Error accessing static field from nested function.

[rbehrends]

This crash is a bit of a doozy, especially as it seems to occur only under very specific circumstances:

$ cat test.dart
class C {
  static List<int> list = List.filled(25, 0);

  void iter() {
    int item(int p) {
      return list[p];
    }
    for (int p = 0; p < 25; p++) {
      item(p);
    }
  }
}

void loop(int steps) {
  var c = C();
  for (int i = 0; i < steps; i++) c.iter();
}

void main() {
  loop(1000);
}
$ dart --version
Dart VM version: 2.8.0-dev.0.0 (Wed Dec 11 10:31:10 2019 +0100) on "macos_x64"
$ dart --enable-experiment=non-nullable --snapshot-kind=app-jit --snapshot=test.snap test.dart && dart --enable-experiment=non-nullable test.snap
Unhandled exception:
NoSuchMethodError: Closure call with mismatched arguments: function '[]'
Receiver: null
Tried calling: [](0)
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
#1      C.iter.item (file:///private/tmp/test.dart:6:18)
#2      C.iter (file:///private/tmp/test.dart:9:11)
#3      loop (file:///private/tmp/test.dart:16:37)
#4      main (file:///private/tmp/test.dart:20:3)
#5      _startIsolate.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:307:19)
#6      _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:174:12)
$ dart --snapshot-kind=app-jit --snapshot=test.snap test.dart && dart test.snap

This bug happens only with --enable-experiment=non-nullable. It happens with dart 2.7.0 and 2.8.0-dev.0.0, but not with dart 2.6.1, and only when generating an app-jit snapshot, not during normal execution.

If you reduce the loop argument enough, eventually the bug also disappears.

If you make the list field non-static, the bug disappears.

If you remove the for loop in iter(), the bug also disappears.

I can trigger the bug on both macOS & Linux.

Edit: reduced the example a bit more and narrowed the conditions where it happens further.

[mkustermann]

@rbehrends Please notice that NNBD is still in implementation phase in the VM.

/cc @crelier @a-siva

[crelier]

@liamappelbe, could this be related to your recent work on late fields?

[rbehrends]

Please notice that NNBD is still in implementation phase in the VM.

Yeah, I know that. I reported it anyway, because it didn't look like something that was simply a gap in the implementation (unlike, say, type inference for map[x] ?? y breaking with NNBD).

[a-siva]

Strange that the error happens only when running from the generated app-jit snapshot, are we not serializing some field or NNBD info into the snapshot?

[crelier]

I am not able to reproduce this issue on linux with the latest build.
Note that we have put safeguards in place recently making sure that the platform dill is consistent in regards to NNBD. In other words, you will be prevented to specify --enable-experiment=non-nullable on a platform not built for it.

Here are my steps:

rm -rf out
./tools/gn.py --mode=debug,release --arch=x64 --nnbd
./tools/build.py --mode=debug --arch=x64 runtime --nnbd
out/DebugX64NNBD/dart --enable-experiment=non-nullable --snapshot-kind=app-jit --snapshot=test.snap test.dart
out/DebugX64NNBD/dart --enable-experiment=non-nullable test.snap

No error reported.

Note that if use a non-nnbd version of dart, I get an error:

out/DebugX64/dart --enable-experiment=non-nullable --snapshot-kind=app-jit --snapshot=test.snap test.dart
VM initialization failed: Snapshot not compatible with the current VM configuration: the snapshot requires 'debug no-dwarf_stack_traces use_bare_instructions' but the VM has 'debug no-dwarf_stack_traces use_bare_instructions nnbd-experiment'

Could that be the problem here?

BTW, @liamappelbe, it looks like the error message has the required and expected features swapped, no?

[crelier]

BTW, @liamappelbe, it looks like the error message has the required and expected features swapped, no?

Never mind, I was not reading the error message correctly. "Snapshot" in the error message does not refer to the snapshot we are trying to generate (requiring nnbd), but to the platform snapshot (missing nnbd) that the VM is running on.

[a-siva]

Tried the test on bleeding edge and it works.