Customizable Redirect Policy about reqwest HOT 4 CLOSED

@Arnavion mind adding exactly what you'd need to the issue?

Well, currently reqwest effectively has a redirect policy of FollowAll because it sets the underlying request policy to FollowNone and has a loop to always follow redirects. I want to be able to control which redirects get followed and which don't, which hyper does by letting me set a FollowIf(fn(&Url) -> bool) policy.

My crate interfaces with a web API that sometimes sends a redirect to a login page instead of a 401/403 response, so by preventing that redirect I'm able to report an equivalent error to the caller of my function instead of trying to deserialize an HTML page as JSON.

I also plan in the future to prevent redirects from taking me to hostnames outside of a whitelist to prevent abuse.

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I've felt that the RedirectPolicy in hyper was too limited, and wanted to use this opportunity to make a better one. To start, the struct won't have it's variants exposed, so new ones can be added. Also, other problems with RedirectPolicy::FollowIf are:

• It must be a standard function, so setting configuration variables at startup and using them is not possible
• It is not possible to check for infinite redirects
• It is not possible to set a number limit

Looking at other implementations for inspiration, there is this in golang's http package:

CheckRedirect func(req *Request, via []*Request) error

This allows a user to a) count the number of previous requests, b) check for duplicate URLs, among other things. If I understand Go enough, I believe that also means the CheckRedirect function could even modify the *Request, but I don't know if that's something desirable or not.

So, the design I've been thinking of looks something like this:

trait Redirect {
    fn redirect(&self, next: &Url, previous: &[Url]) -> bool;
}

impl Client {
    pub fn set_redirect_policy<T: Redirect>(&mut self, policy: T) {
        self.redirect_policy = Box::new(policy);
    }
}

The Redirect trait could perhaps receive Request instead of Urls, if there is use in inspecting the Headers also.

There could be a couple built-ins, such as:

• RedirectPolicy::default() - max redirects 10, look for loops
• RedirectPolicy::none() - always return false

It would also allow custom policies like this:

let max_redirects = config.load('max_redirects').unwrap_or(5);
client.set_redirect_policy(move |_next, previous| {
    previous.len() >= max_redirects    
})

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That sounds good. Maybe you can also add operators to combine redirect policies but that can be done in user code as well.

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I have an implementation in #29.

I ended up not having a trait Redirect, because the compiler had issues infer lifetimes of the &Url and &[Url] arguments passed to closures. Otherwise, it does what was suggested in my previous comment.

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