[DDS] Subscribing to 'Service' stream throws 'Stream already subscribed' with two clients

[annagrin]

I have a test in flutter_tools that connects two clients to the vmService on web that runs with DDS enabled. The first client can subscribe to 'Service' stream successfully. but the second cannot - subscribing to 'Service' causes 'Stream already subscribed' to be thrown.

See the following PR for example:
flutter/flutter#79479

[bkonyi]

It seems like this is caused by a data race that this test just happened to hit. In particular, this code:

await Future.wait(<Future<void>>[
  validateFlutterVersion(client1),
  validateFlutterVersion(client2),
]);

has two separate clients subscribing to the Service stream in validateFlutterVersion. These clients issue their streamListen requests simultaneously, and StreamManager processes them both at roughly the same time, allowing for both requests to bypass the checks we do to avoid having DDS try to subscribe to an existing stream.

We need some form of locking mechanism to ensure streamListen requests are processed atomically or refactor subscribed streams checks in such a way that streams are marked as subscribed by DDS before issuing the streamListen to the VM service.

[annagrin]

@bkonyi looks like this is still failing after re-enabling the flutter test:

07:41 +19 -1: test/web.shard/vm_service_web_test.dart: Clients of flutter run on web with DDS enabled can validate flutter version in parallel [E]                                                     
  streamListen: (-32000) Null check operator used on a null value
  package:vm_service/src/vm_service.dart 1626:45  new _OutstandingRequest
  package:vm_service/src/vm_service.dart 2109:21  VmService._call
  package:vm_service/src/vm_service.dart 2056:7   VmService.streamListen
  test/web.shard/vm_service_web_test.dart 129:16  validateFlutterVersion
  test/web.shard/vm_service_web_test.dart 62:9    main.<fn>.<fn>

DDS version: dds: 2.0.2
DWDS version: dwds: 11.1.2
VMService version: 7.1.1

PR: flutter/flutter#86832
Failing tests:

• https://ci.chromium.org/ui/p/flutter/builders/try/Mac%20web_tool_tests/7333/overview
• https://ci.chromium.org/ui/p/flutter/builders/try/Linux%20web_tool_tests/7679/overview

[bkonyi]

@annagrin that isn't the same error. Can you please file another issue and assign it to me?