Prevent circuit breakers from capturing ambient ExecutionContext (#3284)

Fixes #3275.

Author: Nacl-HCl

src/Polly.Core/CircuitBreaker/Controller/ScheduledTaskExecutor.cs

@@ -11,7 +11,15 @@ internal sealed class ScheduledTaskExecutor : IDisposable
     private readonly SemaphoreSlim _semaphore = new(0);
     private bool _disposed;
 
-    public ScheduledTaskExecutor() => ProcessingTask = Task.Run(StartProcessingAsync);
+    public ScheduledTaskExecutor()
+    {
+        // The processing loop is infrastructure that outlives the code creating the executor,
+        // so it must not capture (and keep alive) the ambient ExecutionContext.
+        using (ExecutionContext.SuppressFlow())
+        {
+            ProcessingTask = Task.Run(StartProcessingAsync);
+        }
+    }
 
     public Task ProcessingTask { get; }
 

test/Polly.Core.Tests/CircuitBreaker/Controller/ScheduledTaskExecutorTests.cs

@@ -1,3 +1,4 @@
+using System.Runtime.CompilerServices;
 using Polly.CircuitBreaker;
 
 namespace Polly.Core.Tests.CircuitBreaker.Controller;
@@ -167,4 +168,38 @@ public async Task Dispose_EnsureNoBackgroundProcessing()
 
         scheduler.ProcessingTask.IsCompleted.ShouldBeTrue();
     }
+
+    [Fact]
+    public void Constructor_DoesNotCaptureAmbientExecutionContext()
+    {
+        // The background processing loop outlives the code that creates the executor,
+        // so it must not capture (and keep alive) the ambient ExecutionContext.
+        var (executor, reference) = CreateExecutorWithAmbientValue();
+
+        for (var i = 0; i < 3; i++)
+        {
+            GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced, blocking: true);
+            GC.WaitForPendingFinalizers();
+        }
+
+        // Keep the executor (and its background processing task) alive for the check:
+        // without the fix, the task's captured ExecutionContext keeps the ambient value alive.
+        GC.KeepAlive(executor);
+        reference.IsAlive.ShouldBeFalse();
+
+        executor.Dispose();
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static (ScheduledTaskExecutor Executor, WeakReference Reference) CreateExecutorWithAmbientValue()
+    {
+        var asyncLocal = new AsyncLocal<object?>();
+        var value = new object();
+        asyncLocal.Value = value;
+
+        var executor = new ScheduledTaskExecutor();
+
+        asyncLocal.Value = null;
+        return (executor, new WeakReference(value));
+    }
 }