fix(otel): only set Scope.Transaction in SentrySpanProcessor when null

src/Sentry.OpenTelemetry/SentrySpanProcessor.cs

@@ -172,7 +172,10 @@ private void CreateRootSpan(Activity data)
             tracer.Contexts.Trace.Origin = OpenTelemetryOrigin;
             tracer.StartTimestamp = data.StartTimeUtc;
         }
-        _hub.ConfigureScope(static (scope, transaction) => scope.Transaction = transaction, transaction);
+        _hub.ConfigureScope(static (scope, transaction) =>
+        {
+            scope.Transaction ??= transaction;
+        }, transaction);
         transaction.SetFused(data);
         _map[data.SpanId] = transaction;
     }

test/Sentry.OpenTelemetry.Tests/SentrySpanProcessorTests.cs

@@ -1012,4 +1012,35 @@ public void ParseOtelSpanDescription_HttpClient()
         description.Should().Be("POST https://example.com/foo");
         source.Should().Be(TransactionNameSource.Custom);
     }
+
+    [Fact]
+    public void OnStart_WithExistingTransactionOnScope_DoesNotOverwriteExistingTransaction()
+    {
+        // Arrange
+        _fixture.Options.Instrumenter = Instrumenter.OpenTelemetry;
+        var sut = _fixture.GetSut();
+
+        var parentContext = new TransactionContext("Program", "Main")
+        {
+            Instrumenter = Instrumenter.OpenTelemetry,
+        };
+        var parent = _fixture.Hub.StartTransaction(parentContext);
+        _fixture.Hub.ConfigureScope(scope => scope.Transaction = parent);
+
+        using var data = Tracer.StartActivity("TestActivity");
+
+        // Act
+        sut.OnStart(data!);
+
+        // Assert
+        data.ParentSpanId.Should().Be(default(ActivitySpanId), $"{nameof(data)} should be a new root Activity, not a child Activity");
+        Assert.True(sut._map.TryGetValue(data.SpanId, out var span));
+        span.Should().BeOfType<TransactionTracer>($"{nameof(data)} should be a new root Transaction, not a child Span");
+
+        object scopeTransaction = null;
+        _fixture.Hub.ConfigureScope(scope => scopeTransaction = scope.Transaction);
+
+        scopeTransaction.Should().BeSameAs(parent,
+            "CreateRootSpan should not overwrite an already-set Scope.Transaction");
+    }
 }