make await blocks fork-aware

packages/svelte/src/internal/client/dom/blocks/await.js

@@ -11,7 +11,9 @@ import {
 	hydrate_node
 } from '../hydration.js';
 import { queue_micro_task } from '../task.js';
+import { should_defer_append } from '../operations.js';
 import { HYDRATION_START_ELSE, UNINITIALIZED } from '../../../../constants.js';
+import { ASYNC, DESTROYED, EFFECT_PRESERVED } from '#client/constants';
 import { is_runes } from '../../context.js';
 import { Batch, current_batch, flushSync, is_flushing_sync } from '../../reactivity/batch.js';
 import { BranchManager } from './branches.js';
@@ -51,11 +53,38 @@ export function await_block(node, get_input, pending_fn, then_fn, catch_fn) {
 
 	var branches = new BranchManager(node);
 
-	block(() => {
+	// The latest run of the block in the real world, and in each fork. A run is superseded
+	// by later runs in the same world; a fork's run joins the real world once it is committed
+	var current = {};
+	/** @type {Map<Batch, {}>} */
+	var forks = new Map();
+
+	/** @param {Batch} batch */
+	var settle = (batch) => {
+		// committed (as opposed to discarded)?
+		if (!batch.is_fork && forks.has(batch)) current = /** @type {{}} */ (forks.get(batch));
+		forks.delete(batch);
+	};
+
+	var effect = block(() => {
 		var batch = /** @type {Batch} */ (current_batch);
 		var input = get_input();
+		var run = {};
 
-		var destroyed = false;
+		// Like other async work, a fork's promise resolves into the fork, so that its pending/then/catch
+		// state stays in the fork until it's committed — unless the block is terminal (has no dependencies),
+		// in which case it can't re-run and is the same in every batch (see `should_defer_append`)
+		var fork = should_defer_append() && batch.is_fork;
+
+		if (fork) {
+			forks.set(batch, run);
+			batch.oncommit(settle);
+			batch.ondiscard(settle);
+		} else {
+			current = run;
+			// supersedes the runs of forks that have since been committed
+			for (const b of forks.keys()) if (!b.is_fork) forks.delete(b);
+		}
 
 		/** Whether or not there was a hydration mismatch. Needs to be a `let` or else it isn't treeshaken out */
 		// @ts-ignore coercing `node` to a `Comment` causes TypeScript and Prettier to fight
@@ -75,22 +104,32 @@ export function await_block(node, get_input, pending_fn, then_fn, catch_fn) {
 			 * @param {() => void} fn
 			 */
 			const resolve = (fn) => {
-				if (destroyed) return;
+				if ((effect.f & DESTROYED) !== 0 || (run !== current && forks.get(batch) !== run)) return;
 
 				resolved = true;
-				// We don't want to restore the previous batch here; {#await} blocks don't follow the async logic
-				// we have elsewhere, instead pending/resolve/fail states are each their own batch so to speak.
-				restore(false);
-				// ...but it might still be set here. That means a `save(...)` has restored it — but that batch will
-				// likely already have been committed by the time it resolves, and this resolve should be processed
-				// in a separate batch. We're not using batch.deactivate()/activate() above because get_input()
-				// could write to sources, which would then incorrectly create a new batch or could mess with
-				// async_derived expecting a current_batch to exist.
-				if (current_batch === batch) {
-					batch.deactivate();
+				// a committed fork's run is now the real world's run (no-op for real runs)
+				if (!batch.is_fork) current = run;
+
+				// A fork's promise resolves into the fork — and once committed, into the batch the fork became (or was
+				// merged into) while that is still pending, so that it lands along with the rest of the fork's state
+				restore(fork);
+				var pending = fork && /** @type {Batch} */ (current_batch).linked;
+
+				if (!pending) {
+					// Otherwise, we don't want to restore the previous batch here; {#await} blocks don't follow the async
+					// logic we have elsewhere, instead pending/resolve/fail states are each their own batch so to speak.
+					// ...but it might still be set here. That means a `save(...)` has restored it — but that batch will
+					// likely already have been committed by the time it resolves, and this resolve should be processed
+					// in a separate batch. We're not using batch.deactivate()/activate() above because get_input()
+					// could write to sources, which would then incorrectly create a new batch or could mess with
+					// async_derived expecting a current_batch to exist.
+					if (fork || current_batch === batch) {
+						current_batch?.deactivate();
+					}
 				}
+
 				// Make sure we have a batch, since the branch manager expects one to exist
-				Batch.ensure();
+				var target = Batch.ensure();
 
 				try {
 					fn();
@@ -98,8 +137,10 @@ export function await_block(node, get_input, pending_fn, then_fn, catch_fn) {
 					unset_context(false);
 
 					// without this, the DOM does not update until two ticks after the promise
-					// resolves, which is unexpected behaviour (and somewhat irksome to test)
-					if (!is_flushing_sync) flushSync();
+					// resolves, which is unexpected behaviour (and somewhat irksome to test).
+					// A pending batch is flushed directly, so that no other work runs while it is active
+					if (pending) target.flush();
+					else if (!is_flushing_sync) flushSync();
 				}
 			};
 
@@ -116,8 +157,8 @@ export function await_block(node, get_input, pending_fn, then_fn, catch_fn) {
 						branches.ensure(CATCH, catch_fn && ((target) => catch_fn(target, error)));
 
 						if (!catch_fn) {
-							// Rethrow the error if no catch block exists
-							throw error.v;
+							// Rethrow the error if no catch block exists (in a fork, `error.v` isn't it)
+							throw e;
 						}
 					});
 				}
@@ -145,9 +186,11 @@ export function await_block(node, get_input, pending_fn, then_fn, catch_fn) {
 			// continue in hydration mode
 			set_hydrating(true);
 		}
+	}, EFFECT_PRESERVED); // branches are created asynchronously: keep the effect even without dependencies
 
-		return () => {
-			destroyed = true;
-		};
-	});
+	// Like the results of async deriveds, the value/error are those of the batch whose run produced
+	// them: the real world writing them must not overtake a fork's own result (see `Batch#overtake`)
+	value.e = error.e = effect;
+	value.f |= ASYNC;
+	error.f |= ASYNC;
 }

packages/svelte/tests/runtime-runes/samples/async-fork-await-block-deferred-commit/_config.js

@@ -0,0 +1,63 @@
+import { fork, tick } from 'svelte';
+import { test } from '../../test';
+
+// A fork that is committed while it still waits for other async work (here: `<p>`) is a pending real batch:
+// its {#await} promise is the real world's promise from then on, and as long as the batch is pending,
+// the promise's states land along with the rest of the fork's state. Once the batch is done, they
+// are their own batches again, like for any real update
+export default test({
+	mode: ['client'],
+	async test({ assert, target, instance }) {
+		instance.settle('p', 0);
+		await tick();
+		instance.settle('aw', 0);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<p>0</p><q>0</q>');
+
+		// resolves before the batch is done; the real world's older promise is obsolete once the fork is committed
+		instance.set({ a: 1 });
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<p>0</p><i>pending</i>');
+		let f = fork(() => instance.set({ b: 10 }));
+		await tick();
+		f.commit();
+		await tick();
+		instance.settle('aw', 11);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<p>0</p><i>pending</i>');
+		instance.settle('aw', 1);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<p>0</p><i>pending</i>');
+		instance.settle('p', 10);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<p>10</p><q>11</q>');
+
+		// resolves after the batch is done
+		f = fork(() => instance.set({ b: 20 }));
+		await tick();
+		f.commit();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<p>10</p><q>11</q>');
+		instance.settle('p', 20);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<p>20</p><i>pending</i>');
+		instance.settle('aw', 21);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<p>20</p><q>21</q>');
+
+		// a real update after the commit supersedes the fork's promise
+		f = fork(() => instance.set({ b: 30 }));
+		await tick();
+		f.commit();
+		await tick();
+		instance.set({ a: 2 });
+		await tick();
+		instance.settle('aw', 31);
+		await tick();
+		instance.settle('aw', 32);
+		await tick();
+		instance.settle('p', 30);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<p>30</p><q>32</q>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-fork-await-block-deferred-commit/main.svelte

@@ -0,0 +1,29 @@
+<script>
+	let a = $state(0);
+	let b = $state(0);
+
+	/** @type {Map<string, Array<() => void>>} */
+	const registry = new Map();
+
+	function f(tag, value) {
+		return new Promise((resolve) => {
+			const key = tag + value;
+			if (!registry.has(key)) registry.set(key, []);
+			registry.get(key).push(() => resolve(value));
+		});
+	}
+
+	export function set(values) {
+		if ('a' in values) a = values.a;
+		if ('b' in values) b = values.b;
+	}
+
+	/** resolve the promises created for `tag` and `value` */
+	export function settle(tag, value) {
+		for (const r of registry.get(tag + value) ?? []) r();
+		registry.delete(tag + value);
+	}
+</script>
+
+<p>{await f('p', b)}</p>
+{#await f('aw', a + b)}<i>pending</i>{:then v}<q>{v}</q>{/await}

packages/svelte/tests/runtime-runes/samples/async-fork-await-block-interleaved/_config.js

@@ -0,0 +1,40 @@
+import { fork, tick } from 'svelte';
+import { test } from '../../test';
+
+// Resolving a fork's {#await} promise must not interfere with real work happening at the same time
+export default test({
+	mode: ['client'],
+	async test({ assert, target, instance }) {
+		instance.settle('a', 0);
+		instance.settle('b', 0);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>0</q><s>0|0</s>');
+
+		// the real update's pending state is queued while the fork's promise resolves:
+		// it must not end up in the fork
+		let f = fork(() => instance.set({ b: 1 }));
+		await tick();
+		instance.set({ a: 1 });
+		instance.settle('b', 1);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<i>pending</i><s>0|1</s>');
+		f.discard();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<i>pending</i><s>0|1</s>');
+		instance.settle('a', 1);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>1</q><s>0|1</s>');
+
+		// a real update right after the fork's promise resolved (in the fork) must still show
+		f = fork(() => instance.set({ b: 2 }));
+		await tick();
+		instance.latest('b').then(() => instance.set({ a: 5 }));
+		instance.settle('b', 2);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<i>pending</i><s>0|5</s>');
+		f.discard();
+		instance.settle('a', 5);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>5</q><s>0|5</s>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-fork-await-block-interleaved/main.svelte

@@ -0,0 +1,38 @@
+<script>
+	let a = $state(0);
+	let b = $state(0);
+
+	/** @type {Map<string, Array<() => void>>} */
+	const registry = new Map();
+	/** @type {Map<string, Promise<number>>} */
+	const promises = new Map();
+
+	function f(tag, value) {
+		const promise = new Promise((resolve) => {
+			const key = tag + value;
+			if (!registry.has(key)) registry.set(key, []);
+			registry.get(key).push(() => resolve(value));
+		});
+		promises.set(tag, promise);
+		return promise;
+	}
+
+	export function set(values) {
+		if ('a' in values) a = values.a;
+		if ('b' in values) b = values.b;
+	}
+
+	/** resolve the promises created for `tag` and `value` */
+	export function settle(tag, value) {
+		for (const r of registry.get(tag + value) ?? []) r();
+		registry.delete(tag + value);
+	}
+
+	/** the latest promise created for `tag` */
+	export function latest(tag) {
+		return promises.get(tag);
+	}
+</script>
+
+{#await f('a', a)}<i>pending</i>{:then v}<q>{v}</q>{/await}
+{#await f('b', b)}<i>pending</i>{:then v}<s>{v}|{a}</s>{/await}

packages/svelte/tests/runtime-runes/samples/async-fork-await-block-pending/_config.js

@@ -0,0 +1,136 @@
+import { fork, tick } from 'svelte';
+import { test } from '../../test';
+
+// The pending state of a fork's {#await} promise, and its result, must not show in the real world unless
+// the fork is committed. Real updates must keep working while a fork is alive, and a promise of the real
+// world must still resolve in the real world when a fork re-runs the block
+export default test({
+	mode: ['client'],
+	async test({ assert, target, instance }) {
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<i>pending</i>');
+		instance.settle(0);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>0</q>');
+
+		// pending and resolved in the fork, then discarded
+		let f = fork(() => instance.set({ b: 1 }));
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>0</q>');
+		instance.settle(1);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>0</q>');
+		f.discard();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>0</q>');
+
+		// rejected in the fork, then discarded
+		f = fork(() => instance.set({ b: -1 }));
+		instance.settle(-1);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>0</q>');
+		f.discard();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>0</q>');
+
+		// For comparison, a real update: pending, then resolved
+		instance.set({ b: 2 });
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<i>pending</i>');
+		instance.settle(2);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>2</q>');
+
+		// committed while pending: shows the pending state, just like a real update would
+		f = fork(() => instance.set({ b: 3 }));
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>2</q>');
+		await f.commit();
+		assert.htmlEqual(target.innerHTML, '<i>pending</i>');
+		instance.settle(3);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>3</q>');
+
+		// a real update while the fork is pending (a + b: real 4 + 3, fork 4 + 10)
+		f = fork(() => instance.set({ b: 10 }));
+		await tick();
+		instance.set({ a: 4 });
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<i>pending</i>');
+		instance.settle(14);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<i>pending</i>');
+		instance.settle(7);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>7</q>');
+		instance.settle(10); // the fork's obsolete promise (from before it saw a = 4)
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>7</q>');
+		await f.commit();
+		assert.htmlEqual(target.innerHTML, '<q>14</q>');
+
+		// a real update while the fork is pending, then discarded (real 5 + 10, fork 5 + 20)
+		f = fork(() => instance.set({ b: 20 }));
+		await tick();
+		instance.set({ a: 5 });
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<i>pending</i>');
+		instance.settle(15);
+		instance.settle(25);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>15</q>');
+		f.discard();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>15</q>');
+		instance.settle(24);
+
+		// a real promise that is still pending when a fork re-runs the block must resolve in the real world
+		instance.set({ a: 6 });
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<i>pending</i>');
+		f = fork(() => instance.set({ b: 30 }));
+		await tick();
+		instance.settle(16);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>16</q>');
+		f.discard();
+		instance.settle(36);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>16</q>');
+
+		// ...also when the fork's promise resolves first, and the fork is committed afterwards
+		instance.set({ a: 7 });
+		await tick();
+		f = fork(() => instance.set({ b: 40 }));
+		instance.settle(47);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<i>pending</i>');
+		instance.settle(17);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>17</q>');
+		await f.commit();
+		assert.htmlEqual(target.innerHTML, '<q>47</q>');
+
+		// ...but once the fork is committed, the real world's older promise is obsolete
+		instance.set({ a: 8 });
+		await tick();
+		f = fork(() => instance.set({ b: 50 }));
+		await tick();
+		await f.commit();
+		assert.htmlEqual(target.innerHTML, '<i>pending</i>');
+		instance.settle(48);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<i>pending</i>');
+		instance.settle(58);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>58</q>');
+
+		// rejected in the fork, then committed
+		f = fork(() => instance.set({ b: -60 }));
+		instance.settle(-52);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>58</q>');
+		await f.commit();
+		assert.htmlEqual(target.innerHTML, '<s>-52</s>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-fork-await-block-pending/main.svelte

@@ -0,0 +1,27 @@
+<script>
+	let a = $state(0);
+	let b = $state(0);
+
+	/** @type {Map<number, Array<() => void>>} */
+	const registry = new Map();
+
+	function f(value) {
+		return new Promise((resolve, reject) => {
+			if (!registry.has(value)) registry.set(value, []);
+			registry.get(value).push(() => (value < 0 ? reject(value) : resolve(value)));
+		});
+	}
+
+	export function set(values) {
+		if ('a' in values) a = values.a;
+		if ('b' in values) b = values.b;
+	}
+
+	/** resolve the promises created for `value` */
+	export function settle(value) {
+		for (const r of registry.get(value) ?? []) r();
+		registry.delete(value);
+	}
+</script>
+
+{#await f(a + b)}<i>pending</i>{:then v}<q>{v}</q>{:catch e}<s>{e}</s>{/await}

packages/svelte/tests/runtime-runes/samples/async-fork-await-block-real-pending/_config.js

@@ -0,0 +1,23 @@
+import { fork, tick } from 'svelte';
+import { test } from '../../test';
+
+// A fork re-running an {#await} block must not make the real world's pending promise obsolete
+export default test({
+	mode: ['client'],
+	async test({ assert, target, instance }) {
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<i>pending</i>');
+
+		const f = fork(() => instance.set_b(1));
+		await tick();
+
+		instance.settle(0);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>0</q>');
+
+		f.discard();
+		instance.settle(1);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>0</q>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-fork-await-block-real-pending/main.svelte

@@ -0,0 +1,25 @@
+<script>
+	let b = $state(0);
+
+	/** @type {Map<number, Array<() => void>>} */
+	const registry = new Map();
+
+	function f(value) {
+		return new Promise((resolve) => {
+			if (!registry.has(value)) registry.set(value, []);
+			registry.get(value).push(() => resolve(value));
+		});
+	}
+
+	export function set_b(v) {
+		b = v;
+	}
+
+	/** resolve the promises created for `value` */
+	export function settle(value) {
+		for (const r of registry.get(value) ?? []) r();
+		registry.delete(value);
+	}
+</script>
+
+{#await f(b)}<i>pending</i>{:then v}<q>{v}</q>{/await}

packages/svelte/tests/runtime-runes/samples/async-fork-await-block-static/_config.js

@@ -0,0 +1,25 @@
+import { fork, tick } from 'svelte';
+import { test } from '../../test';
+
+// An {#await} block without dependencies can't re-run, so it is the same in every batch: if it's
+// created in a fork, a real batch that adopts the fork's branch must see its states as well
+export default test({
+	mode: ['client'],
+	async test({ assert, target, instance }) {
+		const f = fork(() => instance.set({ show: true, x: 1 }));
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<p>0</p>');
+
+		instance.set({ show: true });
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<i>pending</i><p>0</p>');
+
+		instance.settle(5);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>5</q><p>0</p>');
+
+		f.discard();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>5</q><p>0</p>');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-fork-await-block-static/main.svelte

@@ -0,0 +1,19 @@
+<script>
+	let show = $state(false);
+	let x = $state(0);
+
+	/** @type {(value: number) => void} */
+	let resolve;
+	const promise = new Promise((r) => (resolve = r));
+
+	export function set(values) {
+		if ('show' in values) show = values.show;
+		if ('x' in values) x = values.x;
+	}
+
+	export function settle(value) {
+		resolve(value);
+	}
+</script>
+
+{#if show}{#await promise}<i>pending</i>{:then v}<q>{v}</q>{/await}{/if}<p>{x}</p>

packages/svelte/tests/runtime-runes/samples/async-fork-await-block/_config.js

@@ -0,0 +1,89 @@
+import { fork, tick } from 'svelte';
+import { test } from '../../test';
+
+// A fork's {#await} results (then and catch) must not end up in the real world
+// unless the fork is committed
+export default test({
+	mode: ['client'],
+	async test({ assert, target, instance }) {
+		instance.resolve_all();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>0</q>');
+
+		// resolved, then discarded
+		let f = fork(() => instance.set_b(2));
+		instance.resolve_all();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>0</q>');
+		f.discard();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>0</q>');
+		assert.equal(instance.pending(), 0); // no refetch in the real world
+
+		// rejected, then discarded
+		f = fork(() => instance.set_b(-1));
+		instance.resolve_all();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>0</q>');
+		f.discard();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>0</q>');
+
+		// resolved, then committed
+		f = fork(() => instance.set_b(3));
+		instance.resolve_all();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>0</q>');
+		await f.commit();
+		assert.htmlEqual(target.innerHTML, '<q>3</q>');
+		assert.equal(instance.pending(), 0);
+
+		// rejected, then committed
+		f = fork(() => instance.set_b(-2));
+		instance.resolve_all();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>3</q>');
+		await f.commit();
+		assert.htmlEqual(target.innerHTML, '<s>-2</s>');
+
+		// For comparison, a real update: without a pending snippet, the previous
+		// content is removed (a microtask later), and the result shows once it resolves
+		instance.set_b(4);
+		await tick();
+		assert.htmlEqual(target.innerHTML, '');
+		instance.resolve_all();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>4</q>');
+
+		// committed while pending (after the pending state was reached in the fork):
+		// the real world is in the same state as after a real update
+		f = fork(() => instance.set_b(5));
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>4</q>');
+		await f.commit();
+		assert.htmlEqual(target.innerHTML, '');
+		instance.resolve_all();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>5</q>');
+		assert.equal(instance.pending(), 0);
+
+		// committed synchronously (`fork` flushes synchronously, which reaches the pending state)
+		f = fork(() => instance.set_b(6));
+		assert.htmlEqual(target.innerHTML, '<q>5</q>');
+		f.commit();
+		assert.htmlEqual(target.innerHTML, '');
+		instance.resolve_all();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>6</q>');
+		assert.equal(instance.pending(), 0);
+
+		// discarded while pending, resolved afterwards
+		f = fork(() => instance.set_b(7));
+		await tick();
+		f.discard();
+		instance.resolve_all();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<q>6</q>');
+		assert.equal(instance.pending(), 0);
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-fork-await-block/main.svelte

@@ -0,0 +1,24 @@
+<script>
+	let b = $state(0);
+	const registry = [];
+
+	function f(value) {
+		return new Promise((resolve, reject) =>
+			registry.push(() => (value < 0 ? reject(value) : resolve(value)))
+		);
+	}
+
+	export function set_b(v) {
+		b = v;
+	}
+
+	export function resolve_all() {
+		for (const r of registry.splice(0)) r();
+	}
+
+	export function pending() {
+		return registry.length;
+	}
+</script>
+
+{#await f(b) then v}<q>{v}</q>{:catch e}<s>{e}</s>{/await}