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Support fast path reads for mmapped/borrowed data
Often times an AssetLoader receives a AsyncRead implementation that is backed by a memory map or a byte slice. In cases where we're dealing with a format that has a zero-copy parser this introduces an unnecessary copy. We avoid this by special casing the "vfs" and "dvdbnd" asset sources to produce a special `FastPathAssetReader`.
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,132 @@ | ||
| use std::{ | ||
| error::Error, | ||
| future::Future, | ||
| io, | ||
| io::Read, | ||
| marker::PhantomData, | ||
| pin::{pin, Pin}, | ||
| task::Poll, | ||
| }; | ||
|
|
||
| use bevy::{ | ||
| asset::{ | ||
| io::{AssetSourceId, Reader}, | ||
| meta::Settings, | ||
| Asset, AssetLoader, BoxedFuture, LoadContext, | ||
| }, | ||
| reflect::erased_serde::__private::serde::{Deserialize, Serialize}, | ||
| }; | ||
| use futures_lite::{AsyncRead, AsyncReadExt}; | ||
| use memmap2::Mmap; | ||
|
|
||
| pub trait FastPathAsset: Asset + Sized { | ||
| /// The settings type used by this [`AssetLoader`]. | ||
| type Settings: Settings + Default + Serialize + for<'a> Deserialize<'a>; | ||
|
|
||
| /// The type of [error](`std::error::Error`) which could be encountered by this loader. | ||
| type Error: Into<Box<dyn Error + Send + Sync + 'static>> + From<io::Error>; | ||
|
|
||
| fn load_from_bytes<'a>( | ||
| reader: &'a [u8], | ||
| settings: &'a Self::Settings, | ||
| load_context: &'a mut LoadContext, | ||
| ) -> impl Future<Output = Result<Self, Self::Error>> + Send; | ||
| } | ||
|
|
||
| pub struct FastPathAssetLoader<T: FastPathAsset> { | ||
| _phantom: PhantomData<T>, | ||
| extensions: &'static [&'static str], | ||
| } | ||
|
|
||
| impl<T: FastPathAsset> FastPathAssetLoader<T> { | ||
| pub fn new(extensions: &'static [&'static str]) -> Self { | ||
| Self { | ||
| _phantom: PhantomData, | ||
| extensions, | ||
| } | ||
| } | ||
| } | ||
|
|
||
| impl<T: FastPathAsset> AssetLoader for FastPathAssetLoader<T> { | ||
| type Asset = T; | ||
| type Settings = T::Settings; | ||
| type Error = T::Error; | ||
|
|
||
| fn load<'a>( | ||
| &'a self, | ||
| reader: &'a mut Reader, | ||
| settings: &'a Self::Settings, | ||
| load_context: &'a mut LoadContext, | ||
| ) -> BoxedFuture<'a, Result<Self::Asset, Self::Error>> { | ||
| Box::pin(async move { | ||
| let dvdbnd_asset_source_id: AssetSourceId = AssetSourceId::from("dvdbnd"); | ||
| let vfs_asset_source_id: AssetSourceId = AssetSourceId::from("vfs"); | ||
|
|
||
| let source = load_context.asset_path().source(); | ||
| let data = if source == &dvdbnd_asset_source_id || source == &vfs_asset_source_id { | ||
| // SAFETY: This invariant is upheld by the `dvdbnd` and `vfs` asset source | ||
| // implementations. They MUST return an implementation of FastPathReader. | ||
| let reader = unsafe { (reader as *mut Reader).cast::<FastPathReader>().as_mut() }; | ||
| reader.and_then(|r| r.as_bytes()) | ||
| } else { | ||
| None | ||
| }; | ||
|
|
||
| match data { | ||
| None => { | ||
| let mut buffer = Vec::new(); | ||
| reader.read_to_end(&mut buffer).await?; | ||
|
|
||
| T::load_from_bytes(&buffer, settings, load_context).await | ||
| } | ||
| Some(slice) => T::load_from_bytes(slice, settings, load_context).await, | ||
| } | ||
| }) | ||
| } | ||
|
|
||
| fn extensions(&self) -> &[&str] { | ||
| self.extensions | ||
| } | ||
| } | ||
|
|
||
| /// An [`AsyncRead`] implementation that allows consuming Bevy asset loaders to bypass the read | ||
| /// implementation and directly access the data when available. | ||
| pub enum FastPathReader<'a> { | ||
| MemoryMapped(Mmap, usize), | ||
| Reader(Box<dyn AsyncRead + Unpin + Send + Sync + 'a>), | ||
| Slice(&'a [u8]), | ||
| } | ||
|
|
||
| impl<'a> FastPathReader<'a> { | ||
| pub fn as_bytes(&'a self) -> Option<&'a [u8]> { | ||
| match self { | ||
| FastPathReader::Slice(slice) => Some(slice), | ||
| FastPathReader::MemoryMapped(mmap, _) => Some(&mmap[..]), | ||
| FastPathReader::Reader(_) => None, | ||
| } | ||
| } | ||
| } | ||
|
|
||
| impl<'a> AsyncRead for FastPathReader<'a> { | ||
| fn poll_read( | ||
| self: Pin<&mut Self>, | ||
| _cx: &mut std::task::Context<'_>, | ||
| buf: &mut [u8], | ||
| ) -> Poll<io::Result<usize>> { | ||
| match self.get_mut() { | ||
| FastPathReader::Reader(reader) => AsyncRead::poll_read(pin!(reader), _cx, buf), | ||
| FastPathReader::Slice(slice) => Poll::Ready(Read::read(slice, buf)), | ||
| FastPathReader::MemoryMapped(dvd_bnd, ref mut offset) => { | ||
| let mut data = &dvd_bnd[*offset..]; | ||
| let read = match Read::read(&mut data, buf) { | ||
| Ok(length) => length, | ||
| Err(e) => return Poll::Ready(Err(e)), | ||
| }; | ||
|
|
||
| *offset += read; | ||
|
|
||
| Poll::Ready(Ok(read)) | ||
| } | ||
| } | ||
| } | ||
| } |
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