fix: don't rerun async effects unnecessarily

.changeset/wild-mirrors-take.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: don't rerun async effects unnecessarily

packages/svelte/src/internal/client/reactivity/batch.js

@@ -1,4 +1,4 @@
-/** @import { Derived, Effect, Source, Value } from '#client' */
+/** @import { Derived, Effect, Reaction, Source, Value } from '#client' */
 import {
 	BLOCK_EFFECT,
 	BRANCH_EFFECT,
@@ -335,31 +335,43 @@ export class Batch {
 					continue;
 				}
 
+				/** @type {Source[]} */
+				const sources = [];
+
 				for (const [source, value] of this.current) {
 					if (batch.current.has(source)) {
-						if (is_earlier) {
+						if (is_earlier && value !== batch.current.get(source)) {
 							// bring the value up to date
 							batch.current.set(source, value);
 						} else {
-							// later batch has more recent value,
+							// same value or later batch has more recent value,
 							// no need to re-run these effects
 							continue;
 						}
 					}
 
-					mark_effects(source);
+					sources.push(source);
 				}
 
-				if (queued_root_effects.length > 0) {
-					current_batch = batch;
-					const revert = Batch.apply(batch);
-
-					for (const root of queued_root_effects) {
-						batch.#traverse_effect_tree(root);
+				// Only reschedule an async effect if it depends on something else than
+				// the sources that were updated in this batch and that something else changed.
+				const others = [...batch.current.keys()].filter((s) => !this.current.has(s));
+				if (others.length > 0) {
+					for (const source of sources) {
+						mark_effects(source, others);
 					}
 
-					queued_root_effects = [];
-					revert();
+					if (queued_root_effects.length > 0) {
+						current_batch = batch;
+						const revert = Batch.apply(batch);
+
+						for (const root of queued_root_effects) {
+							batch.#traverse_effect_tree(root);
+						}
+
+						queued_root_effects = [];
+						revert();
+					}
 				}
 			}
 
@@ -640,24 +652,46 @@ function flush_queued_effects(effects) {
 
 /**
  * This is similar to `mark_reactions`, but it only marks async/block effects
- * so that these can re-run after another batch has been committed
+ * depending on one of the sources, so that these effects can re-run after
+ * another batch has been committed
  * @param {Value} value
+ * @param {Source[]} sources
  */
-function mark_effects(value) {
+function mark_effects(value, sources) {
 	if (value.reactions !== null) {
 		for (const reaction of value.reactions) {
 			const flags = reaction.f;
 
 			if ((flags & DERIVED) !== 0) {
-				mark_effects(/** @type {Derived} */ (reaction));
-			} else if ((flags & (ASYNC | BLOCK_EFFECT)) !== 0) {
+				mark_effects(/** @type {Derived} */ (reaction), sources);
+			} else if ((flags & (ASYNC | BLOCK_EFFECT)) !== 0 && depends_on(reaction, sources)) {
 				set_signal_status(reaction, DIRTY);
 				schedule_effect(/** @type {Effect} */ (reaction));
 			}
 		}
 	}
 }
 
+/**
+ * @param {Reaction} reaction
+ * @param {Source[]} sources
+ */
+function depends_on(reaction, sources) {
+	if (reaction.deps !== null) {
+		for (const dep of reaction.deps) {
+			if (sources.includes(dep)) {
+				return true;
+			}
+
+			if ((dep.f & DERIVED) !== 0 && depends_on(/** @type {Derived} */ (dep), sources)) {
+				return true;
+			}
+		}
+	}
+
+	return false;
+}
+
 /**
  * @param {Effect} signal
  * @returns {void}

packages/svelte/src/internal/client/reactivity/deriveds.js

@@ -171,6 +171,13 @@ export function async_derived(fn, location) {
 
 				internal_set(signal, value);
 
+				// All prior async derived runs are now stale
+				for (const [b, d] of deferreds) {
+					deferreds.delete(b);
+					if (b === batch) break;
+					d.reject(STALE_REACTION);
+				}
+
 				if (DEV && location !== undefined) {
 					recent_async_deriveds.add(signal);
 

packages/svelte/tests/runtime-runes/samples/async-resolve-stale/_config.js

@@ -20,7 +20,6 @@ export default test({
 		input.value = '12';
 		input.dispatchEvent(new Event('input', { bubbles: true }));
 		await macrotask(6);
-		// TODO this is wrong (separate bug), this should be 3 | 12
-		assert.htmlEqual(target.innerHTML, '<input> 5 | 12');
+		assert.htmlEqual(target.innerHTML, '<input> 3 | 12');
 	}
 });