fix: ignore fork discard() after commit() (#17034)

* fix: ignore fork `discard()` after `commit()`

* fix message

Author: Rich-Harris

.changeset/twenty-onions-attack.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ignore fork `discard()` after `commit()`

documentation/docs/98-reference/.generated/client-errors.md

@@ -149,7 +149,7 @@ This restriction only applies when using the `experimental.async` option, which
 ### fork_discarded
 
 ```
-Cannot commit a fork that was already committed or discarded
+Cannot commit a fork that was already discarded
 ```
 
 ### fork_timing

packages/svelte/messages/client-errors/errors.md

@@ -114,7 +114,7 @@ This restriction only applies when using the `experimental.async` option, which
 
 ## fork_discarded
 
-> Cannot commit a fork that was already committed or discarded
+> Cannot commit a fork that was already discarded
 
 ## fork_timing
 

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

@@ -262,12 +262,12 @@ export function flush_sync_in_effect() {
 }
 
 /**
- * Cannot commit a fork that was already committed or discarded
+ * Cannot commit a fork that was already discarded
  * @returns {never}
  */
 export function fork_discarded() {
 	if (DEV) {
-		const error = new Error(`fork_discarded\nCannot commit a fork that was already committed or discarded\nhttps://svelte.dev/e/fork_discarded`);
+		const error = new Error(`fork_discarded\nCannot commit a fork that was already discarded\nhttps://svelte.dev/e/fork_discarded`);
 
 		error.name = 'Svelte error';
 

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

@@ -913,28 +913,36 @@ export function fork(fn) {
 		e.fork_timing();
 	}
 
-	const batch = Batch.ensure();
+	var batch = Batch.ensure();
 	batch.is_fork = true;
 
-	const settled = batch.settled();
+	var committed = false;
+	var settled = batch.settled();
 
 	flushSync(fn);
 
 	// revert state changes
-	for (const [source, value] of batch.previous) {
+	for (var [source, value] of batch.previous) {
 		source.v = value;
 	}
 
 	return {
 		commit: async () => {
+			if (committed) {
+				await settled;
+				return;
+			}
+
 			if (!batches.has(batch)) {
 				e.fork_discarded();
 			}
 
+			committed = true;
+
 			batch.is_fork = false;
 
 			// apply changes
-			for (const [source, value] of batch.current) {
+			for (var [source, value] of batch.current) {
 				source.v = value;
 			}
 
@@ -945,9 +953,9 @@ export function fork(fn) {
 			// TODO maybe there's a better implementation?
 			flushSync(() => {
 				/** @type {Set<Effect>} */
-				const eager_effects = new Set();
+				var eager_effects = new Set();
 
-				for (const source of batch.current.keys()) {
+				for (var source of batch.current.keys()) {
 					mark_eager_effects(source, eager_effects);
 				}
 
@@ -959,7 +967,7 @@ export function fork(fn) {
 			await settled;
 		},
 		discard: () => {
-			if (batches.has(batch)) {
+			if (!committed && batches.has(batch)) {
 				batches.delete(batch);
 				batch.discard();
 			}