feat(transport): support customizing Channel's async executor (#935)

tonic/src/transport/channel/endpoint.rs

@@ -4,16 +4,19 @@ use super::Channel;
 use super::ClientTlsConfig;
 #[cfg(feature = "tls")]
 use crate::transport::service::TlsConnector;
-use crate::transport::Error;
+use crate::transport::{service::SharedExec, Error, Executor};
 use bytes::Bytes;
 use http::{uri::Uri, HeaderValue};
 use std::{
     convert::{TryFrom, TryInto},
     fmt,
+    future::Future,
+    pin::Pin,
     str::FromStr,
     time::Duration,
 };
 use tower::make::MakeConnection;
+// use crate::transport::E
 
 /// Channel builder.
 ///
@@ -37,6 +40,7 @@ pub struct Endpoint {
     pub(crate) http2_keep_alive_while_idle: Option<bool>,
     pub(crate) connect_timeout: Option<Duration>,
     pub(crate) http2_adaptive_window: Option<bool>,
+    pub(crate) executor: SharedExec,
 }
 
 impl Endpoint {
@@ -263,6 +267,17 @@ impl Endpoint {
         }
     }
 
+    /// Sets the executor used to spawn async tasks.
+    ///
+    /// Uses `tokio::spawn` by default.
+    pub fn executor<E>(mut self, executor: E) -> Self
+    where
+        E: Executor<Pin<Box<dyn Future<Output = ()> + Send>>> + Send + Sync + 'static,
+    {
+        self.executor = SharedExec::new(executor);
+        self
+    }
+
     /// Create a channel from this config.
     pub async fn connect(&self) -> Result<Channel, Error> {
         let mut http = hyper::client::connect::HttpConnector::new();
@@ -396,6 +411,7 @@ impl From<Uri> for Endpoint {
             http2_keep_alive_while_idle: None,
             connect_timeout: None,
             http2_adaptive_window: None,
+            executor: SharedExec::tokio(),
         }
     }
 }

tonic/src/transport/channel/mod.rs

@@ -9,8 +9,9 @@ pub use endpoint::Endpoint;
 #[cfg(feature = "tls")]
 pub use tls::ClientTlsConfig;
 
-use super::service::{Connection, DynamicServiceStream};
+use super::service::{Connection, DynamicServiceStream, SharedExec};
 use crate::body::BoxBody;
+use crate::transport::Executor;
 use bytes::Bytes;
 use http::{
     uri::{InvalidUri, Uri},
@@ -124,10 +125,26 @@ impl Channel {
     pub fn balance_channel<K>(capacity: usize) -> (Self, Sender<Change<K, Endpoint>>)
     where
         K: Hash + Eq + Send + Clone + 'static,
+    {
+        Self::balance_channel_with_executor(capacity, SharedExec::tokio())
+    }
+
+    /// Balance a list of [`Endpoint`]'s.
+    ///
+    /// This creates a [`Channel`] that will listen to a stream of change events and will add or remove provided endpoints.
+    ///
+    /// The [`Channel`] will use the given executor to spawn async tasks.
+    pub fn balance_channel_with_executor<K, E>(
+        capacity: usize,
+        executor: E,
+    ) -> (Self, Sender<Change<K, Endpoint>>)
+    where
+        K: Hash + Eq + Send + Clone + 'static,
+        E: Executor<Pin<Box<dyn Future<Output = ()> + Send>>> + Send + Sync + 'static,
     {
         let (tx, rx) = channel(capacity);
         let list = DynamicServiceStream::new(rx);
-        (Self::balance(list, DEFAULT_BUFFER_SIZE), tx)
+        (Self::balance(list, DEFAULT_BUFFER_SIZE, executor), tx)
     }
 
     pub(crate) fn new<C>(connector: C, endpoint: Endpoint) -> Self
@@ -138,9 +155,11 @@ impl Channel {
         C::Response: AsyncRead + AsyncWrite + HyperConnection + Unpin + Send + 'static,
     {
         let buffer_size = endpoint.buffer_size.unwrap_or(DEFAULT_BUFFER_SIZE);
+        let executor = endpoint.executor.clone();
 
         let svc = Connection::lazy(connector, endpoint);
-        let svc = Buffer::new(Either::A(svc), buffer_size);
+        let (svc, worker) = Buffer::pair(Either::A(svc), buffer_size);
+        executor.execute(Box::pin(worker));
 
         Channel { svc }
     }
@@ -153,25 +172,29 @@ impl Channel {
         C::Response: AsyncRead + AsyncWrite + HyperConnection + Unpin + Send + 'static,
     {
         let buffer_size = endpoint.buffer_size.unwrap_or(DEFAULT_BUFFER_SIZE);
+        let executor = endpoint.executor.clone();
 
         let svc = Connection::connect(connector, endpoint)
             .await
             .map_err(super::Error::from_source)?;
-        let svc = Buffer::new(Either::A(svc), buffer_size);
+        let (svc, worker) = Buffer::pair(Either::A(svc), buffer_size);
+        executor.execute(Box::pin(worker));
 
         Ok(Channel { svc })
     }
 
-    pub(crate) fn balance<D>(discover: D, buffer_size: usize) -> Self
+    pub(crate) fn balance<D, E>(discover: D, buffer_size: usize, executor: E) -> Self
     where
         D: Discover<Service = Connection> + Unpin + Send + 'static,
         D::Error: Into<crate::Error>,
         D::Key: Hash + Send + Clone,
+        E: Executor<futures_core::future::BoxFuture<'static, ()>> + Send + Sync + 'static,
     {
         let svc = Balance::new(discover);
 
         let svc = BoxService::new(svc);
-        let svc = Buffer::new(Either::B(svc), buffer_size);
+        let (svc, worker) = Buffer::pair(Either::B(svc), buffer_size);
+        executor.execute(Box::pin(worker));
 
         Channel { svc }
     }

tonic/src/transport/mod.rs

@@ -99,10 +99,12 @@ pub use self::error::Error;
 #[doc(inline)]
 pub use self::server::{NamedService, Server};
 #[doc(inline)]
-pub use self::service::TimeoutExpired;
+pub use self::service::grpc_timeout::TimeoutExpired;
 pub use self::tls::Certificate;
 pub use hyper::{Body, Uri};
 
+pub(crate) use self::service::executor::Executor;
+
 #[cfg(feature = "tls")]
 #[cfg_attr(docsrs, doc(cfg(feature = "tls")))]
 pub use self::channel::ClientTlsConfig;

tonic/src/transport/service/connection.rs

@@ -39,6 +39,7 @@ impl Connection {
             .http2_initial_connection_window_size(endpoint.init_connection_window_size)
             .http2_only(true)
             .http2_keep_alive_interval(endpoint.http2_keep_alive_interval)
+            .executor(endpoint.executor.clone())
             .clone();
 
         if let Some(val) = endpoint.http2_keep_alive_timeout {

tonic/src/transport/service/executor.rs

@@ -0,0 +1,43 @@
+use futures_core::future::BoxFuture;
+use std::{future::Future, sync::Arc};
+
+pub(crate) use hyper::rt::Executor;
+
+#[derive(Copy, Clone)]
+struct TokioExec;
+
+impl<F> Executor<F> for TokioExec
+where
+    F: Future + Send + 'static,
+    F::Output: Send + 'static,
+{
+    fn execute(&self, fut: F) {
+        tokio::spawn(fut);
+    }
+}
+
+#[derive(Clone)]
+pub(crate) struct SharedExec {
+    inner: Arc<dyn Executor<BoxFuture<'static, ()>> + Send + Sync + 'static>,
+}
+
+impl SharedExec {
+    pub(crate) fn new<E>(exec: E) -> Self
+    where
+        E: Executor<BoxFuture<'static, ()>> + Send + Sync + 'static,
+    {
+        Self {
+            inner: Arc::new(exec),
+        }
+    }
+
+    pub(crate) fn tokio() -> Self {
+        Self::new(TokioExec)
+    }
+}
+
+impl Executor<BoxFuture<'static, ()>> for SharedExec {
+    fn execute(&self, fut: BoxFuture<'static, ()>) {
+        self.inner.execute(fut)
+    }
+}

tonic/src/transport/service/mod.rs

@@ -2,7 +2,8 @@ mod add_origin;
 mod connection;
 mod connector;
 mod discover;
-mod grpc_timeout;
+pub(crate) mod executor;
+pub(crate) mod grpc_timeout;
 mod io;
 mod reconnect;
 mod router;
@@ -14,11 +15,11 @@ pub(crate) use self::add_origin::AddOrigin;
 pub(crate) use self::connection::Connection;
 pub(crate) use self::connector::connector;
 pub(crate) use self::discover::DynamicServiceStream;
+pub(crate) use self::executor::SharedExec;
 pub(crate) use self::grpc_timeout::GrpcTimeout;
 pub(crate) use self::io::ServerIo;
 #[cfg(feature = "tls")]
 pub(crate) use self::tls::{TlsAcceptor, TlsConnector};
 pub(crate) use self::user_agent::UserAgent;
 
-pub use self::grpc_timeout::TimeoutExpired;
 pub use self::router::Routes;