[objective_c] NSInputStream doesn't support run loop scheduling #3752

The NSInputStream returned by toNSInputStream ignores scheduleInRunLoop:forMode:, never sends stream events to its delegate, and hasBytesAvailable returns YES even when no data is buffered. So a reader that schedules the stream and waits for events never receives any.

NSURLSession schedules request body streams, but because hasBytesAvailable returns YES, it calls read:maxLength on its single, process-wide com.apple.NSURLConnectionLoader thread, which waits until the Dart stream provides data. Until then, no other NSURLSession request in the process can make progress. package:cupertino_http sends StreamedRequest bodies this way.

To reproduce, add this to pkgs/objective_c/test/ns_input_stream_test.dart:

test('hasBytesAvailable when scheduled in a run loop', () {
  final controller = StreamController<List<int>>();
  final inputStream = controller.stream.toNSInputStream();
  addTearDown(inputStream.close);
  inputStream.scheduleInRunLoop(
    NSRunLoop.getCurrentRunLoop(),
    forMode: 'kCFRunLoopDefaultMode'.toNSString(),
  );
  inputStream.open();
  expect(inputStream.hasBytesAvailable, isFalse);
});