feat(subscriber) expose server parts (#451)

The `ConsoleLayer` builder provides the user with a console layer and
a server, which is used to start the gRPC server.

However, it may be desireable to expose the instrumentation server together
with other services on the same Tonic router. This was requested
explicitly in #449 428.

Additionally, to add tests which make use of the instrumentation server
(as part of improving test coverage for #450), more flexibility is
needed than what is provided by the current API. Specifically we would
like to connect a client and server via an in memory channel, rather
than a TCP connection.

This change adds an additional method to `console_subscriber::Server`
called `into_parts` which allows the user to access the
`InstrumentServer` directly. The `Aggregator` is also returned and
must be set to run for at least as long as the instrument server. This
allows the aggregator to be spawned wherever the user wishes.

To facilitate the addition of functionality which would result in more
"parts" in the future, `into_parts` returns a non-exhaustive struct,
rather than a tuple of parts.

Closes: #428

console-subscriber/src/aggregator/mod.rs

@@ -22,7 +22,13 @@ mod shrink;
 use self::id_data::{IdData, Include};
 use self::shrink::{ShrinkMap, ShrinkVec};
 
-pub(crate) struct Aggregator {
+/// Aggregates instrumentation traces and prepares state for the instrument
+/// server.
+///
+/// The `Aggregator` is responsible for receiving and organizing the
+/// instrumentated events and preparing the data to be served to a instrument
+/// client.
+pub struct Aggregator {
     /// Channel of incoming events emitted by `TaskLayer`s.
     events: mpsc::Receiver<Event>,
 
@@ -157,7 +163,12 @@ impl Aggregator {
         }
     }
 
-    pub(crate) async fn run(mut self) {
+    /// Runs the aggregator.
+    ///
+    /// This method will start the aggregator loop and should run as long as
+    /// the instrument server is running. If the instrument server stops,
+    /// this future can be aborted.
+    pub async fn run(mut self) {
         let mut publish = tokio::time::interval(self.publish_interval);
         loop {
             let should_send = tokio::select! {

console-subscriber/src/lib.rs

@@ -1,6 +1,6 @@
 #![doc = include_str!("../README.md")]
 use console_api as proto;
-use proto::resources::resource;
+use proto::{instrument::instrument_server::InstrumentServer, resources::resource};
 use serde::Serialize;
 use std::{
     cell::RefCell,
@@ -15,7 +15,10 @@ use std::{
 use thread_local::ThreadLocal;
 #[cfg(unix)]
 use tokio::net::UnixListener;
-use tokio::sync::{mpsc, oneshot};
+use tokio::{
+    sync::{mpsc, oneshot},
+    task::JoinHandle,
+};
 #[cfg(unix)]
 use tokio_stream::wrappers::UnixListenerStream;
 use tracing_core::{
@@ -39,7 +42,7 @@ mod stats;
 pub(crate) mod sync;
 mod visitors;
 
-use aggregator::Aggregator;
+pub use aggregator::Aggregator;
 pub use builder::{Builder, ServerAddr};
 use callsites::Callsites;
 use record::Recorder;
@@ -933,18 +936,16 @@ impl Server {
     ///
     /// [`tonic`]: https://docs.rs/tonic/
     pub async fn serve_with(
-        mut self,
+        self,
         mut builder: tonic::transport::Server,
     ) -> Result<(), Box<dyn std::error::Error + Send + Sync + 'static>> {
-        let aggregate = self
-            .aggregator
-            .take()
-            .expect("cannot start server multiple times");
-        let aggregate = spawn_named(aggregate.run(), "console::aggregate");
         let addr = self.addr.clone();
-        let router = builder.add_service(
-            proto::instrument::instrument_server::InstrumentServer::new(self),
-        );
+        let ServerParts {
+            instrument_server,
+            aggregator,
+        } = self.into_parts();
+        let aggregate = spawn_named(aggregator.run(), "console::aggregate");
+        let router = builder.add_service(instrument_server);
         let res = match addr {
             ServerAddr::Tcp(addr) => {
                 let serve = router.serve(addr);
@@ -960,6 +961,135 @@ impl Server {
         aggregate.abort();
         res?.map_err(Into::into)
     }
+
+    /// Returns the parts needed to spawn a gRPC server and the aggregator that
+    /// supplies it.
+    ///
+    /// Note that a server spawned in this way will disregard any value set by
+    /// [`Builder::server_addr`], as the user becomes responsible for defining
+    /// the address when calling [`Router::serve`].
+    ///
+    /// Additionally, the user of this API must ensure that the [`Aggregator`]
+    /// is running for as long as the gRPC server is. If the server stops
+    /// running, the aggregator task can be aborted.
+    ///
+    /// # Examples
+    ///
+    /// The parts can be used to serve the instrument server together with
+    /// other endpoints from the same gRPC server.
+    ///
+    /// ```
+    /// use console_subscriber::{ConsoleLayer, ServerParts};
+    ///
+    /// # let runtime = tokio::runtime::Builder::new_current_thread()
+    /// #     .enable_all()
+    /// #     .build()
+    /// #     .unwrap();
+    /// # runtime.block_on(async {
+    /// let (console_layer, server) = ConsoleLayer::builder().build();
+    /// let ServerParts {
+    ///     instrument_server,
+    ///     aggregator,
+    ///     ..
+    /// } = server.into_parts();
+    ///
+    /// let aggregator_handle = tokio::spawn(aggregator.run());
+    /// let router = tonic::transport::Server::builder()
+    ///     //.add_service(some_other_service)
+    ///     .add_service(instrument_server);
+    /// let serve = router.serve(std::net::SocketAddr::new(
+    ///     std::net::IpAddr::V4(std::net::Ipv4Addr::new(127, 0, 0, 1)),
+    ///     6669,
+    /// ));
+    ///
+    /// // Finally, spawn the server.
+    /// tokio::spawn(serve);
+    /// # // Avoid a warning that `console_layer` and `aggregator_handle` are unused.
+    /// # drop(console_layer);
+    /// # let mut aggregator_handle = aggregator_handle;
+    /// # aggregator_handle.abort();
+    /// # });
+    /// ```
+    ///
+    /// [`Router::serve`]: fn@tonic::transport::server::Router::serve
+    pub fn into_parts(mut self) -> ServerParts {
+        let aggregator = self
+            .aggregator
+            .take()
+            .expect("cannot start server multiple times");
+
+        let instrument_server = proto::instrument::instrument_server::InstrumentServer::new(self);
+
+        ServerParts {
+            instrument_server,
+            aggregator,
+        }
+    }
+}
+
+/// Server Parts
+///
+/// This struct contains the parts returned by [`Server::into_parts`]. It may contain
+/// further parts in the future, an as such is marked as `non_exhaustive`.
+///
+/// The `InstrumentServer<Server>` can be used to construct a router which
+/// can be added to a [`tonic`] gRPC server.
+///
+/// The `aggregator` is a future which should be running as long as the server is.
+/// Generally, this future should be spawned onto an appropriate runtime and then
+/// aborted if the server gets shut down.
+///
+/// See the [`Server::into_parts`] documentation for usage.
+#[non_exhaustive]
+pub struct ServerParts {
+    /// The instrument server.
+    ///
+    /// See the documentation for [`InstrumentServer`] for details.
+    pub instrument_server: InstrumentServer<Server>,
+
+    /// The aggregator.
+    ///
+    /// Responsible for collecting and preparing traces for the instrument server
+    /// to send its clients.
+    ///
+    /// The aggregator should be [`run`] when the instrument server is started.
+    /// If the server stops running for any reason, the aggregator task can be
+    /// aborted.
+    ///
+    /// [`run`]: fn@crate::Aggregator::run
+    pub aggregator: Aggregator,
+}
+
+/// Aggregator handle.
+///
+/// This object is returned from [`Server::into_parts`]. It can be
+/// used to abort the aggregator task.
+///
+/// The aggregator collects the traces that implement the async runtime
+/// being observed and prepares them to be served by the gRPC server.
+///
+/// Normally, if the server, started with [`Server::serve`] or
+/// [`Server::serve_with`] stops for any reason, the aggregator is aborted,
+/// hoewver, if the server was started with the [`InstrumentServer`] returned
+/// from [`Server::into_parts`], then it is the responsibility of the user
+/// of the API to stop the aggregator task by calling [`abort`] on this
+/// object.
+///
+/// [`abort`]: fn@crate::AggregatorHandle::abort
+pub struct AggregatorHandle {
+    join_handle: JoinHandle<()>,
+}
+
+impl AggregatorHandle {
+    /// Aborts the task running this aggregator.
+    ///
+    /// To avoid having a disconnected aggregator running forever, this
+    /// method should be called when the [`tonic::transport::Server`] started
+    /// with the [`InstrumentServer`] also returned from [`Server::into_parts`]
+    /// stops running.
+    pub fn abort(&mut self) {
+        self.join_handle.abort();
+    }
 }
 
 #[tonic::async_trait]