[Client] Detect main-thread event loop stalls (#7155)

* [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

* [Client] Rename LagMonitor constants to SCREAMING_SNAKE_CASE

Took 15 minutes

* [Client] Discard suspend-spanning gaps in LagMonitor

Windows counts sleep time in its monotonic clock, so a suspend would
fabricate one bogus stall per resume. Reset the clock on application
state changes and drop implausibly huge gaps; extract recordGap() for
testability.

Took 3 minutes

* [Client] Unit test LagMonitor stall recording

Drives recordGap() directly to cover the threshold, plausibility cap,
trim, and clear behavior without timing-dependent waits.

Took 36 seconds

---------

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

tests/lag_monitor_test.cpp

@@ -0,0 +1,103 @@
+#include "client/lag_monitor.h"
+
+#include <QCoreApplication>
+#include <QDateTime>
+#include <QEvent>
+#include <QLoggingCategory>
+#include <gtest/gtest.h>
+
+namespace
+{
+
+/// Timestamps are taken at recording time; allow generous scheduler slack.
+constexpr qint64 TIMESTAMP_SLACK_MS = 10000;
+
+} // namespace
+
+class LagMonitorTest : public ::testing::Test
+{
+protected:
+    LagMonitor monitor;
+};
+
+TEST_F(LagMonitorTest, GapAtOrBelowThresholdIsIgnored)
+{
+    monitor.recordGap(0);
+    monitor.recordGap(LagMonitor::TICK_INTERVAL_MS);
+    monitor.recordGap(LagMonitor::STALL_THRESHOLD_MS);
+
+    EXPECT_TRUE(monitor.recentStalls().isEmpty());
+}
+
+TEST_F(LagMonitorTest, GapAboveThresholdIsRecorded)
+{
+    monitor.recordGap(LagMonitor::STALL_THRESHOLD_MS + 1);
+
+    const QList<LagMonitor::StallRecord> stalls = monitor.recentStalls();
+    ASSERT_EQ(1, stalls.size());
+    EXPECT_EQ(LagMonitor::STALL_THRESHOLD_MS + 1, stalls.first().durationMs);
+}
+
+TEST_F(LagMonitorTest, RecordedTimestampIsFresh)
+{
+    monitor.recordGap(LagMonitor::STALL_THRESHOLD_MS + 1);
+
+    const qint64 now = QDateTime::currentMSecsSinceEpoch();
+    ASSERT_EQ(1, monitor.recentStalls().size());
+    EXPECT_LE(qAbs(monitor.recentStalls().first().timestampMsSinceEpoch - now), TIMESTAMP_SLACK_MS);
+}
+
+TEST_F(LagMonitorTest, RecordsAreTrimmedToMaxOldestFirst)
+{
+    for (int i = 0; i < LagMonitor::MAX_RECORDED_STALLS + 5; ++i) {
+        monitor.recordGap(LagMonitor::STALL_THRESHOLD_MS + 1 + i);
+    }
+
+    const QList<LagMonitor::StallRecord> stalls = monitor.recentStalls();
+    ASSERT_EQ(LagMonitor::MAX_RECORDED_STALLS, stalls.size());
+    EXPECT_EQ(LagMonitor::STALL_THRESHOLD_MS + 6, stalls.first().durationMs);
+    EXPECT_EQ(LagMonitor::STALL_THRESHOLD_MS + 5 + LagMonitor::MAX_RECORDED_STALLS, stalls.last().durationMs);
+}
+
+TEST_F(LagMonitorTest, GapAtPlausibilityCapIsKept)
+{
+    monitor.recordGap(LagMonitor::MAX_PLAUSIBLE_STALL_MS);
+
+    ASSERT_EQ(1, monitor.recentStalls().size());
+    EXPECT_EQ(LagMonitor::MAX_PLAUSIBLE_STALL_MS, monitor.recentStalls().first().durationMs);
+}
+
+TEST_F(LagMonitorTest, GapBeyondPlausibilityCapIsDropped)
+{
+    monitor.recordGap(LagMonitor::MAX_PLAUSIBLE_STALL_MS + 1);
+
+    EXPECT_TRUE(monitor.recentStalls().isEmpty());
+}
+
+TEST_F(LagMonitorTest, ClearStallsEmptiesList)
+{
+    monitor.recordGap(LagMonitor::STALL_THRESHOLD_MS + 1);
+    ASSERT_EQ(1, monitor.recentStalls().size());
+
+    monitor.clearStalls();
+
+    EXPECT_TRUE(monitor.recentStalls().isEmpty());
+}
+
+TEST_F(LagMonitorTest, ApplicationStateChangeDoesNotRecordAStall)
+{
+    QObject probe;
+    QEvent event(QEvent::ApplicationStateChange);
+
+    QCoreApplication::sendEvent(&probe, &event);
+
+    EXPECT_TRUE(monitor.recentStalls().isEmpty());
+}
+
+int main(int argc, char **argv)
+{
+    QLoggingCategory::setFilterRules("lag_monitor.*=false");
+    QCoreApplication app(argc, argv);
+    ::testing::InitGoogleTest(&argc, argv);
+    return RUN_ALL_TESTS();
+}