Repository: rust-lang-nursery/futures-rs (rust)

Issue:
Provide alternative precedences for select!
The `select!` macro currently selects a random future in case multiple futures are ready.
It would be useful to be able to specify the precedence, for example choosing the first future like the existing behavior of `Select`.

This could be implemented by providing a parameter or with a separate wrapper macro such as `select_first!`.

Of course it's possible to work around this by not using the `select!` macro or by calling it multiple times, but it seems useful to have.

Pull request:
Add select_biased! macro
Closes #1805

r? @cramertj 


Patch:
diff --git a/futures-macro/src/lib.rs b/futures-macro/src/lib.rs
index 87833a1b..c629ba15 100644
--- a/futures-macro/src/lib.rs
+++ b/futures-macro/src/lib.rs
@@ -35,3 +35,9 @@ pub fn try_join(input: TokenStream) -> TokenStream {
 pub fn select(input: TokenStream) -> TokenStream {
     crate::select::select(input)
 }
+
+/// The `select_biased!` macro.
+#[proc_macro_hack]
+pub fn select_biased(input: TokenStream) -> TokenStream {
+    crate::select::select_biased(input)
+}
diff --git a/futures-macro/src/select.rs b/futures-macro/src/select.rs
index 29884a0d..82b221e7 100644
--- a/futures-macro/src/select.rs
+++ b/futures-macro/src/select.rs
@@ -136,6 +136,15 @@ fn declare_result_enum(
 
 /// The `select!` macro.
 pub(crate) fn select(input: TokenStream) -> TokenStream {
+    select_inner(input, true)
+}
+
+/// The `select_biased!` macro.
+pub(crate) fn select_biased(input: TokenStream) -> TokenStream {
+    select_inner(input, false)
+}
+
+fn select_inner(input: TokenStream, random: bool) -> TokenStream {
     let parsed = syn::parse_macro_input!(input as Select);
 
     let futures_crate: syn::Path = parsed.futures_crate_path.unwrap_or_else(|| parse_quote!(::futures_util));
@@ -286,6 +295,14 @@ pub(crate) fn select(input: TokenStream) -> TokenStream {
         }
     };
 
+    let shuffle = if random {
+        quote! {
+            #futures_crate::async_await::shuffle(&mut __select_arr);
+        }
+    } else {
+        quote!()
+    };
+
     TokenStream::from(quote! { {
         #enum_item
 
@@ -298,7 +315,7 @@ pub(crate) fn select(input: TokenStream) -> TokenStream {
                 #( #poll_functions )*
 
                 let mut __select_arr = [#( #variant_names ),*];
-                #futures_crate::async_await::shuffle(&mut __select_arr);
+                #shuffle
                 for poller in &mut __select_arr {
                     let poller: &mut &mut dyn FnMut(
                         &mut #futures_crate::task::Context<'_>
diff --git a/futures-util/src/async_await/select_mod.rs b/futures-util/src/async_await/select_mod.rs
index 221b6fdb..edca930a 100644
--- a/futures-util/src/async_await/select_mod.rs
+++ b/futures-util/src/async_await/select_mod.rs
@@ -5,7 +5,7 @@ use proc_macro_hack::proc_macro_hack;
 #[doc(hidden)]
 #[macro_export]
 macro_rules! document_select_macro {
-    ($item:item) => {
+    ($select:item $select_biased:item) => {
         /// Polls multiple futures and streams simultaneously, executing the branch
         /// for the future that finishes first. If multiple futures are ready,
         /// one will be pseudo-randomly selected at runtime. Futures directly
@@ -99,7 +99,7 @@ macro_rules! document_select_macro {
         /// # futures::executor::block_on(async {
         /// use futures::future::FutureExt;
         /// use futures::select;
-        /// use pin_utils::pin_mut;
+        /// use futures::pin_mut;
         ///
         /// // Calling the following async fn returns a Future which does not
         /// // implement Unpin
@@ -153,11 +153,162 @@ macro_rules! document_select_macro {
         /// from inside the `select!` block's branches. This can be used to implement
         /// more complex behavior such as timer resets or writing into the head of
         /// a stream.
-        $item
+        $select
+
+        /// Polls multiple futures and streams simultaneously, executing the branch
+        /// for the future that finishes first. Unlike [`select!`], if multiple futures are ready,
+        /// one will be selected in order of declaration. Futures directly
+        /// passed to `select_biased!` must be `Unpin` and implement `FusedFuture`.
+        ///
+        /// If an expression which yields a `Future` is passed to `select_biased!`
+        /// (e.g. an `async fn` call) instead of a `Future` by name the `Unpin`
+        /// requirement is relaxed, since the macro will pin the resulting `Future`
+        /// on the stack. However the `Future` returned by the expression must
+        /// still implement `FusedFuture`. This difference is presented
+        ///
+        /// Futures and streams which are not already fused can be fused using the
+        /// `.fuse()` method. Note, though, that fusing a future or stream directly
+        /// in the call to `select_biased!` will not be enough to prevent it from being
+        /// polled after completion if the `select_biased!` call is in a loop, so when
+        /// `select_biased!`ing in a loop, users should take care to `fuse()` outside of
+        /// the loop.
+        ///
+        /// `select_biased!` can be used as an expression and will return the return
+        /// value of the selected branch. For this reason the return type of every
+        /// branch in a `select_biased!` must be the same.
+        ///
+        /// This macro is only usable inside of async functions, closures, and blocks.
+        /// It is also gated behind the `async-await` feature of this library, which is
+        /// _not_ activated by default.
+        ///
+        /// # Examples
+        ///
+        /// ```
+        /// # futures::executor::block_on(async {
+        /// use futures::future;
+        /// use futures::select_biased;
+        /// let mut a = future::ready(4);
+        /// let mut b = future::pending::<()>();
+        ///
+        /// let res = select_biased! {
+        ///     a_res = a => a_res + 1,
+        ///     _ = b => 0,
+        /// };
+        /// assert_eq!(res, 5);
+        /// # });
+        /// ```
+        ///
+        /// ```
+        /// # futures::executor::block_on(async {
+        /// use futures::future;
+        /// use futures::stream::{self, StreamExt};
+        /// use futures::select_biased;
+        /// let mut st = stream::iter(vec![2]).fuse();
+        /// let mut fut = future::pending::<()>();
+        ///
+        /// select_biased! {
+        ///     x = st.next() => assert_eq!(Some(2), x),
+        ///     _ = fut => panic!(),
+        /// };
+        /// # });
+        /// ```
+        ///
+        /// As described earlier, `select_biased` can directly select on expressions
+        /// which return `Future`s - even if those do not implement `Unpin`:
+        ///
+        /// ```
+        /// # futures::executor::block_on(async {
+        /// use futures::future::FutureExt;
+        /// use futures::select_biased;
+        ///
+        /// // Calling the following async fn returns a Future which does not
+        /// // implement Unpin
+        /// async fn async_identity_fn(arg: usize) -> usize {
+        ///     arg
+        /// }
+        ///
+        /// let res = select_biased! {
+        ///     a_res = async_identity_fn(62).fuse() => a_res + 1,
+        ///     b_res = async_identity_fn(13).fuse() => b_res,
+        /// };
+        /// assert!(res == 63 || res == 12);
+        /// # });
+        /// ```
+        ///
+        /// If a similar async function is called outside of `select_biased` to produce
+        /// a `Future`, the `Future` must be pinned in order to be able to pass
+        /// it to `select_biased`. This can be achieved via `Box::pin` for pinning a
+        /// `Future` on the heap or the `pin_mut!` macro for pinning a `Future`
+        /// on the stack.
+        ///
+        /// ```
+        /// # futures::executor::block_on(async {
+        /// use futures::future::FutureExt;
+        /// use futures::select_biased;
+        /// use futures::pin_mut;
+        ///
+        /// // Calling the following async fn returns a Future which does not
+        /// // implement Unpin
+        /// async fn async_identity_fn(arg: usize) -> usize {
+        ///     arg
+        /// }
+        ///
+        /// let fut_1 = async_identity_fn(1).fuse();
+        /// let fut_2 = async_identity_fn(2).fuse();
+        /// let mut fut_1 = Box::pin(fut_1); // Pins the Future on the heap
+        /// pin_mut!(fut_2); // Pins the Future on the stack
+        ///
+        /// let res = select_biased! {
+        ///     a_res = fut_1 => a_res,
+        ///     b_res = fut_2 => b_res,
+        /// };
+        /// assert!(res == 1 || res == 2);
+        /// # });
+        /// ```
+        ///
+        /// `select_biased` also accepts a `complete` branch and a `default` branch.
+        /// `complete` will run if all futures and streams have already been
+        /// exhausted. `default` will run if no futures or streams are
+        /// immediately ready. `complete` takes priority over `default` in
+        /// the case where all futures have completed.
+        /// A motivating use-case for passing `Future`s by name as well as for
+        /// `complete` blocks is to call `select_biased!` in a loop, which is
+        /// demonstrated in the following example:
+        ///
+        /// ```
+        /// # futures::executor::block_on(async {
+        /// use futures::future;
+        /// use futures::select_biased;
+        /// let mut a_fut = future::ready(4);
+        /// let mut b_fut = future::ready(6);
+        /// let mut total = 0;
+        ///
+        /// loop {
+        ///     select_biased! {
+        ///         a = a_fut => total += a,
+        ///         b = b_fut => total += b,
+        ///         complete => break,
+        ///         default => panic!(), // never runs (futures run first, then complete)
+        ///     };
+        /// }
+        /// assert_eq!(total, 10);
+        /// # });
+        /// ```
+        ///
+        /// Note that the futures that have been matched over can still be mutated
+        /// from inside the `select_biased!` block's branches. This can be used to implement
+        /// more complex behavior such as timer resets or writing into the head of
+        /// a stream.
+        ///
+        /// [`select!`]: macro.select.html
+        $select_biased
     }
 }
 
 document_select_macro! {
     #[proc_macro_hack(support_nested)]
     pub use futures_macro::select;
+
+    #[proc_macro_hack(support_nested)]
+    pub use futures_macro::select_biased;
 }
diff --git a/futures/src/lib.rs b/futures/src/lib.rs
index 89d098cf..a9419029 100644
--- a/futures/src/lib.rs
+++ b/futures/src/lib.rs
@@ -554,6 +554,7 @@ pub mod inner_macro {
     pub use futures_util::join;
     pub use futures_util::try_join;
     pub use futures_util::select;
+    pub use futures_util::select_biased;
 }
 
 #[cfg(feature = "std")]
@@ -592,4 +593,14 @@ futures_util::document_select_macro! {
             }
         }
     }
+
+    #[macro_export]
+    macro_rules! select_biased { // replace `::futures_util` with `::futures` as the crate path
+        ($($tokens:tt)*) => {
+            $crate::inner_macro::select_biased! {
+                futures_crate_path ( ::futures )
+                $( $tokens )*
+            }
+        }
+    }
 }


Test patch:
diff --git a/futures/tests/async_await_macros.rs b/futures/tests/async_await_macros.rs
index ce48b25a..d2ab30c1 100644
--- a/futures/tests/async_await_macros.rs
+++ b/futures/tests/async_await_macros.rs
@@ -56,6 +56,26 @@ fn select() {
     assert!(ran);
 }
 
+#[test]
+fn select_biased() {
+    use futures::select_biased;
+
+    let (tx1, rx1) = oneshot::channel::<i32>();
+    let (_tx2, rx2) = oneshot::channel::<i32>();
+    tx1.send(1).unwrap();
+    let mut ran = false;
+    block_on(async {
+        select_biased! {
+            res = rx1.fuse() => {
+                assert_eq!(Ok(1), res);
+                ran = true;
+            },
+            _ = rx2.fuse() => unreachable!(),
+        }
+    });
+    assert!(ran);
+}
+
 #[test]
 fn select_streams() {
     let (mut tx1, rx1) = mpsc::channel::<i32>(1);