Rust epoll Wrapper for Event-Driven I/O

High-performance network applications in Linux often leverage epoll for efficient event notification. This challenge requires you to create a safe and idiomatic Rust wrapper around the epoll system interface. This wrapper will abstract away the complexities of direct system calls, providing a Rusty API for registering file descriptors and handling I/O events.

Problem Description

Your task is to implement a Rust Epoll structure that encapsulates the epoll file descriptor and provides methods for managing monitored file descriptors (FDs) and retrieving readiness events. The goal is to create a robust and easy-to-use interface for asynchronous I/O operations.

Key Requirements:

Epoll Structure: Define a struct Epoll that holds the epoll file descriptor.
Initialization: Implement a new() function to create and initialize an Epoll instance. This should involve calling epoll_create1(EPOLL_CLOEXEC).
Registration:
- Implement a register(&mut self, fd: RawFd, event: Event) method to add a file descriptor to the epoll instance.
- This method should use epoll_ctl(EPOLL_CTL_ADD) with the provided fd and event (which includes the event mask).
- The event should be a struct representing the epoll_event structure, containing fields like events (u32) and data (u64). The data field is often used to store an arbitrary identifier associated with the FD.
Modification:
- Implement a modify(&mut self, fd: RawFd, event: Event) method to change the events for an already registered file descriptor. This should use epoll_ctl(EPOLL_CTL_MOD).
Deregistration:
- Implement a unregister(&mut self, fd: RawFd) method to remove a file descriptor from the epoll instance. This should use epoll_ctl(EPOLL_CTL_DEL).
Polling:
- Implement a wait(&self, events: &mut [Event], timeout: i32) method that blocks until at least one event occurs or the timeout expires.
- This method should call epoll_wait() and populate the provided events slice with epoll_event structures.
- It should return the number of events that occurred.
Error Handling: All system calls should be wrapped with proper error handling, returning io::Result.
Safety: Ensure that unsafe code is minimized and correctly justified, especially when dealing with raw file descriptors and FFI calls.
Idiomatic Rust: Utilize Rust's features like Result, traits, and custom types for a clean and safe API.

Event Structure:

Define a struct Event that mirrors the C struct epoll_event. It should have at least two fields:

events: u32: A bitmask representing the events to monitor (e.g., EPOLLIN, EPOLLOUT).
data: u64: A user-defined identifier for the registered file descriptor.

Constants:

You'll need to use several constants defined by the Linux epoll API. For example:

EPOLL_CREATE1: For creating the epoll instance.
EPOLL_CTL_ADD, EPOLL_CTL_MOD, EPOLL_CTL_DEL: For control operations.
EPOLLIN, EPOLLPRI, EPOLLOUT, EPOLLERR, EPOLLHUP, EPOLLET: Event masks.

Examples

Example 1: Basic Registration and Waiting

Let's imagine you have a simple TCP socket.

use std::os::unix::io::RawFd;
use std::io;
// Assume Epoll and Event are defined as per the problem description

fn main() -> io::Result<()> {
    let epoll = Epoll::new()?;
    let socket_fd: RawFd = /* ... get a valid socket FD ... */;

    // Prepare the event to monitor for incoming data
    let mut interest_list = [0u32; 1]; // Placeholder, real usage would be different
    interest_list[0] = EPOLLIN; // Assuming EPOLLIN is defined

    let mut event_data = Event {
        events: EPOLLIN,
        data: socket_fd as u64, // Associate the FD with the event data
    };

    epoll.register(socket_fd, event_data)?;

    // Now, wait for events
    let mut events_buffer = [event_data; 10]; // Buffer to hold returned events
    let num_events = epoll.wait(&mut events_buffer, 1000)?; // Wait for up to 1 second

    if num_events > 0 {
        for i in 0..num_events {
            let ready_event = &events_buffer[i];
            println!("Event on FD: {}, Event mask: {}", ready_event.data, ready_event.events);
            // ... process the event ...
        }
    } else {
        println!("Timeout occurred, no events.");
    }

    Ok(())
}

Explanation: This example shows the typical flow: create an epoll instance, register a file descriptor for read events (EPOLLIN), and then wait for those events to occur. The data field is used here to store the file descriptor itself, allowing easy retrieval of which FD became ready.

Example 2: Modifying and Unregistering an FD

use std::os::unix::io::RawFd;
use std::io;
// Assume Epoll and Event are defined as per the problem description

fn main() -> io::Result<()> {
    let epoll = Epoll::new()?;
    let socket_fd: RawFd = /* ... get a valid socket FD ... */;

    // Initially register for read events
    let read_event = Event {
        events: EPOLLIN,
        data: socket_fd as u64,
    };
    epoll.register(socket_fd, read_event)?;
    println!("Registered for EPOLLIN");

    // Later, decide to also monitor for write events
    let read_write_event = Event {
        events: EPOLLIN | EPOLLOUT, // Assuming EPOLLOUT is defined
        data: socket_fd as u64,
    };
    epoll.modify(socket_fd, read_write_event)?;
    println!("Modified to monitor EPOLLIN and EPOLLOUT");

    // Finally, decide to stop monitoring this FD
    epoll.unregister(socket_fd)?;
    println!("Unregistered FD");

    Ok(())
}

Explanation: This demonstrates the ability to change the monitored events (modify) and to remove an FD from epoll's watch list (unregister).

Constraints

The implementation must be compatible with Linux systems that support epoll.
The wrapper should be thread-safe for wait operations (multiple threads can call wait on the same Epoll instance, though the underlying epoll_wait is not inherently thread-safe for concurrent modification). Registration/modification/unregistration operations from different threads on the same Epoll instance should be synchronized if necessary, though a common pattern is to have one thread managing registrations and another calling wait. For this challenge, assume Epoll::new and Epoll::wait are the primary concerns for concurrent access, and registration/modification/unregistration might be called from a single thread or require external locking if called concurrently.
The data field of the Event struct can be used to store any u64 value, typically a pointer or an index.
The timeout for wait is in milliseconds. A value of -1 indicates an infinite timeout.

Notes

You will need to interact with C functions from the Linux epoll API. This will likely involve using the libc crate or directly calling system calls if you choose to avoid external crates for FFI.
Pay close attention to the epoll_event structure's layout and the types used in the FFI calls.
Consider how to represent the event masks (e.g., EPOLLIN, EPOLLOUT) in a Rusty way, possibly using an enum or bit flags.
The EPOLL_CLOEXEC flag passed to epoll_create1 is important for preventing the epoll file descriptor from being inherited by child processes.
The EPOLLET (edge-triggered) mode is a more advanced feature of epoll. While not strictly required for this challenge, understanding its implications is beneficial. The provided register and modify methods should accommodate this flag.
Think about how to handle potential EINTR errors from epoll_wait (interruption by a signal). A robust implementation would likely retry the epoll_wait call.