[http2] Fix Http2Adapter not redirecting for relative ULRs

plugins/http2_adapter/lib/src/http2_adapter.dart

@@ -196,11 +196,34 @@ class Http2Adapter implements HttpClientAdapter {
     // Handle redirection.
     if (needRedirect) {
       final url = responseHeaders.value('location');
+      if (url == null) {
+        throw DioException.connectionError(
+          requestOptions: options,
+          reason: 'Received redirect without location header.',
+        );
+      }
+      final uri = Uri.parse(url);
       redirects.add(
-        RedirectRecord(statusCode, options.method, Uri.parse(url ?? '')),
+        RedirectRecord(
+          statusCode,
+          options.method,
+          uri,
+        ),
       );
+      final String path;
+      if (uri.hasScheme) {
+        /// This is a full URL which has to be redirected to as is.
+        path = uri.toString();
+      } else {
+        /// This is relative with or without leading slash and is
+        /// resolved against the URL of the original request.
+        path = options.uri.resolveUri(uri).path;
+      }
       return _fetch(
-        options.copyWith(path: url, maxRedirects: --options.maxRedirects),
+        options.copyWith(
+          path: path,
+          maxRedirects: --options.maxRedirects,
+        ),
         list != null ? Stream.fromIterable(list) : null,
         cancelFuture,
         redirects,

plugins/http2_adapter/test/http2_test.dart

@@ -128,13 +128,48 @@ void main() {
     await dio.get('/drip?delay=1&numbytes=1');
   });
 
-  test('request with redirect', () async {
+  group('redirects', () {
     final dio = Dio()
       ..options.baseUrl = 'https://httpbun.com/'
       ..httpClientAdapter = Http2Adapter(ConnectionManager());
 
-    final res = await dio.get('absolute-redirect/2');
-    expect(res.statusCode, 200);
+    test('single', () async {
+      final response = await dio.get(
+        '/redirect',
+        queryParameters: {'url': 'https://httpbun.com/get'},
+        onReceiveProgress: (received, total) {
+          // ignore progress
+        },
+      );
+      expect(response.isRedirect, isTrue);
+
+      // Redirects are not supported in web.
+      // Rhe browser will follow the redirects automatically.
+      expect(response.redirects.length, 1);
+      final ri = response.redirects.first;
+      expect(ri.statusCode, 302);
+      expect(ri.location.path, '/get');
+      expect(ri.method, 'GET');
+    });
+
+    test('multiple', () async {
+      final response = await dio.get(
+        '/redirect/3',
+      );
+      expect(response.isRedirect, isTrue);
+
+      // Redirects are not supported in web.
+      // The browser will follow the redirects automatically.
+      expect(response.redirects.length, 3);
+      final ri = response.redirects.first;
+      expect(ri.statusCode, 302);
+      expect(ri.method, 'GET');
+    });
+
+    test('request with redirect', () async {
+      final res = await dio.get('absolute-redirect/2');
+      expect(res.statusCode, 200);
+    });
   });
 
   test('header value types implicit support', () async {