WASM: Add an internal method to pump the threadpool (#38690)

This can by used by the xharness xunit runner to make sure async tasks are executed.

We also need to tweak how `ThreadPoolTaskScheduler` handles `TaskCreationOptions.LongRunning` since the usual mode of starting a new thread doesn't work, instead we treat it like the option wasn't set and queue the task on the threadpool.

src/coreclr/src/System.Private.CoreLib/src/System/Threading/Thread.CoreCLR.cs

@@ -319,6 +319,8 @@ partial void ThreadNameChanged(string? value)
         [MethodImpl(MethodImplOptions.InternalCall)]
         internal static extern DeserializationTracker GetThreadDeserializationTracker(ref StackCrawlMark stackMark);
 
+        internal const bool IsThreadStartSupported = true;
+
         /// <summary>Returns true if the thread has been started and is not dead.</summary>
         public extern bool IsAlive
         {

src/libraries/System.Private.CoreLib/src/System/Threading/ExecutionContext.cs

@@ -372,7 +372,7 @@ internal static void ResetThreadPoolThread(Thread currentThread)
         [System.Diagnostics.Conditional("DEBUG")]
         internal static void CheckThreadPoolAndContextsAreDefault()
         {
-            Debug.Assert(Thread.CurrentThread.IsThreadPoolThread);
+            Debug.Assert(!Thread.IsThreadStartSupported || Thread.CurrentThread.IsThreadPoolThread); // there are no dedicated threadpool threads on runtimes where we can't start threads
             Debug.Assert(Thread.CurrentThread._executionContext == null, "ThreadPool thread not on Default ExecutionContext.");
             Debug.Assert(Thread.CurrentThread._synchronizationContext == null, "ThreadPool thread not on Default SynchronizationContext.");
         }

src/libraries/System.Private.CoreLib/src/System/Threading/Tasks/ThreadPoolTaskScheduler.cs

@@ -43,7 +43,7 @@ internal ThreadPoolTaskScheduler()
         protected internal override void QueueTask(Task task)
         {
             TaskCreationOptions options = task.Options;
-            if ((options & TaskCreationOptions.LongRunning) != 0)
+            if (Thread.IsThreadStartSupported && (options & TaskCreationOptions.LongRunning) != 0)
             {
                 // Run LongRunning tasks on their own dedicated thread.
                 Thread thread = new Thread(s_longRunningThreadWork);

src/mono/mono/mini/mini-wasm.c

@@ -395,6 +395,9 @@ extern void mono_set_timeout (int t, int d);
 extern void mono_wasm_queue_tp_cb (void);
 G_END_DECLS
 
+extern void mono_wasm_pump_threadpool (void);
+void mono_background_exec (void);
+
 #endif // HOST_WASM
 
 gpointer
@@ -620,12 +623,21 @@ mono_wasm_queue_tp_cb (void)
 #endif
 }
 
+void
+mono_wasm_pump_threadpool (void)
+{
+#ifdef HOST_WASM
+	mono_background_exec ();
+#endif
+}
+
 void
 mono_arch_register_icall (void)
 {
 #ifdef ENABLE_NETCORE
 	mono_add_internal_call_internal ("System.Threading.TimerQueue::SetTimeout", mono_wasm_set_timeout);
 	mono_add_internal_call_internal ("System.Threading.ThreadPool::QueueCallback", mono_wasm_queue_tp_cb);
+	mono_add_internal_call_internal ("System.Threading.ThreadPool::PumpThreadPool", mono_wasm_pump_threadpool);
 #else
 	mono_add_internal_call_internal ("System.Threading.WasmRuntime::SetTimeout", mono_wasm_set_timeout);
 #endif

src/mono/netcore/System.Private.CoreLib/System.Private.CoreLib.csproj

@@ -304,8 +304,12 @@
   <ItemGroup Condition="'$(TargetsBrowser)' == 'true'">
       <Compile Include="$(BclSourcesRoot)\System\Threading\TimerQueue.Browser.Mono.cs" />
       <Compile Include="$(BclSourcesRoot)\System\Threading\ThreadPool.Browser.Mono.cs" />
+      <Compile Include="$(BclSourcesRoot)\System\Threading\Thread.Browser.Mono.cs" />
       <Compile Include="$(LibrariesProjectRoot)\System.Private.CoreLib\src\System\Threading\ThreadPoolBoundHandle.PlatformNotSupported.cs" />
   </ItemGroup>
+  <ItemGroup Condition="'$(TargetsBrowser)' != 'true'">
+      <Compile Include="$(BclSourcesRoot)\System\Threading\Thread.UnixOrWindows.Mono.cs" />
+  </ItemGroup>
   <ItemGroup>
       <Compile Include="$(BclSourcesRoot)\Mono\RuntimeStructs.cs" />
       <Compile Include="$(BclSourcesRoot)\Mono\RuntimeMarshal.cs" />

src/mono/netcore/System.Private.CoreLib/src/System/Threading/Thread.Browser.Mono.cs

@@ -0,0 +1,11 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace System.Threading
+{
+    public partial class Thread
+    {
+        internal const bool IsThreadStartSupported = false;
+    }
+}

src/mono/netcore/System.Private.CoreLib/src/System/Threading/Thread.UnixOrWindows.Mono.cs

@@ -0,0 +1,11 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+namespace System.Threading
+{
+    public partial class Thread
+    {
+        internal const bool IsThreadStartSupported = true;
+    }
+}

src/mono/netcore/System.Private.CoreLib/src/System/Threading/ThreadPool.Browser.Mono.cs

@@ -107,9 +107,13 @@ private static RegisteredWaitHandle RegisterWaitForSingleObject(
         }
 
         [DynamicDependency("Callback")]
+        [DynamicDependency("PumpThreadPool")]
         [MethodImplAttribute(MethodImplOptions.InternalCall)]
         private static extern void QueueCallback();
 
+        [MethodImplAttribute(MethodImplOptions.InternalCall)]
+        private static extern void PumpThreadPool(); // NOTE: this method is called via reflection by test code
+
         private static void Callback()
         {
             _callbackQueued = false;