Regression test: deferred FK violations are checked before commit

tests/integration/query_processing/test_transactions.rs

@@ -208,6 +208,37 @@ fn test_constraint_error_aborts_transaction() {
     assert_eq!(row, vec![Value::Integer(0)]);
 }
 
+#[test]
+/// Regression test for https://github.com/tursodatabase/turso/issues/3784 where dirty pages
+/// were flushed to WAL _before_ deferred FK violations were checked. This resulted in the
+/// violations being persisted to the database, even though the transaction was aborted.
+/// This test ensures that dirty pages are not flushed to WAL until after deferred violations are checked.
+fn test_deferred_fk_violation_rollback_in_autocommit() {
+    let tmp_db = TempDatabase::new("test_deferred_fk_violation_rollback.db", true);
+    let conn = tmp_db.connect_limbo();
+
+    // Enable foreign keys
+    conn.execute("PRAGMA foreign_keys = ON").unwrap();
+
+    // Create parent and child tables with deferred FK constraint
+    conn.execute("CREATE TABLE parent(a PRIMARY KEY)").unwrap();
+    conn.execute("CREATE TABLE child(a, b, FOREIGN KEY(b) REFERENCES parent(a) DEFERRABLE INITIALLY DEFERRED)")
+        .unwrap();
+
+    // This insert should fail because parent(1) doesn't exist
+    // and the deferred FK violation should be caught at statement end in autocommit mode
+    let result = conn.execute("INSERT INTO child VALUES(1,1)");
+    assert!(matches!(result, Err(LimboError::Constraint(_))));
+
+    // Do a truncating checkpoint
+    conn.execute("PRAGMA wal_checkpoint(TRUNCATE)").unwrap();
+
+    // Verify that the child table is empty (the insert was rolled back)
+    let stmt = conn.query("SELECT COUNT(*) FROM child").unwrap().unwrap();
+    let row = helper_read_single_row(stmt);
+    assert_eq!(row, vec![Value::Integer(0)]);
+}
+
 #[test]
 fn test_mvcc_transactions_autocommit() {
     let tmp_db = TempDatabase::new_with_opts(