Don't invoke continuations inline in OnCompleted (#12261)

Author: halter73

src/Servers/Kestrel/Core/src/Internal/Http2/FlowControl/OutputFlowControlAwaitable.cs

@@ -5,6 +5,7 @@
 using System.Diagnostics;
 using System.Runtime.CompilerServices;
 using System.Threading;
+using System.Threading.Tasks;
 
 namespace Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http2.FlowControl
 {
@@ -29,7 +30,7 @@ public void OnCompleted(Action continuation)
             if (ReferenceEquals(_callback, _callbackCompleted) ||
                 ReferenceEquals(Interlocked.CompareExchange(ref _callback, continuation, null), _callbackCompleted))
             {
-                continuation();
+                Task.Run(continuation);
             }
         }
 

src/Servers/Kestrel/Transport.Libuv/src/Internal/LibuvAwaitable.cs

@@ -53,15 +53,12 @@ public void OnCompleted(Action continuation)
         {
             // There should never be a race between IsCompleted and OnCompleted since both operations
             // should always be on the libuv thread
-
-            if (ReferenceEquals(_callback, _callbackCompleted) ||
-                ReferenceEquals(Interlocked.CompareExchange(ref _callback, continuation, null), _callbackCompleted))
+            if (ReferenceEquals(_callback, _callbackCompleted))
             {
-                Debug.Fail($"{typeof(LibuvAwaitable<TRequest>)}.{nameof(OnCompleted)} raced with {nameof(IsCompleted)}, running callback inline.");
-
-                // Just run it inline
-                continuation();
+                Debug.Fail($"{typeof(LibuvAwaitable<TRequest>)}.{nameof(OnCompleted)} raced with {nameof(IsCompleted)}, scheduling callback.");
             }
+
+            _callback = continuation;
         }
 
         public void UnsafeOnCompleted(Action continuation)