fix: Race condition can cause multiple Apollo server initializations under load (#9929)

spec/ParseGraphQLServer.spec.js

@@ -118,6 +118,20 @@ describe('ParseGraphQLServer', () => {
       expect(server3).not.toBe(server2);
       expect(server3).toBe(server4);
     });
+
+    it('should return same server reference when called 100 times in parallel', async () => {
+      parseGraphQLServer.server = undefined;
+
+      // Call _getServer 100 times in parallel
+      const promises = Array.from({ length: 100 }, () => parseGraphQLServer._getServer());
+      const servers = await Promise.all(promises);
+
+      // All resolved servers should be the same reference
+      const firstServer = servers[0];
+      servers.forEach((server, index) => {
+        expect(server).toBe(firstServer);
+      });
+    });
   });
 
   describe('_getGraphQLOptions', () => {

src/GraphQL/ParseGraphQLServer.js

@@ -97,6 +97,14 @@ class ParseGraphQLServer {
     if (schemaRef === newSchemaRef && this._server) {
       return this._server;
     }
+    // It means a parallel _getServer call is already in progress
+    if (this._schemaRefMutex === newSchemaRef) {
+      return this._server;
+    }
+    // Update the schema ref mutex to avoid parallel _getServer calls
+    this._schemaRefMutex = newSchemaRef;
+    const createServer = async () => {
+      try {
         const { schema, context } = await this._getGraphQLOptions();
         const apollo = new ApolloServer({
           csrfPrevention: {
@@ -109,9 +117,18 @@ class ParseGraphQLServer {
           schema,
         });
         await apollo.start();
-    this._server = expressMiddleware(apollo, {
+        return expressMiddleware(apollo, {
           context,
         });
+      } catch (e) {
+        // Reset all mutexes and forward the error
+        this._server = null;
+        this._schemaRefMutex = null;
+        throw e;
+      }
+    }
+    // Do not await so parallel request will wait the same promise ref
+    this._server = createServer();
     return this._server;
   }