doc: example on CORS and multipart for Rust (#112)

* doc: example on CORS and multipart for Rust

* doc: linking to the example from the main README

Author: Kyrremann

README.md

@@ -36,6 +36,7 @@ Table of Contents:
 | **[CORS Node](functions/cors-node/README.md)** <br/> A Node function which allows CORS requests.                                                                                      | node18    | [Serverless Framework] |
 | **[CORS Python](functions/cors-python/README.md)** <br/> A Python function which allows CORS requests.                                                                                | python310 | [Serverless Framework] |
 | **[CORS Rust](functions/cors-rust/README.md)** <br/> A Rust function which allows CORS requests.                                                                                      | rust185   | [Serverless Framework] |
+| **[CORS Multipart Rust](functions/cors-rust-multipart/README.md)** <br/> A Rust function which allows multipart requests with CORS.                                                   | rust185   | [Serverless Framework] |
 | **[Go Hello World](functions/go-hello-world/README.md)** <br/> A simple "hello world" example for Go                                                                                  | go121     | [Serverless Framework] |
 | **[Go SQS Publish](functions/go-mnq-sqs-publish/README.md)** <br/> A Go function to publish messages to SQS.                                                                          | go121     | [Serverless Framework] |
 | **[Go MultiPart Upload to S3](functions/go-upload-file-s3-multipart)** <br/> A function to upload file from form-data to S3.                                                          | go120     | [Serverless Framework] |

functions/cors-rust-multipart/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "cors-rust-multipart"
+version = "0.1.0"
+edition = "2024"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+axum = { version = "0.8.4", features = ["multipart"] }
+http = "1.3.1"

functions/cors-rust-multipart/README.md

@@ -0,0 +1,27 @@
+# Rust CORS multipart example
+
+This function shows how to handle preflight CORS requests that will be sent by a browser when invoking a _multipart_ function.
+
+The example uses totally permissive, open CORS, you will may want to modify this to make it more secure.
+
+## Setup
+
+This examples uses the [Scaleway Serverless Framework Plugin](https://github.com/scaleway/serverless-scaleway-functions).
+
+Once this is set up, you can run:
+
+```console
+npm install
+
+serverless deploy
+```
+
+Then, from the given URL, you can run:
+
+```console
+# Options request
+curl -i -X OPTIONS <function URL>
+
+# Get request
+curl -i -X GET <function URL>
+```

functions/cors-rust-multipart/package.json

@@ -0,0 +1,16 @@
+{
+  "name": "cors-rust-multipart",
+  "version": "1.0.0",
+  "main": "index.js",
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1"
+  },
+  "keywords": [],
+  "author": "",
+  "license": "ISC",
+  "dependencies": {},
+  "devDependencies": {
+    "serverless-scaleway-functions": "^0.4.4"
+  },
+  "description": ""
+}

functions/cors-rust-multipart/serverless.yml

@@ -0,0 +1,18 @@
+service: cors-rust-multipart
+configValidationMode: off
+provider:
+  name: scaleway
+  runtime: rust185
+
+plugins:
+  - serverless-scaleway-functions
+
+package:
+  patterns:
+    - "!node_modules/**"
+    - "!.gitignore"
+    - "!.git/**"
+
+functions:
+  cors-permissive:
+    handler: handler_with_cors

functions/cors-rust-multipart/src/lib.rs

@@ -0,0 +1,65 @@
+use axum::{
+    body::Body, extract::FromRequest, extract::Multipart, extract::Request, response::Response,
+};
+use http::StatusCode;
+
+// See: https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS#the_http_response_headers
+pub fn with_permissive_cors(r: http::response::Builder) -> http::response::Builder {
+    r.header("Access-Control-Allow-Headers", "*")
+        .header("Access-Control-Allow-Methods", "*")
+        .header("Access-Control-Allow-Origin", "*")
+}
+
+pub async fn handler_with_cors(req: Request<Body>) -> Response<Body> {
+    println!("{:?}", req);
+
+    // Check if this is an OPTIONS request
+    if req.method() == http::Method::OPTIONS {
+        return with_permissive_cors(Response::builder())
+            .status(StatusCode::OK)
+            .header("Content-Type", "text/plain")
+            .body(Body::from("This is allowing most CORS requests"))
+            .unwrap();
+    }
+
+    // For POST requests, extract multipart data
+    let (parts, body) = req.into_parts();
+    let body_stream = axum::body::Body::new(body);
+
+    // Reconstruct the request for multipart extraction
+    let request = Request::from_parts(parts, body_stream);
+
+    // Use Axum's built-in multipart extractor
+    match Multipart::from_request(request, &()).await {
+        Ok(multipart) => process_multipart(multipart).await,
+        Err(err) => with_permissive_cors(Response::builder())
+            .status(StatusCode::BAD_REQUEST)
+            .body(Body::from(format!(
+                "Failed to extract multipart data: {}",
+                err
+            )))
+            .unwrap(),
+    }
+}
+
+// Process the multipart data, using Axum's multipart exmaple
+// See: https://github.com/tokio-rs/axum/blob/main/examples/multipart-form/src/main.rs
+async fn process_multipart(mut multipart: Multipart) -> Response<Body> {
+    while let Some(field) = multipart.next_field().await.unwrap() {
+        let name = field.name().unwrap().to_string();
+        let file_name = field.file_name().unwrap().to_string();
+        let content_type = field.content_type().unwrap().to_string();
+        let data = field.bytes().await.unwrap();
+
+        println!(
+            "Length of `{name}` (`{file_name}`: `{content_type}`) is {} bytes",
+            data.len()
+        );
+    }
+
+    with_permissive_cors(Response::builder())
+        .status(StatusCode::OK)
+        .header("Content-Type", "text/plain")
+        .body(Body::from("Multipart data processed successfully"))
+        .unwrap()
+}