fix(uxrce_dds_client): fix session reconnection after agent restart (#26848)

* fix(uxrce_dds_client): fix session reconnection after agent restart

When the Micro XRCE-DDS Agent is restarted (e.g. via systemd or
Docker), the PX4 client stays "Running, disconnected" forever and
never re-establishes the session. Three bugs prevented reconnection:

1. session.on_pong_flag was never reset after being checked. Once set
   to 1 by the first successful pong, it stayed 1 forever, causing
   checkConnectivity() to believe pings were still succeeding even
   after the agent was gone. Fixed by resetting to 0 after reading.

2. _subs->reset() was never called during session teardown. Stale
   uORB file descriptors from the previous session persisted across
   reconnection attempts, causing the new session's publishers to
   malfunction. Fixed by calling _subs->reset() in deleteSession().

3. _connected was not explicitly reset in deleteSession(). While the
   outer loop checked this flag, explicitly clearing it ensures clean
   state for the next reconnection attempt.

Closes #26022

Signed-off-by: Pavel Guzenfeld <pavelgu@gmail.com>

* fix(uxrce_dds_client): clear fds[].events in reset() for reconnection

Without clearing events, init() skips orb_subscribe() on reconnect
because the POLLIN guard (fds[idx].events == 0) is never true again.
This causes all fds to remain at -1, px4_poll() returns 0, TX rate
stays zero, and the client enters a disconnect/reconnect loop.

Reported-by: sansha (tested on Pixhawk 6X with serial transport)

* ci: re-trigger SITL tests (flaky offboard test)

* docs(uxrce_dds_client): explain why reset() clears fds[].events

---------

Signed-off-by: Pavel Guzenfeld <pavelgu@gmail.com>

src/modules/uxrce_dds_client/dds_topics.h.em

@@ -120,6 +120,7 @@ void SendTopicsSubs::reset() {
 		send_subscriptions[idx].data_writer = uxr_object_id(0, UXR_INVALID_ID);
 		orb_unsubscribe(fds[idx].fd);
 		fds[idx].fd = -1;
+		fds[idx].events = 0;  // force re-subscribe on reconnect (init() skips when events != 0)
 	}
 };
 

src/modules/uxrce_dds_client/uxrce_dds_client.cpp

@@ -386,6 +386,11 @@ void UxrceddsClient::deleteSession(uxrSession *session)
 		_session_created = false;
 	}
 
+	if (_subs) {
+		_subs->reset();
+	}
+
+	_connected = false;
 	_last_payload_tx_rate = 0;
 	_timesync.reset_filter();
 }
@@ -709,6 +714,7 @@ void UxrceddsClient::run()
 			/* PONG_IN_SESSION_STATUS */
 			if (session.on_pong_flag == 1) {
 				_had_ping_reply = true;
+				session.on_pong_flag = 0;
 			}
 
 			// Calculate the payload tx/rx rate for connectivity monitoring
@@ -720,6 +726,7 @@ void UxrceddsClient::run()
 			perf_end(_loop_perf);
 		}
 
+		PX4_INFO("session disconnected, attempting to reconnect...");
 		deleteSession(&session);
 	}
 }