manage emscripten event loop from PortableThreadPool.WorkerThread

make sure to keep the thread alive after setting up the semaphore
wait.
Cleanup the thread when exiting

src/mono/System.Private.CoreLib/src/System/Threading/PortableThreadPool.WorkerThread.Browser.Threads.Mono.cs

@@ -1,7 +1,9 @@
 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Diagnostics.CodeAnalysis;
 using System.Diagnostics.Tracing;
+using System.Runtime.CompilerServices;
 
 namespace System.Threading
 {
@@ -65,9 +67,25 @@ private static void WorkerThreadStart()
 
                 LowLevelLock threadAdjustmentLock = threadPoolInstance._threadAdjustmentLock;
                 SemaphoreWaitState state = new(threadPoolInstance, threadAdjustmentLock) { SpinWait = true };
+                // set up the callbacks for semaphore waits, tell
+                // emscripten to keep the thread alive, and return to
+                // the JS event loop.
                 WaitForWorkLoop(s_semaphore, state);
+                EmscriptenKeepalivePush();
+                EmscriptenUnwindToJsEventLoop();
             }
 
+            [MethodImpl(MethodImplOptions.InternalCall)]
+            private static extern void EmscriptenKeepalivePush();
+            [MethodImpl(MethodImplOptions.InternalCall)]
+            private static extern void EmscriptenKeepalivePop();
+            [DoesNotReturn]
+            [MethodImpl(MethodImplOptions.InternalCall)]
+            private static extern void EmscriptenUnwindToJsEventLoop();
+            [DoesNotReturn]
+            [MethodImpl(MethodImplOptions.InternalCall)]
+            private static extern void MonoThreadExit(); // FIXME: this is not Emscripten, it's just mono_thread_exit();
+
             private static readonly Action<LowLevelJSSemaphore, object?> s_WorkLoopSemaphoreSuccess = new(WorkLoopSemaphoreSuccess);
             private static readonly Action<LowLevelJSSemaphore, object?> s_WorkLoopSemaphoreTimedOut = new(WorkLoopSemaphoreTimedOut);
 
@@ -90,8 +108,8 @@ private static void WorkLoopSemaphoreTimedOut(LowLevelJSSemaphore semaphore, obj
                 if (WorkerTimedOutMaybeStop(state.ThreadPoolInstance, state.ThreadAdjustmentLock)) {
                     // we're done, kill the thread.
 
-                    // emscriptenKeepalivePop();
-                    // emscriptenThreadExit();
+                    EmscriptenKeepalivePop();
+                    MonoThreadExit();
                     return;
                 } else {
                     // more work showed up while we were shutting down, go around one more time

src/mono/mono/metadata/icall-decl.h

@@ -189,6 +189,10 @@ ICALL_EXPORT gpointer ves_icall_System_Threading_LowLevelJSSemaphore_InitInterna
 ICALL_EXPORT void     ves_icall_System_Threading_LowLevelJSSemaphore_DeleteInternal (gpointer sem_ptr);
 ICALL_EXPORT void   ves_icall_System_Threading_LowLevelJSSemaphore_PrepareWaitInternal (gpointer sem_ptr, gint32 timeout_ms, gpointer success_cb, gpointer timeout_cb, gpointer gchandle, gpointer user_data);
 ICALL_EXPORT void     ves_icall_System_Threading_LowLevelJSSemaphore_ReleaseInternal (gpointer sem_ptr, gint32 count);
+
+ICALL_EXPORT void ves_icall_System_Threading_PortableThreadPool_WorkerThread_EmscriptenKeepalivePop (void);
+ICALL_EXPORT void ves_icall_System_Threading_PortableThreadPool_WorkerThread_EmscriptenKeepalivePush (void);
+ICALL_EXPORT void ves_icall_System_Threading_PortableThreadPool_WorkerThread_EmscriptenUnwindToJsEventLoop (void);
 #endif
 
 #ifdef TARGET_AMD64

src/mono/mono/metadata/icall-def.h

@@ -571,8 +571,8 @@ NOHANDLES(ICALL(ILOCK_23, "Read(long&)", ves_icall_System_Threading_Interlocked_
 ICALL_TYPE(JSSEM, "System.Threading.LowLevelJSSemaphore", JSSEM_1)
 NOHANDLES(ICALL(JSSEM_1, "DeleteInternal", ves_icall_System_Threading_LowLevelJSSemaphore_DeleteInternal))
 NOHANDLES(ICALL(JSSEM_2, "InitInternal", ves_icall_System_Threading_LowLevelJSSemaphore_InitInternal))
-NOHANDLES(ICALL(JSSEM_3, "ReleaseInternal", ves_icall_System_Threading_LowLevelJSSemaphore_ReleaseInternal))
-NOHANDLES(ICALL(JSSEM_4, "PrepareWaitInternal", ves_icall_System_Threading_LowLevelJSSemaphore_PrepareWaitInternal))
+NOHANDLES(ICALL(JSSEM_3, "PrepareWaitInternal", ves_icall_System_Threading_LowLevelJSSemaphore_PrepareWaitInternal))
+NOHANDLES(ICALL(JSSEM_4, "ReleaseInternal", ves_icall_System_Threading_LowLevelJSSemaphore_ReleaseInternal))
 #endif
 
 ICALL_TYPE(LIFOSEM, "System.Threading.LowLevelLifoSemaphore", LIFOSEM_1)
@@ -590,6 +590,14 @@ HANDLES(MONIT_7, "Monitor_wait", ves_icall_System_Threading_Monitor_Monitor_wait
 NOHANDLES(ICALL(MONIT_8, "get_LockContentionCount", ves_icall_System_Threading_Monitor_Monitor_LockContentionCount))
 HANDLES(MONIT_9, "try_enter_with_atomic_var", ves_icall_System_Threading_Monitor_Monitor_try_enter_with_atomic_var, void, 4, (MonoObject, guint32, MonoBoolean, MonoBoolean_ref))
 
+#if defined(HOST_BROWSER) && !defined(DISABLE_THREADS)
+ICALL_TYPE(TPOOL_WORKER, "System.Threading.PortableThreadPool/WorkerThread", TPOOL_WORKER_1)
+NOHANDLES(ICALL(TPOOL_WORKER_1, "EmscriptenKeepalivePop", ves_icall_System_Threading_PortableThreadPool_WorkerThread_EmscriptenKeepalivePop))
+NOHANDLES(ICALL(TPOOL_WORKER_2, "EmscriptenKeepalivePush", ves_icall_System_Threading_PortableThreadPool_WorkerThread_EmscriptenKeepalivePush))
+NOHANDLES(ICALL(TPOOL_WORKER_3, "EmscriptenUnwindToJsEventLoop", ves_icall_System_Threading_PortableThreadPool_WorkerThread_EmscriptenUnwindToJsEventLoop))
+NOHANDLES(ICALL(TPOOL_WORKER_4, "MonoThreadExit", mono_thread_exit))
+#endif
+
 ICALL_TYPE(THREAD, "System.Threading.Thread", THREAD_1)
 HANDLES(THREAD_1, "ClrState", ves_icall_System_Threading_Thread_ClrState, void, 2, (MonoInternalThread, guint32))
 HANDLES(ITHREAD_2, "FreeInternal", ves_icall_System_Threading_InternalThread_Thread_free_internal, void, 1, (MonoInternalThread))

src/mono/mono/metadata/threads.c

@@ -91,6 +91,10 @@ mono_native_thread_join_handle (HANDLE thread_handle, gboolean close_handle);
 #include <errno.h>
 #endif
 
+#if defined(HOST_BROWSER) && !defined(DISABLE_THREADS)
+#include <emscripten/eventloop.h>
+#endif
+
 #include "icall-decl.h"
 
 /*#define THREAD_DEBUG(a) do { a; } while (0)*/
@@ -4991,4 +4995,23 @@ ves_icall_System_Threading_LowLevelJSSemaphore_ReleaseInternal (gpointer sem_ptr
 	LifoJSSemaphore *sem = (LifoJSSemaphore *)sem_ptr;
 	mono_lifo_js_semaphore_release (sem, count);
 }
+
+void
+ves_icall_System_Threading_PortableThreadPool_WorkerThread_EmscriptenKeepalivePop (void)
+{
+	emscripten_runtime_keepalive_pop();
+}
+
+void
+ves_icall_System_Threading_PortableThreadPool_WorkerThread_EmscriptenKeepalivePush (void)
+{
+	emscripten_runtime_keepalive_push();
+}
+
+void
+ves_icall_System_Threading_PortableThreadPool_WorkerThread_EmscriptenUnwindToJsEventLoop (void)
+{
+	emscripten_unwind_to_js_event_loop();
+}
+
 #endif /* HOST_BROWSER && !DISABLE_THREADS */