AUT-5433: Return combined error when both session store reads fail

Adds DualStoreError with typed primary and secondary properties so callers can identify which stores failed easier.

Author: sw-mattw

src/config/dual-session-store.ts

@@ -2,6 +2,15 @@ import { type SessionData, Store } from "express-session";
 import { logger } from "../utils/logger.js";
 import { isDeepStrictEqual } from "node:util";
 
+export class DualStoreError extends Error {
+  constructor(
+    public readonly primary: Error,
+    public readonly secondary: Error
+  ) {
+    super("Both session stores failed");
+  }
+}
+
 export class DualSessionStore extends Store {
   constructor(
     private readonly primary: Store,
@@ -32,7 +41,16 @@ export class DualSessionStore extends Store {
 
       this.secondary.get(sid, (secondaryErr, secondarySession) => {
         if (primaryErr) {
-          cb(secondaryErr, secondarySession);
+          if (secondaryErr) {
+            logger.warn(
+              { err: secondaryErr, sid, store: this.secondaryLabel },
+              "Secondary session read failed"
+            );
+          }
+          const error = secondaryErr
+            ? new DualStoreError(primaryErr, secondaryErr)
+            : null;
+          cb(error, secondarySession);
           return;
         }
 

test/unit/dual-session-store.test.ts

@@ -1,6 +1,9 @@
 import { afterEach, beforeEach, describe, it } from "mocha";
 import { expect, sinon } from "../utils/test-utils.js";
-import { DualSessionStore } from "../../src/config/dual-session-store.js";
+import {
+  DualSessionStore,
+  DualStoreError,
+} from "../../src/config/dual-session-store.js";
 import { type SessionData, Store } from "express-session";
 
 describe("DualSessionStore", () => {
@@ -47,14 +50,18 @@ describe("DualSessionStore", () => {
       });
     });
 
-    it("should return secondary error when both primary and secondary fail", (done) => {
+    it("should return combined error when both primary and secondary fail", (done) => {
+      const primaryErr = new Error("primary failure");
       const secondaryErr = new Error("secondary failure");
-      primary.get = sinon.fake((_sid, cb) => cb(new Error("primary failure")));
+      primary.get = sinon.fake((_sid, cb) => cb(primaryErr));
       secondary.get = sinon.fake((_sid, cb) => cb(secondaryErr));
       store = new DualSessionStore(primary, secondary, "Redis", "DynamoDB");
 
       store.get(sid, (err) => {
-        expect(err).to.equal(secondaryErr);
+        expect(err).to.be.instanceOf(DualStoreError);
+        expect(err.message).to.equal("Both session stores failed");
+        expect(err.primary).to.equal(primaryErr);
+        expect(err.secondary).to.equal(secondaryErr);
         done();
       });
     });