feat(mpsc): initial sync and async channel APIs (#2)

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

Cargo.toml

@@ -13,4 +13,12 @@ default = ["std"]
 [dependencies]
 
 [dev-dependencies]
-loom = "0.5"
+loom = { version = "0.5", features = ["checkpoint", "futures"] }
+# So that we can use `poll_fn` in tests.
+futures-util = "0.3"
+
+[profile.test]
+opt-level = 3
+
+[patch.crates-io]
+loom = { git = "https://github.com/tokio-rs/loom", branch = "master"}

src/lib.rs

@@ -1,48 +1,25 @@
 #![cfg_attr(not(feature = "std"), no_std)]
-
+#![cfg_attr(docsrs, feature(doc_cfg))]
 use core::{fmt, mem::MaybeUninit, ops::Index};
 
-#[cfg(feature = "alloc")]
-extern crate alloc;
-
-macro_rules! test_println {
-    ($($arg:tt)*) => {
-        if cfg!(test) {
-            if crate::util::panicking() {
-                // getting the thread ID while panicking doesn't seem to play super nicely with loom's
-                // mock lazy_static...
-                println!("[PANIC {:>17}:{:<3}] {}", file!(), line!(), format_args!($($arg)*))
-            } else {
-                println!("[{:?} {:>17}:{:<3}] {}", crate::loom::thread::current().id(), file!(), line!(), format_args!($($arg)*))
-            }
-        }
-    }
-}
-
-macro_rules! test_dbg {
-    ($e:expr) => {
-        match $e {
-            e => {
-                #[cfg(test)]
-                test_println!("{} = {:?}", stringify!($e), &e);
-                e
-            }
-        }
-    };
-}
+#[macro_use]
+mod macros;
 
 mod loom;
 mod util;
 
-#[cfg(feature = "alloc")]
-mod thingbuf;
-#[cfg(feature = "alloc")]
-pub use self::thingbuf::ThingBuf;
-#[cfg(feature = "alloc")]
-mod stringbuf;
+feature! {
+    #![feature = "alloc"]
+    extern crate alloc;
 
-#[cfg(feature = "alloc")]
-pub use stringbuf::{StaticStringBuf, StringBuf};
+    mod thingbuf;
+    pub use self::thingbuf::ThingBuf;
+
+    mod stringbuf;
+    pub use stringbuf::{StaticStringBuf, StringBuf};
+
+    pub mod mpsc;
+}
 
 mod static_thingbuf;
 pub use self::static_thingbuf::StaticThingBuf;
@@ -55,6 +32,14 @@ use crate::{
     util::{Backoff, CachePadded},
 };
 
+pub struct Ref<'slot, T> {
+    slot: &'slot Slot<T>,
+    new_state: usize,
+}
+
+#[derive(Debug)]
+pub struct AtCapacity(pub(crate) usize);
+
 #[derive(Debug)]
 struct Core {
     head: CachePadded<AtomicUsize>,
@@ -65,15 +50,7 @@ struct Core {
     capacity: usize,
 }
 
-pub struct Ref<'slot, T> {
-    slot: &'slot Slot<T>,
-    new_state: usize,
-}
-
-#[derive(Debug)]
-pub struct AtCapacity(usize);
-
-pub struct Slot<T> {
+struct Slot<T> {
     value: UnsafeCell<MaybeUninit<T>>,
     state: AtomicUsize,
 }

src/loom.rs

@@ -6,7 +6,7 @@ mod inner {
         pub use loom::sync::atomic::*;
         pub use std::sync::atomic::Ordering;
     }
-    pub(crate) use loom::{cell::UnsafeCell, hint, thread};
+    pub(crate) use loom::{cell::UnsafeCell, future, hint, sync, thread};
 
     pub(crate) fn model(f: impl Fn() + Sync + Send + 'static) {
         let iteration = core::sync::atomic::AtomicUsize::new(0);
@@ -72,6 +72,13 @@ mod inner {
 #[cfg(not(test))]
 mod inner {
     #![allow(dead_code)]
+    pub(crate) mod sync {
+        pub use core::sync::*;
+
+        #[cfg(feature = "alloc")]
+        pub use alloc::sync::*;
+    }
+
     pub(crate) use core::sync::atomic;
 
     #[cfg(feature = "std")]

src/macros.rs

@@ -0,0 +1,67 @@
+macro_rules! test_println {
+    ($($arg:tt)*) => {
+        if cfg!(test) {
+            if crate::util::panic::panicking() {
+                // getting the thread ID while panicking doesn't seem to play super nicely with loom's
+                // mock lazy_static...
+                println!("[PANIC {:>17}:{:<3}] {}", file!(), line!(), format_args!($($arg)*))
+            } else {
+                println!("[{:?} {:>17}:{:<3}] {}", crate::loom::thread::current().id(), file!(), line!(), format_args!($($arg)*))
+            }
+        }
+    }
+}
+
+macro_rules! test_dbg {
+    ($e:expr) => {
+        match $e {
+            e => {
+                #[cfg(test)]
+                test_println!("{} = {:?}", stringify!($e), &e);
+                e
+            }
+        }
+    };
+}
+
+macro_rules! feature {
+    (
+        #![$meta:meta]
+        $($item:item)*
+    ) => {
+        $(
+            #[cfg($meta)]
+            #[cfg_attr(docsrs, doc(cfg($meta)))]
+            $item
+        )*
+    }
+}
+
+#[allow(unused_macros)]
+macro_rules! unreachable_unchecked {
+    ($($arg:tt)+) => {
+        crate::unreachable_unchecked!(@inner , format_args!(": {}", format_args!($($arg)*)))
+    };
+
+    () => {
+        crate::unreachable_unchecked!(@inner ".")
+    };
+
+    (@inner $msg:expr) => {
+        #[cfg(debug_assertions)] {
+            panic!(
+                "internal error: entered unreachable code{}\n\n\
+                /!\\ EXTREMELY SERIOUS WARNING /!\\\n
+                This code should NEVER be entered; in release mode, this would \
+                have been an `unreachable_unchecked` hint. The fact that this \
+                occurred means something VERY bad is going on. \n\
+                Please contact the `thingbuf` maintainers immediately. Sorry!",
+                $msg,
+            );
+        }
+        #[cfg(not(debug_assertions))]
+        unsafe {
+            core::hint::unreachable_unchecked();
+        }
+    };
+}

src/mpsc.rs

@@ -0,0 +1,31 @@
+//! Multi-producer, single-consumer channels using [`ThingBuf`](crate::ThingBuf).
+//!
+//! The default MPSC channel returned by the [`channel`] function is
+//! _asynchronous_: receiving from the channel is an `async fn`, and the
+//! receiving task willwait when there are no messages in the channel.
+//!
+//! If the "std" feature flag is enabled, this module also provides a
+//! synchronous channel, in the [`sync`] module. The synchronous  receiver will
+//! instead wait for new messages by blocking the current thread. Naturally,
+//! this requires the Rust standard library. A synchronous channel
+//! can be constructed using the [`sync::channel`] function.
+mod async_impl;
+pub use self::async_impl::*;
+
+feature! {
+    #![feature = "std"]
+    pub mod sync;
+}
+
+#[derive(Debug)]
+#[non_exhaustive]
+pub enum TrySendError {
+    AtCapacity(crate::AtCapacity),
+    Closed(Closed),
+}
+
+#[derive(Debug)]
+pub struct Closed(pub(crate) ());
+
+#[cfg(test)]
+mod tests;