rust: add demo Tonic server (#4318)

Summary:
This patch defines a simple “demo” service with one RPC, which adds a
sequence of numbers. It includes a Tonic server for this service to
demonstrate the end-to-end setup.

Test Plan:
In one shell, run `bazel run -c opt //tensorboard/data/server`. In
another shell, use `grpc_cli` to send an RPC to localhost port 6789:

```
grpc_cli --channel_creds_type=insecure \
    --protofiles tensorboard/data/server/demo.proto \
    call localhost:6789 Demo.Add "term: 2 term: 2"
```

This should print a response like `sum: 4`. On my machine, it completes
in 5.2 ± 2.6 ms (methodology: run `hyperfine` on the above command).
This seems reasonably fast given that it has to establish a connection,
whereas a Python gRPC client will keep a connection open. It’s also well
below the 40ms magic number for `TCP_NODELAY` issues.

wchargin-branch: rust-demo-tonic-server

tensorboard/data/server/BUILD

@@ -6,7 +6,10 @@ licenses(["notice"])
 
 # Protocol buffer packages (as in `package foo.bar;` directives) that we need
 # to compile to Rust bindings.
-_proto_packages = ["tensorboard"]
+_proto_packages = [
+    "demo",
+    "tensorboard",
+]
 
 # Generated files with Rust protobuf bindings. These only exist in the build
 # graph, not in the source tree. The file name pattern is specified by Prost
@@ -69,11 +72,17 @@ rust_binary(
     name = "server",
     srcs = ["main.rs"],
     edition = "2018",
+    deps = [
+        ":rustboard_core",
+        "//third_party/rust:tokio",
+        "//third_party/rust:tonic",
+    ],
 )
 
 genrule(
     name = "gen_protos",
     srcs = [
+        ":demo.proto",
         "//tensorboard/compat/proto:proto_srcs",
     ],
     outs = _genproto_files,

tensorboard/data/server/demo.proto

@@ -0,0 +1,15 @@
+syntax = "proto3";
+
+package demo;
+
+service Demo {
+  rpc Add(AddRequest) returns (AddResponse);
+}
+
+message AddRequest {
+  repeated int32 term = 1;
+}
+
+message AddResponse {
+  int32 sum = 1;
+}

tensorboard/data/server/gen_protos_tool.rs

@@ -29,7 +29,13 @@ fn main() -> std::io::Result<()> {
     tonic_build::configure()
         .out_dir(&out_dir)
         .format(false) // don't run `rustfmt`; shouldn't be needed to build
-        .compile(&["tensorboard/compat/proto/event.proto"], &["."])
+        .compile(
+            &[
+                "tensorboard/compat/proto/event.proto",
+                "tensorboard/data/server/demo.proto",
+            ],
+            &["."],
+        )
         .expect("compile_protos");
     Ok(())
 }

tensorboard/data/server/lib.rs

@@ -25,6 +25,10 @@ mod scripted_reader;
 /// Protocol buffer bindings.
 #[allow(clippy::all)]
 pub mod proto {
+    /// Bindings for `package demo`, used for a demo Tonic server.
+    pub mod demo {
+        include!("demo.pb.rs");
+    }
     /// Bindings for `package tensorboard`, containing standard TensorFlow protos.
     pub mod tensorboard {
         include!("tensorboard.pb.rs");

tensorboard/data/server/main.rs

@@ -13,4 +13,38 @@ See the License for the specific language governing permissions and
 limitations under the License.
 ==============================================================================*/
 
-fn main() {}
+#![allow(clippy::needless_update)] // https://github.com/rust-lang/rust-clippy/issues/6323
+
+use tonic::{transport::Server, Request, Response};
+
+use rustboard_core::proto::demo as pb;
+
+#[derive(Debug, Default)]
+struct DemoHandler;
+
+#[tonic::async_trait]
+impl pb::demo_server::Demo for DemoHandler {
+    async fn add(
+        &self,
+        request: Request<pb::AddRequest>,
+    ) -> Result<Response<pb::AddResponse>, tonic::Status> {
+        let request = request.into_inner();
+        let sum: i32 = request.term.into_iter().sum();
+        let response = pb::AddResponse {
+            sum,
+            ..Default::default()
+        };
+        Ok(Response::new(response))
+    }
+}
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let addr = "[::0]:6789".parse::<std::net::SocketAddr>()?;
+    let handler = DemoHandler::default();
+    Server::builder()
+        .add_service(pb::demo_server::DemoServer::new(handler))
+        .serve(addr)
+        .await?;
+    Ok(())
+}