Set IsBackground property to true on libuv Threads for non-debug builds

- If libuv doesn't shutdown as expected, the process will still stop. Thanks @benaadams!
- Address other minor PR feedback.

src/Microsoft.AspNetCore.Server.Kestrel/Http/ConnectionManager.cs

@@ -23,7 +23,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Http
         {
             if (_connectionStopTasks != null)
             {
-                throw new InvalidOperationException(nameof(WalkConnectionsAndClose) + " cannot be called twice.");
+                throw new InvalidOperationException($"{nameof(WalkConnectionsAndClose)} cannot be called twice.");
             }
 
             _connectionStopTasks = new List<Task>();
@@ -44,7 +44,7 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Http
         {
             if (_connectionStopTasks == null)
             {
-                throw new InvalidOperationException(nameof(WalkConnectionsAndClose) + " must be called first.");
+                throw new InvalidOperationException($"{nameof(WalkConnectionsAndClose)} must be called first.");
             }
 
             return Task.WhenAll(_connectionStopTasks);

src/Microsoft.AspNetCore.Server.Kestrel/Http/Frame.cs

@@ -189,12 +189,11 @@ namespace Microsoft.AspNetCore.Server.Kestrel.Http
 
         protected FrameRequestHeaders FrameRequestHeaders => _frameHeaders.RequestHeaders;
 
-        public Frame InitializeHeaders()
+        public void InitializeHeaders()
         {
             _frameHeaders = HttpComponentFactory.CreateHeaders(DateHeaderValueManager);
             RequestHeaders = _frameHeaders.RequestHeaders;
             ResponseHeaders = _frameHeaders.ResponseHeaders;
-            return this;
         }
 
 

src/Microsoft.AspNetCore.Server.Kestrel/Infrastructure/KestrelThread.cs

@@ -79,6 +79,12 @@ namespace Microsoft.AspNetCore.Server.Kestrel
 
         public void Stop(TimeSpan timeout)
         {
+#if !DEBUG
+            // Mark the thread as being as unimportant to keeping the process alive.
+            // Don't do this for debug builds, so we know if the thread isn't terminating.
+            _thread.IsBackground = true;
+#endif
+
             if (!_initCompleted)
             {
                 return;

test/Microsoft.AspNetCore.Server.KestrelTests/FrameResponseHeadersTests.cs

@@ -26,8 +26,8 @@ namespace Microsoft.AspNetCore.Server.KestrelTests
                 ServerInformation = serverInformation,
                 HttpComponentFactory = new HttpComponentFactory(serverInformation)
         };
-            var frame = new Frame<object>(application: null, context: connectionContext)
-                            .InitializeHeaders();
+            var frame = new Frame<object>(application: null, context: connectionContext);
+            frame.InitializeHeaders();
 
             IDictionary<string, StringValues> headers = frame.ResponseHeaders;
 
@@ -60,8 +60,8 @@ namespace Microsoft.AspNetCore.Server.KestrelTests
                 ServerInformation = serverInformation,
                 HttpComponentFactory = new HttpComponentFactory(serverInformation)
             };
-            var frame = new Frame<object>(application: null, context: connectionContext)
-                            .InitializeHeaders();
+            var frame = new Frame<object>(application: null, context: connectionContext);
+            frame.InitializeHeaders();
 
             Assert.True(frame.ResponseHeaders.Count > 0);
 

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

@@ -3,10 +3,7 @@
 
 using System;
 using System.Threading;
-using Microsoft.AspNetCore.Server.Kestrel;
 using Microsoft.AspNetCore.Server.Kestrel.Http;
-using Microsoft.AspNetCore.Server.Kestrel.Infrastructure;
-using Microsoft.AspNetCore.Server.Kestrel.Networking;
 
 namespace Microsoft.AspNetCore.Server.KestrelTests.TestHelpers
 {