fix: correctly handle proxied signal writes before reads (#10612)

* fix: correctly handle proxied signal writes before reads

* managed

* Update packages/svelte/src/internal/client/proxy.js

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

.changeset/rude-ghosts-tickle.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: correctly handle proxied signal writes before reads

packages/svelte/src/internal/client/proxy.js

@@ -6,7 +6,8 @@ import {
 	updating_derived,
 	batch_inspect,
 	current_component_context,
-	set_ignore_mutation_validation
+	set_ignore_mutation_validation,
+	untrack
 } from './runtime.js';
 import { effect_active } from './reactivity/computations.js';
 import {
@@ -261,10 +262,21 @@ const state_proxy_handler = {
 		return has;
 	},
 
-	set(target, prop, value) {
+	set(target, prop, value, receiver) {
 		const metadata = target[STATE_SYMBOL];
-		const s = metadata.s.get(prop);
-		if (s !== undefined) set(s, proxy(value, metadata.i, metadata.o));
+		let s = metadata.s.get(prop);
+		// If we haven't yet created a source for this property, we need to ensure
+		// we do so otherwise if we read it later, then the write won't be tracked and
+		// the heuristics of effects will be different vs if we had read the proxied
+		// object property before writing to that property.
+		if (s === undefined && effect_active()) {
+			// the read creates a signal
+			untrack(() => receiver[prop]);
+			s = metadata.s.get(prop);
+		}
+		if (s !== undefined) {
+			set(s, proxy(value, metadata.i, metadata.o));
+		}
 		const is_array = metadata.a;
 		const not_has = !(prop in target);
 

packages/svelte/src/internal/client/runtime.js

@@ -744,6 +744,7 @@ export function get(signal) {
 			current_untracked_writes !== null &&
 			current_effect !== null &&
 			(current_effect.f & CLEAN) !== 0 &&
+			(current_effect.f & MANAGED) === 0 &&
 			current_untracked_writes.includes(signal)
 		) {
 			set_signal_status(current_effect, DIRTY);
@@ -975,7 +976,8 @@ export function set_signal_value(signal, value) {
 			!ignore_mutation_validation &&
 			current_effect !== null &&
 			current_effect.c === null &&
-			(current_effect.f & CLEAN) !== 0
+			(current_effect.f & CLEAN) !== 0 &&
+			(current_effect.f & MANAGED) === 0
 		) {
 			if (current_dependencies !== null && current_dependencies.includes(signal)) {
 				set_signal_status(current_effect, DIRTY);

packages/svelte/tests/signals/test.ts

@@ -8,6 +8,7 @@ import {
 } from '../../src/internal/client/reactivity/computations';
 import { source } from '../../src/internal/client/reactivity/sources';
 import type { ComputationSignal } from '../../src/internal/client/types';
+import { proxy } from '../../src/internal/client/proxy';
 
 /**
  * @param runes runes mode
@@ -333,4 +334,25 @@ describe('signals', () => {
 			assert.equal(errored, true);
 		};
 	});
+
+	test('schedules rerun when writing to signal before reading it', (runes) => {
+		if (!runes) return () => {};
+
+		const value = proxy({ arr: [] });
+		user_effect(() => {
+			value.arr = [];
+			value.arr;
+		});
+
+		return () => {
+			let errored = false;
+			try {
+				$.flushSync();
+			} catch (e: any) {
+				assert.include(e.message, 'ERR_SVELTE_TOO_MANY_UPDATES');
+				errored = true;
+			}
+			assert.equal(errored, true);
+		};
+	});
 });