sample_rand now added to DSC from BaggageHeaders

src/Sentry/BaggageHeader.cs

@@ -26,7 +26,7 @@ private BaggageHeader(IEnumerable<KeyValuePair<string, string>> members) =>
 
     // We can safely return a dictionary of Sentry members, as we are in control over the keys added.
     // Just to be safe though, we'll group by key and only take the first of each one.
-    internal IReadOnlyDictionary<string, string> GetSentryMembers() =>
+    internal Dictionary<string, string> GetSentryMembers() =>
         Members
             .Where(kvp => kvp.Key.StartsWith(SentryKeyPrefix))
             .GroupBy(kvp => kvp.Key, kvp => kvp.Value)

src/Sentry/DynamicSamplingContext.cs

@@ -1,3 +1,4 @@
+using Sentry.Internal;
 using Sentry.Internal.Extensions;
 
 namespace Sentry;
@@ -111,6 +112,21 @@ private DynamicSamplingContext(
             return null;
         }
 
+        // See https://develop.sentry.dev/sdk/telemetry/traces/#propagated-random-value
+        if (items.TryGetValue("sample_rand", out var sampleRand))
+        {
+            if (!double.TryParse(sampleRand, NumberStyles.Float, CultureInfo.InvariantCulture, out var rand) ||
+                 rand is < 0.0 or >= 1.0)
+            {
+                return null;
+            }
+        }
+        else
+        {
+            var seed = FnvHash.ComputeHash(traceId);
+            var rand = new Random(seed).NextDouble() * rate;
+            items.Add("sample_rand", rand.ToString("N4", CultureInfo.InvariantCulture));
+        }
         return new DynamicSamplingContext(items);
     }
 

test/Sentry.Tests/DynamicSamplingContextTests.cs

@@ -142,6 +142,104 @@ public void CreateFromBaggage_SampleRate_TooHigh()
         Assert.Null(dsc);
     }
 
+    [Fact]
+    public void CreateFromBaggage_SampleRand_Invalid()
+    {
+        var baggage = BaggageHeader.Create(new List<KeyValuePair<string, string>>
+        {
+            {"sentry-trace_id", "43365712692146d08ee11a729dfbcaca"},
+            {"sentry-public_key", "d4d82fc1c2c4032a83f3a29aa3a3aff"},
+            {"sentry-sample_rate", "1.0"},
+            {"sentry-sample_rand", "not-a-number"},
+        });
+
+        var dsc = baggage.CreateDynamicSamplingContext();
+
+        Assert.Null(dsc);
+    }
+
+    [Fact]
+    public void CreateFromBaggage_SampleRand_TooLow()
+    {
+        var baggage = BaggageHeader.Create(new List<KeyValuePair<string, string>>
+        {
+            {"sentry-trace_id", "43365712692146d08ee11a729dfbcaca"},
+            {"sentry-public_key", "d4d82fc1c2c4032a83f3a29aa3a3aff"},
+            {"sentry-sample_rate", "1.0"},
+            {"sentry-sample_rand", "-0.1"}
+        });
+
+        var dsc = baggage.CreateDynamicSamplingContext();
+
+        Assert.Null(dsc);
+    }
+
+    [Fact]
+    public void CreateFromBaggage_SampleRand_TooHigh()
+    {
+        var baggage = BaggageHeader.Create(new List<KeyValuePair<string, string>>
+        {
+            {"sentry-trace_id", "43365712692146d08ee11a729dfbcaca"},
+            {"sentry-public_key", "d4d82fc1c2c4032a83f3a29aa3a3aff"},
+            {"sentry-sample_rate", "1.0"},
+            {"sentry-sample_rand", "1.0"} // Must be less than 1
+        });
+
+        var dsc = baggage.CreateDynamicSamplingContext();
+
+        Assert.Null(dsc);
+    }
+
+    [Fact]
+    public void CreateFromBaggage_NotSampledNoSampleRand_GeneratesSampleRand()
+    {
+        var baggage = BaggageHeader.Create(new List<KeyValuePair<string, string>>
+        {
+            {"sentry-trace_id", "43365712692146d08ee11a729dfbcaca"},
+            {"sentry-public_key", "d4d82fc1c2c4032a83f3a29aa3a3aff"},
+            {"sentry-sample_rate", "0.5"}
+        });
+
+        var dsc = baggage.CreateDynamicSamplingContext();
+
+        using var scope = new AssertionScope();
+        Assert.NotNull(dsc);
+        var sampleRandItem = Assert.Contains("sample_rand", dsc.Items);
+        var sampleRand = double.Parse(sampleRandItem, NumberStyles.Float, CultureInfo.InvariantCulture);
+        Assert.True(sampleRand >= 0.0);
+        Assert.True(sampleRand < 1.0);
+    }
+
+    [Theory]
+    [InlineData("true")]
+    public void CreateFromBaggage_SampledNoSampleRand_GeneratesConsistentSampleRand(string sampled)
+    {
+        var baggage = BaggageHeader.Create(new List<KeyValuePair<string, string>>
+        {
+            {"sentry-trace_id", "43365712692146d08ee11a729dfbcaca"},
+            {"sentry-public_key", "d4d82fc1c2c4032a83f3a29aa3a3aff"},
+            {"sentry-sample_rate", "0.5"},
+            {"sentry-sampled", sampled},
+        });
+
+        var dsc = baggage.CreateDynamicSamplingContext();
+
+        using var scope = new AssertionScope();
+        Assert.NotNull(dsc);
+        var sampleRandItem = Assert.Contains("sample_rand", dsc.Items);
+        var sampleRand = double.Parse(sampleRandItem, NumberStyles.Float, CultureInfo.InvariantCulture);
+        if (sampled == "true")
+        {
+            Assert.True(sampleRand >= 0.0);
+            Assert.True(sampleRand < 0.5);
+        }
+        else
+        {
+            Assert.True(sampleRand >= 0.5);
+            Assert.True(sampleRand < 1.0);
+        }
+    }
+
     [Fact]
     public void CreateFromBaggage_Sampled_MalFormed()
     {
@@ -186,6 +284,7 @@ public void CreateFromBaggage_Valid_Complete()
             {"sentry-public_key", "d4d82fc1c2c4032a83f3a29aa3a3aff"},
             {"sentry-sampled", "true"},
             {"sentry-sample_rate", "1.0"},
+            {"sentry-sample_rand", "0.1234"},
             {"sentry-release", "[email protected]+abc"},
             {"sentry-environment", "production"},
             {"sentry-user_segment", "Group B"},
@@ -200,6 +299,7 @@ public void CreateFromBaggage_Valid_Complete()
         Assert.Equal("d4d82fc1c2c4032a83f3a29aa3a3aff", Assert.Contains("public_key", dsc.Items));
         Assert.Equal("true", Assert.Contains("sampled", dsc.Items));
         Assert.Equal("1.0", Assert.Contains("sample_rate", dsc.Items));
+        Assert.Equal("0.1234", Assert.Contains("sample_rand", dsc.Items));
         Assert.Equal("[email protected]+abc", Assert.Contains("release", dsc.Items));
         Assert.Equal("production", Assert.Contains("environment", dsc.Items));
         Assert.Equal("Group B", Assert.Contains("user_segment", dsc.Items));