fix: protect UnsampledTransaction against double-Dispose

src/Sentry/Internal/UnsampledTransaction.cs

@@ -21,6 +21,7 @@ internal sealed class UnsampledTransaction : NoOpTransaction
     private readonly IHub _hub;
     private readonly ITransactionContext _context;
     private readonly SentryOptions? _options;
+    private InterlockedBoolean _isFinished;
 
     public UnsampledTransaction(IHub hub, ITransactionContext context)
     {
@@ -32,7 +33,6 @@ public UnsampledTransaction(IHub hub, ITransactionContext context)
 
     internal DynamicSamplingContext? DynamicSamplingContext { get; set; }
 
-    private bool _isFinished;
     public override bool IsFinished => _isFinished;
 
     public override IReadOnlyCollection<ISpan> Spans => _spans;
@@ -63,11 +63,14 @@ public override string Operation
 
     public override void Finish()
     {
-        _options?.LogDebug("Finishing unsampled transaction");
-
         // Ensure the transaction is really cleared from the scope
         // See: https://github.com/getsentry/sentry-dotnet/issues/4198
-        _isFinished = true;
+        if (_isFinished.Exchange(true))
+        {
+            return;
+        }
+
+        _options?.LogDebug("Finishing unsampled transaction");
 
         // Clear the transaction from the scope and regenerate the Propagation Context, so new events don't have a
         // trace context that is "older" than the transaction that just finished

test/Sentry.Tests/Internals/UnsampledTransactionTests.cs

@@ -2,6 +2,45 @@ namespace Sentry.Tests.Internals;
 
 public class UnsampledTransactionTests
 {
+    [Fact]
+    public void Dispose_Unfinished_Finishes()
+    {
+        // Arrange
+        var hub = Substitute.For<IHub>();
+        ITransactionContext context = new TransactionContext("TestTransaction", "TestOperation",
+            new SentryTraceHeader(SentryId.Create(), SpanId.Create(), false)
+        );
+        var transaction = new UnsampledTransaction(hub, context);
+
+        // Act
+        transaction.Dispose();
+
+        // Assert
+        transaction.IsFinished.Should().BeTrue();
+        hub.Received(1).ConfigureScope(Arg.Any<Action<Scope, UnsampledTransaction>>(),
+            Arg.Is<UnsampledTransaction>(t => ReferenceEquals(t, transaction)));
+    }
+
+    [Fact]
+    public void Dispose_Finished_DoesNothing()
+    {
+        // Arrange
+        var hub = Substitute.For<IHub>();
+        ITransactionContext context = new TransactionContext("TestTransaction", "TestOperation",
+            new SentryTraceHeader(SentryId.Create(), SpanId.Create(), false)
+        );
+        var transaction = new UnsampledTransaction(hub, context);
+        transaction.Finish();
+
+        // Act
+        transaction.Dispose();
+
+        // Assert
+        transaction.IsFinished.Should().BeTrue();
+        hub.Received(1).ConfigureScope(Arg.Any<Action<Scope, UnsampledTransaction>>(),
+            Arg.Is<UnsampledTransaction>(t => ReferenceEquals(t, transaction)));
+    }
+
     [Fact]
     public void StartChild_CreatesSpan_IsTrackedByParent()
     {