[Client] Detect main-thread event loop stalls

LagMonitor ticks the GUI event loop every 500 ms and records gaps
beyond 2 s as stalls, warning with their duration and keeping a
bounded ring of recent records for diagnostics. Measurement uses a
monotonic QElapsedTimer so wall-clock steps and suspend do not
fabricate stalls; recorded timestamps stay in wall time for
correlating with user reports.

Took 1 minute

Took 13 minutes


Took 2 minutes

cockatrice/CMakeLists.txt

@@ -15,6 +15,7 @@ set(cockatrice_SOURCES
     src/client/network/update/client/client_update_checker.cpp
     src/client/network/update/client/release_channel.cpp
     src/client/network/update/card_spoiler/spoiler_background_updater.cpp
+    src/client/lag_monitor.cpp
     src/client/sound_engine.cpp
     src/client/settings/cache_settings.cpp
     src/client/settings/card_counter_settings.cpp

cockatrice/src/client/lag_monitor.cpp

@@ -0,0 +1,43 @@
+#include "lag_monitor.h"
+
+#include <QTimer>
+
+LagMonitor::LagMonitor(QObject *parent) : QObject(parent)
+{
+    timer = new QTimer(this);
+    timer->setInterval(TickIntervalMs);
+    connect(timer, &QTimer::timeout, this, &LagMonitor::checkTick);
+    tickClock.start();
+    timer->start();
+}
+
+QList<LagMonitor::StallRecord> LagMonitor::recentStalls() const
+{
+    return stalls;
+}
+
+void LagMonitor::clearStalls()
+{
+    stalls.clear();
+}
+
+void LagMonitor::checkTick()
+{
+    const qint64 gap = tickClock.restart();
+
+    if (gap <= StallThresholdMs) {
+        return;
+    }
+
+    StallRecord record;
+    record.timestampMsSinceEpoch = QDateTime::currentMSecsSinceEpoch(); // wall time, for log correlation
+    record.durationMs = gap;
+
+    stalls.append(record);
+    while (stalls.size() > MaxRecordedStalls) {
+        stalls.removeFirst();
+    }
+
+    qCWarning(LagMonitorLog, "Event loop stalled for %lld ms (threshold: %d ms)", static_cast<long long>(gap),
+              StallThresholdMs);
+}

cockatrice/src/client/lag_monitor.h

@@ -0,0 +1,66 @@
+/**
+ * @file lag_monitor.h
+ * @ingroup Client
+ */
+
+#ifndef LAG_MONITOR_H
+#define LAG_MONITOR_H
+
+#include <QDateTime>
+#include <QElapsedTimer>
+#include <QList>
+#include <QLoggingCategory>
+#include <QObject>
+
+inline Q_LOGGING_CATEGORY(LagMonitorLog, "lag_monitor");
+
+class QTimer;
+
+/**
+ * @brief Detects main-thread event loop stalls ("UI freezes") from the inside.
+ *
+ * A timer is expected to fire every TickIntervalMs of wall time. When the
+ * observed gap greatly exceeds that interval, some other task blocked the
+ * event loop for roughly the overshooting duration. This is what separates
+ * "my client froze" from "the network is lagging" in user reports.
+ *
+ * Healthy operation costs one timer wakeup per tick and two integer
+ * comparisons. Allocations happen only when a stall is actually recorded.
+ */
+class LagMonitor : public QObject
+{
+    Q_OBJECT
+
+public:
+    struct StallRecord
+    {
+        qint64 timestampMsSinceEpoch = 0; ///< when the stalled period ended
+        qint64 durationMs = 0;            ///< approximate length of the freeze
+    };
+
+    static constexpr int TickIntervalMs = 500;
+    static constexpr int StallThresholdMs = 2000;
+    static constexpr int MaxRecordedStalls = 32;
+
+    explicit LagMonitor(QObject *parent = nullptr);
+
+    /**
+     * @brief Stalls recorded during this session, oldest first.
+     *
+     * Intended consumers are log output and the diagnostics export. The list
+     * holds at most MaxRecordedStalls entries.
+     */
+    QList<StallRecord> recentStalls() const;
+
+    void clearStalls();
+
+private slots:
+    void checkTick();
+
+private:
+    QTimer *timer;
+    QElapsedTimer tickClock; ///< monotonic clock, so wall clock steps and suspend do not fabricate stalls
+    QList<StallRecord> stalls;
+};
+
+#endif

cockatrice/src/interface/window_main.h

@@ -25,6 +25,7 @@
 #ifndef WINDOW_H
 #define WINDOW_H
 
+#include "../client/lag_monitor.h"
 #include "connection_controller/remote_connection_controller.h"
 #include "widgets/dialogs/dlg_local_game_options.h"
 
@@ -145,6 +146,7 @@ private:
     WndSets *wndSets;
     ConnectionController *connectionController;
     LocalServer *localServer;
+    LagMonitor lagMonitor; ///< watches the main thread for event loop stalls
     bool bHasActivated, askedForDbUpdater;
     QProcess *cardUpdateProcess;
     DlgViewLog *logviewDialog;