Awaiter Stack Traces

One of the common challenges associated with debugging asynchronous code is that stack traces do not reference the code which led to the exception. The context is lost when execution cross asynchronous gap.

Consider the following code:

Future<int> inner() async {
  await null;  // asynchronous gap
  print(StackTrace.current); // (*)
  return 0;
}
 
Future<int> outer() async {
  int process(int v) {
    return v + 1;
  }
 
  return await inner().then(process);
}
 
void main() async {
  await outer();
}

Producing synchronous stack trace at the line marked (*) will yield the following:

#0 inner
#1 _SuspendState._createAsyncCallbacks.thenCallback
#2 _RootZone.runUnary
#3 _SuspendState._awaitNotFuture.run
#4 _microtaskLoop
#5 _startMicrotaskLoop
#6 _startMicrotaskLoop
#7 _runPendingImmediateCallback
#8 _RawReceivePort._handleMessage

Only a single frame corresponds to user code (#0 inner) and the rest are dart:async internals. Nothing in this stack trace mentions outer or main, which called inner. This makes diagnosing issues based on a stack trace much harder.

To address this problem runtime system augments synchronous portion of the stack trace with an awaiter stack trace. Each awaiter frame represents a closure or a suspended async function which will be invoked when the currently running asynchronous computation completes.

This support allows runtime system to produce the following output for the example given above:

#0  inner
<asynchronous suspension>
#1  outer.process
<asynchronous suspension>
#2  outer
<asynchronous suspension>
#3  main
<asynchronous suspension>

Algorithm

To recover awaiter stack trace runtime follows through a chain of _Future, _StreamController and SuspendState objects. The following diagram illustrates the path it takes to produce asynchronous stack trace in our initial example:

Each awaiter frame is a pair of (closure, nextFrame):

• closure is a listener which will be invoked when the future this frame is waiting on completes.
  • This might be one of the callbacks associated with suspendable functions internals, e.g. _SuspendState.thenCallback which resumes execution after the await.
• next is an object representing the next awaiter frame, which is waiting for the completion of this awaiter frame.

Unwinding rules can be summarised as follows:

• If at any point closure has a captured variable marked with ‘@pragma('vm:awaiter-link’)variable then the value of that variable will be used asnextFrame.
• IfnextFrameis a_SuspendStatethen_SuspendState.function_datagives us_FutureImplor_AsyncStarStreamControllerto look at.
• IfnextFrameis_FutureImplthen we can take the first_FutureListenerinlistenersand then the next frame is(listener.callback, listener.result).
• IfnextFrameis_AsyncStarStreamControllerthen we getasyncStarStreamController.controller.subscription._onData, which should give us an instance of_StreamIterator, which inside contains a_FutureImplon whichawait for` is waiting.

Awaiter unwinding is implemented in by [dart::StackTraceUtils::CollectFrames] in [runtime/vm/stack_trace.cc].

‘@pragma('vm:awaiter-link’)`

Dart code which does not use async/async* functions and instead uses callbacks and lower-level primitives can integrate with awaiter frame unwinding by annotating variables which link to the next awaiter frame with ‘@pragma('vm:awaiter-link’)`.

Consider the following variation of the example:

Future<int> outer() {
  final completer = Completer<int>();
 
  int process(int v) {
    completer.complete(v);
  }
 
  inner().then(process);
 
  return completer.future;
}

Running this would produce the following stack trace:

#0 inner
<asynchronous suspension>
#1 outer.process
<asynchronous suspension>

Runtime is unable to unwind the awaiter stack past process. However if completer is annotated with ‘@pragma('vm:awaiter-link’)` then unwinder will know where to continue:

Future<int> outer() {
  @pragma('vm:awaiter-link')
  final completer = Completer<int>();
  // ...
}

#0      inner
<asynchronous suspension>
#1      outer.process
<asynchronous suspension>
#2      main
<asynchronous suspension>

vm:awaiter-link can be used in dart:async internals to avoid hardcoding recognition of specific methods into the runtime system, see for example _SuspendState.thenCallback or Future.timeout implementations.