[lldb] [llgs] Fix `?` packet response for running threads

[mgorny]

Summary

Fix the response to ? packet for threads that are running at the time
(in non-stop mode). The previous code would wrongly send or queue
an empty response for them.

Sponsored by: The FreeBSD Foundation

lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationServerLLGS.cpp

 }
 
 void GDBRemoteCommunicationServerLLGS::EnqueueStopReplyPackets(
     lldb::tid_t thread_to_skip) {
   if (!m_non_stop)
     return;
 
   for (NativeThreadProtocol &listed_thread : m_current_process->Threads()) {
-    if (listed_thread.GetID() != thread_to_skip)
-      m_stop_notification_queue.push_back(
-          PrepareStopReplyPacketForThread(listed_thread).GetString().str());
+    if (listed_thread.GetID() != thread_to_skip) {
+      StreamString stop_reply = PrepareStopReplyPacketForThread(listed_thread);
+      if (!stop_reply.Empty())
+        m_stop_notification_queue.push_back(stop_reply.GetString().str());
+    }
   }
 }
 
 void GDBRemoteCommunicationServerLLGS::HandleInferiorState_Exited(
     NativeProcessProtocol *process) {
   assert(process && "process cannot be NULL");
 
   Log *log = GetLog(LLDBLog::Process);
llvm::erase_if(m_stop_notification_queue, [](const std::string &x) {
       return x.front() != 'W' && x.front() != 'X';
     });
 
     if (m_current_process) {
       // Queue stop reply packets for all active threads.  Start with
       // the current thread (for clients that don't actually support multiple
       // stop reasons).
       NativeThreadProtocol *thread = m_current_process->GetCurrentThread();
-      if (thread)
-        m_stop_notification_queue.push_back(
-            PrepareStopReplyPacketForThread(*thread).GetString().str());
+      if (thread) {
+        StreamString stop_reply = PrepareStopReplyPacketForThread(*thread);
+        if (!stop_reply.Empty())
+          m_stop_notification_queue.push_back(stop_reply.GetString().str());
+      }
       EnqueueStopReplyPackets(thread ? thread->GetID()
                                      : LLDB_INVALID_THREAD_ID);
     }
 
     // If the notification queue is empty (i.e. everything is running), send OK.
     if (m_stop_notification_queue.empty())
       return SendOKResponse();
 

lldb/test/API/tools/lldb-server/TestNonStop.py

def test_stdio(self):
         self.assertIn(ret["O_content"], b"message 2\r\n")
 
         self.reset_test_sequence()
         self.test_sequence.add_log_lines(
             ["read packet: $vStopped#00",
              "send packet: $OK#00",
              ], True)
         self.expect_gdbremote_sequence()
+
+    @add_test_categories(["llgs"])
+    def test_stop_reason_while_running(self):
+        self.build()
+        self.set_inferior_startup_launch()
+        procs = self.prep_debug_monitor_and_inferior(
+                inferior_args=["thread:new", "thread:new", "stop", "sleep:15"])
+        self.test_sequence.add_log_lines(
+            ["read packet: $QNonStop:1#00",
+             "send packet: $OK#00",
+             # stop is used to synchronize starting threads
+             "read packet: $c#63",
+             "send packet: $OK#00",
+             {"direction": "send", "regex": "%Stop:T.*"},
+             "read packet: $c#63",
+             "send packet: $OK#00",
+             "read packet: $?#00",
+             "send packet: $OK#00",
+             ], True)
+        self.expect_gdbremote_sequence()