Support staged rollout of migration to Rust Crypto (#12184)

* Rust migration staged rollout

* Phased rollout unit tests

playwright/e2e/crypto/staged-rollout.spec.ts

@@ -0,0 +1,64 @@
+/*
+Copyright 2024 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { test, expect } from "../../element-web-test";
+import { logIntoElement } from "./utils";
+
+test.describe("Migration of existing logins", () => {
+    test("Test migration of existing logins when rollout is 100%", async ({
+        page,
+        context,
+        app,
+        credentials,
+        homeserver,
+    }, workerInfo) => {
+        test.skip(workerInfo.project.name === "Rust Crypto", "This test only works with Rust crypto.");
+        await page.goto("/#/login");
+
+        let featureRustCrypto = false;
+        let stagedRolloutPercent = 0;
+
+        await context.route(`http://localhost:8080/config.json*`, async (route) => {
+            const json = {};
+            json["features"] = {
+                feature_rust_crypto: featureRustCrypto,
+            };
+            json["setting_defaults"] = {
+                "RustCrypto.staged_rollout_percent": stagedRolloutPercent,
+            };
+            await route.fulfill({ json });
+        });
+
+        await logIntoElement(page, homeserver, credentials);
+
+        await app.settings.openUserSettings("Help & About");
+        await expect(page.getByText("Crypto version: Olm")).toBeVisible();
+
+        featureRustCrypto = true;
+
+        await page.reload();
+
+        await app.settings.openUserSettings("Help & About");
+        await expect(page.getByText("Crypto version: Olm")).toBeVisible();
+
+        stagedRolloutPercent = 100;
+
+        await page.reload();
+
+        await app.settings.openUserSettings("Help & About");
+        await expect(page.getByText("Crypto version: Rust SDK")).toBeVisible();
+    });
+});