Resource Acquisition Is Initialization (RAII) in Rust: File Handling

RAII (Resource Acquisition Is Initialization) is a powerful programming idiom where resource acquisition is tied to the lifetime of an object. In Rust, this is naturally achieved through its ownership and borrowing system. This challenge focuses on implementing RAII to safely manage file resources, ensuring files are properly closed even in the presence of errors or panics.

Problem Description

You are tasked with creating a FileGuard struct in Rust that encapsulates file handling and guarantees the file is closed when the FileGuard instance goes out of scope. The FileGuard should:

1. Acquire a file: Take a file path as input and attempt to open the file in read-only mode.
2. Handle errors: If the file cannot be opened, return a Result indicating the error.
3. Ensure closure: When the FileGuard instance is dropped (goes out of scope), the file should be automatically closed.
4. Provide file descriptor: Offer a method to retrieve the file descriptor (as a u32) associated with the opened file. This is necessary for interacting with the file.
5. Prevent double-free: Ensure that the file is not closed more than once.

Examples

Example 1:

Input: "my_file.txt" (file exists and is readable)
Output: Ok(FileGuard { fd: 3 })  (assuming file descriptor 3 is assigned)
Explanation: The file "my_file.txt" is successfully opened, and a `FileGuard` is created with the file descriptor 3.

Example 2:

Input: "nonexistent_file.txt" (file does not exist)
Output: Err(std::io::Error { kind: NotFound, details: None })
Explanation: The file "nonexistent_file.txt" cannot be opened, and an `io::Error` with the `NotFound` kind is returned.

Example 3:

Input: "read_only_file.txt" (file exists, but permissions prevent reading)
Output: Err(std::io::Error { kind: PermissionDenied, details: None })
Explanation: The file "read_only_file.txt" cannot be opened due to permission issues, and an `io::Error` with the `PermissionDenied` kind is returned.

Constraints

• The file should be opened in read-only mode (OpenOptions::new().read(true).open).
• The FileGuard should use std::fs::File internally.
• The FileGuard should implement the Drop trait to ensure automatic file closure.
• The get_fd method should return Err(std::io::Error) if the file hasn't been successfully opened.
• Error handling should use the standard std::io::Error type.
• The file descriptor returned by get_fd should be a u32.

Notes

• Consider using std::os::unix::io::AsRawFd to obtain the file descriptor from the std::fs::File.
• The Drop trait is crucial for RAII. The destructor will be automatically called when the FileGuard goes out of scope.
• Think about how to handle potential errors during file opening and ensure they are propagated correctly.
• This exercise demonstrates a fundamental aspect of Rust's memory safety and resource management. Proper RAII implementation prevents resource leaks and improves code reliability.
• You do not need to implement any file reading or writing functionality; the focus is solely on acquiring and releasing the file resource.