fix: capacitor iOS Buffer read error (#933)

Author: stevensJourney

.changeset/rude-squids-unite.md

@@ -0,0 +1,5 @@
+---
+'@powersync/capacitor': patch
+---
+
+Fixed "Error in reading buffer" error on iOS when connecting via WebSocket.

packages/capacitor/src/PowerSyncDatabase.ts

@@ -1,5 +1,6 @@
 import { Capacitor } from '@capacitor/core';
 import {
+  BucketStorageAdapter,
   DBAdapter,
   MEMORY_TRIGGER_CLAIM_MANAGER,
   PowerSyncBackendConnector,
@@ -11,8 +12,8 @@ import {
   WebRemote
 } from '@powersync/web';
 import { CapacitorSQLiteAdapter } from './adapter/CapacitorSQLiteAdapter.js';
+import { CapacitorBucketStorageAdapter } from './sync/CapacitorBucketStorageAdapter.js';
 import { CapacitorStreamingSyncImplementation } from './sync/CapacitorSyncImplementation.js';
-
 /**
  * PowerSyncDatabase class for managing database connections and sync implementations.
  * This extends the WebPowerSyncDatabase to provide platform-specific implementations
@@ -69,6 +70,14 @@ export class PowerSyncDatabase extends WebPowerSyncDatabase {
     }
   }
 
+  protected generateBucketStorageAdapter(): BucketStorageAdapter {
+    if (this.isNativeCapacitorPlatform) {
+      return new CapacitorBucketStorageAdapter(this.database, this.logger);
+    } else {
+      return super.generateBucketStorageAdapter();
+    }
+  }
+
   protected generateSyncStreamImplementation(
     connector: PowerSyncBackendConnector,
     options: RequiredAdditionalConnectionOptions

packages/capacitor/src/sync/CapacitorBucketStorageAdapter.ts

@@ -0,0 +1,22 @@
+import { PowerSyncControlCommand, SqliteBucketStorage } from '@powersync/web';
+
+export class CapacitorBucketStorageAdapter extends SqliteBucketStorage {
+  control(op: PowerSyncControlCommand, payload: string | Uint8Array | ArrayBuffer | null): Promise<string> {
+    if (payload instanceof Uint8Array && (payload as any)['_isBuffer'] == true) {
+      /**
+       * The Buffer polyfill, used in @powersync/common, is a Uint8Array subclass which defines additional fields like
+       * `_isBuffer` and `parent` on its `prototype`. The additional fields are serialized when passed through the native bridge.
+       * The Capacitor Community SQLite lib expects a dictionarty of indexes to numerical bytes.
+       * The additiona fields (which are not an index to numerical byte mapping) cause the parsing logic in the SQLite lib to throw an error:
+       *  "Error in reading buffer".
+       *
+       * Re-wrapping the same backing buffer as a plain Uint8Array removes the Buffer subclass wrapper
+       * while keeping the same underlying bytes. This creates a new view, not a byte copy, so the
+       * overhead should be minimal.
+       */
+      payload = new Uint8Array(payload.buffer, payload.byteOffset, payload.byteLength);
+    }
+
+    return super.control(op, payload);
+  }
+}