rt: add tokio::task::Builder (#3881)

Adds a builder API for spawning tasks. Initially, this enables the caller to name the spawned
task in order to provide better visibility into all tasks in the system.

tokio/src/runtime/handle.rs

@@ -145,7 +145,7 @@ impl Handle {
         F::Output: Send + 'static,
     {
         #[cfg(all(tokio_unstable, feature = "tracing"))]
-        let future = crate::util::trace::task(future, "task");
+        let future = crate::util::trace::task(future, "task", None);
         self.spawner.spawn(future)
     }
 
@@ -170,6 +170,15 @@ impl Handle {
     /// # }
     #[cfg_attr(tokio_track_caller, track_caller)]
     pub fn spawn_blocking<F, R>(&self, func: F) -> JoinHandle<R>
+    where
+        F: FnOnce() -> R + Send + 'static,
+        R: Send + 'static,
+    {
+        self.spawn_blocking_inner(func, None)
+    }
+
+    #[cfg_attr(tokio_track_caller, track_caller)]
+    pub(crate) fn spawn_blocking_inner<F, R>(&self, func: F, name: Option<&str>) -> JoinHandle<R>
     where
         F: FnOnce() -> R + Send + 'static,
         R: Send + 'static,
@@ -187,6 +196,7 @@ impl Handle {
                 "task",
                 kind = %"blocking",
                 function = %std::any::type_name::<F>(),
+                task.name = %name.unwrap_or_default(),
                 spawn.location = %format_args!("{}:{}:{}", location.file(), location.line(), location.column()),
             );
             #[cfg(not(tokio_track_caller))]
@@ -194,10 +204,15 @@ impl Handle {
                 target: "tokio::task",
                 "task",
                 kind = %"blocking",
+                task.name = %name.unwrap_or_default(),
                 function = %std::any::type_name::<F>(),
             );
             fut.instrument(span)
         };
+
+        #[cfg(not(all(tokio_unstable, feature = "tracing")))]
+        let _ = name;
+
         let (task, handle) = task::joinable(fut);
         let _ = self.blocking_spawner.spawn(task, &self);
         handle

tokio/src/task/builder.rs

@@ -0,0 +1,105 @@
+#![allow(unreachable_pub)]
+use crate::util::error::CONTEXT_MISSING_ERROR;
+use crate::{runtime::context, task::JoinHandle};
+use std::future::Future;
+
+/// Factory which is used to configure the properties of a new task.
+///
+/// Methods can be chained in order to configure it.
+///
+/// Currently, there is only one configuration option:
+///
+/// - [`name`], which specifies an associated name for
+///   the task
+///
+/// There are three types of task that can be spawned from a Builder:
+/// - [`spawn_local`] for executing futures on the current thread
+/// - [`spawn`] for executing [`Send`] futures on the runtime
+/// - [`spawn_blocking`] for executing blocking code in the
+///   blocking thread pool.
+///
+/// ## Example
+///
+/// ```no_run
+/// use tokio::net::{TcpListener, TcpStream};
+///
+/// use std::io;
+///
+/// async fn process(socket: TcpStream) {
+///     // ...
+/// # drop(socket);
+/// }
+///
+/// #[tokio::main]
+/// async fn main() -> io::Result<()> {
+///     let listener = TcpListener::bind("127.0.0.1:8080").await?;
+///
+///     loop {
+///         let (socket, _) = listener.accept().await?;
+///
+///         tokio::task::Builder::new()
+///             .name("tcp connection handler")
+///             .spawn(async move {
+///                 // Process each socket concurrently.
+///                 process(socket).await
+///             });
+///     }
+/// }
+/// ```
+#[derive(Default, Debug)]
+pub struct Builder<'a> {
+    name: Option<&'a str>,
+}
+
+impl<'a> Builder<'a> {
+    /// Creates a new task builder.
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    /// Assigns a name to the task which will be spawned.
+    pub fn name(&self, name: &'a str) -> Self {
+        Self { name: Some(name) }
+    }
+
+    /// Spawns a task on the executor.
+    ///
+    /// See [`task::spawn`](crate::task::spawn) for
+    /// more details.
+    #[cfg_attr(tokio_track_caller, track_caller)]
+    pub fn spawn<Fut>(self, future: Fut) -> JoinHandle<Fut::Output>
+    where
+        Fut: Future + Send + 'static,
+        Fut::Output: Send + 'static,
+    {
+        super::spawn::spawn_inner(future, self.name)
+    }
+
+    /// Spawns a task on the current thread.
+    ///
+    /// See [`task::spawn_local`](crate::task::spawn_local)
+    /// for more details.
+    #[cfg_attr(tokio_track_caller, track_caller)]
+    pub fn spawn_local<Fut>(self, future: Fut) -> JoinHandle<Fut::Output>
+    where
+        Fut: Future + 'static,
+        Fut::Output: 'static,
+    {
+        super::local::spawn_local_inner(future, self.name)
+    }
+
+    /// Spawns blocking code on the blocking threadpool.
+    ///
+    /// See [`task::spawn_blocking`](crate::task::spawn_blocking)
+    /// for more details.
+    #[cfg_attr(tokio_track_caller, track_caller)]
+    pub fn spawn_blocking<Function, Output>(self, function: Function) -> JoinHandle<Output>
+    where
+        Function: FnOnce() -> Output + Send + 'static,
+        Output: Send + 'static,
+    {
+        context::current()
+            .expect(CONTEXT_MISSING_ERROR)
+            .spawn_blocking_inner(function, self.name)
+    }
+}

tokio/src/task/local.rs

@@ -297,7 +297,14 @@ cfg_rt! {
         F: Future + 'static,
         F::Output: 'static,
     {
-        let future = crate::util::trace::task(future, "local");
+        spawn_local_inner(future, None)
+    }
+
+    pub(super) fn spawn_local_inner<F>(future: F, name: Option<&str>) -> JoinHandle<F::Output>
+    where F: Future + 'static,
+          F::Output: 'static
+    {
+        let future = crate::util::trace::task(future, "local", name);
         CURRENT.with(|maybe_cx| {
             let cx = maybe_cx
                 .expect("`spawn_local` called from outside of a `task::LocalSet`");
@@ -381,7 +388,7 @@ impl LocalSet {
         F: Future + 'static,
         F::Output: 'static,
     {
-        let future = crate::util::trace::task(future, "local");
+        let future = crate::util::trace::task(future, "local", None);
         let (task, handle) = unsafe { task::joinable_local(future) };
         self.context.tasks.borrow_mut().queue.push_back(task);
         self.context.shared.waker.wake();

tokio/src/task/mod.rs

@@ -299,4 +299,9 @@ cfg_rt! {
 
     mod unconstrained;
     pub use unconstrained::{unconstrained, Unconstrained};
+
+    cfg_trace! {
+        mod builder;
+        pub use builder::Builder;
+    }
 }

tokio/src/task/spawn.rs

@@ -1,6 +1,4 @@
-use crate::runtime;
-use crate::task::JoinHandle;
-use crate::util::error::CONTEXT_MISSING_ERROR;
+use crate::{task::JoinHandle, util::error::CONTEXT_MISSING_ERROR};
 
 use std::future::Future;
 
@@ -124,14 +122,22 @@ cfg_rt! {
     /// error[E0391]: cycle detected when processing `main`
     /// ```
     #[cfg_attr(tokio_track_caller, track_caller)]
-    pub fn spawn<T>(task: T) -> JoinHandle<T::Output>
+    pub fn spawn<T>(future: T) -> JoinHandle<T::Output>
     where
         T: Future + Send + 'static,
         T::Output: Send + 'static,
     {
-        let spawn_handle = runtime::context::spawn_handle()
-        .expect(CONTEXT_MISSING_ERROR);
-        let task = crate::util::trace::task(task, "task");
+        spawn_inner(future, None)
+    }
+
+    #[cfg_attr(tokio_track_caller, track_caller)]
+    pub(super) fn spawn_inner<T>(future: T, name: Option<&str>) -> JoinHandle<T::Output>
+    where
+        T: Future + Send + 'static,
+        T::Output: Send + 'static,
+    {
+        let spawn_handle = crate::runtime::context::spawn_handle().expect(CONTEXT_MISSING_ERROR);
+        let task = crate::util::trace::task(future, "task", name);
         spawn_handle.spawn(task)
     }
 }

tokio/src/util/trace.rs

@@ -4,7 +4,7 @@ cfg_trace! {
 
         #[inline]
         #[cfg_attr(tokio_track_caller, track_caller)]
-        pub(crate) fn task<F>(task: F, kind: &'static str) -> Instrumented<F> {
+        pub(crate) fn task<F>(task: F, kind: &'static str, name: Option<&str>) -> Instrumented<F> {
             use tracing::instrument::Instrument;
             #[cfg(tokio_track_caller)]
             let location = std::panic::Location::caller();
@@ -14,12 +14,14 @@ cfg_trace! {
                 "task",
                 %kind,
                 spawn.location = %format_args!("{}:{}:{}", location.file(), location.line(), location.column()),
+                task.name = %name.unwrap_or_default()
             );
             #[cfg(not(tokio_track_caller))]
             let span = tracing::trace_span!(
                 target: "tokio::task",
                 "task",
                 %kind,
+                task.name = %name.unwrap_or_default()
             );
             task.instrument(span)
         }
@@ -29,7 +31,7 @@ cfg_trace! {
 cfg_not_trace! {
     cfg_rt! {
         #[inline]
-        pub(crate) fn task<F>(task: F, _: &'static str) -> F {
+        pub(crate) fn task<F>(task: F, _: &'static str, _name: Option<&str>) -> F {
             // nop
             task
         }

tokio/tests/task_builder.rs

@@ -0,0 +1,67 @@
+#[cfg(all(tokio_unstable, feature = "tracing"))]
+mod tests {
+    use std::rc::Rc;
+    use tokio::{
+        task::{Builder, LocalSet},
+        test,
+    };
+
+    #[test]
+    async fn spawn_with_name() {
+        let result = Builder::new()
+            .name("name")
+            .spawn(async { "task executed" })
+            .await;
+
+        assert_eq!(result.unwrap(), "task executed");
+    }
+
+    #[test]
+    async fn spawn_blocking_with_name() {
+        let result = Builder::new()
+            .name("name")
+            .spawn_blocking(|| "task executed")
+            .await;
+
+        assert_eq!(result.unwrap(), "task executed");
+    }
+
+    #[test]
+    async fn spawn_local_with_name() {
+        let unsend_data = Rc::new("task executed");
+        let result = LocalSet::new()
+            .run_until(async move {
+                Builder::new()
+                    .name("name")
+                    .spawn_local(async move { unsend_data })
+                    .await
+            })
+            .await;
+
+        assert_eq!(*result.unwrap(), "task executed");
+    }
+
+    #[test]
+    async fn spawn_without_name() {
+        let result = Builder::new().spawn(async { "task executed" }).await;
+
+        assert_eq!(result.unwrap(), "task executed");
+    }
+
+    #[test]
+    async fn spawn_blocking_without_name() {
+        let result = Builder::new().spawn_blocking(|| "task executed").await;
+
+        assert_eq!(result.unwrap(), "task executed");
+    }
+
+    #[test]
+    async fn spawn_local_without_name() {
+        let unsend_data = Rc::new("task executed");
+        let result = LocalSet::new()
+            .run_until(async move { Builder::new().spawn_local(async move { unsend_data }).await })
+            .await;
+
+        assert_eq!(*result.unwrap(), "task executed");
+    }
+}