[Network] Measure real server round-trip times (#7153)

* [Network] Measure real server round-trip times

Time each command container from send to response with QElapsedTimer,
aggregate samples in a fixed-size ring buffer (last/median/p95/max),
and emit aggregated pingStatsUpdated at most once per second so the
hot path stays free of signal traffic. Stats are cleared on
disconnect. Forward the signal through ConnectionController for UI
consumers. Unit-tested in latency_tracker_test.

Took 37 minutes

Took 4 minutes

# Commit time for manual adjustment:
# Took 8 minutes

* Move params to struct, more informative debug

Took 56 seconds

Took 53 seconds

Took 2 minutes

Took 33 seconds

---------

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

tests/latency_tracker_test.cpp

@@ -0,0 +1,149 @@
+#include <gtest/gtest.h>
+#include <libcockatrice/network/client/abstract/latency_tracker.h>
+
+TEST(LatencyTrackerTest, EmptyTrackerYieldsZeroedStats)
+{
+    LatencyTracker tracker;
+
+    const auto stats = tracker.stats();
+
+    EXPECT_EQ(0, stats.sampleCount);
+    EXPECT_EQ(0, stats.lastMs);
+    EXPECT_EQ(0, stats.medianMs);
+    EXPECT_EQ(0, stats.p95Ms);
+    EXPECT_EQ(0, stats.maxMs);
+}
+
+TEST(LatencyTrackerTest, SingleSampleIsEveryStatistic)
+{
+    LatencyTracker tracker;
+    tracker.addSample(42);
+
+    const auto stats = tracker.stats();
+
+    EXPECT_EQ(1, stats.sampleCount);
+    EXPECT_EQ(42, stats.lastMs);
+    EXPECT_EQ(42, stats.medianMs);
+    EXPECT_EQ(42, stats.p95Ms);
+    EXPECT_EQ(42, stats.maxMs);
+}
+
+TEST(LatencyTrackerTest, OddSizedWindowMedianAndP95)
+{
+    LatencyTracker tracker;
+    for (qint64 ms : {qint64(50), qint64(10), qint64(30), qint64(20), qint64(40)}) {
+        tracker.addSample(ms);
+    }
+
+    const auto stats = tracker.stats();
+
+    EXPECT_EQ(5, stats.sampleCount);
+    EXPECT_EQ(40, stats.lastMs);
+    EXPECT_EQ(30, stats.medianMs);
+    // nearest-rank p95 of 5 samples: ceil(4.75) = 5th smallest
+    EXPECT_EQ(50, stats.p95Ms);
+    EXPECT_EQ(50, stats.maxMs);
+}
+
+TEST(LatencyTrackerTest, EvenSizedWindowMedianIsAverageOfMiddleTwo)
+{
+    LatencyTracker tracker;
+    for (qint64 ms : {qint64(10), qint64(20), qint64(30), qint64(40)}) {
+        tracker.addSample(ms);
+    }
+
+    const auto stats = tracker.stats();
+
+    EXPECT_EQ(4, stats.sampleCount);
+    EXPECT_EQ(25, stats.medianMs);
+    // nearest-rank p95 of 4 samples: ceil(3.8) = 4th smallest
+    EXPECT_EQ(40, stats.p95Ms);
+}
+
+TEST(LatencyTrackerTest, WindowEvictsOldestSamples)
+{
+    LatencyTracker tracker;
+    for (int i = 0; i <= 99; ++i) {
+        tracker.addSample(i);
+    }
+
+    const auto stats = tracker.stats();
+
+    EXPECT_EQ(LatencyTracker::WindowSize, stats.sampleCount);
+    EXPECT_EQ(99, stats.lastMs);
+    EXPECT_EQ(99, stats.maxMs);
+    // window now contains 36..99 (64 samples)
+    EXPECT_EQ(67, stats.medianMs); // (67 + 68) / 2 with integer division
+    EXPECT_EQ(96, stats.p95Ms);    // ceil(0.95 * 64) - 1 = index 60 -> 36 + 60
+}
+
+TEST(LatencyTrackerTest, ClearResetsAllState)
+{
+    LatencyTracker tracker;
+    for (int i = 0; i <= 99; ++i) {
+        tracker.addSample(i);
+    }
+
+    tracker.clear();
+    const auto cleared = tracker.stats();
+    EXPECT_EQ(0, cleared.sampleCount);
+
+    tracker.addSample(7);
+    const auto stats = tracker.stats();
+    EXPECT_EQ(1, stats.sampleCount);
+    EXPECT_EQ(7, stats.lastMs);
+    EXPECT_EQ(7, stats.medianMs);
+}
+
+TEST(LatencyTrackerTest, LastSampleSurvivesWraparound)
+{
+    LatencyTracker tracker;
+    for (int i = 0; i < LatencyTracker::WindowSize; ++i) {
+        tracker.addSample(i);
+    }
+    tracker.addSample(1000);
+
+    EXPECT_EQ(1000, tracker.stats().lastMs);
+}
+
+TEST(LatencyTrackerTest, RecentSamplesAreChronologicalOldestFirst)
+{
+    LatencyTracker tracker;
+    for (qint64 ms : {qint64(50), qint64(10), qint64(30)}) {
+        tracker.addSample(ms);
+    }
+
+    const QList<qint64> samples = tracker.recentSamples();
+
+    EXPECT_EQ((QList<qint64>{50, 10, 30}), samples);
+}
+
+TEST(LatencyTrackerTest, RecentSamplesFollowRingBufferWraparound)
+{
+    LatencyTracker tracker;
+    for (int i = 0; i <= 99; ++i) {
+        tracker.addSample(i);
+    }
+
+    const QList<qint64> samples = tracker.recentSamples();
+
+    ASSERT_EQ(LatencyTracker::WindowSize, samples.size());
+    EXPECT_EQ(36, samples.first());
+    EXPECT_EQ(99, samples.last());
+}
+
+TEST(LatencyTrackerTest, RecentSamplesEmptyOnFreshAndClearedTracker)
+{
+    LatencyTracker tracker;
+    EXPECT_TRUE(tracker.recentSamples().isEmpty());
+
+    tracker.addSample(5);
+    tracker.clear();
+    EXPECT_TRUE(tracker.recentSamples().isEmpty());
+}
+
+int main(int argc, char **argv)
+{
+    ::testing::InitGoogleTest(&argc, argv);
+    return RUN_ALL_TESTS();
+}