Added HubTests

Author: jamescrosswell

src/Sentry/Internal/Hub.cs

@@ -44,8 +44,7 @@ internal Hub(
         ISystemClock? clock = null,
         IInternalScopeManager? scopeManager = null,
         RandomValuesFactory? randomValuesFactory = null,
-        IReplaySession? replaySession = null,
-        BackpressureMonitor? backpressureMonitor = null)
+        IReplaySession? replaySession = null)
     {
         if (string.IsNullOrWhiteSpace(options.Dsn))
         {
@@ -60,9 +59,10 @@ internal Hub(
         _randomValuesFactory = randomValuesFactory ?? new SynchronizedRandomValuesFactory();
         _sessionManager = sessionManager ?? new GlobalSessionManager(options);
         _clock = clock ?? SystemClock.Clock;
-        _options.BackpressureMonitor = _options.EnableBackpressureHandling
-            ? backpressureMonitor ?? new BackpressureMonitor(_options.DiagnosticLogger, clock)
-            : null;
+        if (_options.EnableBackpressureHandling && _options.BackpressureMonitor is null)
+        {
+            _options.BackpressureMonitor = new BackpressureMonitor(_options.DiagnosticLogger, clock);
+        }
         client ??= new SentryClient(options, randomValuesFactory: _randomValuesFactory, sessionManager: _sessionManager);
         _replaySession = replaySession ?? ReplaySession.Instance;
         ScopeManager = scopeManager ?? new SentryScopeManager(options, client);

test/Sentry.Tests/HubTests.cs

@@ -714,6 +714,48 @@ public void StartTransaction_DynamicSamplingContextWithSampleRate_UsesSampleRate
         transactionTracer.DynamicSamplingContext.Should().BeSameAs(dsc);
     }
 
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public void StartTransaction_Backpressure_Downsamples(bool usesTracesSampler)
+    {
+        // Arrange
+        var transactionContext = new TransactionContext("name", "operation");
+
+        var clock = new MockClock(DateTimeOffset.UtcNow);
+        var backpressureMonitor = new BackpressureMonitor(null, clock, startImmediately: false);
+        backpressureMonitor.SetDownsampleLevel(1);
+        _fixture.Options.BackpressureMonitor = backpressureMonitor;
+        var sampleRate = 0.5f;
+        var expectedDownsampledRate = sampleRate * backpressureMonitor.DownsampleFactor;
+        if (usesTracesSampler)
+        {
+            _fixture.Options.TracesSampler = _ => sampleRate;
+        }
+        else
+        {
+            _fixture.Options.TracesSampleRate = sampleRate;
+        }
+
+        var hub = _fixture.GetSut();
+
+        // Act
+        var transaction = hub.StartTransaction(transactionContext, new Dictionary<string, object>());
+
+        switch (transaction)
+        {
+            // Assert
+            case TransactionTracer tracer:
+                tracer.SampleRate.Should().Be(expectedDownsampledRate);
+                break;
+            case UnsampledTransaction unsampledTransaction:
+                unsampledTransaction.SampleRate.Should().Be(expectedDownsampledRate);
+                break;
+            default:
+                throw new Exception("Unexpected transaction type.");
+        }
+    }
+
     // overwrite the 'sample_rate' of the Dynamic Sampling Context (DSC) when a sampling decisions is made in the downstream SDK
     // 1. overwrite when 'TracesSampler' reaches a sampling decision
     // 2. keep when a sampling decision has been made upstream (via 'TransactionContext.IsSampled')