fix: overwrite DSC sample_rate when sampling

CHANGELOG.md

@@ -1,5 +1,11 @@
 # Changelog
 
+## Unreleased
+
+### Fixes
+
+- Update `sample_rate` of _Dynamic Sampling Context (DSC)_ when making sampling decisions ([#4374](https://github.com/getsentry/sentry-dotnet/pull/4374))
+
 ## 5.13.0
 
 ### Features
@@ -1252,7 +1258,7 @@ There are some functional differences when publishing Native AOT:
 
 ### Fixes
 
-- Resolved an isse where the SDK would throw an exception while attempting to set the DynamicSamplingContext but the context exists already. ([#2592](https://github.com/getsentry/sentry-dotnet/pull/2592))
+- Resolved an issue where the SDK would throw an exception while attempting to set the DynamicSamplingContext but the context exists already. ([#2592](https://github.com/getsentry/sentry-dotnet/pull/2592))
 
 ### Dependencies
 

src/Sentry/DynamicSamplingContext.cs

@@ -98,6 +98,24 @@ private DynamicSamplingContext(SentryId traceId,
 
     public BaggageHeader ToBaggageHeader() => BaggageHeader.Create(Items, useSentryPrefix: true);
 
+    public DynamicSamplingContext WithSampleRate(double sampleRate)
+    {
+        if (Items.TryGetValue("sample_rate", out var dscSampleRate))
+        {
+            if (double.TryParse(dscSampleRate, NumberStyles.Float, CultureInfo.InvariantCulture, out var rate))
+            {
+                if (Math.Abs(rate - sampleRate) > double.Epsilon)
+                {
+                    var items = Items.ToDictionary(kvp => kvp.Key, kvp => kvp.Value);
+                    items["sample_rate"] = sampleRate.ToString(CultureInfo.InvariantCulture);
+                    return new DynamicSamplingContext(items);
+                }
+            }
+        }
+
+        return this;
+    }
+
     public DynamicSamplingContext WithReplayId(IReplaySession? replaySession)
     {
         if (replaySession?.ActiveReplayId is not { } replayId || replayId == SentryId.Empty)

src/Sentry/Internal/Hub.cs

@@ -173,8 +173,8 @@ internal ITransactionTracer StartTransaction(
 
         bool? isSampled = null;
         double? sampleRate = null;
-        var sampleRand = dynamicSamplingContext?.Items.TryGetValue("sample_rand", out var dscsampleRand) ?? false
-            ? double.Parse(dscsampleRand, NumberStyles.Float, CultureInfo.InvariantCulture)
+        var sampleRand = dynamicSamplingContext?.Items.TryGetValue("sample_rand", out var dscSampleRand) ?? false
+            ? double.Parse(dscSampleRand, NumberStyles.Float, CultureInfo.InvariantCulture)
             : SampleRandHelper.GenerateSampleRand(context.TraceId.ToString());
 
         // TracesSampler runs regardless of whether a decision has already been made, as it can be used to override it.
@@ -188,14 +188,26 @@ internal ITransactionTracer StartTransaction(
             {
                 // The TracesSampler trumps all other sampling decisions (even the trace header)
                 sampleRate = samplerSampleRate;
-                isSampled = SampleRandHelper.IsSampled(sampleRand, sampleRate.Value);
+                isSampled = SampleRandHelper.IsSampled(sampleRand, samplerSampleRate);
+
+                // Ensure the actual sampleRate is set on the provided DSC (if any) when the TracesSampler reached a sampling decision
+                dynamicSamplingContext = dynamicSamplingContext?.WithSampleRate(samplerSampleRate);
             }
         }
 
         // If the sampling decision isn't made by a trace sampler we check the trace header first (from the context) or
         // finally fallback to Random sampling if the decision has been made by no other means
-        sampleRate ??= _options.TracesSampleRate ?? 0.0;
-        isSampled ??= context.IsSampled ?? SampleRandHelper.IsSampled(sampleRand, sampleRate.Value);
+        if (isSampled == null)
+        {
+            sampleRate = _options.TracesSampleRate ?? 0.0;
+            isSampled = context.IsSampled ?? SampleRandHelper.IsSampled(sampleRand, sampleRate.Value);
+
+            if (context.IsSampled is null && _options.TracesSampleRate is not null)
+            {
+                // Ensure the actual sampleRate is set on the provided DSC (if any) when not IsSampled upstream but the TracesSampleRate reached a sampling decision
+                dynamicSamplingContext = dynamicSamplingContext?.WithSampleRate(_options.TracesSampleRate.Value);
+            }
+        }
 
         // Make sure there is a replayId (if available) on the provided DSC (if any).
         dynamicSamplingContext = dynamicSamplingContext?.WithReplayId(_replaySession);

src/Sentry/TransactionTracer.cs

@@ -226,7 +226,6 @@ internal TransactionTracer(IHub hub, string name, string operation, TransactionN
     /// </summary>
     internal TransactionTracer(IHub hub, ITransactionContext context, TimeSpan? idleTimeout = null)
     {
-        Debug.Assert(context.IsSampled ?? true, "context.IsSampled should always be true when creating a TransactionTracer");
         _hub = hub;
         _options = _hub.GetSentryOptions();
         Name = context.Name;

test/Sentry.AspNetCore.Tests/SentryTracingMiddlewareTests.cs

@@ -316,7 +316,7 @@ public async Task Baggage_header_propagates_to_outbound_requests(bool shouldProp
                 "sentry-trace_id=75302ac48a024bde9a3b3734a82e36c8, " +
                 "sentry-public_key=d4d82fc1c2c4032a83f3a29aa3a3aff, " +
                 "sentry-sample_rand=0.1234, " +
-                "sentry-sample_rate=0.5";
+                "sentry-sample_rate=1";
         }
         else
         {
@@ -395,13 +395,18 @@ public async Task Baggage_header_propagates_to_outbound_requests(bool shouldProp
     [Fact]
     public async Task Baggage_header_sets_dynamic_sampling_context()
     {
-        // incoming baggage header
-        const string baggage =
+        const string incomingBaggageHeader =
             "sentry-trace_id=75302ac48a024bde9a3b3734a82e36c8, " +
             "sentry-public_key=d4d82fc1c2c4032a83f3a29aa3a3aff, " +
             "sentry-sample_rand=0.1234, " +
             "sentry-sample_rate=0.5";
 
+        const string expectedBaggageHeader =
+            "sentry-trace_id=75302ac48a024bde9a3b3734a82e36c8, " +
+            "sentry-public_key=d4d82fc1c2c4032a83f3a29aa3a3aff, " +
+            "sentry-sample_rand=0.1234, " +
+            "sentry-sample_rate=1";
+
         // Arrange
         TransactionTracer transaction = null;
 
@@ -440,7 +445,7 @@ public async Task Baggage_header_sets_dynamic_sampling_context()
         {
             Headers =
             {
-                {"baggage", baggage}
+                {"baggage", incomingBaggageHeader}
             }
         };
 
@@ -449,7 +454,7 @@ public async Task Baggage_header_sets_dynamic_sampling_context()
         // Assert
         var dsc = transaction?.DynamicSamplingContext;
         Assert.NotNull(dsc);
-        Assert.Equal(baggage, dsc.ToBaggageHeader().ToString());
+        Assert.Equal(expectedBaggageHeader, dsc.ToBaggageHeader().ToString());
     }
 
     [Fact]

test/Sentry.OpenTelemetry.Tests/SentrySpanProcessorTests.cs

@@ -141,7 +141,8 @@ public void OnStart_Transaction_With_DynamicSamplingContext()
         {
             actual.Items["trace_id"].Should().Be(expected["trace_id"]);
             actual.Items["public_key"].Should().Be(expected["public_key"]);
-            actual.Items["sample_rate"].Should().Be(expected["sample_rate"]);
+            actual.Items["sample_rate"].Should().NotBe(expected["sample_rate"]);
+            actual.Items["sample_rate"].Should().Be(_fixture.Options.TracesSampleRate.ToString());
         }
     }