fix(OpenTelemetry): fuse Activity weakly so never-ended spans are pruned

src/Sentry.OpenTelemetry/SentrySpanProcessor.cs

@@ -134,13 +134,15 @@ private void CreateChildSpan(Activity data, ISpan parentSpan, SpanId? parentSpan
         };
 
         var span = parentSpan.StartChild(context);
-        // Used to filter out spans that are not recorded when finishing a transaction
-        span.SetFused(data);
+        // Fuse the Activity via a WeakReference so _map does not pin it, letting PruneFilteredSpans evict
+        // never-ended spans. #3198 intended weak refs here but SetFused stores the value strongly.
+        SetFusedActivity(span, data);
         if (span is SpanTracer spanTracer)
         {
             spanTracer.Origin = OpenTelemetryOrigin;
             spanTracer.StartTimestamp = data.StartTimeUtc;
-            spanTracer.IsFiltered = () => spanTracer.GetFused<Activity>() is { IsAllDataRequested: false, Recorded: false };
+            // Used to filter out spans that are not recorded when finishing a transaction.
+            spanTracer.IsFiltered = () => GetFusedActivity(spanTracer) is { IsAllDataRequested: false, Recorded: false };
         }
         _map[data.SpanId] = span;
     }
@@ -176,7 +178,8 @@ private void CreateRootSpan(Activity data)
         {
             scope.Transaction ??= transaction;
         }, transaction);
-        transaction.SetFused(data);
+        // Fuse weakly so _map does not pin the Activity.
+        SetFusedActivity(transaction, data);
         _map[data.SpanId] = transaction;
     }
 
@@ -304,16 +307,28 @@ internal void PruneFilteredSpans(bool force = false)
         foreach (var mappedItem in _map)
         {
             var (spanId, span) = mappedItem;
-            var activity = span.GetFused<Activity>();
-            // Also prune when the activity has been GC'd (weak ref returns null): the activity is gone, so it
-            // can never call OnEnd, and the span will never be removed otherwise — causing a memory leak.
+            var activity = GetFusedActivity(span);
+            // Prune when the activity has been GC'd (weak ref returns null): it can no longer call OnEnd, so
+            // the span would otherwise stay in _map forever.
             if (activity is null or { Recorded: false, IsAllDataRequested: false })
             {
                 _map.TryRemove(spanId, out _);
             }
         }
     }
 
+    private const string ActivityPropertyName = "Activity";
+
+    // Fuses the Activity onto the span via a WeakReference so _map does not pin it.
+    private static void SetFusedActivity(ISpan span, Activity data) =>
+        span.SetFused(ActivityPropertyName, new WeakReference<Activity>(data));
+
+    // Returns the fused Activity, or null once it has been garbage-collected.
+    private static Activity? GetFusedActivity(ISpan span) =>
+        span.GetFused<WeakReference<Activity>>(ActivityPropertyName) is { } weakRef && weakRef.TryGetTarget(out var activity)
+            ? activity
+            : null;
+
     private bool NeedsPruning()
     {
         var lastPruned = Interlocked.Read(ref _lastPruned);

test/Sentry.OpenTelemetry.Tests/SentrySpanProcessorTests.cs

@@ -965,6 +965,25 @@ public void PruneFilteredSpans_GarbageCollectedActivity_Pruned()
         Assert.False(sut._map.TryGetValue(spanId, out _));
     }
 
+    [Fact]
+    public void OnStart_FusesActivityWeakly()
+    {
+        // Arrange
+        _fixture.Options.Instrumenter = Instrumenter.OpenTelemetry;
+        var sut = _fixture.GetSut();
+
+        using var activity = Tracer.StartActivity("test")!;
+
+        // Act
+        sut.OnStart(activity);
+
+        // The Activity must be fused via a WeakReference, not strongly. A strong reference is pinned by
+        // _map, which prevents GC and defeats PruneFilteredSpans, leaking never-ended spans.
+        sut._map.TryGetValue(activity.SpanId, out var span).Should().BeTrue();
+        span.GetFused<WeakReference<Activity>>("Activity").Should().NotBeNull();
+        span.GetFused<Activity>("Activity").Should().BeNull("the Activity must not be fused with a strong reference");
+    }
+
     [Fact]
     public void PruneFilteredSpans_RecentlyPruned_DoesNothing()
     {