use super::BlobReader;
use pin_project_lite::pin_project;
use std::io;
use std::task::Poll;
use tokio::io::AsyncRead;
use tracing::{debug, instrument};
pin_project! {
/// This implements [tokio::io::AsyncSeek] for and [tokio::io::AsyncRead] by
/// simply skipping over some bytes, keeping track of the position.
/// It fails whenever you try to seek backwards.
///
/// ## Pinning concerns:
///
/// [NaiveSeeker] is itself pinned by callers, and we do not need to concern
/// ourselves regarding that.
///
/// Though, its fields as per
/// <https://doc.rust-lang.org/std/pin/#pinning-is-not-structural-for-field>
/// can be pinned or unpinned.
///
/// So we need to go over each field and choose our policy carefully.
///
/// The obvious cases are the bookkeeping integers we keep in the structure,
/// those are private and not shared to anyone, we never build a
/// `Pin<&mut X>` out of them at any point, therefore, we can safely never
/// mark them as pinned. Of course, it is expected that no developer here
/// attempt to `pin!(self.pos)` to pin them because it makes no sense. If
/// they have to become pinned, they should be marked `#[pin]` and we need
/// to discuss it.
///
/// So the bookkeeping integers are in the right state with respect to their
/// pinning status. The projection should offer direct access.
///
/// On the `r` field, i.e. a `BufReader<R>`, given that
/// <https://docs.rs/tokio/latest/tokio/io/struct.BufReader.html#impl-Unpin-for-BufReader%3CR%3E>
/// is available, even a `Pin<&mut BufReader<R>>` can be safely moved.
///
/// The only care we should have regards the internal reader itself, i.e.
/// the `R` instance, see that Tokio decided to `#[pin]` it too:
/// <https://docs.rs/tokio/latest/src/tokio/io/util/buf_reader.rs.html#29>
///
/// In general, there's no `Unpin` instance for `R: tokio::io::AsyncRead`
/// (see <https://docs.rs/tokio/latest/tokio/io/trait.AsyncRead.html>).
///
/// Therefore, we could keep it unpinned and pin it in every call site
/// whenever we need to call `poll_*` which can be confusing to the non-
/// expert developer and we have a fair share amount of situations where the
/// [BufReader] instance is naked, i.e. in its `&mut BufReader<R>`
/// form, this is annoying because it could lead to expose the naked `R`
/// internal instance somehow and would produce a risk of making it move
/// unexpectedly.
///
/// We choose the path of the least resistance as we have no reason to have
/// access to the raw `BufReader<R>` instance, we just `#[pin]` it too and
/// enjoy its `poll_*` safe APIs and push the unpinning concerns to the
/// internal implementations themselves, which studied the question longer
/// than us.
pub struct NaiveSeeker<R: tokio::io::AsyncRead> {
#[pin]
r: tokio::io::BufReader<R>,
pos: u64,
bytes_to_skip: u64,
}
}
impl<R: tokio::io::AsyncRead> NaiveSeeker<R> {
pub fn new(r: R) -> Self {
NaiveSeeker {
r: tokio::io::BufReader::new(r),
pos: 0,
bytes_to_skip: 0,
}
}
}
impl<R: tokio::io::AsyncRead> tokio::io::AsyncRead for NaiveSeeker<R> {
fn poll_read(
self: std::pin::Pin<&mut Self>,
cx: &mut std::task::Context<'_>,
buf: &mut tokio::io::ReadBuf<'_>,
) -> Poll<std::io::Result<()>> {
// The amount of data read can be determined by the increase
// in the length of the slice returned by `ReadBuf::filled`.
let filled_before = buf.filled().len();
let this = self.project();
let pos: &mut u64 = this.pos;
match this.r.poll_read(cx, buf) {
Poll::Ready(a) => {
let bytes_read = buf.filled().len() - filled_before;
*pos += bytes_read as u64;
Poll::Ready(a)
}
Poll::Pending => Poll::Pending,
}
}
}
impl<R: tokio::io::AsyncRead> tokio::io::AsyncBufRead for NaiveSeeker<R> {
fn poll_fill_buf(
self: std::pin::Pin<&mut Self>,
cx: &mut std::task::Context<'_>,
) -> Poll<io::Result<&[u8]>> {
self.project().r.poll_fill_buf(cx)
}
fn consume(self: std::pin::Pin<&mut Self>, amt: usize) {
let this = self.project();
this.r.consume(amt);
let pos: &mut u64 = this.pos;
*pos += amt as u64;
}
}
impl<R: tokio::io::AsyncRead> tokio::io::AsyncSeek for NaiveSeeker<R> {
#[instrument(skip(self))]
fn start_seek(
self: std::pin::Pin<&mut Self>,
position: std::io::SeekFrom,
) -> std::io::Result<()> {
let absolute_offset: u64 = match position {
io::SeekFrom::Start(start_offset) => {
if start_offset < self.pos {
return Err(io::Error::new(
io::ErrorKind::Unsupported,
format!("can't seek backwards ({} -> {})", self.pos, start_offset),
));
} else {
start_offset
}
}
// we don't know the total size, can't support this.
io::SeekFrom::End(_end_offset) => {
return Err(io::Error::new(
io::ErrorKind::Unsupported,
"can't seek from end",
));
}
io::SeekFrom::Current(relative_offset) => {
if relative_offset < 0 {
return Err(io::Error::new(
io::ErrorKind::Unsupported,
"can't seek backwards relative to current position",
));
} else {
self.pos + relative_offset as u64
}
}
};
debug!(absolute_offset=?absolute_offset, "seek");
// we already know absolute_offset is larger than self.pos
debug_assert!(
absolute_offset >= self.pos,
"absolute_offset {} is larger than self.pos {}",
absolute_offset,
self.pos
);
// calculate bytes to skip
*self.project().bytes_to_skip = absolute_offset - self.pos;
Ok(())
}
#[instrument(skip(self))]
fn poll_complete(
mut self: std::pin::Pin<&mut Self>,
cx: &mut std::task::Context<'_>,
) -> Poll<std::io::Result<u64>> {
if self.bytes_to_skip == 0 {
// return the new position (from the start of the stream)
return Poll::Ready(Ok(self.pos));
}
// discard some bytes, until pos is where we want it to be.
// We create a buffer that we'll discard later on.
let mut buf = [0; 1024];
// Loop until we've reached the desired seek position. This is done by issuing repeated
// `poll_read` calls. If the data is not available yet, we will yield back to the executor
// and wait to be polled again.
loop {
// calculate the length we want to skip at most, which is either a max
// buffer size, or the number of remaining bytes to read, whatever is
// smaller.
let bytes_to_skip = std::cmp::min(self.bytes_to_skip as usize, buf.len());
let mut read_buf = tokio::io::ReadBuf::new(&mut buf[..bytes_to_skip]);
match self.as_mut().poll_read(cx, &mut read_buf) {
Poll::Ready(_a) => {
let bytes_read = read_buf.filled().len() as u64;
if bytes_read == 0 {
return Poll::Ready(Err(io::Error::new(
io::ErrorKind::UnexpectedEof,
format!(
"tried to skip {} bytes, but only was able to skip {} until reaching EOF",
bytes_to_skip, bytes_read
),
)));
}
// calculate bytes to skip
let bytes_to_skip = self.bytes_to_skip - bytes_read;
*self.as_mut().project().bytes_to_skip = bytes_to_skip;
if bytes_to_skip == 0 {
return Poll::Ready(Ok(self.pos));
}
}
Poll::Pending => return Poll::Pending,
};
}
}
}
impl<R: tokio::io::AsyncRead + Send + Unpin + 'static> BlobReader for NaiveSeeker<R> {}
#[cfg(test)]
mod tests {
use super::NaiveSeeker;
use std::io::{Cursor, SeekFrom};
use tokio::io::{AsyncReadExt, AsyncSeekExt};
/// This seek requires multiple `poll_read` as we use a 1024 bytes internal
/// buffer when doing the seek.
/// This ensures we don't hang indefinitely.
#[tokio::test]
async fn seek() {
let buf = vec![0u8; 4096];
let reader = Cursor::new(&buf);
let mut seeker = NaiveSeeker::new(reader);
seeker.seek(SeekFrom::Start(4000)).await.unwrap();
}
#[tokio::test]
async fn seek_read() {
let mut buf = vec![0u8; 2048];
buf.extend_from_slice(&[1u8; 2048]);
buf.extend_from_slice(&[2u8; 2048]);
let reader = Cursor::new(&buf);
let mut seeker = NaiveSeeker::new(reader);
let mut read_buf = vec![0u8; 1024];
seeker.read_exact(&mut read_buf).await.expect("must read");
assert_eq!(read_buf.as_slice(), &[0u8; 1024]);
seeker
.seek(SeekFrom::Current(1024))
.await
.expect("must seek");
seeker.read_exact(&mut read_buf).await.expect("must read");
assert_eq!(read_buf.as_slice(), &[1u8; 1024]);
seeker
.seek(SeekFrom::Start(2 * 2048))
.await
.expect("must seek");
seeker.read_exact(&mut read_buf).await.expect("must read");
assert_eq!(read_buf.as_slice(), &[2u8; 1024]);
}
}
