util: Add TokioContext future (#2791)

tokio-macros/src/entry.rs

@@ -1,3 +1,5 @@
+#![allow(clippy::unnecessary_lazy_evaluations)]
+
 use proc_macro::TokenStream;
 use quote::quote;
 use std::num::NonZeroUsize;

tokio-util/Cargo.toml

@@ -29,6 +29,7 @@ full = ["codec", "udp", "compat"]
 compat = ["futures-io",]
 codec = ["tokio/stream"]
 udp = ["tokio/udp"]
+rt = ["tokio/rt-core"]
 
 [dependencies]
 tokio = { version = "0.2.5", path = "../tokio" }

tokio-util/src/cfg.rs

@@ -18,6 +18,16 @@ macro_rules! cfg_compat {
     }
 }
 
+macro_rules! cfg_rt {
+    ($($item:item)*) => {
+        $(
+            #[cfg(feature = "rt")]
+            #[cfg_attr(docsrs, doc(cfg(feature = "rt")))]
+            $item
+        )*
+    }
+}
+
 macro_rules! cfg_udp {
     ($($item:item)*) => {
         $(

tokio-util/src/context.rs

@@ -0,0 +1,78 @@
+//! Tokio context aware futures utilities.
+//!
+//! This module includes utilities around integrating tokio with other runtimes
+//! by allowing the context to be attached to futures. This allows spawning
+//! futures on other executors while still using tokio to drive them. This
+//! can be useful if you need to use a tokio based library in an executor/runtime
+//! that does not provide a tokio context.
+
+use pin_project_lite::pin_project;
+use std::{
+    future::Future,
+    pin::Pin,
+    task::{Context, Poll},
+};
+use tokio::runtime::Handle;
+
+pin_project! {
+    /// `TokioContext` allows connecting a custom executor with the tokio runtime.
+    ///
+    /// It contains a `Handle` to the runtime. A handle to the runtime can be
+    /// obtain by calling the `Runtime::handle()` method.
+    pub struct TokioContext<F> {
+        #[pin]
+        inner: F,
+        handle: Handle,
+    }
+}
+
+impl<F: Future> Future for TokioContext<F> {
+    type Output = F::Output;
+
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+        let me = self.project();
+        let handle = me.handle;
+        let fut = me.inner;
+
+        handle.enter(|| fut.poll(cx))
+    }
+}
+
+/// Trait extension that simplifies bundling a `Handle` with a `Future`.
+pub trait HandleExt {
+    /// Convenience method that takes a Future and returns a `TokioContext`.
+    ///
+    /// # Example: calling Tokio Runtime from a custom ThreadPool
+    ///
+    /// ```no_run
+    /// use tokio_util::context::HandleExt;
+    /// use tokio::time::{delay_for, Duration};
+    ///
+    /// let mut rt = tokio::runtime::Builder::new()
+    ///     .threaded_scheduler()
+    ///     .enable_all()
+    ///     .build().unwrap();
+    ///
+    /// let rt2 = tokio::runtime::Builder::new()
+    ///     .threaded_scheduler()
+    ///     .build().unwrap();
+    ///
+    /// let fut = delay_for(Duration::from_millis(2));
+    ///
+    /// rt.block_on(
+    ///     rt2
+    ///         .handle()
+    ///         .wrap(async { delay_for(Duration::from_millis(2)).await }),
+    /// );
+    ///```
+    fn wrap<F: Future>(&self, fut: F) -> TokioContext<F>;
+}
+
+impl HandleExt for Handle {
+    fn wrap<F: Future>(&self, fut: F) -> TokioContext<F> {
+        TokioContext {
+            inner: fut,
+            handle: self.clone(),
+        }
+    }
+}

tokio-util/src/lib.rs

@@ -35,3 +35,7 @@ cfg_udp! {
 cfg_compat! {
     pub mod compat;
 }
+
+cfg_rt! {
+    pub mod context;
+}

tokio-util/tests/context.rs

@@ -0,0 +1,29 @@
+#![warn(rust_2018_idioms)]
+#![cfg(feature = "rt")]
+
+use tokio::runtime::Builder;
+use tokio::time::*;
+use tokio_util::context::HandleExt;
+
+#[test]
+fn tokio_context_with_another_runtime() {
+    let mut rt1 = Builder::new()
+        .threaded_scheduler()
+        .core_threads(1)
+        // no timer!
+        .build()
+        .unwrap();
+    let rt2 = Builder::new()
+        .threaded_scheduler()
+        .core_threads(1)
+        .enable_all()
+        .build()
+        .unwrap();
+
+    // Without the `HandleExt.wrap()` there would be a panic because there is
+    // no timer running, since it would be referencing runtime r1.
+    let _ = rt1.block_on(
+        rt2.handle()
+            .wrap(async move { delay_for(Duration::from_millis(2)).await }),
+    );
+}

tokio/src/lib.rs

@@ -2,7 +2,9 @@
 #![allow(
     clippy::cognitive_complexity,
     clippy::large_enum_variant,
-    clippy::needless_doctest_main
+    clippy::needless_doctest_main,
+    clippy::match_like_matches_macro,
+    clippy::stable_sort_primitive
 )]
 #![warn(
     missing_debug_implementations,

tokio/tests/macros_select.rs

@@ -1,4 +1,5 @@
-#![allow(clippy::blacklisted_name)]
+#![allow(clippy::blacklisted_name, clippy::stable_sort_primitive)]
+
 use tokio::sync::{mpsc, oneshot};
 use tokio::task;
 use tokio_test::{assert_ok, assert_pending, assert_ready};

tokio/tests/rt_common.rs

@@ -1,4 +1,4 @@
-#![allow(clippy::needless_range_loop)]
+#![allow(clippy::needless_range_loop, clippy::stable_sort_primitive)]
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "full")]
 

tokio/tests/stream_stream_map.rs

@@ -1,3 +1,5 @@
+#![allow(clippy::stable_sort_primitive)]
+
 use tokio::stream::{self, pending, Stream, StreamExt, StreamMap};
 use tokio::sync::mpsc;
 use tokio_test::{assert_ok, assert_pending, assert_ready, task};
@@ -213,17 +215,17 @@ fn new_capacity_zero() {
     let map = StreamMap::<&str, stream::Pending<()>>::new();
     assert_eq!(0, map.capacity());
 
-    let keys = map.keys().collect::<Vec<_>>();
-    assert!(keys.is_empty());
+    let mut keys = map.keys();
+    assert!(keys.next().is_none());
 }
 
 #[test]
 fn with_capacity() {
     let map = StreamMap::<&str, stream::Pending<()>>::with_capacity(10);
     assert!(10 <= map.capacity());
 
-    let keys = map.keys().collect::<Vec<_>>();
-    assert!(keys.is_empty());
+    let mut keys = map.keys();
+    assert!(keys.next().is_none());
 }
 
 #[test]

tokio/tests/sync_broadcast.rs

@@ -1,4 +1,4 @@
-#![allow(clippy::cognitive_complexity)]
+#![allow(clippy::cognitive_complexity, clippy::match_like_matches_macro)]
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "sync")]
 

tokio/tests/time_delay_queue.rs

@@ -1,4 +1,4 @@
-#![allow(clippy::blacklisted_name)]
+#![allow(clippy::blacklisted_name, clippy::stable_sort_primitive)]
 #![warn(rust_2018_idioms)]
 #![cfg(feature = "full")]