WIP

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

@@ -134,11 +134,9 @@ export function async_derived(fn, location) {
 				}
 
 				if (fork !== null) {
-					fork?.enable();
+					fork.run(() => {
-					flush_sync(() => {
 						internal_set(signal, v);
 					});
-					fork?.disable();
 				} else {
 					internal_set(signal, v);
 				}

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

@@ -1,5 +1,8 @@
 /** @import { Effect, Source } from '#client' */
 
+import { flush_sync } from '../runtime.js';
+import { internal_set } from './sources.js';
+
 /** @type {Set<Fork>} */
 const forks = new Set();
 
@@ -29,14 +32,64 @@ export class Fork {
 		}
 	}
 
-	enable() {
+	/**
-		active_fork = this;
+	 *
-		// TODO revert other forks
+	 * @param {() => void} fn
+	 */
+	flush(fn) {
+		var values = new Map();
+
+		for (const fork of forks) {
+			if (fork === this) continue;
+
+			for (const [source, previous] of fork.previous) {
+				if (this.previous.has(source)) continue;
+
+				values.set(source, source.v);
+				source.v = previous;
+				// internal_set(source, previous);
+			}
 		}
 
-	disable() {
+		try {
+			fn();
+		} finally {
+			for (const [source, value] of values) {
+				// internal_set(source, value);
+				source.v = value;
+			}
+		}
+	}
+
+	remove() {
+		forks.delete(this);
+
+		for (var fork of forks) {
+			if (fork.id < this.id) {
+				// other fork is older than this
+				for (var source of this.previous.keys()) {
+					fork.previous.delete(source);
+				}
+			} else {
+				// other fork is newer than this
+				for (var source of fork.previous.keys()) {
+					if (this.previous.has(source)) {
+						fork.previous.set(source, source.v);
+					}
+				}
+			}
+		}
+	}
+
+	/**
+	 * @param {() => void} fn
+	 */
+	run(fn) {
+		active_fork = this;
+
+		flush_sync(fn);
+
 		active_fork = null;
-		// TODO restore state
 	}
 
 	increment() {
@@ -52,7 +105,12 @@ export class Fork {
 	}
 
 	static ensure() {
-		return (active_fork ??= new Fork());
+		if (active_fork === null) {
+			active_fork = new Fork();
+			forks.add(active_fork); // TODO figure out where we remove this
+		}
+
+		return active_fork;
 	}
 
 	static unset() {

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

@@ -703,10 +703,12 @@ function flush_queued_root_effects(root_effects) {
 				effect.f ^= CLEAN;
 			}
 
+			var fork = /** @type {Fork} */ (active_fork);
 			var collected_effects = process_effects(effect);
 
-			if (/** @type {Fork} */ (active_fork).settled()) {
+			if (fork.settled()) {
 				flush_queued_effects(collected_effects);
+				fork.remove();
 			}
 		}
 	} finally {