sync: remove try_recv() from mpsc types (#3263)

The mpsc `try_recv()` functions have an issue where a sent message
happens-before a call to `try_recv()` but `try_recv()` returns `None`.
Fixing this is non-trivial, so the function is removed for 1.0. When the
bug is fixed, the function can be added back.

Closes #2020

tokio/src/sync/mpsc/bounded.rs

@@ -1,6 +1,9 @@
 use crate::sync::batch_semaphore::{self as semaphore, TryAcquireError};
 use crate::sync::mpsc::chan;
-use crate::sync::mpsc::error::{SendError, TryRecvError, TrySendError};
+#[cfg(unix)]
+#[cfg(any(feature = "signal", feature = "process"))]
+use crate::sync::mpsc::error::TryRecvError;
+use crate::sync::mpsc::error::{SendError, TrySendError};
 
 cfg_time! {
     use crate::sync::mpsc::error::SendTimeoutError;
@@ -194,7 +197,9 @@ impl<T> Receiver<T> {
     ///
     /// Compared with recv, this function has two failure cases instead of
     /// one (one for disconnection, one for an empty buffer).
-    pub fn try_recv(&mut self) -> Result<T, TryRecvError> {
+    #[cfg(unix)]
+    #[cfg(any(feature = "signal", feature = "process"))]
+    pub(crate) fn try_recv(&mut self) -> Result<T, TryRecvError> {
         self.chan.try_recv()
     }
 

tokio/src/sync/mpsc/chan.rs

@@ -2,7 +2,6 @@ use crate::loom::cell::UnsafeCell;
 use crate::loom::future::AtomicWaker;
 use crate::loom::sync::atomic::AtomicUsize;
 use crate::loom::sync::Arc;
-use crate::sync::mpsc::error::TryRecvError;
 use crate::sync::mpsc::list;
 use crate::sync::notify::Notify;
 
@@ -259,21 +258,29 @@ impl<T, S: Semaphore> Rx<T, S> {
             }
         })
     }
+}
 
-    /// Receives the next value without blocking
-    pub(crate) fn try_recv(&mut self) -> Result<T, TryRecvError> {
-        use super::block::Read::*;
-        self.inner.rx_fields.with_mut(|rx_fields_ptr| {
-            let rx_fields = unsafe { &mut *rx_fields_ptr };
-            match rx_fields.list.pop(&self.inner.tx) {
-                Some(Value(value)) => {
-                    self.inner.semaphore.add_permit();
-                    Ok(value)
+feature! {
+    #![all(unix, any(feature = "signal", feature = "process"))]
+
+    use crate::sync::mpsc::error::TryRecvError;
+
+    impl<T, S: Semaphore> Rx<T, S> {
+        /// Receives the next value without blocking
+        pub(crate) fn try_recv(&mut self) -> Result<T, TryRecvError> {
+            use super::block::Read::*;
+            self.inner.rx_fields.with_mut(|rx_fields_ptr| {
+                let rx_fields = unsafe { &mut *rx_fields_ptr };
+                match rx_fields.list.pop(&self.inner.tx) {
+                    Some(Value(value)) => {
+                        self.inner.semaphore.add_permit();
+                        Ok(value)
+                    }
+                    Some(Closed) => Err(TryRecvError::Closed),
+                    None => Err(TryRecvError::Empty),
                 }
-                Some(Closed) => Err(TryRecvError::Closed),
-                None => Err(TryRecvError::Empty),
-            }
-        })
+            })
+        }
     }
 }
 

tokio/src/sync/mpsc/error.rs

@@ -67,32 +67,36 @@ impl Error for RecvError {}
 
 // ===== TryRecvError =====
 
-/// This enumeration is the list of the possible reasons that try_recv
-/// could not return data when called.
-#[derive(Debug, PartialEq)]
-pub enum TryRecvError {
-    /// This channel is currently empty, but the Sender(s) have not yet
-    /// disconnected, so data may yet become available.
-    Empty,
-    /// The channel's sending half has been closed, and there will
-    /// never be any more data received on it.
-    Closed,
-}
+feature! {
+    #![all(unix, any(feature = "signal", feature = "process"))]
+
+    /// This enumeration is the list of the possible reasons that try_recv
+    /// could not return data when called.
+    #[derive(Debug, PartialEq)]
+    pub(crate) enum TryRecvError {
+        /// This channel is currently empty, but the Sender(s) have not yet
+        /// disconnected, so data may yet become available.
+        Empty,
+        /// The channel's sending half has been closed, and there will
+        /// never be any more data received on it.
+        Closed,
+    }
 
-impl fmt::Display for TryRecvError {
-    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(
-            fmt,
-            "{}",
-            match self {
-                TryRecvError::Empty => "channel empty",
-                TryRecvError::Closed => "channel closed",
-            }
-        )
+    impl fmt::Display for TryRecvError {
+        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+            write!(
+                fmt,
+                "{}",
+                match self {
+                    TryRecvError::Empty => "channel empty",
+                    TryRecvError::Closed => "channel closed",
+                }
+            )
+        }
     }
-}
 
-impl Error for TryRecvError {}
+    impl Error for TryRecvError {}
+}
 
 cfg_time! {
     // ===== SendTimeoutError =====

tokio/src/sync/mpsc/unbounded.rs

@@ -1,6 +1,6 @@
 use crate::loom::sync::atomic::AtomicUsize;
 use crate::sync::mpsc::chan;
-use crate::sync::mpsc::error::{SendError, TryRecvError};
+use crate::sync::mpsc::error::SendError;
 
 use std::fmt;
 use std::task::{Context, Poll};
@@ -152,21 +152,6 @@ impl<T> UnboundedReceiver<T> {
         crate::future::block_on(self.recv())
     }
 
-    /// Attempts to return a pending value on this receiver without blocking.
-    ///
-    /// This method will never block the caller in order to wait for data to
-    /// become available. Instead, this will always return immediately with
-    /// a possible option of pending data on the channel.
-    ///
-    /// This is useful for a flavor of "optimistic check" before deciding to
-    /// block on a receiver.
-    ///
-    /// Compared with recv, this function has two failure cases instead of
-    /// one (one for disconnection, one for an empty buffer).
-    pub fn try_recv(&mut self) -> Result<T, TryRecvError> {
-        self.chan.try_recv()
-    }
-
     /// Closes the receiving half of a channel, without dropping it.
     ///
     /// This prevents any further messages from being sent on the channel while

tokio/tests/sync_mpsc.rs

@@ -5,7 +5,7 @@
 use std::thread;
 use tokio::runtime::Runtime;
 use tokio::sync::mpsc;
-use tokio::sync::mpsc::error::{TryRecvError, TrySendError};
+use tokio::sync::mpsc::error::TrySendError;
 use tokio_test::task;
 use tokio_test::{
     assert_err, assert_ok, assert_pending, assert_ready, assert_ready_err, assert_ready_ok,
@@ -385,44 +385,6 @@ fn unconsumed_messages_are_dropped() {
     assert_eq!(1, Arc::strong_count(&msg));
 }
 
-#[test]
-fn try_recv() {
-    let (tx, mut rx) = mpsc::channel(1);
-    match rx.try_recv() {
-        Err(TryRecvError::Empty) => {}
-        _ => panic!(),
-    }
-    tx.try_send(42).unwrap();
-    match rx.try_recv() {
-        Ok(42) => {}
-        _ => panic!(),
-    }
-    drop(tx);
-    match rx.try_recv() {
-        Err(TryRecvError::Closed) => {}
-        _ => panic!(),
-    }
-}
-
-#[test]
-fn try_recv_unbounded() {
-    let (tx, mut rx) = mpsc::unbounded_channel();
-    match rx.try_recv() {
-        Err(TryRecvError::Empty) => {}
-        _ => panic!(),
-    }
-    tx.send(42).unwrap();
-    match rx.try_recv() {
-        Ok(42) => {}
-        _ => panic!(),
-    }
-    drop(tx);
-    match rx.try_recv() {
-        Err(TryRecvError::Closed) => {}
-        _ => panic!(),
-    }
-}
-
 #[test]
 fn blocking_recv() {
     let (tx, mut rx) = mpsc::channel::<u8>(1);