support file operations in single threaded context (#10312)

# Objective

- Fixes #10209 
- Assets should work in single threaded

## Solution

- In single threaded mode, don't use `async_fs` but fallback on
`std::fs` with a thin layer to mimic the async API
- file `file_asset.rs` is the async imps from `mod.rs`
- file `sync_file_asset.rs` is the same with `async_fs` APIs replaced by
`std::fs`
- which module is used depends on the `multi-threaded` feature

---------

Co-authored-by: Carter Anderson <mcanders1@gmail.com>

crates/bevy_asset/src/io/file/file_asset.rs

@@ -0,0 +1,231 @@
+use crate::io::{
+    get_meta_path, AssetReader, AssetReaderError, AssetWriter, AssetWriterError, PathStream,
+    Reader, Writer,
+};
+use async_fs::{read_dir, File};
+use bevy_utils::BoxedFuture;
+use futures_lite::StreamExt;
+
+use std::path::Path;
+
+use super::{FileAssetReader, FileAssetWriter};
+
+impl AssetReader for FileAssetReader {
+    fn read<'a>(
+        &'a self,
+        path: &'a Path,
+    ) -> BoxedFuture<'a, Result<Box<Reader<'a>>, AssetReaderError>> {
+        Box::pin(async move {
+            let full_path = self.root_path.join(path);
+            match File::open(&full_path).await {
+                Ok(file) => {
+                    let reader: Box<Reader> = Box::new(file);
+                    Ok(reader)
+                }
+                Err(e) => {
+                    if e.kind() == std::io::ErrorKind::NotFound {
+                        Err(AssetReaderError::NotFound(full_path))
+                    } else {
+                        Err(e.into())
+                    }
+                }
+            }
+        })
+    }
+
+    fn read_meta<'a>(
+        &'a self,
+        path: &'a Path,
+    ) -> BoxedFuture<'a, Result<Box<Reader<'a>>, AssetReaderError>> {
+        let meta_path = get_meta_path(path);
+        Box::pin(async move {
+            let full_path = self.root_path.join(meta_path);
+            match File::open(&full_path).await {
+                Ok(file) => {
+                    let reader: Box<Reader> = Box::new(file);
+                    Ok(reader)
+                }
+                Err(e) => {
+                    if e.kind() == std::io::ErrorKind::NotFound {
+                        Err(AssetReaderError::NotFound(full_path))
+                    } else {
+                        Err(e.into())
+                    }
+                }
+            }
+        })
+    }
+
+    fn read_directory<'a>(
+        &'a self,
+        path: &'a Path,
+    ) -> BoxedFuture<'a, Result<Box<PathStream>, AssetReaderError>> {
+        Box::pin(async move {
+            let full_path = self.root_path.join(path);
+            match read_dir(&full_path).await {
+                Ok(read_dir) => {
+                    let root_path = self.root_path.clone();
+                    let mapped_stream = read_dir.filter_map(move |f| {
+                        f.ok().and_then(|dir_entry| {
+                            let path = dir_entry.path();
+                            // filter out meta files as they are not considered assets
+                            if let Some(ext) = path.extension().and_then(|e| e.to_str()) {
+                                if ext.eq_ignore_ascii_case("meta") {
+                                    return None;
+                                }
+                            }
+                            let relative_path = path.strip_prefix(&root_path).unwrap();
+                            Some(relative_path.to_owned())
+                        })
+                    });
+                    let read_dir: Box<PathStream> = Box::new(mapped_stream);
+                    Ok(read_dir)
+                }
+                Err(e) => {
+                    if e.kind() == std::io::ErrorKind::NotFound {
+                        Err(AssetReaderError::NotFound(full_path))
+                    } else {
+                        Err(e.into())
+                    }
+                }
+            }
+        })
+    }
+
+    fn is_directory<'a>(
+        &'a self,
+        path: &'a Path,
+    ) -> BoxedFuture<'a, std::result::Result<bool, AssetReaderError>> {
+        Box::pin(async move {
+            let full_path = self.root_path.join(path);
+            let metadata = full_path
+                .metadata()
+                .map_err(|_e| AssetReaderError::NotFound(path.to_owned()))?;
+            Ok(metadata.file_type().is_dir())
+        })
+    }
+}
+
+impl AssetWriter for FileAssetWriter {
+    fn write<'a>(
+        &'a self,
+        path: &'a Path,
+    ) -> BoxedFuture<'a, Result<Box<Writer>, AssetWriterError>> {
+        Box::pin(async move {
+            let full_path = self.root_path.join(path);
+            if let Some(parent) = full_path.parent() {
+                async_fs::create_dir_all(parent).await?;
+            }
+            let file = File::create(&full_path).await?;
+            let writer: Box<Writer> = Box::new(file);
+            Ok(writer)
+        })
+    }
+
+    fn write_meta<'a>(
+        &'a self,
+        path: &'a Path,
+    ) -> BoxedFuture<'a, Result<Box<Writer>, AssetWriterError>> {
+        Box::pin(async move {
+            let meta_path = get_meta_path(path);
+            let full_path = self.root_path.join(meta_path);
+            if let Some(parent) = full_path.parent() {
+                async_fs::create_dir_all(parent).await?;
+            }
+            let file = File::create(&full_path).await?;
+            let writer: Box<Writer> = Box::new(file);
+            Ok(writer)
+        })
+    }
+
+    fn remove<'a>(
+        &'a self,
+        path: &'a Path,
+    ) -> BoxedFuture<'a, std::result::Result<(), AssetWriterError>> {
+        Box::pin(async move {
+            let full_path = self.root_path.join(path);
+            async_fs::remove_file(full_path).await?;
+            Ok(())
+        })
+    }
+
+    fn remove_meta<'a>(
+        &'a self,
+        path: &'a Path,
+    ) -> BoxedFuture<'a, std::result::Result<(), AssetWriterError>> {
+        Box::pin(async move {
+            let meta_path = get_meta_path(path);
+            let full_path = self.root_path.join(meta_path);
+            async_fs::remove_file(full_path).await?;
+            Ok(())
+        })
+    }
+
+    fn remove_directory<'a>(
+        &'a self,
+        path: &'a Path,
+    ) -> BoxedFuture<'a, std::result::Result<(), AssetWriterError>> {
+        Box::pin(async move {
+            let full_path = self.root_path.join(path);
+            async_fs::remove_dir_all(full_path).await?;
+            Ok(())
+        })
+    }
+
+    fn remove_empty_directory<'a>(
+        &'a self,
+        path: &'a Path,
+    ) -> BoxedFuture<'a, std::result::Result<(), AssetWriterError>> {
+        Box::pin(async move {
+            let full_path = self.root_path.join(path);
+            async_fs::remove_dir(full_path).await?;
+            Ok(())
+        })
+    }
+
+    fn remove_assets_in_directory<'a>(
+        &'a self,
+        path: &'a Path,
+    ) -> BoxedFuture<'a, std::result::Result<(), AssetWriterError>> {
+        Box::pin(async move {
+            let full_path = self.root_path.join(path);
+            async_fs::remove_dir_all(&full_path).await?;
+            async_fs::create_dir_all(&full_path).await?;
+            Ok(())
+        })
+    }
+
+    fn rename<'a>(
+        &'a self,
+        old_path: &'a Path,
+        new_path: &'a Path,
+    ) -> BoxedFuture<'a, std::result::Result<(), AssetWriterError>> {
+        Box::pin(async move {
+            let full_old_path = self.root_path.join(old_path);
+            let full_new_path = self.root_path.join(new_path);
+            if let Some(parent) = full_new_path.parent() {
+                async_fs::create_dir_all(parent).await?;
+            }
+            async_fs::rename(full_old_path, full_new_path).await?;
+            Ok(())
+        })
+    }
+
+    fn rename_meta<'a>(
+        &'a self,
+        old_path: &'a Path,
+        new_path: &'a Path,
+    ) -> BoxedFuture<'a, std::result::Result<(), AssetWriterError>> {
+        Box::pin(async move {
+            let old_meta_path = get_meta_path(old_path);
+            let new_meta_path = get_meta_path(new_path);
+            let full_old_path = self.root_path.join(old_meta_path);
+            let full_new_path = self.root_path.join(new_meta_path);
+            if let Some(parent) = full_new_path.parent() {
+                async_fs::create_dir_all(parent).await?;
+            }
+            async_fs::rename(full_old_path, full_new_path).await?;
+            Ok(())
+        })
+    }
+}