Make all calls to ThreadPool.QueueUserWorkItem through IThreadPool

- This allows SocketOutputTests to cause QUWI to exec synchronously
- To avoid allocations, the logger can't be captured making it "unsafe"

src/Microsoft.AspNetCore.Server.Kestrel/Internal/Http/SocketOutput.cs

@@ -288,7 +288,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Http
 
             if (blockToReturn != null)
             {
-                ThreadPool.QueueUserWorkItem(_returnBlocks, blockToReturn);
+                _threadPool.UnsafeRun(_returnBlocks, blockToReturn);
             }
         }
 
@@ -542,8 +542,8 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Http
 
         private class WriteContext
         {
-            private static WaitCallback _returnWrittenBlocks = (state) => ReturnWrittenBlocks((MemoryPoolBlock)state);
-            private static WaitCallback _completeWrite = (state) => ((WriteContext)state).CompleteOnThreadPool();
+            private static readonly WaitCallback _returnWrittenBlocks = (state) => ReturnWrittenBlocks((MemoryPoolBlock)state);
+            private static readonly WaitCallback _completeWrite = (state) => ((WriteContext)state).CompleteOnThreadPool();
 
             private SocketOutput Self;
             private UvWriteReq _writeReq;
@@ -658,7 +658,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Http
                 }
                 else
                 {
-                    ThreadPool.QueueUserWorkItem(_completeWrite, this);
+                    Self._threadPool.UnsafeRun(_completeWrite, this);
                 }
             }
 
@@ -697,7 +697,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Http
                 }
                 block.Next = null;
 
-                ThreadPool.QueueUserWorkItem(_returnWrittenBlocks, _lockedStart.Block);
+                Self._threadPool.UnsafeRun(_returnWrittenBlocks, _lockedStart.Block);
             }
 
             private static void ReturnWrittenBlocks(MemoryPoolBlock block)

src/Microsoft.AspNetCore.Server.Kestrel/Internal/Infrastructure/IThreadPool.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Threading;
 using System.Threading.Tasks;
 
 namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Infrastructure
@@ -12,5 +13,6 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Infrastructure
         void Cancel(TaskCompletionSource<object> tcs);
         void Error(TaskCompletionSource<object> tcs, Exception ex);
         void Run(Action action);
+        void UnsafeRun(WaitCallback action, object state);
     }
 }

src/Microsoft.AspNetCore.Server.Kestrel/Internal/Infrastructure/LoggingThreadPool.cs

@@ -63,6 +63,11 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Internal.Infrastructure
             ThreadPool.QueueUserWorkItem(_runAction, action);
         }
 
+        public void UnsafeRun(WaitCallback action, object state)
+        {
+            ThreadPool.QueueUserWorkItem(action, state);
+        }
+
         public void Complete(TaskCompletionSource<object> tcs)
         {
             ThreadPool.QueueUserWorkItem(_completeTcs, tcs);

test/Microsoft.AspNetCore.Server.KestrelTests/TestHelpers/SynchronousThreadPool.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.Threading;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Server.Kestrel.Internal.Infrastructure;
 
@@ -28,5 +29,10 @@ namespace Microsoft.AspNetCore.Server.KestrelTests.TestHelpers
         {
             action();
         }
+
+        public void UnsafeRun(WaitCallback action, object state)
+        {
+            action(state);
+        }
     }
 }