329 lines
12 KiB
Rust
329 lines
12 KiB
Rust
//! This module contains two functions whose sole purpose is to allow a single
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//! producer multiple consumer (SPMC) system using the filesystem as an
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//! intermediate.
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//!
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//! Consider the scenario where two clients, A and B, request the same uncached
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//! file, one after the other. In a typical caching system, both requests would
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//! result in a cache miss, and both requests would then be proxied from
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//! upstream. But, we can do better. We know that by the time one request
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//! begins, there should be a file on disk for us to read from. Why require
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//! subsequent requests to read from upstream, when we can simply fetch one and
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//! read from the filesystem that we know will have the exact same data?
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//! Instead, we can just read from the filesystem and just inform all readers
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//! when the file is done. This is beneficial to both downstream and upstream as
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//! upstream no longer needs to process duplicate requests and sequential cache
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//! misses are treated as closer as a cache hit.
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use actix_web::error::PayloadError;
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use bytes::{Buf, Bytes, BytesMut};
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use futures::{Future, Stream, StreamExt};
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use log::debug;
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use once_cell::sync::Lazy;
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use serde::{Deserialize, Serialize};
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use sodiumoxide::crypto::secretstream::Header;
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use std::collections::HashMap;
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use std::error::Error;
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use std::fmt::Display;
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use std::io::SeekFrom;
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use std::num::NonZeroU32;
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use std::path::{Path, PathBuf};
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use std::pin::Pin;
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use std::task::{Context, Poll};
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use tokio::fs::{create_dir_all, remove_file, File};
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use tokio::io::{AsyncRead, AsyncSeekExt, AsyncWriteExt, BufReader, ReadBuf};
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use tokio::sync::mpsc::Sender;
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use tokio::sync::watch::{channel, Receiver};
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use tokio::sync::RwLock;
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use tokio_stream::wrappers::WatchStream;
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use tokio_util::codec::{BytesCodec, FramedRead};
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use super::{BoxedImageStream, CacheKey, CacheStreamItem, ImageMetadata, InnerStream};
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#[derive(Serialize, Deserialize)]
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pub enum OnDiskMetadata {
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Encrypted(Header, ImageMetadata),
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Plaintext(ImageMetadata),
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}
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/// Keeps track of files that are currently being written to.
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///
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/// Why is this necessary? Consider the following situation:
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///
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/// Client A requests file `foo.png`. We construct a transparent file stream,
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/// and now the file is being streamed into and from.
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///
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/// Client B requests the same file `foo.png`. A naive implementation would
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/// attempt to either read directly the file as it sees the file existing. This
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/// is problematic as the file could still be written to. If Client B catches
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/// up to Client A's request, then Client B could receive a broken image, as it
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/// thinks it's done reading the file.
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///
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/// We effectively use `WRITING_STATUS` as a status relay to ensure concurrent
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/// reads to the file while it's being written to will wait for writing to be
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/// completed.
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static WRITING_STATUS: Lazy<RwLock<HashMap<PathBuf, Receiver<WritingStatus>>>> =
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Lazy::new(|| RwLock::new(HashMap::new()));
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/// Attempts to lookup the file on disk, returning a byte stream if it exists.
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/// Note that this could return two types of streams, depending on if the file
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/// is in progress of being written to.
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pub(super) async fn read_file(
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path: &Path,
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) -> Option<Result<(InnerStream, Option<Header>, ImageMetadata), std::io::Error>> {
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let std_file = std::fs::File::open(path).ok()?;
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let file = File::from_std(std_file.try_clone().ok()?);
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let metadata = {
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let mut de = serde_json::Deserializer::from_reader(std_file);
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ImageMetadata::deserialize(&mut de).ok()?
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};
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// False positive lint, `file` is used in both cases, which means that it's
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// not possible to move this into a map_or_else without cloning `file`.
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#[allow(clippy::option_if_let_else)]
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let stream = if let Some(status) = WRITING_STATUS.read().await.get(path).map(Clone::clone) {
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InnerStream::Concurrent(ConcurrentFsStream::from_file(
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file,
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WatchStream::new(status),
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))
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} else {
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InnerStream::Completed(FramedRead::new(BufReader::new(file), BytesCodec::new()))
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};
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Some(Ok((stream, None, metadata)))
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}
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/// Writes the metadata and input stream (in that order) to a file, returning a
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/// stream that reads from that file. Accepts a db callback function that is
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/// provided the number of bytes written, and an optional on-complete callback
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/// that is called with a completed cache entry.
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pub(super) async fn write_file<Fut, DbCallback>(
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path: &Path,
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cache_key: CacheKey,
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mut byte_stream: BoxedImageStream,
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metadata: ImageMetadata,
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db_callback: DbCallback,
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on_complete: Option<Sender<(CacheKey, Bytes, ImageMetadata, usize)>>,
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) -> Result<(InnerStream, Option<Header>), std::io::Error>
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where
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Fut: 'static + Send + Sync + Future<Output = ()>,
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DbCallback: 'static + Send + Sync + FnOnce(u32) -> Fut,
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{
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let (tx, rx) = channel(WritingStatus::NotDone);
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let mut file = {
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let mut write_lock = WRITING_STATUS.write().await;
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let parent = path.parent().expect("The path to have a parent");
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create_dir_all(parent).await?;
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let file = File::create(path).await?; // we need to make sure the file exists and is truncated.
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write_lock.insert(path.to_path_buf(), rx.clone());
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file
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};
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let metadata_string = serde_json::to_string(&metadata).expect("serialization to work");
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let metadata_size = metadata_string.len();
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// need owned variant because async lifetime
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let path_buf = path.to_path_buf();
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tokio::spawn(async move {
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let path_buf = path_buf; // moves path buf into async
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let mut errored = false;
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let mut bytes_written: u32 = 0;
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let mut acc_bytes = BytesMut::new();
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let accumulate = on_complete.is_some();
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file.write_all(metadata_string.as_bytes()).await?;
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while let Some(bytes) = byte_stream.next().await {
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if let Ok(mut bytes) = bytes {
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if accumulate {
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acc_bytes.extend(&bytes);
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}
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loop {
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match file.write(&bytes).await? {
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0 => break,
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n => {
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bytes.advance(n);
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bytes_written += n as u32;
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// We don't care if we don't have receivers
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let _ = tx.send(WritingStatus::NotDone);
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}
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}
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}
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} else {
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errored = true;
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break;
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}
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}
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if errored {
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// It's ok if the deleting the file fails, since we truncate on
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// create anyways, but it should be best effort.
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//
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// We don't care about the result of the call.
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std::mem::drop(remove_file(&path_buf).await);
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} else {
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file.flush().await?;
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file.sync_all().await?; // we need metadata
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debug!("writing to file done");
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}
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{
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let mut write_lock = WRITING_STATUS.write().await;
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// This needs to be written atomically with the write lock, else
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// it's possible we have an inconsistent state
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//
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// We don't really care if we have no receivers
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if errored {
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let _ = tx.send(WritingStatus::Error);
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} else {
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let _ = tx.send(WritingStatus::Done(bytes_written));
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}
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write_lock.remove(&path_buf);
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}
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tokio::spawn(db_callback(bytes_written));
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if let Some(sender) = on_complete {
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tokio::spawn(async move {
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sender
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.send((
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cache_key,
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acc_bytes.freeze(),
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metadata,
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bytes_written as usize,
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))
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.await
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});
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}
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// We don't ever check this, so the return value doesn't matter
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Ok::<_, std::io::Error>(())
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});
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Ok((
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InnerStream::Concurrent(
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ConcurrentFsStream::new(path, metadata_size, WatchStream::new(rx)).await?,
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),
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None,
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))
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}
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pub struct ConcurrentFsStream {
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/// The File to read from
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file: Pin<Box<BufReader<File>>>,
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/// The channel to get updates from. The writer must send its status, else
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/// this reader will never complete.
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receiver: Pin<Box<WatchStream<WritingStatus>>>,
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/// The number of bytes the reader has read
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bytes_read: u32,
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/// The number of bytes that the writer has reported it has written. If the
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/// writer has not reported yet, this value is None.
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bytes_total: Option<NonZeroU32>,
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}
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impl ConcurrentFsStream {
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async fn new(
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path: &Path,
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seek: usize,
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receiver: WatchStream<WritingStatus>,
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) -> Result<Self, std::io::Error> {
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let mut file = File::open(path).await?;
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file.seek(SeekFrom::Start(seek as u64)).await?;
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Ok(Self::from_file(file, receiver))
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}
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fn from_file(file: File, receiver: WatchStream<WritingStatus>) -> Self {
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Self {
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file: Box::pin(BufReader::new(file)),
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receiver: Box::pin(receiver),
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bytes_read: 0,
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bytes_total: None,
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}
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}
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}
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/// Represents some upstream error.
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#[derive(Debug)]
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pub struct UpstreamError;
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impl Error for UpstreamError {}
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impl Display for UpstreamError {
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fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
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write!(f, "An upstream error occurred")
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}
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}
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impl Stream for ConcurrentFsStream {
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type Item = CacheStreamItem;
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fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
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// First, try to read from the file...
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// TODO: Might be more efficient to have a larger buffer
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let mut bytes = [0; 4 * 1024].to_vec();
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let mut buffer = ReadBuf::new(&mut bytes);
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match self.file.as_mut().poll_read(cx, &mut buffer) {
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Poll::Ready(Ok(_)) => (),
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Poll::Ready(Err(_)) => return Poll::Ready(Some(Err(UpstreamError))),
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Poll::Pending => return Poll::Pending,
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}
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// At this point, we know that we "successfully" read some amount of
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// data. Let's see if there's actual data in there...
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let filled = buffer.filled().len();
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if filled == 0 {
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// Filled is zero, which indicates two situations:
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// 1. We are actually done.
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// 2. We read to the EOF while the writer is still writing to it.
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// To handle the second case, we need to see the status of the
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// writer, and if it's done writing yet.
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if let Poll::Ready(Some(WritingStatus::Done(n))) =
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self.receiver.as_mut().poll_next_unpin(cx)
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{
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self.bytes_total = Some(NonZeroU32::new(n).expect("Stored a 0 byte image?"))
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}
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// Okay, now we know if we've read enough bytes or not. If the
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// writer hasn't told use that it's done yet, then we know that
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// there must be more bytes to read from.
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if let Some(bytes_total) = self.bytes_total {
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if bytes_total.get() == self.bytes_read {
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// We matched the number of bytes the writer said it wrote,
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// so we're finally done
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return Poll::Ready(None);
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}
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}
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// We haven't read enough bytes, so just return an empty bytes and
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// have the executor request some bytes some time in the future.
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//
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// This case might be solved by io_uring, but for now this is this
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// the best we can do.
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Poll::Ready(Some(Ok(Bytes::new())))
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} else {
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// We have data! Give it to the reader!
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self.bytes_read += filled as u32;
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bytes.truncate(filled);
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Poll::Ready(Some(Ok(bytes.into())))
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}
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}
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}
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impl From<UpstreamError> for actix_web::Error {
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#[inline]
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fn from(_: UpstreamError) -> Self {
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PayloadError::Incomplete(None).into()
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}
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}
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#[derive(Debug, Clone, Copy)]
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enum WritingStatus {
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NotDone,
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Done(u32),
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Error,
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}
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