@ def process_request req
           @nonstop = false
         end
 
+        capture_debuggee_stdio
         load_extensions req
 
       when 'configurationDone'
@@ -397,6 +400,7 @@ def process_request req
         terminate = args.fetch("terminateDebuggee", false)
 
         SESSION.clear_all_breakpoints
+        restore_debuggee_stdio
         send_response req
 
         if SESSION.in_subsession?
@@ -506,6 +510,56 @@ def puts result = ""
       send_event 'output', category: 'console', output: "#{result&.chomp}\n"
     end
 
+    def capture_debuggee_stdio
+      return if @stdio_captured
+
+      @original_stdout = $stdout
+      @original_stderr = $stderr
+
+      @stdout_reader, @stdout_writer = IO.pipe
+      @stderr_reader, @stderr_writer = IO.pipe
+
+      @stdout_writer.sync = true
+      @stderr_writer.sync = true
+
+      $stdout = @stdout_writer
+      $stderr = @stderr_writer
+      @stdio_captured = true
+
+      @stdout_monitor = start_monitor_thread(@stdout_reader, 'stdout')
+      @stderr_monitor = start_monitor_thread(@stderr_reader, 'stderr')
+    end
+
+    def restore_debuggee_stdio
+      return unless @stdio_captured
+
+      $stdout = @original_stdout if @original_stdout
+      $stderr = @original_stderr if @original_stderr
+
+      [@stdout_writer, @stderr_writer]
+        .filter { |writer| !writer&.closed? }
+        .each(&:close)
+
+      [@stdout_monitor, @stderr_monitor].each { |monitor| monitor&.join }
+      [@stdout_reader, @stderr_reader].each { |reader| reader&.close unless reader&.closed? }
+
+      @stdio_captured = false
+    end
+
+    private
+
+    def start_monitor_thread(reader, category)
+      Thread.new do
+        reader.each_line do |line|
+          send_event 'output', category: category, output: line
+        end
+      rescue IOError, Errno::EBADF
+        # Pipe closed, exit gracefully
+      end
+    end
+
+    public
+
     def ignore_output_on_suspend?
       true
     end

test/protocol/stdio_capture_dap_test.rb

@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+require_relative '../support/protocol_test_case'
+
+module DEBUGGER__
+  class StdioCaptureDAPTest < ProtocolTestCase
+    PROGRAM = <<~RUBY
+       1| $stdout.puts "stdout message"
+       2| $stderr.puts "stderr message"
+       3| a = 1
+    RUBY
+
+    def test_stdout_captured_as_output_event
+      run_protocol_scenario PROGRAM, cdp: false do
+        req_add_breakpoint 3
+        req_continue
+
+        stdout_event = find_response :event, 'output', 'V<D'
+        assert_equal 'stdout', stdout_event.dig(:body, :category)
+        assert_match(/stdout message/, stdout_event.dig(:body, :output))
+
+        req_terminate_debuggee
+      end
+    end
+
+    def test_stderr_captured_as_output_event
+      run_protocol_scenario PROGRAM, cdp: false do
+        req_add_breakpoint 3
+        req_continue
+
+        find_response :event, 'output', 'V<D'
+        stderr_event = find_response :event, 'output', 'V<D'
+        assert_equal 'stderr', stderr_event.dig(:body, :category)
+        assert_match(/stderr message/, stderr_event.dig(:body, :output))
+
+        req_terminate_debuggee
+      end
+    end
+  end
+end