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();
 }