File upload and download in Rust

Many web applications, especially user-facing ones, provide an interface for uploading files. This can range from simple files, such as an avatar picture, to complex items such as ultra-secure, cryptographically signed contracts.

In any case, for many web services, this is a must-have — just as critical as the ability to download or access these files again once they are uploaded.

In this guide, we’ll demonstrate how to implement file upload and download in a Rust web application. We’ll use the warp web framework, but the basics will mostly apply to other async web frameworks.

There’s a nifty crate for dealing with async multipart streams called mpart-async, which streams incoming files instead of loading them into memory completely, as warp does at this time (there is an issue for this).

However, for the purpose of this tutorial, we’ll use warp’s built-in method for handling multipart requests. If your use case involves dealing with huge files you need to stream through or validate before loading into memory, the aforementioned mpart-async crate is a sensible way to go for now.

We’ll build an example web application that enables users to upload .pdf and .png files up to 5 MB. Each file will be placed in the local ./files folder with a randomly generated name and made available for download using the GET /files/$filename endpoint.

Setup

To follow along, all you need is a reasonably recent Rust installation (1.39+) and a tool to send HTTP requests, such as cURL.

First, create a new Rust project.

cargo new rust-upload-download-example
cd rust-upload-download-example

Next, edit the Cargo.toml file and add the dependencies you’ll need.

[dependencies]
tokio = { version = "0.2.21", features = ["macros", "rt-threaded", "fs"] }
warp = "0.2.3"
uuid = { version = "0.8", features = ["v4"] }
futures = { version = "=0.3.5", default-features = false }
bytes = "0.5.6"

We’re using warp to build the web service, which uses Tokio underneath. The other dependencies are used to handle the file uploads with warp. With uuid, we’ll create unique names for the uploaded files and the futures and bytes crates will help us deal with the incoming file stream.

Web service

Let’s start by creating a basic Warp web application that lets users download files from a local folder.

#[tokio::main]
async fn main() {
    let download_route = warp::path("files").and(warp::fs::dir("./files/"));

    let router = download_route.recover(handle_rejection);
    println!("Server started at localhost:8080");
    warp::serve(router).run(([0, 0, 0, 0], 8080)).await;
}

async fn handle_rejection(err: Rejection) -> std::result::Result<impl Reply, Infallible> {
    let (code, message) = if err.is_not_found() {
        (StatusCode::NOT_FOUND, "Not Found".to_string())
    } else if err.find::<warp::reject::PayloadTooLarge>().is_some() {
        (StatusCode::BAD_REQUEST, "Payload too large".to_string())
    } else {
        eprintln!("unhandled error: {:?}", err);
        (
            StatusCode::INTERNAL_SERVER_ERROR,
            "Internal Server Error".to_string(),
        )
    };

    Ok(warp::reply::with_status(message, code))
}

In this code snippet, we define a download_route at GET /files, which, using Warp’s fs::dir filter, serves files from the given path. That’s all we have to do to implement file downloads.

Of course, depending on your specific use case and the size of the files you’re dealing with — or if you have security concerns — the logic for downloading files will be more complex than this. But it’s adequate for our example.

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Uploading files

Next, let’s look at the upload route definition.

// in main
let upload_route = warp::path("upload")
    .and(warp::post())
    .and(warp::multipart::form().max_length(5_000_000))
    .and_then(upload);

let router = upload_route.or(download_route).recover(handle_rejection);

The upload route is a POST endpoint. We use Warp’s multipart::form() filter to pass through multipart requests.

We can also define a max length here. You might have noticed in the handle_rejection error handling method that we explicitly handle the PayloadTooLarge error. This error is triggered when a payload exceeds this limit.

Finally, let’s check out the upload handler. This is the core piece of this small application, and it’s a bit more complex, so we’re going to go over it step by step.

async fn upload(form: FormData) -> Result<impl Reply, Rejection> {
    let parts: Vec<Part> = form.try_collect().await.map_err(|e| {
        eprintln!("form error: {}", e);
        warp::reject::reject()
    })?;

We immediately see the FormData in the function signature. This is actually a warp::multipart::FormData, which is a stream of multipart Part elements. Since we’re dealing with a futures::Stream, we can use the TryStreamExt trait for some helpers. In this case, we use the try_collect function to gather the whole stream into a collection asynchronously, logging the error if this fails.

To understand the next part, let’s look at an example request we might send to this server using cURL.

curl --location --request POST 'http://localhost:8080/upload' \
--header 'Content-Type: multipart/form-data' \
--form '[email protected]/home/somewhere/picture.png'

As you can see in the --form option, we define our file with the name file. We could also add additional parameters, such as a file name or some additional metadata.

The next step is to iterate over our Parts collected above to see if there is a file field.

    for p in parts {
        if p.name() == "file" {
            let content_type = p.content_type();
...

We iterate over the parts we got and, if one is called file, we’ll assume it’s a file. Now we need to make sure it has the correct content type — in this case, PDF or PNG — and process it further.

            let file_ending;
            match content_type {
                Some(file_type) => match file_type {
                    "application/pdf" => {
                        file_ending = "pdf";
                    }
                    "image/png" => {
                        file_ending = "png";
                    }
                    v => {
                        eprintln!("invalid file type found: {}", v);
                        return Err(warp::reject::reject());
                    }
                },
                None => {
                    eprintln!("file type could not be determined");
                    return Err(warp::reject::reject());
                }
            }

We can use the .content_type() method of Part to check the actual file type. rust-mime-sniffer is another useful crate you could use to detect the file type.

We also set the file_ending based on the file type, so we can append it to the file we want to create later on.

If the file is of an unknown type or doesn’t have a known type, we log the error and return.

The next step is to convert the Part into a byte vector we can actually write to disk.

            let value = p
                .stream()
                .try_fold(Vec::new(), |mut vec, data| {
                    vec.put(data);
                    async move { Ok(vec) }
                })
                .await
                .map_err(|e| {
                    eprintln!("reading file error: {}", e);
                    warp::reject::reject()
                })?;

At this point, we can use Part‘s .stream() or .data() methods to get to the underlying bytes::Buf containing the data.

In this example, we turn the whole part into a stream again and, using another helper from TryStreamExt called .try_fold(), concatenate all the buffers to one Vec<u8> with all our data.

This try_fold is essentially similar to any other .reduce() or .fold() function, except that it’s asynchronous. We define an initial value (an empty vector) and then add each piece of data to it.

If any of this fails, we again log the error and return.

At this point, we parsed and validated the incoming file and have the data ready to be written to disk, which is the final step.

            let file_name = format!("./files/{}.{}", Uuid::new_v4().to_string(), file_ending);
            tokio::fs::write(&file_name, value).await.map_err(|e| {
                eprint!("error writing file: {}", e);
                warp::reject::reject()
            })?;
            println!("created file: {}", file_name);
        }
    }
    Ok("success")
}

First, we create a randomly generated, unique file name using the Uuid crate and add the above-calculated file_ending. Then, using Tokio’s fs::write, which is an asynchronous equivalent to std::fs::write, we write the data to a file with the generated file name. If it works out, we log the file name and return a success message to the caller.

Let’s try it using the following commands.

# first, upload some png
curl --location --request POST 'http://localhost:8080/upload' \
--header 'Content-Type: multipart/form-data' \
--form '[email protected]/home/somewhere/picture.png'

# check the logs for the file name, or go into the ./files folder
created file: ./files/7d678724-9480-489e-8a33-57e1ae5adb4d.png

# request the file and pipe it to a new png
curl http://localhost:8080/files/7d678724-9480-489e-8a33-57e1ae5adb4d.png > new_picture.png

It works! Fantastic. And all that in less than a hundred lines of Rust code with some very basic error handling and input validation using just a couple of crates.

You can find the full example code at GitHub.

Conclusion

Efficiently and robustly handling file uploads in a web service is not an easy task, but the Rust ecosystem provides all the tools to do so, even with options to asynchronously stream files for additional speed and flexibility.

The above example is a starting point for a real-world implementation. While it’s used in production systems, there are more things to consider. That said, the fundamentals are already there in the Rust web ecosystem to create great upload and download experiences for your users.

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