Rocket FromRequest: Triggering Outcome::Failure

Hey guys! Building APIs with Rocket, a Rust web framework, can be super exciting, especially when you're diving into request guards. These guards are like the bouncers of your API, ensuring only the right requests get through. One common task is implementing authorization checks. But what happens when the check fails? How do you trigger that Outcome::Failure in your FromRequest implementation? Let's break it down and make sure your request guards are robust and your API is secure.

Understanding Request Guards and FromRequest

Before we get into the nitty-gritty, let's quickly recap what request guards and the FromRequest trait are all about. In Rocket, request guards are a powerful mechanism for validating incoming requests before they even reach your handlers. Think of them as middleware, but more integrated into Rocket's type system. The FromRequest trait is the heart of this system. When you implement FromRequest for a type, you're telling Rocket how to construct that type from an incoming request. This could involve inspecting headers, cookies, query parameters, or anything else you can get your hands on from the Request object. The Outcome enum plays a crucial role here, signaling the result of your request guard.

Diving Deep into FromRequest and Outcome

The FromRequest trait has a single method, from_request, which takes a &Request and returns an Outcome. The Outcome enum has three variants:

• Outcome::Success(T): Indicates that the request guard was successful, and the value of type T (your guard type) is ready to be used in the handler.
• Outcome::Failure(Status, E): Signals that the request guard failed. Status is a Rocket Status code (like 401 Unauthorized or 403 Forbidden), and E is an error type that provides more details about the failure.
• Outcome::Forward(Data): Tells Rocket to skip this request guard and try the next one. This is useful when a guard doesn't apply to a particular route or when you want to chain guards together.

So, our mission is to figure out how to make our from_request function return Outcome::Failure when our authorization checks don't pass. Let's get into some code examples!

Scenarios for Triggering Outcome::Failure

Okay, let's explore some common scenarios where you'd want to trigger Outcome::Failure in your request guard. These usually revolve around authentication and authorization, but the principles apply to any kind of request validation.

1. Missing Authorization Header

The most basic scenario is when the required authorization header is missing altogether. If you're expecting an Authorization header, and it's not there, you definitely want to reject the request.

2. Invalid Authorization Header Format

Sometimes the header is present, but it's not in the format you expect. For example, you might be expecting a Bearer token, but the header contains something else, or it's malformed.

3. Invalid Token

This is where you'd check the actual token against your authentication system. If the token is expired, revoked, or simply doesn't match a valid user, you'll trigger a failure.

4. Insufficient Permissions

Even if the user is authenticated, they might not have the necessary permissions to access a particular resource. This is where you'd implement role-based or permission-based authorization checks.

Code Examples: Implementing Outcome::Failure

Alright, let's get our hands dirty with some code! We'll walk through a few examples, each demonstrating a different scenario for triggering Outcome::Failure. We will start by creating a simple example of missing authorization header then build on the example for invalid authorization header format and finally an example of invalid token

Example 1: Missing Authorization Header

Let's start with the simplest case: the Authorization header is missing. Here's how you'd implement a request guard that checks for this:

use rocket::request::{self, Request, FromRequest, Outcome};
use rocket::http::Status;

#[derive(Debug)]
pub struct ApiKey(String);

#[derive(Debug)]
pub enum ApiKeyError {
    Missing,
    Invalid,
}

#[rocket::async_trait]
impl<'r> FromRequest<'r> for ApiKey {
    type Error = ApiKeyError;

    async fn from_request(req: &'r Request<'_>) -> request::Outcome<Self, Self::Error> {
        match req.headers().get_one(