Awaiting for a Future returning a class that implements Finalizable in a for-in loop causes a crash

[halildurmus]

import 'dart:ffi';

class Foo implements Finalizable {}

Future<Foo> bar() => Future.value(Foo());

void main() async {
  for (final element in [await bar()]) {
    print(element);
  }
}

Running the above program causes this crash:

Null check operator used on a null value
#0      BinaryPrinter._getVariableIndex (package:kernel/binary/ast_to_binary.dart:83:17)
#1      BinaryPrinter.visitVariableGet (package:kernel/binary/ast_to_binary.dart:1575:17)
#2      VariableGet.accept (package:kernel/ast.dart:4443:44)
#3      BinaryPrinter.writeNode (package:kernel/binary/ast_to_binary.dart:446:10)
#4      BinaryPrinter.writeNodeList (package:kernel/binary/ast_to_binary.dart:347:7)
#5      BinaryPrinter.visitArguments (package:kernel/binary/ast_to_binary.dart:1873:5)
#6      Arguments.accept (package:kernel/ast.dart:5734:38)
#7      BinaryPrinter.writeArgumentsNode (package:kernel/binary/ast_to_binary.dart:460:10)
#8      BinaryPrinter.visitStaticInvocation (package:kernel/binary/ast_to_binary.dart:1856:5)
#9      StaticInvocation.accept (package:kernel/ast.dart:7017:44)
#10     BinaryPrinter.writeNode (package:kernel/binary/ast_to_binary.dart:446:10)
#11     BinaryPrinter.visitExpressionStatement (package:kernel/binary/ast_to_binary.dart:2185:5)
#12     ExpressionStatement.accept (package:kernel/ast.dart:9703:43)
#13     BinaryPrinter.writeNode (package:kernel/binary/ast_to_binary.dart:446:10)
#14     BinaryPrinter.writeNodeList (package:kernel/binary/ast_to_binary.dart:347:7)
#15     BinaryPrinter.visitBlockExpression (package:kernel/binary/ast_to_binary.dart:2150:5)
#16     BlockExpression.accept (package:kernel/ast.dart:9336:44)
#17     BinaryPrinter.writeNode (package:kernel/binary/ast_to_binary.dart:446:10)
#18     BinaryPrinter.visitAwaitExpression (package:kernel/binary/ast_to_binary.dart:2121:5)
#19     AwaitExpression.accept (package:kernel/ast.dart:9085:44)
#20     BinaryPrinter.writeNode (package:kernel/binary/ast_to_binary.dart:446:10)
#21     BinaryPrinter.writeNodeList (package:kernel/binary/ast_to_binary.dart:347:7)
#22     BinaryPrinter.visitArguments (package:kernel/binary/ast_to_binary.dart:1873:5)
#23     Arguments.accept (package:kernel/ast.dart:5734:38)
#24     BinaryPrinter.writeArgumentsNode (package:kernel/binary/ast_to_binary.dart:460:10)
#25     BinaryPrinter.visitStaticInvocation (package:kernel/binary/ast_to_binary.dart:1856:5)
#26     StaticInvocation.accept (package:kernel/ast.dart:7017:44)
#27     BinaryPrinter.writeNode (package:kernel/binary/ast_to_binary.dart:446:10)
#28     BinaryPrinter.visitInstanceGet (package:kernel/binary/ast_to_binary.dart:1621:5)
#29     InstanceGet.accept (package:kernel/ast.dart:4822:44)
#30     BinaryPrinter.writeNode (package:kernel/binary/ast_to_binary.dart:446:10)
#31     BinaryPrinter.writeOptionalNode (package:kernel/binary/ast_to_binary.dart:545:7)
#32     BinaryPrinter.writeVariableDeclaration (package:kernel/binary/ast_to_binary.dart:2400:5)
#33     BinaryPrinter.visitVariableDeclaration (package:kernel/binary/ast_to_binary.dart:2386:5)
#34     VariableDeclaration.accept (package:kernel/ast.dart:11346:43)
#35     BinaryPrinter.writeNode (package:kernel/binary/ast_to_binary.dart:446:10)
#36     BinaryPrinter.writeNodeList (package:kernel/binary/ast_to_binary.dart:347:7)
#37     BinaryPrinter.visitBlock (package:kernel/binary/ast_to_binary.dart:2196:5)
#38     Block.accept (package:kernel/ast.dart:9759:43)
#39     BinaryPrinter.writeNode (package:kernel/binary/ast_to_binary.dart:446:10)
#40     BinaryPrinter.writeNodeList (package:kernel/binary/ast_to_binary.dart:347:7)
#41     BinaryPrinter.visitBlock (package:kernel/binary/ast_to_binary.dart:2196:5)
#42     Block.accept (package:kernel/ast.dart:9759:43)
#43     BinaryPrinter.writeNode (package:kernel/binary/ast_to_binary.dart:446:10)
#44     BinaryPrinter.writeOptionalNode (package:kernel/binary/ast_to_binary.dart:545:7)
#45     BinaryPrinter.visitFunctionNode (package:kernel/binary/ast_to_binary.dart:1559:5)
#46     FunctionNode.accept (package:kernel/ast.dart:4186:38)
#47     BinaryPrinter.writeFunctionNode (package:kernel/binary/ast_to_binary.dart:453:10)
#48     BinaryPrinter.visitProcedure (package:kernel/binary/ast_to_binary.dart:1388:5)
#49     Procedure.accept (package:kernel/ast.dart:3578:40)
#50     BinaryPrinter.writeProcedureNode (package:kernel/binary/ast_to_binary.dart:474:10)
#51     BinaryPrinter.writeProcedureNodeList (package:kernel/binary/ast_to_binary.dart:356:7)
#52     BinaryPrinter.visitLibrary (package:kernel/binary/ast_to_binary.dart:1132:5)
#53     Library.accept (package:kernel/ast.dart:585:38)
#54     BinaryPrinter.writeLibraryNode (package:kernel/binary/ast_to_binary.dart:467:10)
#55     BinaryPrinter.writeLibraries (package:kernel/binary/ast_to_binary.dart:780:9)
#56     BinaryPrinter.writeComponentFile.<anonymous closure> (package:kernel/binary/ast_to_binary.dart:619:7)
#57     Timeline.timeSync (dart:developer/timeline.dart:166:22)
#58     BinaryPrinter.writeComponentFile (package:kernel/binary/ast_to_binary.dart:600:14)
#59     serializeComponent (file:///C:/b/s/w/ir/x/w/sdk/pkg/vm/bin/kernel_service.dart:1036:11)
#60     _processLoadRequest (file:///C:/b/s/w/ir/x/w/sdk/pkg/vm/bin/kernel_service.dart:947:41)
<asynchronous suspension>

Dart version:

Dart SDK version: 3.0.0-272.0.dev (dev) (Fri Feb 24 15:27:56 2023 -0800) on "windows_x64"

If class Foo does not implement the Finalizable or the expression await bar() is stored in a variable:

  ...

  final result = await bar();
  for (final element in [result]) {
    print(element);
  }

... the program no longer crashes.

[mraleph]

/cc @dcharkes

[dcharkes]

Thanks @halildurmus for the report!

I can reproduce this issue.

We have some strange interaction going on between the sync transformer and the Finalizable-transformer:

Block (Block({
  {
    Iterator<Foo> :sync-for-iterator = _GrowableList._literal1<Foo>(await block {
      final Future<Foo> :expressionValueWrappedFinalizable = bar();
      reachabilityFence(element);
    } => :expressionValueWrappedFinalizable).{Iterable.iterator};
    for (; :sync-for-iterator.{Iterator.moveNext}(); ) {
      final Foo element = :sync-for-iterator.{Iterator.current};
      {
        print(element);
        reachabilityFence(element);
      }
    }
  }
}))

We end up with a reachabilityFence of the element before the element is defined.

I'll investigate what the proper solution is.

[dcharkes]

Fix underway: https://dart-review.googlesource.com/c/sdk/+/286182

[halildurmus]

Thank you, @dcharkes

[dcharkes]

More funny code:

import 'dart:ffi';

class Foo implements Finalizable {
  Foo? next;
}

Future<Foo> bar() => Future.value(Foo());

void main() async {
  for (Foo? element = await bar(); element != null; element = element.next) {
    print(element);
  }
}

But here it's actually correct to use element in the expressions inside the for definition. 😄