fix: preload hangs forever after cleanup (#1576) (#1606)

* test: cover re-preloading a live query after cleanup

Add a regression test asserting that a live query loads its data again
when it is preloaded after the live query and its source collection were
cleaned up (e.g. when switching data sets at runtime).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* fix: reset live query error state on sync restart so preload recovers after cleanup (#1576)

A source collection that is cleaned up while a live query depends on it
pushes the live query into an error state and latches `isInErrorState`.
That flag was never reset, so when sync restarted via preload() after
cleanup (e.g. switching profiles without a page refresh),
updateLiveQueryStatus() returned early, markReady() was never called and
the preload promise hung forever.

Reset `isInErrorState` at the start of each sync session so the live
query can become ready again.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: kevin-dp

.changeset/preload-after-cleanup-1576.md

@@ -0,0 +1,7 @@
+---
+'@tanstack/db': patch
+---
+
+Fix live query `preload()` hanging forever after a source collection was cleaned up (#1576)
+
+When a source collection is cleaned up while a live query depends on it, the live query transitions to an error state and latches an internal `isInErrorState` flag. That flag was never reset, so restarting sync (e.g. calling `preload()` again after cleanup when switching profiles) left the live query unable to become ready and the returned promise never resolved. The flag is now cleared at the start of each sync session so the live query can recover.

packages/db/src/query/live/collection-config-builder.ts

@@ -596,6 +596,8 @@ export class CollectionConfigBuilder<
   private syncFn(config: SyncMethods<TResult>) {
     // Store reference to the live query collection for error state transitions
     this.liveQueryCollection = config.collection
+    // Reset error state from any previous sync session so a restarted sync can become ready again.
+    this.isInErrorState = false
     // Store config and syncState as instance properties for the duration of this sync session
     this.currentSyncConfig = config
 

packages/db/tests/query/live-query-collection.test.ts

@@ -594,6 +594,33 @@ describe(`createLiveQueryCollection`, () => {
     finalSubscription.unsubscribe()
   })
 
+  it(`loads its data again when preloaded after the live query and its source collection were cleaned up`, async () => {
+    const activeUsers = createLiveQueryCollection({
+      query: (q) =>
+        q
+          .from({ user: usersCollection })
+          .where(({ user }) => eq(user.active, true)),
+    })
+
+    await activeUsers.preload()
+    expect(activeUsers.status).toBe(`ready`)
+    expect(activeUsers.size).toBe(2)
+
+    // Tear down the source collection and the live query, e.g. when switching
+    // to a different data set at runtime. Cleaning up a source collection puts
+    // the dependent live query into an error state.
+    await usersCollection.cleanup()
+    expect(activeUsers.status).toBe(`error`)
+
+    await activeUsers.cleanup()
+    expect(activeUsers.status).toBe(`cleaned-up`)
+
+    // Preloading again restarts sync and resolves once the data is loaded.
+    await activeUsers.preload()
+    expect(activeUsers.status).toBe(`ready`)
+    expect(activeUsers.size).toBe(2)
+  })
+
   it(`should handle temporal values correctly in live queries`, async () => {
     // Define a type with temporal values
     type Task = {