fix race condition issue

lib/serializable-tx.ts

@@ -0,0 +1,47 @@
+/**
+ * Helper transaksi Serializable + retry. Single source of truth supaya semua
+ * operasi yang rawan race (join/cancel trip, refund, payment) konsisten:
+ * - Isolation level Serializable → Postgres SSI mendeteksi konflik.
+ * - Retry otomatis saat serialization failure (Prisma error code `P2034`).
+ */
+import { Prisma } from "@/app/generated/prisma/client";
+import { prisma } from "@/lib/prisma";
+
+const SERIAL_TX_ATTEMPTS = 6;
+
+export function isSerializationConflict(err: unknown): boolean {
+  return (
+    typeof err === "object" &&
+    err !== null &&
+    "code" in err &&
+    (err as { code: string }).code === "P2034"
+  );
+}
+
+/**
+ * Jalankan `fn` dalam transaksi Serializable. Kalau gagal karena konflik
+ * serialisasi, ulang sampai `SERIAL_TX_ATTEMPTS` kali. Error bisnis (non-P2034)
+ * langsung dilempar tanpa retry.
+ */
+export async function runSerializable<T>(
+  fn: (tx: Prisma.TransactionClient) => Promise<T>,
+  fallbackMessage = "Operasi sedang ramai. Coba lagi sebentar."
+): Promise<T> {
+  let lastErr: unknown;
+  for (let attempt = 0; attempt < SERIAL_TX_ATTEMPTS; attempt++) {
+    try {
+      return await prisma.$transaction(fn, {
+        isolationLevel: Prisma.TransactionIsolationLevel.Serializable,
+        maxWait: 5000,
+        timeout: 15000,
+      });
+    } catch (e) {
+      lastErr = e;
+      if (isSerializationConflict(e) && attempt < SERIAL_TX_ATTEMPTS - 1) {
+        continue;
+      }
+      throw e;
+    }
+  }
+  throw lastErr instanceof Error ? lastErr : new Error(fallbackMessage);
+}