From 2ff38d4cf3f19ab53ae1f734e0e8545fb7035b8a Mon Sep 17 00:00:00 2001
From: David Kondrad <david.a.kondrad@jci.com>
Date: Sat, 25 Jan 2020 12:21:55 -0500
Subject: [PATCH] Internals: Scheduler: Fix infinite loop in flush (#4316)

---
 src/runtime/internal/scheduler.ts                |  7 ++++---
 .../lifecycle-onmount-infinite-loop/Child.svelte |  1 +
 .../lifecycle-onmount-infinite-loop/_config.js   |  6 ++++++
 .../lifecycle-onmount-infinite-loop/main.svelte  | 16 ++++++++++++++++
 4 files changed, 27 insertions(+), 3 deletions(-)
 create mode 100644 test/runtime/samples/lifecycle-onmount-infinite-loop/Child.svelte
 create mode 100644 test/runtime/samples/lifecycle-onmount-infinite-loop/_config.js
 create mode 100644 test/runtime/samples/lifecycle-onmount-infinite-loop/main.svelte

diff --git a/src/runtime/internal/scheduler.ts b/src/runtime/internal/scheduler.ts
index 04c1264ab1d2..1ce255b21772 100644
--- a/src/runtime/internal/scheduler.ts
+++ b/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($$) {
diff --git a/test/runtime/samples/lifecycle-onmount-infinite-loop/Child.svelte b/test/runtime/samples/lifecycle-onmount-infinite-loop/Child.svelte
new file mode 100644
index 000000000000..ef16875b6448
--- /dev/null
+++ b/test/runtime/samples/lifecycle-onmount-infinite-loop/Child.svelte
@@ -0,0 +1 @@
+Child
diff --git a/test/runtime/samples/lifecycle-onmount-infinite-loop/_config.js b/test/runtime/samples/lifecycle-onmount-infinite-loop/_config.js
new file mode 100644
index 000000000000..a76a2686ac27
--- /dev/null
+++ b/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);
+	}
+};
diff --git a/test/runtime/samples/lifecycle-onmount-infinite-loop/main.svelte b/test/runtime/samples/lifecycle-onmount-infinite-loop/main.svelte
new file mode 100644
index 000000000000..1fa4263e39ea
--- /dev/null
+++ b/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>