From 7025957256bba6bac5467be828b1d04432d0c28c Mon Sep 17 00:00:00 2001
From: MinhxNguyen7 <mxnguye1@uci.edu>
Date: Sat, 26 Aug 2023 15:06:44 +0700
Subject: [PATCH] Lock and retry for grades cache

---
 apps/antalmanac/src/lib/helpers.ts | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/apps/antalmanac/src/lib/helpers.ts b/apps/antalmanac/src/lib/helpers.ts
index 25ea77d23..34d2af0ec 100644
--- a/apps/antalmanac/src/lib/helpers.ts
+++ b/apps/antalmanac/src/lib/helpers.ts
@@ -229,7 +229,7 @@ export interface Grades {
     gradeNPCount: number;
 }
 
-const gradesCache: { [key: string]: Grades } = {};
+const gradesCache: { [key: string]: Grades | null } = {};
 
 /*
  * Query the PeterPortal GraphQL API for a course's grades with caching
@@ -240,14 +240,18 @@ const gradesCache: { [key: string]: Grades } = {};
  *
  * @returns Grades
  */
-export async function queryGrades(deptCode: string, courseNumber: string, instructor = '') {
+export async function queryGrades(deptCode: string, courseNumber: string, instructor = ''): Promise<Grades> {
     instructor = instructor.replace('STAFF', '').trim(); // Ignore STAFF
     const instructorFilter = instructor ? `instructor: "${instructor}"` : '';
 
     const cacheKey = deptCode + courseNumber + instructor;
 
     if (gradesCache[cacheKey]) {
-        return gradesCache[cacheKey];
+        // If cache is undefined, there's a request in progress
+        while (gradesCache[cacheKey] !== null) {
+            await new Promise((resolve) => setTimeout(resolve, 200)); // Wait before checking cache again
+        }
+        return gradesCache[cacheKey] as Grades;
     }
 
     const queryString = `{