Fix postgres op_id sequence intialization

.changeset/sixty-melons-bow.md

@@ -0,0 +1,7 @@
+---
+'@powersync/service-module-postgres-storage': patch
+'@powersync/service-core-tests': patch
+'@powersync/service-image': patch
+---
+
+[Postgres Storage] Fix op_id_sequence intialization edge case

modules/module-postgres-storage/src/storage/PostgresSyncRulesStorage.ts

@@ -336,7 +336,7 @@ export class PostgresSyncRulesStorage
 
     await callback(batch);
     await batch.flush();
-    if (batch.last_flushed_op) {
+    if (batch.last_flushed_op != null) {
       return { flushed_op: batch.last_flushed_op };
     } else {
       return null;

modules/module-postgres-storage/src/storage/batch/PostgresBucketBatch.ts

@@ -208,13 +208,7 @@ export class PostgresBucketBatch
       return null;
     }
 
-    const currentSequence = await this.db.sql`
-      SELECT
-        LAST_VALUE AS value
-      FROM
-        op_id_sequence;
-    `.first<{ value: bigint }>();
-    return currentSequence!.value;
+    return this.getLastOpIdSequence(this.db);
   }
 
   async drop(sourceTables: storage.SourceTable[]): Promise<storage.FlushedResult | null> {
@@ -262,13 +256,7 @@ export class PostgresBucketBatch
     const lastOp = await this.withReplicationTransaction(async (db) => {
       resumeBatch = await this.replicateBatch(db, batch);
 
-      const sequence = await db.sql`
-        SELECT
-          LAST_VALUE AS value
-        FROM
-          op_id_sequence;
-      `.first<{ value: bigint }>();
-      return sequence!.value;
+      return this.getLastOpIdSequence(db);
     });
 
     // null if done, set if we need another flush
@@ -895,6 +883,23 @@ export class PostgresBucketBatch
       `.execute();
     }
   }
+
+  private async getLastOpIdSequence(db: lib_postgres.AbstractPostgresConnection) {
+    // When no op_id has been generated, last_value = 1 and nextval() will be 1.
+    // To cater for this case, we check is_called, and default to 0 if no value has been generated.
+    const sequence = await db.sql`
+      SELECT
+        (
+          CASE
+            WHEN is_called THEN last_value
+            ELSE 0
+          END
+        ) AS value
+      FROM
+        op_id_sequence;
+    `.first<{ value: bigint }>();
+    return sequence!.value;
+  }
 }
 
 /**

packages/service-core-tests/src/tests/register-data-storage-tests.ts

@@ -1878,4 +1878,54 @@ bucket_definitions:
       }
     });
   });
+
+  test('op_id initialization edge case', async () => {
+    // Test syncing a batch of data that is small in count,
+    // but large enough in size to be split over multiple returned chunks.
+    // Similar to the above test, but splits over 1MB chunks.
+    const sync_rules = test_utils.testRules(
+      `
+  bucket_definitions:
+    global:
+      data:
+        - SELECT id FROM test
+        - SELECT id FROM test_ignore WHERE false
+  `
+    );
+    await using factory = await generateStorageFactory();
+    const bucketStorage = factory.getInstance(sync_rules);
+
+    const sourceTable = test_utils.makeTestTable('test', ['id']);
+    const sourceTableIgnore = test_utils.makeTestTable('test_ignore', ['id']);
+
+    const result1 = await bucketStorage.startBatch(test_utils.BATCH_OPTIONS, async (batch) => {
+      // This saves a record to current_data, but not bucket_data.
+      // This causes a checkpoint to be created without increasing the op_id sequence.
+      await batch.save({
+        sourceTable: sourceTableIgnore,
+        tag: storage.SaveOperationTag.INSERT,
+        after: {
+          id: 'test1'
+        },
+        afterReplicaId: test_utils.rid('test1')
+      });
+    });
+
+    const checkpoint1 = result1!.flushed_op;
+
+    const result2 = await bucketStorage.startBatch(test_utils.BATCH_OPTIONS, async (batch) => {
+      await batch.save({
+        sourceTable: sourceTable,
+        tag: storage.SaveOperationTag.INSERT,
+        after: {
+          id: 'test2'
+        },
+        afterReplicaId: test_utils.rid('test2')
+      });
+    });
+
+    const checkpoint2 = result2!.flushed_op;
+    // we expect 0n and 1n, or 1n and 2n.
+    expect(checkpoint2).toBeGreaterThan(checkpoint1);
+  });
 }