From ba0bdc7aa19feb183094aec7b60b7081d3218bf7 Mon Sep 17 00:00:00 2001
From: Simon H <5968653+dummdidumm@users.noreply.github.com>
Date: Fri, 16 Feb 2024 17:41:42 +0100
Subject: [PATCH] chore: document and test signals edge case (#10228)

---
 .../svelte/src/internal/client/runtime.js     |  6 ++++-
 packages/svelte/tests/signals/test.ts         | 27 ++++++++++++++++---
 2 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/packages/svelte/src/internal/client/runtime.js b/packages/svelte/src/internal/client/runtime.js
index cf55be1f5e..c47f0d4638 100644
--- a/packages/svelte/src/internal/client/runtime.js
+++ b/packages/svelte/src/internal/client/runtime.js
@@ -74,7 +74,11 @@ export let current_effect = null;
 /** @type {null | import('./types.js').Signal[]} */
 let current_dependencies = null;
 let current_dependencies_index = 0;
-/** @type {null | import('./types.js').Signal[]} */
+/**
+ * Tracks writes that the effect it's executed in doesn't listen to yet,
+ * so that the dependency can be added to the effect later on if it then reads it
+ * @type {null | import('./types.js').Signal[]}
+ */
 let current_untracked_writes = null;
 /** @type {null | import('./types.js').SignalDebug} */
 let last_inspected_signal = null;
diff --git a/packages/svelte/tests/signals/test.ts b/packages/svelte/tests/signals/test.ts
index 7ba75deb49..d34cd7f06c 100644
--- a/packages/svelte/tests/signals/test.ts
+++ b/packages/svelte/tests/signals/test.ts
@@ -7,7 +7,7 @@ import type { ComputationSignal } from '../../src/internal/client/types';
  * @param fn A function that returns a function because we first need to setup all the signals
  * 			 and then execute the test in order to simulate a real component
  */
-function run_test(runes: boolean, fn: () => () => void) {
+function run_test(runes: boolean, fn: (runes: boolean) => () => void) {
 	return () => {
 		// Create a component context to test runes vs legacy mode
 		$.push({}, runes);
@@ -15,7 +15,7 @@ function run_test(runes: boolean, fn: () => () => void) {
 		let execute: any;
 		const signal = $.render_effect(
 			() => {
-				execute = fn();
+				execute = fn(runes);
 			},
 			null,
 			true,
@@ -27,7 +27,7 @@ function run_test(runes: boolean, fn: () => () => void) {
 	};
 }
 
-function test(text: string, fn: () => any) {
+function test(text: string, fn: (runes: boolean) => any) {
 	it(`${text} (legacy mode)`, run_test(false, fn));
 	it(`${text} (runes mode)`, run_test(true, fn));
 }
@@ -262,4 +262,25 @@ describe('signals', () => {
 			assert.deepEqual(count.c, null);
 		};
 	});
+
+	test('schedules rerun when writing to signal before reading it', (runes) => {
+		if (!runes) return () => {};
+
+		const value = $.source({ count: 0 });
+		$.user_effect(() => {
+			$.set(value, { count: 0 });
+			$.get(value);
+		});
+
+		return () => {
+			let errored = false;
+			try {
+				$.flushSync();
+			} catch (e: any) {
+				assert.include(e.message, 'ERR_SVELTE_TOO_MANY_UPDATES');
+				errored = true;
+			}
+			assert.equal(errored, true);
+		};
+	});
 });