Devtool extension authoring: EvalOnDartLibrary, how to recursively inspect an Object without causing `Sentinel <expired>` due to race conditions?

[rrousselGit]

Hello!

I think I'm missing something with regards to EvalOnDartLibrary. I fail to see how it is possible to fully map out an object without ever risking an <expired> Sentinel or not loosing the ability to inspect private members.

I am aware that one trick is to do (pseudo code):

final id = eval('globalMap.register(<expression to evaluate>)')

try {
  final content = eval('globalMap.get("$id").<field I want to read>');
} finally {
  eval('globalMap.unregister("$id")');
}

This prevents the <expired>. But afaik, prevents reading private members on the inspected object, due to globalMap and the private members being defined in different libraries.
(also, this will forcibly trigger late fields ; which can be undesired)

If we instead do:

final value= eval('<expression to evaluate>')
final content = eval('that.<field I want to read>', scope: {'that': value.id});

Then we risk that value will expire before content is resolved.

Is there something I'm missing here? Or do we need a new feature to solve this?

[elliette]

Hi @rrousselGit - I don't think you're missing anything, but I'm not too familiar with the EvalOnDartLibrary so would need to do more investigation to confirm. I'm happy to review a PR for this.

FYI @srawlins (for visibility into extension author requests)

[rrousselGit]

Is there maybe a way to create a Ref that cannot expire? 🤔

[elliette]

Is there maybe a way to create a Ref that cannot expire? 🤔

Question for @bkonyi - I'm guessing this would require some changes to the vm_service's handling of InstanceRefs?

[rrousselGit]

Alternatively I wonder we could have a scope-like, where instead of Ref IDs, we'd pass code + a library URI.

So we could do:

eval(
  'refCache.get($id)._field'
  expressions: {
    'refCache': (uri: 'package:my_package/src/ref_cache.dart', code: 'MyPackageDevtool.instance')
  }
)

This would allow reading private fields using the Map cache trick, without expired errors (assuming vm_service handles it internally)

[rrousselGit]

Or maybe something similar to MongoDB/Firebase SQL, with "pipelines" to bundle multiple Evals in a single instruction

// Returns the last object in the eval chain
InstanceRef ref = await evalPipeline([
  Eval(libraryUri:  'package:my_package/src/ref_cache.dart', code: 'MyPackageDevtool.instance'),
  // $prev is a variable injected in the code, referring to the previous "pipeline". So here MyPackageDevtool.instance
  Eval(libraryUri: 'package:example_app/counter.dart', '$prev.get($counterId)._increment()'),
])