perf(dynamite_runtime): Increase response body reading speed

[provokateurin]

The download performance was really bad when implementing the largefile litmus tests in #513. I don't have exact numbers, but it seems like the speed is about 50-75% better with this. I assume that it either was doing memory copies (which get really slow once you have a big response body) or it just had a big overhead due to the iterator.

[Leptopoda]

I could imagine this coming from the toList needing to iterate over every byte in the end.
We should probably use a generator function (sync* and yield).

How did you test this? I'd like to take a look on a few old devices that are severely memory limited.

[provokateurin]

This is how I discovered it: https://github.com/provokateurin/nextcloud-neon/blob/d32084f192dd0da5493f8e2646749058baedcd3c/packages/nextcloud/test/litmus_test.dart#L231
I don't know if there is a good way to intentionally limit the memory of your dart process.

[Leptopoda]

void main(final List<String> args) async {
  final largefileSize = pow(10, 8).toInt();

  final value = Uint8List(largefileSize);
  final stream = Stream.value(value.map((final e) => e).toList());

  final stopwatch = Stopwatch()..start();

  /// 2960ms
  final data = await stream.expand((final element) => element).toList();
  final result = Uint8List.fromList(data);

  /// 1250ms
  final result = Uint8List.fromList([
    for (final element in await stream.toList()) ...[
      ...element,
    ],
  ]);

  /// 859ms
  final result = Uint8List.fromList([
    for (final element in await stream.toList()) ...element,
  ]);

  /// 180 ms
  final buffer = BytesBuilder();
  for (final element in await stream.toList()) {
    buffer.add(element);
  }
  final result = buffer.toBytes();

  stopwatch.stop();
  print(stopwatch.elapsedMilliseconds);
}

not a scientific result by any means as I had my usual programs running in the background. Runs where consistent within 5% in subsequent runs.
I also want to highlight test 2 and 3 as we use this pattern in many places throughout the app. The last test was also the only one not causing an OOM kill with 10^9 as the size.

[provokateurin]

Interesting! So it can be improved even more 👍

[Leptopoda]

@provokateurin what do you think about this?

[provokateurin]

LGTM