File.openRead does not respect back-pressure

[mraleph]

File.openRead returns _FileStream which does not pause reading even if subscription to the stream is paused. See

sdk/sdk/lib/io/file_impl.dart

Lines 61 to 75 in 4dd1522

	_controller = new StreamController<Uint8List>(
	sync: true,
	onListen: _start,
	onResume: _readBlock,
	onCancel: () {
	_unsubscribed = true;
	return _closeFile();
	},
	);
	return _controller.stream.listen(
	onData,
	onError: onError,
	onDone: onDone,
	cancelOnError: cancelOnError,
	);

This means it does not respect the back-pressure. Consider the situation when you are piping a huge file into the socket (e.g. file.openRead().pipe(socket)) - the whole file will end up being buffered in memory because even though socket will pause once send buffer is full _FileStream will continue reading.

cc @brianquinlan

[mraleph]

I noticed that the comment here mentions "paused" - so I blamed the implementation. Indeed we properly handled pausing at some point but then lost it - this was broken by 364bd9e

[lrhn]

That change doesn't look like it's intended to not handle pausing, it just does it differently.
(Rather than cache the current block outside of the controller while paused, it puts it into the controller eagerly, but then doesn't request another block until resumed. It doesn't have an onPause callback because the only thing it did was set a flag that was equivalent to controller.isPaused.)

I can't see why it'd be broken, but state management is hard.

[mraleph]

Yeah, you are right. readBlock only issues another read if controller is not paused. My bad. I have tested it and it seems to work.