Would like support for a VM Service query for null-safety mode (mixed, sound)

[jonahwilliams]

The flutter tool would like to perform analytics tracking of hot reload performance based on mixed vs sound. The tool can sort of guestimate sound-ness by checking command line flags and parsing the entrypoint language version, or by performing an expression evaluation - but since the VM should already know it would be great if this could be surfaced through the VM Service API. It would be great if dwds could also support this query, too (@grouma ). FYI @jacob314 might have needed this too.

@bkonyi

[bkonyi]

Both the --enable-experiments and --strong-null-safety flags should appear in the FlagList returned by getFlagList now.

[jonahwilliams]

I'm not seeing sound-null-safety set to true. I tried reading values from the flag list using hello_world.

Step 1: update examples/hello_world

diff --git a/examples/hello_world/lib/main.dart b/examples/hello_world/lib/main.dart
index 3cefcf3662..a3a48e6e10 100644
--- a/examples/hello_world/lib/main.dart
+++ b/examples/hello_world/lib/main.dart
@@ -2,6 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+// @dart = 2.11
 import 'package:flutter/widgets.dart';
 
 void main() =>

Step 2: print out flag value during flutter run

diff --git a/packages/flutter_tools/lib/src/resident_runner.dart b/packages/flutter_tools/lib/src/resident_runner.dart
index ac8c6ac639..f3d141ee47 100644
--- a/packages/flutter_tools/lib/src/resident_runner.dart
+++ b/packages/flutter_tools/lib/src/resident_runner.dart
@@ -513,6 +513,11 @@ class FlutterDevice {
 
   Future<void> initLogReader() async {
     final vm_service.VM vm = await vmService.getVM();
+    final vm_service.FlagList flagList = await vmService.getFlagList();
+    final vm_service.Flag soundNullSafety = flagList.flags.firstWhere((vm_service.Flag flag) {
+      return flag.name == 'sound_null_safety';
+    }, orElse: () => null);
+    print('IS SOUND NULL SAFETY ENABLED: ${soundNullSafety?.valueAsString}|${soundNullSafety?.modified}');
     final DeviceLogReader logReader = await device.getLogReader(app: package);
     logReader.appPid = vm.pid;
   }

Step 3:

flutter run --sound-null-safety --enable-experiment=non-nullable
...
IS SOUND NULL SAFETY ENABLED: false|false

[grouma]

For the web we explicitly return an empty array. @jakemac53 and @annagrin is there a way to tell when the Frontend Server / build_web_compilers is run with null safety? We could even do an evaluation on the running app if necessary. Maybe @nshahan has some ideas.

[nshahan]

Edited to reflect the way this works now. I forgot we made some changes here to the bootstrapping.

It seems like it would make more sense to retrieve the runtime mode elsewhere before the app starts running because we actually need to know and set the mode before we run main() and chose the right dart_sdk.js file.

That said the old standby test is [1, 2, null] is List<int> should be true in weak more and false in sound mode.

[grouma]

Is this something we should / could add in the DDC metadata?

[nshahan]

I was talking with @annagrin about this topic earlier. She is investigating some refactoring of the infrastructure that needs to happen so the expression evaluation can use the correct mode. The case is a little different between frontend_server and build_web_compilers.

In the build_web_compilers case DDC could probably deduce the mode based on the dart sdk .dill file that is provided when compiling modules. As long as everything is working correctly the sdk .dill mode should match the version of the dart_sdk.js that is used when running. We could emit the mode in every metadata file.

I don't know exactly what happens in the frontend server setup or what emits the metadata file. Does this still hit the code path in DDC that writes the metadata file?

[grouma]

Does this still hit the code path in DDC that writes the metadata file?

That is my understanding. @annagrin would know best though.