add fallbackmutex

backend/src/models/util/FallBackMutex.ts

@@ -0,0 +1,27 @@
+import { AsyncFailable, HasSuccess } from 'picsur-shared/dist/types';
+
+const fallBackMap: Record<string, Promise<unknown>> = {};
+
+export async function FallBackMutex<
+  MF extends () => AsyncFailable<O>,
+  FF extends () => AsyncFailable<unknown>,
+  O,
+>(key: string, mainFunc: MF, fallBackFunc: FF): AsyncFailable<O> {
+  const try_it = await mainFunc();
+  if (HasSuccess(try_it)) return try_it;
+
+  if (fallBackMap[key] !== undefined) {
+    await fallBackMap[key];
+    return FallBackMutex(key, mainFunc, fallBackFunc);
+  }
+
+  fallBackMap[key] = fallBackFunc();
+  fallBackMap[key]
+    .then(() => {
+      delete fallBackMap[key];
+    })
+    .catch(() => {
+      delete fallBackMap[key];
+    });
+  return FallBackMutex(key, mainFunc, fallBackFunc);
+}