Auto merge of #35048 - tmiasko:monotonic-wait-timeout, r=alexcrichton

Use monotonic time in condition variables.

Configure condition variables to use monotonic time using
pthread_condattr_setclock on systems where this is possible.
This fixes the issue when thread waiting on condition variable is
woken up too late when system time is moved backwards.

Author: bors

src/libstd/sync/condvar.rs

@@ -80,10 +80,14 @@ impl Condvar {
     /// notified.
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn new() -> Condvar {
-        Condvar {
+        let mut c = Condvar {
             inner: box sys::Condvar::new(),
             mutex: AtomicUsize::new(0),
+        };
+        unsafe {
+            c.inner.init();
         }
+        c
     }
 
     /// Blocks the current thread until this condition variable receives a
@@ -138,6 +142,10 @@ impl Condvar {
     /// differences that may not cause the maximum amount of time
     /// waited to be precisely `ms`.
     ///
+    /// Note that the best effort is made to ensure that the time waited is
+    /// measured with a monotonic clock, and not affected by the changes made to
+    /// the system time.
+    ///
     /// The returned boolean is `false` only if the timeout is known
     /// to have elapsed.
     ///
@@ -162,6 +170,10 @@ impl Condvar {
     /// preemption or platform differences that may not cause the maximum
     /// amount of time waited to be precisely `dur`.
     ///
+    /// Note that the best effort is made to ensure that the time waited is
+    /// measured with a monotonic clock, and not affected by the changes made to
+    /// the system time.
+    ///
     /// The returned `WaitTimeoutResult` value indicates if the timeout is
     /// known to have elapsed.
     ///

src/libstd/sys/common/condvar.rs

@@ -27,6 +27,13 @@ impl Condvar {
     /// first used with any of the functions below.
     pub const fn new() -> Condvar { Condvar(imp::Condvar::new()) }
 
+    /// Prepares the condition variable for use.
+    ///
+    /// This should be called once the condition variable is at a stable memory
+    /// address.
+    #[inline]
+    pub unsafe fn init(&mut self) { self.0.init() }
+
     /// Signals one waiter on this condition variable to wake up.
     #[inline]
     pub unsafe fn notify_one(&self) { self.0.notify_one() }

src/libstd/sys/unix/condvar.rs

@@ -10,22 +10,43 @@
 
 use cell::UnsafeCell;
 use libc;
-use ptr;
 use sys::mutex::{self, Mutex};
-use time::{Instant, Duration};
+use time::Duration;
 
 pub struct Condvar { inner: UnsafeCell<libc::pthread_cond_t> }
 
 unsafe impl Send for Condvar {}
 unsafe impl Sync for Condvar {}
 
+const TIMESPEC_MAX: libc::timespec = libc::timespec {
+    tv_sec: <libc::time_t>::max_value(),
+    tv_nsec: 1_000_000_000 - 1,
+};
+
 impl Condvar {
     pub const fn new() -> Condvar {
         // Might be moved and address is changing it is better to avoid
         // initialization of potentially opaque OS data before it landed
         Condvar { inner: UnsafeCell::new(libc::PTHREAD_COND_INITIALIZER) }
     }
 
+    #[cfg(any(target_os = "macos", target_os = "ios", target_os = "android"))]
+    pub unsafe fn init(&mut self) {}
+
+    #[cfg(not(any(target_os = "macos", target_os = "ios", target_os = "android")))]
+    pub unsafe fn init(&mut self) {
+        use mem;
+        let mut attr: libc::pthread_condattr_t = mem::uninitialized();
+        let r = libc::pthread_condattr_init(&mut attr);
+        assert_eq!(r, 0);
+        let r = libc::pthread_condattr_setclock(&mut attr, libc::CLOCK_MONOTONIC);
+        assert_eq!(r, 0);
+        let r = libc::pthread_cond_init(self.inner.get(), &attr);
+        assert_eq!(r, 0);
+        let r = libc::pthread_condattr_destroy(&mut attr);
+        assert_eq!(r, 0);
+    }
+
     #[inline]
     pub unsafe fn notify_one(&self) {
         let r = libc::pthread_cond_signal(self.inner.get());
@@ -44,10 +65,45 @@ impl Condvar {
         debug_assert_eq!(r, 0);
     }
 
+    // This implementation is used on systems that support pthread_condattr_setclock
+    // where we configure condition variable to use monotonic clock (instead of
+    // default system clock). This approach avoids all problems that result
+    // from changes made to the system time.
+    #[cfg(not(any(target_os = "macos", target_os = "ios", target_os = "android")))]
+    pub unsafe fn wait_timeout(&self, mutex: &Mutex, dur: Duration) -> bool {
+        use mem;
+
+        let mut now: libc::timespec = mem::zeroed();
+        let r = libc::clock_gettime(libc::CLOCK_MONOTONIC, &mut now);
+        assert_eq!(r, 0);
+
+        // Nanosecond calculations can't overflow because both values are below 1e9.
+        let nsec = dur.subsec_nanos() as libc::c_long + now.tv_nsec as libc::c_long;
+        // FIXME: Casting u64 into time_t could truncate the value.
+        let sec = (dur.as_secs() as libc::time_t)
+            .checked_add((nsec / 1_000_000_000) as libc::time_t)
+            .and_then(|s| s.checked_add(now.tv_sec));
+        let nsec = nsec % 1_000_000_000;
+
+        let timeout = sec.map(|s| {
+            libc::timespec { tv_sec: s, tv_nsec: nsec }
+        }).unwrap_or(TIMESPEC_MAX);
+
+        let r = libc::pthread_cond_timedwait(self.inner.get(), mutex::raw(mutex),
+                                            &timeout);
+        assert!(r == libc::ETIMEDOUT || r == 0);
+        r == 0
+    }
+
+
     // This implementation is modeled after libcxx's condition_variable
     // https://github.com/llvm-mirror/libcxx/blob/release_35/src/condition_variable.cpp#L46
     // https://github.com/llvm-mirror/libcxx/blob/release_35/include/__mutex_base#L367
+    #[cfg(any(target_os = "macos", target_os = "ios", target_os = "android"))]
     pub unsafe fn wait_timeout(&self, mutex: &Mutex, dur: Duration) -> bool {
+        use ptr;
+        use time::Instant;
+
         // First, figure out what time it currently is, in both system and
         // stable time.  pthread_cond_timedwait uses system time, but we want to
         // report timeout based on stable time.
@@ -66,12 +122,7 @@ impl Condvar {
             s.checked_add(seconds)
         }).map(|s| {
             libc::timespec { tv_sec: s, tv_nsec: nsec }
-        }).unwrap_or_else(|| {
-            libc::timespec {
-                tv_sec: <libc::time_t>::max_value(),
-                tv_nsec: 1_000_000_000 - 1,
-            }
-        });
+        }).unwrap_or(TIMESPEC_MAX);
 
         // And wait!
         let r = libc::pthread_cond_timedwait(self.inner.get(), mutex::raw(mutex),

src/libstd/sys/windows/condvar.rs

@@ -24,6 +24,9 @@ impl Condvar {
         Condvar { inner: UnsafeCell::new(c::CONDITION_VARIABLE_INIT) }
     }
 
+    #[inline]
+    pub unsafe fn init(&mut self) {}
+
     #[inline]
     pub unsafe fn wait(&self, mutex: &Mutex) {
         let r = c::SleepConditionVariableSRW(self.inner.get(),