add e2e testing (#167)

Reviewed-on: #167

integration_test/helpers/oauth.dart

@@ -0,0 +1,68 @@
+import "package:http/http.dart" as http;
+
+import "fakes.dart";
+
+/// Fake platforms that do the browser's part of OAuth on Continuwuity, logging
+/// in as [username], then hand the redirect back to Nexus.
+class OAuthFakes(final String username, final String password) {
+  final links = FakeAppLinks();
+  late final launcher = FakeUrlLauncher(onLaunch);
+
+  Future<void> onLaunch(Uri url) async {
+    if (!url.queryParameters.containsKey("code_challenge")) return;
+
+    final redirect = await authorize(url);
+    // Nexus listens for the redirect after the launch returns
+    await Future.delayed(const .new(milliseconds: 500));
+    links.open(redirect);
+  }
+
+  /// Logs in on [authorizationUrl], then confirms the grant, returning the URI
+  /// Continuwuity redirects back to the app with.
+  Future<Uri> authorize(Uri authorizationUrl) async {
+    final client = http.Client();
+    String? cookie;
+
+    Future<Uri> followRedirect(
+      String method,
+      Uri url, [
+      Map<String, String>? form,
+    ]) async {
+      final request = http.Request(method, url)
+        ..followRedirects = false
+        ..headers.addAll({
+          // Continuwuity only accepts its forms from top-level navigations
+          "Sec-Fetch-Site": "same-origin",
+          "Sec-Fetch-Mode": "navigate",
+          "Sec-Fetch-Dest": "document",
+          "Cookie": ?cookie,
+        });
+      if (form != null) request.bodyFields = form;
+
+      final response = await http.Response.fromStream(
+        await client.send(request),
+      );
+      if (response.headers["set-cookie"] case final setCookie?) {
+        cookie = setCookie.split(";").first;
+      }
+
+      return switch ((response.statusCode, response.headers["location"])) {
+        (302 || 303 || 307, final location?) => url.resolve(location),
+        (final status, _) => throw StateError(
+          "Expected $method $url to redirect, got $status: ${response.body}",
+        ),
+      };
+    }
+
+    try {
+      final loginUrl = await followRedirect("GET", authorizationUrl);
+      final grantUrl = await followRedirect("POST", loginUrl, {
+        "identifier": username,
+        "password": password,
+      });
+      return await followRedirect("POST", grantUrl, {"stage": "confirm"});
+    } finally {
+      client.close();
+    }
+  }
+}