From 41fe826b109bbad61c1171b48f28086eb80d5bc3 Mon Sep 17 00:00:00 2001
From: James Crosswell <james.crosswell@gmail.com>
Date: Wed, 13 May 2026 12:52:54 +1200
Subject: [PATCH] fix: prevent race condition when span count approaches the
 1000-span limit

Wraps AddChildSpan in a lock so concurrent span creation cannot exceed
the limit or set IsSampled incorrectly under contention.

Fixes #5173

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
---
 src/Sentry/TransactionTracer.cs                | 18 +++++++++++-------
 .../Protocol/SentryTransactionTests.cs         | 15 +++++++++++++++
 2 files changed, 26 insertions(+), 7 deletions(-)

diff --git a/src/Sentry/TransactionTracer.cs b/src/Sentry/TransactionTracer.cs
index e4ce907607..8bad607205 100644
--- a/src/Sentry/TransactionTracer.cs
+++ b/src/Sentry/TransactionTracer.cs
@@ -166,6 +166,7 @@ public IReadOnlyList<string> Fingerprint
     public IReadOnlyDictionary<string, string> Tags => _tags;
 
     private readonly ConcurrentBagLite<ISpan> _spans = new();
+    private readonly Lock _childSpanLock = new();
 
     /// <inheritdoc />
     public IReadOnlyCollection<ISpan> Spans => _spans;
@@ -296,14 +297,17 @@ internal ISpan StartChild(SpanId? spanId, SpanId parentSpanId, string operation,
 
     private void AddChildSpan(SpanTracer span)
     {
-        // Limit spans to 1000
-        var isOutOfLimit = _spans.Count >= 1000;
-        span.IsSampled = isOutOfLimit ? false : IsSampled;
-
-        if (!isOutOfLimit)
+        lock (_childSpanLock)
         {
-            _spans.Add(span);
-            _activeSpanTracker.Push(span);
+            // Limit spans to 1000
+            var isOutOfLimit = _spans.Count >= 1000;
+            span.IsSampled = isOutOfLimit ? false : IsSampled;
+
+            if (!isOutOfLimit)
+            {
+                _spans.Add(span);
+                _activeSpanTracker.Push(span);
+            }
         }
     }
 
diff --git a/test/Sentry.Tests/Protocol/SentryTransactionTests.cs b/test/Sentry.Tests/Protocol/SentryTransactionTests.cs
index 604e1cdda7..d8c2695385 100644
--- a/test/Sentry.Tests/Protocol/SentryTransactionTests.cs
+++ b/test/Sentry.Tests/Protocol/SentryTransactionTests.cs
@@ -378,6 +378,21 @@ public void StartChild_Limit_Maintained()
         spans.Count(s => s.IsSampled == true).Should().Be(1000);
     }
 
+    [Fact]
+    public void StartChild_Limit_Maintained_Concurrently()
+    {
+        // Arrange
+        var transaction = new TransactionTracer(DisabledHub.Instance, "my name", "my op");
+        var spans = new ConcurrentBag<ISpan>();
+
+        // Act
+        Parallel.For(0, 5000, i => spans.Add(transaction.StartChild("span " + i)));
+
+        // Assert
+        transaction.Spans.Should().HaveCount(1000);
+        spans.Count(s => s.IsSampled == true).Should().Be(1000);
+    }
+
     [Fact]
     public void StartChild_SamplingInherited_True()
     {