DAP: use Mutex when sending response

When debug.gem tries to send response from multiple threads, the socket connection is closed. I confirmed this bug when using custom request from vscode-rdbg

Author: ono-max

lib/debug/server_dap.rb

@@ -125,6 +125,7 @@ def self.local_fs_map_set map
     def dap_setup bytes
       CONFIG.set_config no_color: true
       @seq = 0
+      @mutex = Mutex.new
 
       case self
       when UI_UnixDomainServer
@@ -212,7 +213,11 @@ def send **kw
       if sock = @sock
         kw[:seq] = @seq += 1
         str = JSON.dump(kw)
-        sock.write "Content-Length: #{str.bytesize}\r\n\r\n#{str}"
+        @mutex.synchronize do
+          unless sock.closed?
+            sock.write "Content-Length: #{str.bytesize}\r\n\r\n#{str}"
+          end
+        end
         show_protocol '<', str
       end
     end

test/support/protocol_test_case.rb

@@ -93,6 +93,10 @@ def attach_to_cdp_server
 
     def req_dap_disconnect(terminate_debuggee:)
       send_dap_request 'disconnect', restart: false, terminateDebuggee: terminate_debuggee
+      unless terminate_debuggee
+        close_dap_writer
+        res = find_response :event, 'terminated', 'V<D'
+      end
       close_reader
     end
 
@@ -438,6 +442,10 @@ def close_reader
       end
     end
 
+    def close_dap_writer
+      @sock.close_write
+    end
+
     HOST = '127.0.0.1'
 
     JAVASCRIPT_TYPE_TO_CLASS_MAPS = {