runtime (js/wasm): avoid scheduler recursion

src/runtime/runtime_wasm_js.go

@@ -6,13 +6,20 @@ import "unsafe"
 
 type timeUnit float64 // time in milliseconds, just like Date.now() in JavaScript
 
+// wasmRunning is used to track whether wasm is currently running to avoid nested scheduling.
+var wasmRunning bool
+
 //export _start
 func _start() {
+	wasmRunning = true
+
 	// These need to be initialized early so that the heap can be initialized.
 	heapStart = uintptr(unsafe.Pointer(&heapStartSymbol))
 	heapEnd = uintptr(wasm_memory_size(0) * wasmPageSize)
 
 	run()
+
+	wasmRunning = false
 }
 
 var handleEvent func()
@@ -24,15 +31,27 @@ func setEventHandler(fn func()) {
 
 //export resume
 func resume() {
+	prevRunning := wasmRunning
+	wasmRunning = true
 	go func() {
 		handleEvent()
 	}()
-	scheduler()
+	if !prevRunning {
+		// Nothing is currently running, so we can safely invoke the scheduler.
+		scheduler()
+	}
+	wasmRunning = prevRunning
 }
 
 //export go_scheduler
 func go_scheduler() {
+	if wasmRunning {
+		return
+	}
+
+	wasmRunning = true
 	scheduler()
+	wasmRunning = false
 }
 
 func ticksToNanoseconds(ticks timeUnit) int64 {

src/runtime/scheduler.go

@@ -116,9 +116,18 @@ func addSleepTask(t *task.Task, duration timeUnit) {
 	*q = t
 }
 
+var schedulerRunning bool
+
 // Run the scheduler until all tasks have finished.
 func scheduler() {
 	// Main scheduler loop.
+	if schedulerDebug {
+		if schedulerRunning {
+			runtimePanic("nested scheduler")
+		}
+
+		schedulerRunning = true
+	}
 	var now timeUnit
 	for !schedulerDone {
 		scheduleLog("")
@@ -143,7 +152,7 @@ func scheduler() {
 			if sleepQueue == nil {
 				if asyncScheduler {
 					// JavaScript is treated specially, see below.
-					return
+					break
 				}
 				waitForEvents()
 				continue
@@ -170,6 +179,9 @@ func scheduler() {
 		scheduleLogTask("  run:", t)
 		t.Resume()
 	}
+	if schedulerDebug {
+		schedulerRunning = false
+	}
 }
 
 func Gosched() {