feat(examples): add grpc-web example (#710)

Creates a server using tonic-web and a client that uses a regular
`hyper::Client` and issues a regular HTTP/1.1 request.

examples/src/grpc-web/client.rs

@@ -0,0 +1,90 @@
+use bytes::{Buf, BufMut, Bytes, BytesMut};
+use hello_world::{HelloReply, HelloRequest};
+use http::header::{ACCEPT, CONTENT_TYPE};
+
+pub mod hello_world {
+    tonic::include_proto!("helloworld");
+}
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let msg = HelloRequest {
+        name: "Bob".to_string(),
+    };
+
+    // a good old http/1.1 request
+    let request = http::Request::builder()
+        .version(http::Version::HTTP_11)
+        .method(http::Method::POST)
+        .uri("http://127.0.0.1:3000/helloworld.Greeter/SayHello")
+        .header(CONTENT_TYPE, "application/grpc-web")
+        .header(ACCEPT, "application/grpc-web")
+        .body(hyper::Body::from(encode_body(msg)))
+        .unwrap();
+
+    let client = hyper::Client::new();
+
+    let response = client.request(request).await.unwrap();
+
+    assert_eq!(
+        response.headers().get(CONTENT_TYPE).unwrap(),
+        "application/grpc-web+proto"
+    );
+
+    let body = response.into_body();
+    let reply = decode_body::<HelloReply>(body).await;
+
+    println!("REPLY={:?}", reply);
+
+    Ok(())
+}
+
+// one byte for the compression flag plus four bytes for the length
+const GRPC_HEADER_SIZE: usize = 5;
+
+fn encode_body<T>(msg: T) -> Bytes
+where
+    T: prost::Message,
+{
+    let mut buf = BytesMut::with_capacity(1024);
+
+    // first skip past the header
+    // cannot write it yet since we don't know the size of the
+    // encoded message
+    buf.reserve(GRPC_HEADER_SIZE);
+    unsafe {
+        buf.advance_mut(GRPC_HEADER_SIZE);
+    }
+
+    // write the message
+    msg.encode(&mut buf).unwrap();
+
+    // now we know the size of encoded message and can write the
+    // header
+    let len = buf.len() - GRPC_HEADER_SIZE;
+    {
+        let mut buf = &mut buf[..GRPC_HEADER_SIZE];
+
+        // compression flag, 0 means "no compression"
+        buf.put_u8(0);
+
+        buf.put_u32(len as u32);
+    }
+
+    buf.split_to(len + GRPC_HEADER_SIZE).freeze()
+}
+
+async fn decode_body<T>(body: hyper::Body) -> T
+where
+    T: Default + prost::Message,
+{
+    let mut body = hyper::body::to_bytes(body).await.unwrap();
+
+    // ignore the compression flag
+    body.advance(1);
+
+    let len = body.get_u32();
+    let msg = T::decode(&mut body.split_to(len as usize)).unwrap();
+
+    msg
+}

examples/src/grpc-web/server.rs

@@ -0,0 +1,49 @@
+use tonic::{transport::Server, Request, Response, Status};
+
+use hello_world::greeter_server::{Greeter, GreeterServer};
+use hello_world::{HelloReply, HelloRequest};
+
+pub mod hello_world {
+    tonic::include_proto!("helloworld");
+}
+
+#[derive(Default)]
+pub struct MyGreeter {}
+
+#[tonic::async_trait]
+impl Greeter for MyGreeter {
+    async fn say_hello(
+        &self,
+        request: Request<HelloRequest>,
+    ) -> Result<Response<HelloReply>, Status> {
+        println!("Got a request from {:?}", request.remote_addr());
+
+        let reply = hello_world::HelloReply {
+            message: format!("Hello {}!", request.into_inner().name),
+        };
+        Ok(Response::new(reply))
+    }
+}
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    tracing_subscriber::fmt::init();
+
+    let addr = "127.0.0.1:3000".parse().unwrap();
+
+    let greeter = MyGreeter::default();
+    let greeter = GreeterServer::new(greeter);
+    let greeter = tonic_web::config()
+        .allow_origins(vec!["127.0.0.1"])
+        .enable(greeter);
+
+    println!("GreeterServer listening on {}", addr);
+
+    Server::builder()
+        .accept_http1(true)
+        .add_service(greeter)
+        .serve(addr)
+        .await?;
+
+    Ok(())
+}