Mutation conformance oracle for sync adapters, and the write bugs it found

.changeset/rxdb-ordered-writes.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/rxdb-db-collection': patch
+---
+
+Persist writes to RxDB in the order the application made them. A write now starts only after the earlier write on the same collection fulfills or rejects. Before, a later write could reach RxDB while an earlier one was still pending. A rejected delete now rejects with RxDB's `RxError` for the write error, as a rejected insert does, instead of with an array of write errors.

.changeset/rxdb-reject-missing-update.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/rxdb-db-collection': patch
+---
+
+Reject an update whose document no longer exists in RxDB, for example because another tab deleted it while the update waited. The update now fails with `UpdateKeyNotFoundError` and its optimistic change rolls back. Before, the adapter skipped it and reported success, so the write was silently lost.

.changeset/rxdb-require-17-6.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/rxdb-db-collection': patch
+---
+
+Require RxDB 17.6.0 or later. In RxDB 17.1.0 to 17.5.0, a storage write that throws stops RxDB's incremental write queue (pubkey/rxdb#9025), so the adapter's update never settles and every later update on the collection waits forever. RxDB 17.6.0 fixes this, and the adapter's update now rejects with the storage error.

.changeset/trailbase-ordered-confirmed-writes.md

@@ -0,0 +1,10 @@
+---
+'@tanstack/trailbase-db-collection': patch
+---
+
+Fix four write bugs.
+
+- Removing a field in an update (for example `draft.note = undefined`) now sends `null`, so TrailBase clears the column. Before, the update omitted the field, and TrailBase kept the old value. The field's `serialize` function is not called for a removal.
+- Writes now reach TrailBase in the order the application made them. A write starts only after TrailBase has answered the earlier write on the same collection. It does not wait for the earlier write's change event.
+- A transaction's update and delete requests now go out one at a time, in mutation order. Before, they went out together, so after one was rejected the next write could start while another request was still unanswered. A rejection now ends the transaction, and its later requests are not sent.
+- A write is now confirmed only by its own change event. Before, an earlier event for the same record could confirm a later write, so its optimistic value dropped and the earlier value showed until the later event arrived. This change also removes the adapter's periodic cleanup timer.

docs/collections/trailbase-collection.md

@@ -52,7 +52,7 @@ The `trailBaseCollectionOptions` function accepts the following options:
 
 - `schema`: [Standard Schema](https://standardschema.dev) compatible schema (e.g., Zod, Effect) for client-side validation
 - `parse`: Object mapping field names to parsing functions that transform data coming from TrailBase
-- `serialize`: Object mapping field names to serialization functions that transform data going to TrailBase
+- `serialize`: Object mapping field names to serialization functions that transform data going to TrailBase. An update that removes a field (for example `draft.tags = undefined`) sends `null`, which clears the column, without calling that field's function. A `NOT NULL` column rejects such an update.
 - `onInsert`: Handler function called when items are inserted
 - `onUpdate`: Handler function called when items are updated
 - `onDelete`: Handler function called when items are deleted