bspeice.github.io/content/articles/2016-10-22-rustic-repodcasting.md

Title: A Rustic Re-Podcasting Server (Part 1) Date: 2016-10-22 Category: Blog Tags: Rust, nutone Authors: Bradlee Speice Summary: [//]: <> "Modified: "

I listen to a lot of Drum and Bass music, because it's beautiful music. And there's a particular site, Bassdrive.com that hosts a lot of great content. Specifically, the archives section of the site has a list of the past shows that you can download and listen to. The issue is, it's just a giant list of links to download. I'd really like this in a podcast format to take with me on the road, etc.

So I wrote the elektricity web application to actually accomplish all that. Whenever you request a feed, it goes out to Bassdrive, processes all the links on a page, and serves up some fresh, tasty RSS to satisfy your ears. I hosted it on Heroku using the free tier because it's really not resource-intensive at all.

The issue so far is that I keep running out of free tier hours during a month because my podcasting application likes to have a server scan for new episodes constantly. Not sure why it's doing that, but I don't have a whole lot of control over it. It's a phenomenal application otherwise.

My (over-engineered) solution: Re-write the application using the Rust programming language. I'd like to run this on a small hacker board I own, and doing this in Rust would allow me to easily cross-compile it. Plus, I've been very interested in the Rust language for a while and this would be a great opportunity to really learn it well. The code is available here as development progresses.

The Setup

We'll be using the iron library to handle the server, and hyper to fetch the data we need from elsewhere on the interwebs. HTML5Ever allows us to ingest the content that will be coming from Bassdrive, and finally, output is done with handlebars-rust.

It will ultimately be interesting to see how much more work must be done to actually get this working over another language like Python. Coming from a dynamic state of mind it's super easy to just chain stuff together, ship it out, and call it a day. I think I'm going to end up getting much dirtier trying to write all of this out.

Issue 1: Strings

Strings in Rust are hard. I acknowledge Python can get away with some things that make strings super easy (and Python 3 has gotten better at cracking down on some bad cases, str <-> bytes specifically), but Rust is hard.

Let's take for example the 404 error handler I'm trying to write. The result should be incredibly simple: All I want is to echo back Didn't find URL: <url>. Shouldn't be that hard right? In Python I'd just do something like:

def echo_handler(request):
    return "You're visiting: {}".format(request.uri)

And we'd call it a day. Rust isn't so simple. Let's start with the trivial examples people post online:

fn hello_world(req: &mut Request) -> IronResult<Response> {
    Ok(Response::with((status::Ok, "You found the server!")))
}

Doesn't look too bad right? In fact, it's essentially the same as the Python version! All we need to do is just send back a string of some form. So, we look up the documentation for Request and see a url field that will contain what we want. Let's try the first iteration:

fn hello_world(req: &mut Request) -> IronResult<Response> {
    Ok(Response::with((status::Ok, "You found the URL: " + req.url)))
}

Which yields the error:

error[E0369]: binary operation `+` cannot be applied to type `&'static str`

OK, what's going on here? Time to start Googling for "concatenate strings in Rust". That's what we want to do right? Concatenate a static string and the URL.

After Googling, we come across a helpful concat! macro that looks really nice! Let's try that one:

fn hello_world(req: &mut Request) -> IronResult<Response> {
    Ok(Response::with((status::Ok, concat!("You found the URL: ", req.url))))
}

And the error:

error: expected a literal

Turns out Rust actually blows up because the concat! macro expects us to know at compile time what req.url is. Which, in my outsider opinion, is a bit strange. println! and format!, etc., all handle values they don't know at compile time. Why can't concat!? By any means, we need a new plan of attack. How about we try formatting strings?

fn hello_world(req: &mut Request) -> IronResult<Response> {
    Ok(Response::with((status::Ok, format!("You found the URL: {}", req.url))))
}

And at long last, it works. Onwards!

Issue 2: Fighting with the borrow checker

Rust's single coolest feature is how the compiler can guarantee safety in your program. As long as you don't use unsafe pointers in Rust, you're guaranteed safety. And not having truly manual memory management is really cool; I'm totally OK with never having to write malloc() again.

That said, even the Rust documentation makes a specific note:

Many new users to Rust experience something we like to call ‘fighting with the borrow checker’, where the Rust compiler refuses to compile a program that the author thinks is valid.

If you have to put it in the documentation, it's not a helpful note: it's hazing.

So now that we have a handler which works with information from the request, we want to start making something that looks like an actual web application. The router provided by iron isn't terribly difficult so I won't cover it. Instead, the thing that had me stumped for a couple hours was trying to dynamically create routes.

The unfortunate thing with Rust (in my limited experience at the moment) is that there is a severe lack of non-trivial examples. Using the router is easy when you want to give an example of a static function. But how do you you start working on things that are a bit more complex?

We're going to cover that here. Our first try: creating a function which returns other functions. This is a principle called currying. We set up a function that allows us to keep some data in scope for another function to come later.

fn build_handler(message: String) -> Fn(&mut Request) -> IronResult<Response> {
    move |_: &mut Request| {
        Ok(Response::with((status::Ok, message)))
    }
}

We've simply set up a function that returns another anonymous function with the message parameter scoped in. If you compile this, you get not 1, not 2, but 5 new errors. 4 of them are the same though:

error[E0277]: the trait bound `for<'r, 'r, 'r> std::ops::Fn(&'r mut iron::Request<'r, 'r>) -> std::result::Result<iron::Response, iron::IronError> + 'static: std::marker::Sized` is not satisfied

...oookay. I for one, am not going to spend time trying to figure out what's going on there.

And it is here that I will save the audience many hours of frustrated effort. At this point, I decided to switch from iron to pure hyper since using hyper would give me a much simpler API. All I would have to do is build a function that took two parameters as input, and we're done. That said, it ultimately posed many more issues because I started getting into a weird fight with the 'static lifetime and being a Rust newbie I just gave up on trying to understand it.

Instead, we will abandon (mostly) the curried function attempt, and instead take advantage of something Rust actually intends us to use: struct and trait.

Remember when I talked about a lack of non-trivial examples on the Internet? This is what I was talking about. I could only find one example of this available online, and it was incredibly complex and contained code we honestly don't need or care about. There was no documentation of how to build routes that didn't use static functions, etc. But, I'm assuming you don't really care about my whining, so let's get to it.

The iron documentation mentions the Handler trait as being something we can implement. Does the function signature for that handle() method look familiar? It's what we've been working with so far.

The principle is that we need to define a new struct to hold our data, then implement that handle() method to return the result. Something that looks like this might do:

struct EchoHandler {
    message: String
}

impl Handler for EchoHandler {
    fn handle(&self, _: &mut Request) -> IronResult<Response> {
        Ok(Response::with((status::Ok, self.message)))
    }
}

// Later in the code when we set up the router...
let echo = EchoHandler {
    message: "Is it working yet?"
}
router.get("/", echo.handle, "index");

We attempt to build a struct, and give its handle method off to the router so the router knows what to do.

You guessed it, more errors:

error: attempted to take value of method `handle` on type `EchoHandler`

Now, the Rust compiler is actually a really nice fellow, and offers us help:

help: maybe a `()` to call it is missing? If not, try an anonymous function

We definitely don't want to call that function, so maybe try an anonymous function as it recommends?

router.get("/", |req: &mut Request| echo.handle(req), "index");

Another error:

error[E0373]: closure may outlive the current function, but it borrows `echo`, which is owned by the current function

Another helpful message:

help: to force the closure to take ownership of `echo` (and any other referenced variables), use the `move` keyword

We're getting closer though! Let's implement this change:

router.get("/", move |req: &mut Request| echo.handle(req), "index");

And here's where things get strange:

error[E0507]: cannot move out of borrowed content
  --> src/main.rs:18:40
   |
18 |         Ok(Response::with((status::Ok, self.message)))
   |                                        ^^^^ cannot move out of borrowed content

Now, this took me another couple hours to figure out. I'm going to explain it, but keep this in mind: Rust only allows one reference at a time (exceptions apply of course).

When we attempt to use self.message as it has been created in the earlier struct, we essentially are trying to give it away to another piece of code. Rust's semantics then state that we may no longer access it unless it is returned to us (which iron's code does not do). There are two ways to fix this:

1. Only give away references (i.e. &self.message instead of self.message) instead of transferring ownership
2. Make a copy of the underlying value which will be safe to give away

I didn't know these were the two options originally, so I hope this helps the audience out. Because iron won't accept a reference, we are forced into the second option: making a copy. To do so, we just need to change the function to look like this:

Ok(Response::with((status::Ok, self.message.clone())))

Not so bad, huh? My only complaint is that it took so long to figure out exactly what was going on.

And now we have a small server that we can configure dynamically. At long last.

Final sidenote: You can actually do this without anonymous functions. Just change the router line to: router.get("/", echo, "index");

Rust's type system seems to figure out that we want to use the handle() method.

Conclusion

After a good long days' work, we now have the routing functionality set up on our application. We should be able to scale this pretty well in the future: the RSS content we need to deliver in the future can be treated as a string, so the building blocks are in place.

There are two important things I learned starting with Rust today:

1. Rust is a new language, and while the code is high-quality, the mindshare is coming.
2. I'm a terrible programmer.

Number 1 is pretty obvious and not surprising to anyone. Number two caught me off guard. I've gotten used to having either a garbage collector (Java, Python, etc.) or playing a little fast and loose with scoping rules (C, C++). You don't have to worry about object lifetime there. With Rust, it's forcing me to fully understand and use well the memory in my applications. In the final mistake I fixed (using .clone()) I would have been fine in C++ to just give away that reference and never use it again. I wouldn't have run into a "use-after-free" error, but I would have potentially been leaking memory. Rust forced me to be incredibly precise about how I use it.

All said I'm excited for using Rust more. I think it's super cool, it's just going to take me a lot longer to do this than I originally thought.