Fix idle transaction finish race conditions

Co-Authored-By: Cursor Agent <agent@noreply.example.com>

Author: cursoragent

src/Sentry/TransactionTracer.cs

@@ -287,6 +287,7 @@ private void OnIdleTimeout()
         if (shouldDiscard)
         {
             _options?.LogDebug("Idle transaction '{0}' has no child spans. Discarding.", SpanId);
+            EndTimestamp ??= _stopwatch.CurrentDateTimeOffset;
             _idleTimer?.Dispose();
             _hub.ConfigureScope(static (scope, tracer) => scope.ResetTransaction(tracer), this);
             return;
@@ -378,7 +379,15 @@ internal void ChildSpanFinished()
         // Only restart the idle timer when there are no more active (unfinished) child spans
         if (_activeSpanTracker.PeekActive() == null)
         {
-            _idleTimer?.Start(_idleTimeout.Value);
+            try
+            {
+                _idleTimer?.Start(_idleTimeout.Value);
+            }
+            catch (ObjectDisposedException)
+            {
+                // Finish() may dispose the timer concurrently between the _hasFinished check and Start().
+                // Swallow the exception — the transaction is already finishing.
+            }
         }
     }
 

test/Sentry.Tests/TransactionTracerTests.cs

@@ -6,6 +6,29 @@ public class TransactionTracerTests
 {
     private static readonly TimeSpan AnyTimeout = TimeSpan.FromSeconds(30);
 
+    private sealed class ThrowOnDisposedTimer : ISentryTimer
+    {
+        private bool _isDisposed;
+
+        public ThrowOnDisposedTimer(Action _)
+        {
+        }
+
+        public void Start(TimeSpan timeout)
+        {
+            if (_isDisposed)
+            {
+                throw new ObjectDisposedException(nameof(ThrowOnDisposedTimer));
+            }
+        }
+
+        public void Cancel()
+        {
+        }
+
+        public void Dispose() => _isDisposed = true;
+    }
+
     private static (TransactionTracer transaction, MockTimer timer) CreateIdleTransaction(
         IHub hub, string name = "name", string op = "op")
     {
@@ -122,13 +145,39 @@ public void IdleTimeout_NoChildSpans_TransactionIsDiscarded()
     {
         // Given an auto-generated UI event transaction with no child spans
         var hub = Substitute.For<IHub>();
-        var (_, timer) = CreateIdleTransaction(hub);
+        var (transaction, timer) = CreateIdleTransaction(hub);
 
         // When the idleTimeout fires
         timer.Fire();
 
         // Then the SDK discards the transaction (does not capture it)
         hub.DidNotReceive().CaptureTransaction(Arg.Any<SentryTransaction>());
+        transaction.IsFinished.Should().BeTrue();
+        transaction.EndTimestamp.Should().NotBeNull();
+    }
+
+    [Fact]
+    public void ChildSpanFinished_DisposedIdleTimer_DoesNotThrow()
+    {
+        // Given an idle transaction whose timer has been disposed after the finished-state check
+        var hub = Substitute.For<IHub>();
+        ThrowOnDisposedTimer? disposedTimer = null;
+        var transaction = new TransactionTracer(
+            hub,
+            new TransactionContext("name", "op"),
+            idleTimeout: AnyTimeout,
+            timerFactory: callback =>
+            {
+                disposedTimer = new ThrowOnDisposedTimer(callback);
+                return disposedTimer;
+            });
+        disposedTimer!.Dispose();
+
+        // When ChildSpanFinished attempts to restart the disposed timer
+        var action = () => transaction.ChildSpanFinished();
+
+        // Then no ObjectDisposedException escapes
+        action.Should().NotThrow();
     }
 
     [Fact]