wasm: don't block //go:wasmexport because of running goroutines

This fixes bug https://github.com/tinygo-org/tinygo/issues/4874.

src/runtime/runtime_wasmentry.go

@@ -92,6 +92,10 @@ func wasmExportRun(done *bool) {
 //
 // This function is not called when the scheduler is disabled.
 func wasmExportExit() {
+	// Signal to the scheduler that it should return, since this call to a
+	// //go:wasmexport function has exited.
+	schedulerExit = true
+
 	task.Pause()
 
 	// TODO: we could cache the allocated stack so we don't have to keep

src/runtime/scheduler_cooperative.go

@@ -30,6 +30,10 @@ const hasParallelism = false
 // Set to true after main.main returns.
 var mainExited bool
 
+// Set to true when the scheduler should exit after the next switch to the
+// scheduler. This is a special case for //go:wasmexport.
+var schedulerExit bool
+
 // Queues used by the scheduler.
 var (
 	runqueue           task.Queue
@@ -214,6 +218,13 @@ func scheduler(returnAtDeadlock bool) {
 		// Run the given task.
 		scheduleLogTask("  run:", t)
 		t.Resume()
+
+		// The last call to Resume() was a signal to stop the scheduler since a
+		// //go:wasmexport function returned.
+		if GOARCH == "wasm" && schedulerExit {
+			schedulerExit = false // reset the signal
+			return
+		}
 	}
 }
 

src/runtime/scheduler_none.go

@@ -12,6 +12,9 @@ const hasParallelism = false
 // Set to true after main.main returns.
 var mainExited bool
 
+// dummy flag, not used without scheduler
+var schedulerExit bool
+
 // run is called by the program entry point to execute the go program.
 // With the "none" scheduler, init and the main function are invoked directly.
 func run() {