Context switching can affect timer triggering

src/Libraries/Microsoft.Extensions.TimeProvider.Testing/FakeTimeProvider.cs

@@ -194,6 +194,8 @@ internal void AddWaiter(Waiter waiter, long dueTime)
         WakeWaiters();
     }
 
+    internal event EventHandler? GateOpening;
+
     private void WakeWaiters()
     {
         if (Interlocked.CompareExchange(ref _wakeWaitersGate, 1, 0) == 1)
@@ -234,13 +236,14 @@ private void WakeWaiters()
                         candidate = waiter;
                     }
                 }
-            }
 
-            if (candidate == null)
-            {
-                // didn't find a candidate to wake, we're done
-                _wakeWaitersGate = 0;
-                return;
+                if (candidate == null)
+                {
+                    // didn't find a candidate to wake, we're done
+                    GateOpening?.Invoke(this, EventArgs.Empty);
+                    _wakeWaitersGate = 0;
+                    return;
+                }
             }
 
             var oldTicks = _now.Ticks;

test/Libraries/Microsoft.Extensions.TimeProvider.Testing.Tests/FakeTimeProviderTests.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System;
+using System.Collections.Generic;
 using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.Extensions.Time.Testing;
@@ -361,4 +362,31 @@ public void AdvanceTimeInCallback()
 
         Assert.True(true, "Yay, we didn't enter an infinite loop!");
     }
+
+    [Fact]
+    public void ShouldResetGateUnderLock_PreventingContextSwitching_AffectionOnTimerCallback()
+    {
+        // Arrange
+        var provider = new FakeTimeProvider { AutoAdvanceAmount = TimeSpan.FromSeconds(2) };
+        var calls = new List<object?>();
+        using var timer = provider.CreateTimer(calls.Add, "timer-1", TimeSpan.FromSeconds(3), TimeSpan.Zero);
+        var th = new Thread(() => provider.GetUtcNow());
+        provider.GateOpening += (_, _) =>
+        {
+            if (!th.IsAlive)
+            {
+                th.Start();
+            }
+
+            // use a timeout to prevent deadlock
+            th.Join(TimeSpan.FromMilliseconds(200));
+        };
+
+        // Act
+        provider.GetUtcNow();
+        th.Join();
+
+        // Assert
+        Assert.Single(calls);
+    }
 }