perf(app): run miner shutdown off the UI thread

At quit the UI thread drained and joined the mining-control thread and
then called the miner's blocking stop with a 3 s budget. Both ran on the
UI thread, so the window could stall for up to that long before the
shutdown screen rendered.

- Quit now only signals the mining-control thread (no join) on the UI
  thread. The blocking teardown — join the control thread, then stop the
  miner — moves to a background thread, so xmrig still has a single
  driver but the UI thread never blocks on it.
- On the daemon-stop path that teardown runs first on the shutdown
  thread, so the miner is fully stopped before the daemon stop begins.
  On the disconnect-only quit paths it runs on its own thread, which
  the final shutdown joins before tearing xmrig down, so there is never
  a second concurrent join of the control thread.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app.cpp

@@ -175,16 +175,29 @@ void App::postMiningControl(std::function<void()> job)
 
 // Signal the mining-control thread to stop and join it. Called at shutdown BEFORE xmrig_manager_ is stopped
 // or destroyed, so no control job runs concurrently with teardown. Idempotent.
-void App::stopMiningControlThread()
+void App::signalMiningControlStop()
 {
     {
         std::lock_guard<std::mutex> lk(mining_ctl_mutex_);
         mining_ctl_stop_ = true;
     }
     mining_ctl_cv_.notify_all();
+}
+
+void App::stopMiningControlThread()
+{
+    signalMiningControlStop();
     if (mining_ctl_thread_.joinable()) mining_ctl_thread_.join();
 }
 
+// Blocking miner teardown, run OFF the UI thread at quit. Joins the control thread first (waits out any in-flight
+// start/stop job) so this is the sole driver of xmrig_manager_, then stops the miner.
+void App::finishMinerShutdown()
+{
+    if (mining_ctl_thread_.joinable()) mining_ctl_thread_.join();
+    if (xmrig_manager_ && xmrig_manager_->isRunning()) xmrig_manager_->stop(3000);
+}
+
 namespace {
 // How often auto-balance re-evaluates the pool while active. Long, because switching
 // restarts the miner (drops in-flight shares + reconnect); the incumbent stickiness
@@ -6591,14 +6604,12 @@ void App::beginShutdown()
         http_worker_->requestStop();
     }
 
-    // Drain + join the mining-control thread FIRST so no async start/stop job runs while we tear the miner
-    // down here (avoids two threads driving xmrig_manager_ during shutdown). (M-03 cluster)
-    stopMiningControlThread();
-
-    // Stop xmrig pool miner before stopping the daemon
+    // Signal the mining-control thread now (no join: the UI thread must not block). The blocking
+    // join-control-thread -> xmrig stop(3000) teardown runs on a background thread (finishMinerShutdown), so
+    // xmrig_manager_ still has a single driver. (M-03 cluster)
+    signalMiningControlStop();
     if (xmrig_manager_ && xmrig_manager_->isRunning()) {
         shutdown_status_ = TR("appx_stopping_pool_miner");
-        xmrig_manager_->stop(3000);
     }
     
         DEBUG_LOGF("beginShutdown: starting (daemon_controller_=%s)\n",
@@ -6614,7 +6625,10 @@ void App::beginShutdown()
         if (settings_) {
             settings_->save();
         }
-        shutdown_complete_ = true;
+        miner_stop_thread_ = std::thread([this]() {
+            finishMinerShutdown();
+            shutdown_complete_ = true;
+        });
         return;
     }
     
@@ -6628,7 +6642,10 @@ void App::beginShutdown()
         if (shutdownDecision.action == daemon::DaemonController::ShutdownAction::DisconnectOnly) {
             DEBUG_LOGF("beginShutdown: %s, skipping daemon stop\n", shutdownDecision.logReason);
             shutdown_status_ = shutdownDecision.status;
-            shutdown_complete_ = true;
+            miner_stop_thread_ = std::thread([this]() {
+                finishMinerShutdown();
+                shutdown_complete_ = true;
+            });
             return;
         }
         shutdown_status_ = shutdownDecision.status;
@@ -6641,6 +6658,7 @@ void App::beginShutdown()
     // keeps rendering the shutdown screen (like SilentDragonX's
     // modal "Please wait" dialog).
     shutdown_thread_ = std::thread([this]() {
+        finishMinerShutdown();   // miner fully stopped before the daemon stop begins
         DEBUG_LOGF("shutdown thread: calling stopEmbeddedDaemon()\n");
         shutdown_status_ = TR("appx_sending_stop_command_to_daemon");
         // Deliver stop and wait for the node to exit: no time limit, never killed unasked. The shutdown
@@ -7735,7 +7753,11 @@ void App::shutdown()
 
     // Ensure the mining-control thread is stopped + joined (idempotent; beginShutdown already did it on the
     // normal quit path, but shutdown() can also run without it). Must precede xmrig_manager_ teardown.
-    stopMiningControlThread();
+    // The background miner teardown (disconnect-only quit paths) is joined first: it is the one driver.
+    if (miner_stop_thread_.joinable()) miner_stop_thread_.join();
+    // After beginShutdown() the control thread is joined by finishMinerShutdown() (on shutdown_thread_ or
+    // miner_stop_thread_); joining it here too would be two concurrent joins.
+    if (!shutting_down_) stopMiningControlThread();
     abortRpcBusyCanary();
     if (canary_thread_.joinable()) canary_thread_.join();
 

src/app.h

@@ -1196,7 +1196,9 @@ private:
     std::deque<std::function<void()>> mining_ctl_queue_;
     bool mining_ctl_stop_ = false;
     void postMiningControl(std::function<void()> job);  // enqueue a blocking xmrig op onto the FIFO thread
+    void signalMiningControlStop();                     // signal only (no join): safe on the UI thread
     void stopMiningControlThread();                     // signal + join the control thread (shutdown)
+    void finishMinerShutdown();                         // join control thread -> xmrig stop(3000); background only
     // Builds the pool config on the main thread and posts stop(if running)+start to the control thread.
     // retargetOnly: a thread-count change, decided on the control thread (never starts a miner).
     void postPoolMinerJob(int threads, bool retargetOnly);
@@ -1262,6 +1264,7 @@ private:
     bool address_list_dirty_ = false;   // P8: dedup rebuildAddressList
     GuardedStatus shutdown_status_;   // thread-safe: written by the shutdown thread, read by the UI (M-05)
     std::thread shutdown_thread_;
+    std::thread miner_stop_thread_;   // disconnect-only quit paths: miner teardown off the UI thread
     // Confirm-before-stopping-daemon-mid-witness-rebuild guard (see beginShutdown / renderDaemonStopConfirm)
     bool pending_shutdown_confirm_ = false;       // a quit is deferred, waiting to open the confirm modal
     bool daemon_stop_confirm_open_ = false;       // the confirm modal is currently showing