ElementR: Check key backup when user identity changes (#3760)

Fixes vector-im/element-web#26244

spec/unit/rust-crypto/rust-crypto.spec.ts

@@ -850,6 +850,44 @@ describe("RustCrypto", () => {
             expect(userVerificationStatus.wasCrossSigningVerified()).toBeFalsy();
         });
     });
+
+    describe("onUserIdentityUpdated", () => {
+        it("raises KeyBackupStatus event when identify change", async () => {
+            const mockHttpApi = new MatrixHttpApi(new TypedEventEmitter<HttpApiEvent, HttpApiEventHandlerMap>(), {
+                baseUrl: "http://server/",
+                prefix: "",
+                onlyData: true,
+            });
+
+            const olmMachine = {
+                getIdentity: jest.fn(),
+                // Force the backup to be trusted by the olmMachine
+                verifyBackup: jest.fn().mockResolvedValue({ trusted: jest.fn().mockReturnValue(true) }),
+                isBackupEnabled: jest.fn().mockReturnValue(true),
+                getBackupKeys: jest.fn(),
+                enableBackupV1: jest.fn(),
+            } as unknown as Mocked<RustSdkCryptoJs.OlmMachine>;
+
+            const rustCrypto = new RustCrypto(
+                olmMachine,
+                mockHttpApi,
+                testData.TEST_USER_ID,
+                testData.TEST_DEVICE_ID,
+                {} as ServerSideSecretStorage,
+                {} as CryptoCallbacks,
+            );
+
+            // Return the key backup
+            fetchMock.get("path:/_matrix/client/v3/room_keys/version", testData.SIGNED_BACKUP_DATA);
+
+            // Wait for the key backup to be available
+            const keyBackupStatusPromise = new Promise<boolean>((resolve) =>
+                rustCrypto.once(CryptoEvent.KeyBackupStatus, resolve),
+            );
+            await rustCrypto.onUserIdentityUpdated(new RustSdkCryptoJs.UserId(testData.TEST_USER_ID));
+            expect(await keyBackupStatusPromise).toBe(true);
+        });
+    });
 });
 
 /** build a basic RustCrypto instance for testing

src/rust-crypto/rust-crypto.ts

@@ -1339,6 +1339,12 @@ export class RustCrypto extends TypedEventEmitter<RustCryptoEvents, RustCryptoEv
     public async onUserIdentityUpdated(userId: RustSdkCryptoJs.UserId): Promise<void> {
         const newVerification = await this.getUserVerificationStatus(userId.toString());
         this.emit(CryptoEvent.UserTrustStatusChanged, userId.toString(), newVerification);
+
+        // If our own user identity has changed, we may now trust the key backup where we did not before.
+        // So, re-check the key backup status and enable it if available.
+        if (userId.toString() === this.userId) {
+            await this.checkKeyBackupAndEnable();
+        }
     }
 
     /**