Internals: Scheduler: Fix infinite loop in flush (sveltejs#4316)

src/runtime/internal/scheduler.ts

@@ -31,8 +31,8 @@ export function add_flush_callback(fn) {
 	flush_callbacks.push(fn);
 }
 
+const seen_callbacks = new Set();
 export function flush() {
-	const seen_callbacks = new Set();
 
 	do {
 		// first, call beforeUpdate functions
@@ -52,10 +52,10 @@ export function flush() {
 			const callback = render_callbacks[i];
 
 			if (!seen_callbacks.has(callback)) {
-				callback();
-
 				// ...so guard against infinite loops
 				seen_callbacks.add(callback);
+
+				callback();
 			}
 		}
 
@@ -67,6 +67,7 @@ export function flush() {
 	}
 
 	update_scheduled = false;
+	seen_callbacks.clear();
 }
 
 function update($$) {

test/runtime/samples/lifecycle-onmount-infinite-loop/Child.svelte

@@ -0,0 +1 @@
+Child

test/runtime/samples/lifecycle-onmount-infinite-loop/_config.js

@@ -0,0 +1,6 @@
+export default {
+	test({ assert, component }) {
+		const { count } = component;
+		assert.deepEqual(count, 1);
+	}
+};

test/runtime/samples/lifecycle-onmount-infinite-loop/main.svelte

@@ -0,0 +1,16 @@
+<script>
+	import { onMount } from 'svelte';
+	import Child from './Child.svelte';
+
+	let root;
+	export let count = 0;
+
+	onMount(() => {
+		if (count < 5) {
+			count++;
+			new Child({ target: root });
+		}
+	});
+</script>
+
+<div bind:this={root}></div>