Implementing an Epoll Wrapper in Rust

This challenge asks you to create a Rust wrapper around the Linux epoll system call. epoll is a powerful mechanism for efficiently monitoring multiple file descriptors for I/O events, crucial for building high-performance network servers and other I/O-bound applications. Your wrapper should provide a safe and convenient Rust interface to epoll's functionality.

Problem Description

You are to implement a basic Epoll wrapper in Rust that allows users to create an epoll instance, add file descriptors to monitor, wait for events, and remove file descriptors. The wrapper should handle error conditions gracefully and provide a safe abstraction over the underlying C API.

Key Requirements:

• Creation: A function to create a new Epoll instance. This should initialize the epoll data structure.
• Adding File Descriptors: A method to add file descriptors to the epoll instance for monitoring. This should accept a file descriptor and an event mask (e.g., EPOLLIN, EPOLLOUT, EPOLLEXIT, EPOLLET).
• Removing File Descriptors: A method to remove a file descriptor from the epoll instance.
• Waiting for Events: A method to wait for events on the monitored file descriptors. This method should block until at least one event is available. It should return a vector of EpollEvent structs, each representing an event on a monitored file descriptor.
• Error Handling: The wrapper should handle potential errors from the underlying epoll calls (e.g., ENOMEM, EBADF) and propagate them appropriately.
• Safety: The wrapper should be memory-safe and prevent common pitfalls associated with raw epoll usage.

Expected Behavior:

• The Epoll instance should be initialized correctly.
• Adding a file descriptor should register it with epoll.
• Removing a file descriptor should unregister it from epoll.
• wait() should block until events are available and return a vector of EpollEvent structs.
• Error conditions should be handled gracefully and propagated.

Important Edge Cases to Consider:

• Invalid file descriptors (e.g., closed file descriptors).
• Resource exhaustion (e.g., ENOMEM when creating the epoll instance or adding file descriptors).
• Signals interrupting the wait() call. (While not strictly required to handle signals, the code should not panic if a signal interrupts wait()).
• File descriptors already registered with epoll.
• Adding the same file descriptor multiple times.

Examples

Example 1:

Input: Create an Epoll instance, add file descriptor 3 with EPOLLIN, wait for 1 second.
Output: A vector of EpollEvent structs, potentially empty if no events occurred within 1 second.
Explanation: The Epoll instance is created, file descriptor 3 is registered to monitor for incoming data. The wait function blocks for up to 1 second, returning any events detected on file descriptor 3.

Example 2:

Input: Create an Epoll instance, add file descriptor 4 with EPOLLOUT, wait indefinitely.  Then remove file descriptor 4.
Output: A vector of EpollEvent structs (potentially empty), followed by no further events after removal.
Explanation: The Epoll instance is created, file descriptor 4 is registered to monitor for outgoing data. The wait function blocks until an event occurs on file descriptor 4. After the event, file descriptor 4 is removed, and subsequent calls to wait will return empty vectors.

Example 3: (Edge Case)

Input: Create an Epoll instance, attempt to add an invalid file descriptor (-1) with EPOLLIN, wait for events.
Output: An error is returned when attempting to add the invalid file descriptor. The wait function should not be called.
Explanation: Attempting to add an invalid file descriptor should result in an error being returned immediately, preventing further operations on the invalid descriptor.

Constraints

• The code must be written in safe Rust, avoiding unsafe blocks as much as possible. If unsafe is necessary, it must be clearly justified and carefully managed.
• The wrapper should be compatible with standard Linux systems that support epoll.
• The wait() function should accept a timeout in seconds.
• The EpollEvent struct should contain at least the file descriptor and the event mask.
• The code should be reasonably efficient, avoiding unnecessary allocations or copies.
• The code should compile without warnings.

Notes

• You will need to use the libc crate to access the epoll system calls.
• Consider using Rust's error handling mechanisms (e.g., Result) to propagate errors.
• Think about how to represent the epoll event mask using Rust's bitwise operators.
• Focus on providing a clear and concise API that is easy to use and understand.
• This is a simplified wrapper; a production-ready wrapper would likely include more features (e.g., edge-triggered mode, signal masking). This challenge focuses on the core functionality.
• Consider using a struct to encapsulate the epoll instance and its associated data.