Crate async_file 

Asynchronous file I/O operations with priority handling.

async_file provides a simple yet powerful API for performing asynchronous file operations in Rust. It closely follows the standard library’s file API design while adding async support and priority-based scheduling.

Features

• Async Operations: All file operations are asynchronous, allowing for non-blocking I/O
• Priority Scheduling: Every operation accepts a priority parameter for fine-grained control
• Memory Safety: Uses an opaque Data type to safely handle OS-managed memory allocations
• Platform Agnostic: Backend-agnostic API with a default std implementation

Quick Start

use async_file::{File, Priority};

// Open a file with unit test priority
let file = File::open("/dev/zero", Priority::unit_test()).await?;

// Read up to 1KB of data
let data = file.read(1024, Priority::unit_test()).await?;
println!("Read {} bytes", data.len());

Architecture Overview

Opaque Type Design

The library uses opaque wrapper types that hide platform-specific implementations:

• File: Wraps platform file handles behind a unified async interface
• Data: Encapsulates OS-managed memory buffers for safe async I/O
• Metadata: Provides file information in a platform-agnostic way
• Error: Wraps platform-specific error types

This design ensures API stability while allowing platform-specific optimizations.

Single Operation Constraint

Important: Only one operation may be in-flight at a time per file handle.

This constraint:

• Prevents race conditions on file position
• Simplifies the implementation
• Avoids many classes of concurrency bugs
• Matches typical file I/O patterns

Attempting concurrent operations on the same file handle will result in undefined behavior.

Memory Management Strategy

The library uses an opaque Data type instead of user-provided buffers. This design:

• Prevents use-after-free bugs: If an async operation is cancelled (by dropping the future), the OS might still write to the buffer. OS-managed allocation prevents this.
• Enables platform optimizations: Different platforms can use their optimal memory allocation strategies.
• Simplifies the API: Users don’t need to manage buffer lifetimes across await points.

Common Usage Patterns

Reading a File Completely

use async_file::{File, Priority};

// For small files, use read_all()
let file = File::open("config.txt", Priority::highest_async()).await?;
let contents = file.read_all(Priority::highest_async()).await?;

// Convert to String if needed
let text = String::from_utf8(contents.into_boxed_slice().into_vec())
    .expect("Invalid UTF-8");

Sequential Reading with Seeking

use async_file::{File, Priority};
use std::io::SeekFrom;

let mut file = File::open("/dev/zero", Priority::unit_test()).await?;

// Read header (first 128 bytes)
let header = file.read(128, Priority::unit_test()).await?;

// Skip to data section at byte 1024
file.seek(SeekFrom::Start(1024), Priority::unit_test()).await?;

// Read data
let data = file.read(4096, Priority::unit_test()).await?;

Checking File Existence Before Opening

use async_file::{exists, File, Priority};

let path = "important.dat";

if exists(path, Priority::unit_test()).await {
    let file = File::open(path, Priority::highest_async()).await?;
    // Process file...
} else {
    eprintln!("File not found: {}", path);
}

Priority-Based Operations

use async_file::{File, Priority};

// Critical system file - use highest priority
let system_file = File::open("/critical/system.conf",
    Priority::highest_async()).await?;

// Background logging - use low priority
// For other priority levels, use Priority::new()
// Priority::new(0.2) for low priority tasks

// User-facing operation - use high priority
// Priority::new(0.8) for high priority tasks

// Unit tests - use dedicated test priority
let test_file = File::open("test_fixture.txt",
    Priority::unit_test()).await?;

Platform Support

• Unix/Linux/macOS: Uses blocking crate to run std::fs operations in a thread pool
• WASM: Uses web fetch API for remote file access (requires set_default_origin)
• Windows: Same as Unix implementation using blocking crate

Design Philosophy

This library enforces that only one operation may be in-flight at a time per file handle. This constraint simplifies the implementation and prevents many classes of concurrency bugs.

The library uses opaque types (File, Data, Metadata) that wrap platform-specific implementations, providing a clean abstraction layer while maintaining efficiency.

Structs

Data

An opaque buffer type that holds data read from files.

Error

An error that can occur during file operations.

File

A handle to an open file for asynchronous I/O operations.

Metadata

Metadata information about a file.

Functions

exists

Tests if a file or directory exists at the given path.

set_default_origin

Sets the default origin for WASM file operations.

Type Aliases

Priority

A priority value for scheduling file operations.