main.rs

use bytes::Bytes;
use std::pin::Pin;
use std::task::{Context, Poll};
use std::io::BufReader;
use std::io::Read;
use futures_util::ready;
use futures_core::Future;

struct StreamBody {
    reader: BufReader<File>,
    buffer: Vec<u8>,
}

impl StreamBody {
    fn new(file: File, chunk_size: usize) -> Self {
        let mut reader = BufReader::new(file);
        let mut buffer = vec![0u8; chunk_size];
        StreamBody {
            reader,
            buffer,
        }
    }
}

impl futures_core::stream::Stream for StreamBody {
    type Item = Result<Bytes, ResponseError>;
    fn poll_next(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Option<Self::Item>> {
        match self.reader.read(&mut self.buffer) {
            Ok(count) => {
                if count > 0 {
                    Poll::Ready(Some(Ok(Bytes::copy_from_slice(&self.buffer[..count]))))
                } else {
                    Poll::Ready(None)
                }
            },
            Err(e) => Poll::Ready(None)
        }
    }
}