Repository: tokio-rs/tokio
File: tokio/src/fs/read_uring.rs (Rust)

Code before:
    }

    loop {
        if buf.len() == buf.capacity() && buf.capacity() == start_cap {
            // The buffer might be an exact fit. Let's read into a probe buffer
            // and see if it returns `Ok(0)`. If so, we've avoided an
            // unnecessary increasing of the capacity. But if not, append the
            // probe buffer to the primary buffer and let its capacity grow.
            let (size_read, r_fd, r_buf) = small_probe_read(fd, buf, offset).await?;

            if size_read == 0 {
                return Ok(r_buf);
            }

            buf = r_buf;
            fd = r_fd;
            offset += size_read as u64;
        }

        // buf is full, need more capacity
        if buf.len() == buf.capacity() {
            if consecutive_short_reads > 1 {
                buf.try_reserve(PROBE_SIZE.saturating_mul(consecutive_short_reads))?;
            } else {
                buf.try_reserve(PROBE_SIZE)?;
            }
        }

        // doesn't matter if we have a valid size_hint or not, if we do more
        // than 2 consecutive_short_reads, gradually increase the buffer
        // capacity to read more data at a time

        // prepare the spare capacity to be read into
        let spare = buf.capacity() - buf.len();
        let buf_len = cmp::min(spare, MAX_READ_SIZE);

        // SAFETY: buf_len cannot be greater than u32::MAX because max_read_size
        // is u32::MAX
        let mut read_len = buf_len as u32;

        loop {
            // read into spare capacity
            let (res, r_fd, mut r_buf) = Op::read(fd, buf, read_len, offset).await;

            match res {
                Ok(0) => return Ok(r_buf),
                Ok(size_read) => {
                    let new_len = size_read as usize + r_buf.len();
                    // SAFETY: We didn't read more than what as reserved
                    // as capacity, the _size_read was initialized by the kernel
                    // via a mutable pointer

Review comment (suggestion):
I don't understand why there is this `consecutive_short_reads` logic. When you reserve, it will double the capacity internally so we do not need this logic.

Code after:
            fd = r_fd;
        }

        // buf is full, need more capacity
        if buf.len() == buf.capacity() {
            buf.try_reserve(PROBE_SIZE)?;
        }

        // prepare the spare capacity to be read into
        let buf_len = usize::min(buf.spare_capacity_mut().len(), MAX_READ_SIZE);

        // buf_len cannot be greater than u32::MAX because MAX_READ_SIZE
        // is less than u32::MAX
        let read_len = u32::try_from(buf_len).expect("buf_len must always fit in u32");

        // read into spare capacity
        let (r_fd, r_buf, is_eof) = op_read(fd, buf, &mut offset, read_len).await?;

        if is_eof {
            return Ok(r_buf);
        }

        fd = r_fd;
        buf = r_buf;
    }
}

async fn small_probe_read(
    fd: OwnedFd,
    mut buf: Vec<u8>,
    offset: &mut u64,
) -> io::Result<(OwnedFd, Vec<u8>, bool)> {
    let read_len = PROBE_SIZE_U32;

    let mut temp_arr = [0; PROBE_SIZE];
    // we don't call this function if buffer's length < PROBE_SIZE
    let back_bytes_len = buf.len() - PROBE_SIZE;

    temp_arr.copy_from_slice(&buf[back_bytes_len..]);

    // We're decreasing the length of the buffer and len is greater
    // than PROBE_SIZE. So we can read into the discarded length
    buf.truncate(back_bytes_len);

    let (r_fd, mut r_buf, is_eof) = op_read(fd, buf, offset, read_len).await?;
    // If `size_read` returns zero due to reasons such as buffer's exact fit,
    // then this `try_reserve` does not perform allocation.
    r_buf.try_reserve(PROBE_SIZE)?;
    r_buf.splice(back_bytes_len..back_bytes_len, temp_arr);

    Ok((r_fd, r_buf, is_eof))