make environment shutdown asynchronous

src/csharp/Grpc.Core.Tests/GrpcEnvironmentTest.cs

@@ -49,7 +49,7 @@ namespace Grpc.Core.Tests
             {
                 Assert.IsNotNull(env.CompletionQueues.ElementAt(i));
             }
-            GrpcEnvironment.Release();
+            GrpcEnvironment.ReleaseAsync().Wait();
         }
 
         [Test]
@@ -58,8 +58,8 @@ namespace Grpc.Core.Tests
             var env1 = GrpcEnvironment.AddRef();
             var env2 = GrpcEnvironment.AddRef();
             Assert.AreSame(env1, env2);
-            GrpcEnvironment.Release();
-            GrpcEnvironment.Release();
+            GrpcEnvironment.ReleaseAsync().Wait();
+            GrpcEnvironment.ReleaseAsync().Wait();
         }
 
         [Test]
@@ -68,10 +68,10 @@ namespace Grpc.Core.Tests
             Assert.AreEqual(0, GrpcEnvironment.GetRefCount());
 
             var env1 = GrpcEnvironment.AddRef();
-            GrpcEnvironment.Release();
+            GrpcEnvironment.ReleaseAsync().Wait();
 
             var env2 = GrpcEnvironment.AddRef();
-            GrpcEnvironment.Release();
+            GrpcEnvironment.ReleaseAsync().Wait();
 
             Assert.AreNotSame(env1, env2);
         }
@@ -80,7 +80,7 @@ namespace Grpc.Core.Tests
         public void ReleaseWithoutAddRef()
         {
             Assert.AreEqual(0, GrpcEnvironment.GetRefCount());
-            Assert.Throws(typeof(InvalidOperationException), () => GrpcEnvironment.Release());
+            Assert.ThrowsAsync(typeof(InvalidOperationException), async () => await GrpcEnvironment.ReleaseAsync());
         }
 
         [Test]

src/csharp/Grpc.Core.Tests/Internal/CompletionQueueSafeHandleTest.cs

@@ -48,7 +48,7 @@ namespace Grpc.Core.Internal.Tests
             GrpcEnvironment.AddRef();
             var cq = CompletionQueueSafeHandle.Create();
             cq.Dispose();
-            GrpcEnvironment.Release();
+            GrpcEnvironment.ReleaseAsync().Wait();
         }
 
         [Test]
@@ -59,7 +59,7 @@ namespace Grpc.Core.Internal.Tests
             cq.Shutdown();
             var ev = cq.Next();
             cq.Dispose();
-            GrpcEnvironment.Release();
+            GrpcEnvironment.ReleaseAsync().Wait();
             Assert.AreEqual(CompletionQueueEvent.CompletionType.Shutdown, ev.type);
             Assert.AreNotEqual(IntPtr.Zero, ev.success);
             Assert.AreEqual(IntPtr.Zero, ev.tag);

src/csharp/Grpc.Core.Tests/PInvokeTest.cs

@@ -65,7 +65,7 @@ namespace Grpc.Core.Tests
                     cq.Dispose();
                 });
 
-            GrpcEnvironment.Release();
+            GrpcEnvironment.ReleaseAsync().Wait();
         }
 
         /// <summary>

src/csharp/Grpc.Core/Channel.cs

@@ -220,7 +220,7 @@ namespace Grpc.Core
 
             handle.Dispose();
 
-            await Task.Run(() => GrpcEnvironment.Release()).ConfigureAwait(false);
+            await GrpcEnvironment.ReleaseAsync().ConfigureAwait(false);
         }
 
         internal ChannelSafeHandle Handle

src/csharp/Grpc.Core/GrpcEnvironment.cs

@@ -35,6 +35,7 @@ using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Runtime.InteropServices;
+using System.Threading.Tasks;
 using Grpc.Core.Internal;
 using Grpc.Core.Logging;
 using Grpc.Core.Utils;
@@ -79,21 +80,26 @@ namespace Grpc.Core
         }
 
         /// <summary>
-        /// Decrements the reference count for currently active environment and shuts down the gRPC environment if reference count drops to zero.
-        /// (and blocks until the environment has been fully shutdown).
+        /// Decrements the reference count for currently active environment and asynchronously shuts down the gRPC environment if reference count drops to zero.
         /// </summary>
-        internal static void Release()
+        internal static async Task ReleaseAsync()
         {
+            GrpcEnvironment instanceToShutdown = null;
             lock (staticLock)
             {
                 GrpcPreconditions.CheckState(refCount > 0);
                 refCount--;
                 if (refCount == 0)
                 {
-                    instance.Close();
+                    instanceToShutdown = instance;
                     instance = null;
                 }
             }
+
+            if (instanceToShutdown != null)
+            {
+                await instanceToShutdown.ShutdownAsync();
+            }
         }
 
         internal static int GetRefCount()
@@ -223,13 +229,13 @@ namespace Grpc.Core
         /// <summary>
         /// Shuts down this environment.
         /// </summary>
-        private void Close()
+        private async Task ShutdownAsync()
         {
             if (isClosed)
             {
                 throw new InvalidOperationException("Close has already been called");
             }
-            threadPool.Stop();
+            await threadPool.StopAsync().ConfigureAwait(false);
             GrpcNativeShutdown();
             isClosed = true;
 

src/csharp/Grpc.Core/Internal/GrpcThreadPool.cs

@@ -35,6 +35,7 @@ using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Threading;
+using System.Threading.Tasks;
 using Grpc.Core.Logging;
 using Grpc.Core.Utils;
 
@@ -53,6 +54,8 @@ namespace Grpc.Core.Internal
         readonly int poolSize;
         readonly int completionQueueCount;
 
+        bool stopRequested;
+
         IReadOnlyCollection<CompletionQueueSafeHandle> completionQueues;
 
         /// <summary>
@@ -84,15 +87,21 @@ namespace Grpc.Core.Internal
             }
         }
 
-        public void Stop()
+        public Task StopAsync()
         {
             lock (myLock)
             {
+                GrpcPreconditions.CheckState(!stopRequested, "Stop already requested.");
+                stopRequested = true;
+
                 foreach (var cq in completionQueues)
                 {
                     cq.Shutdown();
                 }
+            }
 
+            return Task.Run(() =>
+            {
                 foreach (var thread in threads)
                 {
                     thread.Join();
@@ -102,7 +111,7 @@ namespace Grpc.Core.Internal
                 {
                     cq.Dispose();
                 }
-            }
+            });
         }
 
         internal IReadOnlyCollection<CompletionQueueSafeHandle> CompletionQueues

src/csharp/Grpc.Core/Server.cs

@@ -169,7 +169,7 @@ namespace Grpc.Core
             await shutdownTcs.Task.ConfigureAwait(false);
             DisposeHandle();
 
-            await Task.Run(() => GrpcEnvironment.Release()).ConfigureAwait(false);
+            await GrpcEnvironment.ReleaseAsync().ConfigureAwait(false);
         }
 
         /// <summary>
@@ -194,7 +194,7 @@ namespace Grpc.Core
             await shutdownTcs.Task.ConfigureAwait(false);
             DisposeHandle();
 
-            await Task.Run(() => GrpcEnvironment.Release()).ConfigureAwait(false);
+            await GrpcEnvironment.ReleaseAsync().ConfigureAwait(false);
         }
 
         internal void AddCallReference(object call)