Schedule the user callback from unmanaged code

	When possible, schedule the user callback from unmanaged code. We pass
	it from the IO pool to the regular pool after our internal callback is
	finished.

mcs/class/System/System.Net.Sockets/Socket_2_1.cs

@@ -250,14 +250,7 @@ namespace System.Net.Sockets {
 					if (sac != null)
 						Socket.socket_pool_queue (sac, worker.result);
 				}
-
-				if (callback != null)
-					ThreadPool.QueueUserWorkItem (CB, null);
-			}
-
-			void CB (object unused)
-			{
-				callback (this);
+				// IMPORTANT: 'callback', if any is scheduled from unmanaged code
 			}
 
 			SocketAsyncCall GetDelegate (Worker worker, SocketOperation op)

mono/metadata/threadpool.c

@@ -1254,8 +1254,8 @@ mono_thread_pool_init ()
 #endif
 }
 
-void
-icall_append_io_job (MonoObject *target, MonoSocketAsyncResult *state)
+static MonoAsyncResult *
+create_simple_asyncresult (MonoObject *target, MonoObject *state)
 {
 	MonoDomain *domain = mono_domain_get ();
 	MonoAsyncResult *ares;
@@ -1264,6 +1264,15 @@ icall_append_io_job (MonoObject *target, MonoSocketAsyncResult *state)
 	ares = (MonoAsyncResult *) mono_object_new (domain, mono_defaults.asyncresult_class);
 	MONO_OBJECT_SETREF (ares, async_delegate, target);
 	MONO_OBJECT_SETREF (ares, async_state, state);
+	return ares;
+}
+
+void
+icall_append_io_job (MonoObject *target, MonoSocketAsyncResult *state)
+{
+	MonoAsyncResult *ares;
+
+	ares = create_simple_asyncresult (target, (MonoObject *) state);
 	socket_io_add (ares, state);
 }
 
@@ -1828,6 +1837,7 @@ async_invoke_thread (gpointer data)
 	for (;;) {
 		MonoAsyncResult *ar;
 		gboolean is_io_task;
+		gboolean is_socket;
 		int n_naps = 0;
 
 		is_io_task = FALSE;
@@ -1836,8 +1846,10 @@ async_invoke_thread (gpointer data)
 			InterlockedIncrement (&tp->busy_threads);
 #ifndef DISABLE_SOCKETS
 			is_io_task = (strcmp (((MonoObject *) data)->vtable->klass->name, "AsyncResult"));
+			is_socket = FALSE;
 			if (is_io_task) {
 				MonoSocketAsyncResult *state = (MonoSocketAsyncResult *) data;
+				is_socket = (is_io_task && !strcmp (((MonoObject *) data)->vtable->klass->name, "SocketAsyncResult"));
 				ar = state->ares;
 				switch (state->operation) {
 				case AIO_OP_RECEIVE:
@@ -1896,6 +1908,16 @@ async_invoke_thread (gpointer data)
 							exit (255);
 						}
 					}
+					if (is_socket && tp->is_io) {
+						MonoSocketAsyncResult *state = (MonoSocketAsyncResult *) data;
+
+						if (state->completed && state->callback) {
+							MonoAsyncResult *cb_ares;
+							cb_ares = create_simple_asyncresult ((MonoObject *) state->callback,
+												(MonoObject *) state);
+							icall_append_job ((MonoObject *) cb_ares);
+						}
+					}
 					mono_domain_set (mono_get_root_domain (), TRUE);
 				}
 				mono_thread_pop_appdomain_ref ();