Spawn no more than 1 iframe in web tests. Using multiple iframes in DDK mode...

Spawn no more than 1 iframe in web tests. Using multiple iframes in DDK mode times out tests. Also set concurrency to 1 to guarantee that the test runner does not even attempt to parallelize tests. (#40630)

Spawn no more than 1 iframe in web tests. Using multiple iframes in DDK mode times out tests. Also set concurrency to 1 to guarantee that the test runner does not even attempt to parallelize tests.

dev/bots/test.dart

@@ -468,9 +468,9 @@ Future<void> _runTests() async {
 Future<void> _runWebTests() async {
   // TODO(yjbanov): re-enable when web test cirrus flakiness is resolved
   await _runFlutterWebTest(path.join(flutterRoot, 'packages', 'flutter'), tests: <String>[
+    'test/foundation/',
+    'test/physics/',
     // TODO(yjbanov): re-enable when flakiness is resolved
-    // 'test/foundation/',
-    // 'test/physics/',
     // 'test/rendering/',
     // 'test/services/',
     // 'test/painting/',
@@ -801,6 +801,8 @@ Future<void> _runFlutterWebTestBatch(String workingDirectory, {
 }) async {
   final List<String> args = <String>[
     'test',
+    if (_getCiProvider() == CiProviders.cirrus)
+      '--concurrency=1',  // do not parallelize on Cirrus to reduce flakiness
     '-v',
     '--platform=chrome',
     ...?flutterTestArgs,

packages/flutter_tools/lib/src/test/flutter_web_platform.dart

@@ -412,8 +412,8 @@ class BrowserManager {
     })
       ..cancel();
 
-    // Whenever we get a message, no matter which child channel it's for, we the
-    // know browser is still running code which means the user isn't debugging.
+    // Whenever we get a message, no matter which child channel it's for, we know
+    // the browser is still running code which means the user isn't debugging.
     _channel = MultiChannel<dynamic>(
       webSocket.cast<String>().transform(jsonDocument).changeStream((Stream<Object> stream) {
         return stream.map((Object message) {
@@ -453,7 +453,13 @@ class BrowserManager {
   /// loaded in the same browser. However, the browser can only load so many at
   /// once, and we want a timeout in case they fail so we only wait for so many
   /// at once.
-  final Pool _pool = Pool(8);
+  // The number 1 is chosen to disallow multiple iframes in the same browser. This
+  // is because in some environments, such as Cirrus CI, tests end up stuck and
+  // time out eventually. The exact reason for timeouts is unknown, but the
+  // hypothesis is that we were the first ones to attempt to run DDK-compiled
+  // tests concurrently in the browser. DDK is known to produce an order of
+  // magnitude bigger and somewhat slower code, which may overload the browser.
+  final Pool _pool = Pool(1);
 
   /// The ID of the next suite to be loaded.
   ///
@@ -544,7 +550,7 @@ class BrowserManager {
         this, null, _browser.remoteDebuggerUri, _onRestartController.stream);
   }
 
-  /// Tells the browser the load a test suite from the URL [url].
+  /// Tells the browser to load a test suite from the URL [url].
   ///
   /// [url] should be an HTML page with a reference to the JS-compiled test
   /// suite. [path] is the path of the original test suite file, which is used