Auto merge of rust-lang#33748 - emilio:mpsc-recv-timeout, r=alexcrichton

std: sync: Implement recv_timeout()

This is an attempt to implement rust-lang/rfcs#962.

I'm not sure about if a change like this would require an rfc or something like
that, and this surely needs a lot more testing, but I wanted to take some eyes
on it before following.

cc @metajack @asajeffrey servo/servo#11279 servo/servo#11283

r? @aturon

src/libstd/sync/mpsc/blocking.rs

@@ -16,6 +16,7 @@ use sync::Arc;
 use marker::{Sync, Send};
 use mem;
 use clone::Clone;
+use time::Instant;
 
 struct Inner {
     thread: Thread,
@@ -74,7 +75,6 @@ impl SignalToken {
     pub unsafe fn cast_from_usize(signal_ptr: usize) -> SignalToken {
         SignalToken { inner: mem::transmute(signal_ptr) }
     }
-
 }
 
 impl WaitToken {
@@ -83,4 +83,16 @@ impl WaitToken {
             thread::park()
         }
     }
+
+    /// Returns true if we wake up normally, false otherwise.
+    pub fn wait_max_until(self, end: Instant) -> bool {
+        while !self.inner.woken.load(Ordering::SeqCst) {
+            let now = Instant::now();
+            if now >= end {
+                return false;
+            }
+            thread::park_timeout(end - now)
+        }
+        true
+    }
 }