chore(examples): add helloworld-client-blocking (#179)

Add an example of using a tonic client in a synchronous environment.

tonic-examples/Cargo.toml

@@ -14,6 +14,10 @@ path = "src/helloworld/server.rs"
 name = "helloworld-client"
 path = "src/helloworld/client.rs"
 
+[[bin]]
+name = "helloworld-client-blocking"
+path = "src/helloworld/client-blocking.rs"
+
 [[bin]]
 name = "routeguide-server"
 path = "src/routeguide/server.rs"

tonic-examples/src/helloworld/client-blocking.rs

@@ -0,0 +1,56 @@
+use tokio::runtime::{Builder, Runtime};
+
+pub mod hello_world {
+    tonic::include_proto!("helloworld");
+}
+
+use hello_world::{greeter_client::GreeterClient, HelloReply, HelloRequest};
+
+type StdError = Box<dyn std::error::Error + Send + Sync + 'static>;
+type Result<T, E = StdError> = ::std::result::Result<T, E>;
+
+// The order of the fields in this struct is important. The runtime must be the first field and the
+// client must be the last field so that when `BlockingClient` is dropped the client is dropped
+// before the runtime. Not doing this will result in a deadlock when dropped.
+struct BlockingClient {
+    rt: Runtime,
+    client: GreeterClient<tonic::transport::Channel>,
+}
+
+impl BlockingClient {
+    pub fn connect<D>(dst: D) -> Result<Self, tonic::transport::Error>
+    where
+        D: std::convert::TryInto<tonic::transport::Endpoint>,
+        D::Error: Into<StdError>,
+    {
+        let mut rt = Builder::new()
+            .basic_scheduler()
+            .enable_all()
+            .build()
+            .unwrap();
+        let client = rt.block_on(GreeterClient::connect(dst))?;
+
+        Ok(Self { rt, client })
+    }
+
+    pub fn say_hello(
+        &mut self,
+        request: impl tonic::IntoRequest<HelloRequest>,
+    ) -> Result<tonic::Response<HelloReply>, tonic::Status> {
+        self.rt.block_on(self.client.say_hello(request))
+    }
+}
+
+fn main() -> Result<()> {
+    let mut client = BlockingClient::connect("http://[::1]:50051")?;
+
+    let request = tonic::Request::new(HelloRequest {
+        name: "Tonic".into(),
+    });
+
+    let response = client.say_hello(request)?;
+
+    println!("RESPONSE={:?}", response);
+
+    Ok(())
+}