Implement std::condition_variable via pthread_cond_clockwait() where available

std::condition_variable is currently implemented via
pthread_cond_timedwait() on systems that use pthread. This is
problematic, since that function waits by default on CLOCK_REALTIME
and libc++ does not provide any mechanism to change from this
default.

Due to this, regardless of if condition_variable::wait_until() is
called with a chrono::system_clock or chrono::steady_clock parameter,
condition_variable::wait_until() will wait using CLOCK_REALTIME. This
is not accurate to the C++ standard as calling
condition_variable::wait_until() with a chrono::steady_clock parameter
should use CLOCK_MONOTONIC.

This is particularly problematic because CLOCK_REALTIME is a bad
choice as it is subject to discontinuous time adjustments, that may
cause condition_variable::wait_until() to immediately timeout or wait
indefinitely.

This change fixes this issue with a new POSIX function,
pthread_cond_clockwait() proposed on
http://austingroupbugs.net/view.php?id=1216. The new function is
similar to pthread_cond_timedwait() with the addition of a clock
parameter that allows it to wait using either CLOCK_REALTIME or
CLOCK_MONOTONIC, thus allowing condition_variable::wait_until() to
wait using CLOCK_REALTIME for chrono::system_clock and CLOCK_MONOTONIC
for chrono::steady_clock.

pthread_cond_clockwait() is implemented in glibc (2.30 and later) and
Android's bionic (Android API version 30 and later).

This change additionally makes wait_for() and wait_until() with clocks
other than chrono::system_clock use CLOCK_MONOTONIC.<Paste>

llvm-svn: 372016

Author: DanAlbert

libcxx/include/__config

@@ -1087,6 +1087,12 @@ _LIBCPP_FUNC_VIS extern "C" void __sanitizer_annotate_contiguous_container(
 #  endif // _LIBCPP_HAS_THREAD_API
 #endif // _LIBCPP_HAS_NO_THREADS
 
+#if defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
+#  if (defined(__ANDROID__) && __ANDROID_API__ >= 30) || _LIBCPP_GLIBC_PREREQ(2, 30)
+#    define _LIBCPP_HAS_COND_CLOCKWAIT
+#  endif
+#endif
+
 #if defined(_LIBCPP_HAS_NO_THREADS) && defined(_LIBCPP_HAS_THREAD_API_PTHREAD)
 #error _LIBCPP_HAS_THREAD_API_PTHREAD may only be defined when \
        _LIBCPP_HAS_NO_THREADS is not defined.

libcxx/include/__mutex_base

@@ -15,6 +15,7 @@
 #include <system_error>
 #include <__threading_support>
 
+#include <time.h>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #pragma GCC system_header
@@ -337,23 +338,75 @@ public:
 private:
     void __do_timed_wait(unique_lock<mutex>& __lk,
        chrono::time_point<chrono::system_clock, chrono::nanoseconds>) _NOEXCEPT;
+#if defined(_LIBCPP_HAS_COND_CLOCKWAIT)
+    void __do_timed_wait(unique_lock<mutex>& __lk,
+       chrono::time_point<chrono::steady_clock, chrono::nanoseconds>) _NOEXCEPT;
+#endif
+    template <class _Clock>
+    void __do_timed_wait(unique_lock<mutex>& __lk,
+       chrono::time_point<_Clock, chrono::nanoseconds>) _NOEXCEPT;
 };
 #endif // !_LIBCPP_HAS_NO_THREADS
 
-template <class _To, class _Rep, class _Period>
+template <class _Rep, class _Period>
 inline _LIBCPP_INLINE_VISIBILITY
 typename enable_if
 <
-    chrono::__is_duration<_To>::value,
-    _To
+    is_floating_point<_Rep>::value,
+    chrono::nanoseconds
 >::type
-__ceil(chrono::duration<_Rep, _Period> __d)
+__safe_nanosecond_cast(chrono::duration<_Rep, _Period> __d)
 {
     using namespace chrono;
-    _To __r = duration_cast<_To>(__d);
-    if (__r < __d)
-        ++__r;
-    return __r;
+    using __ratio = ratio_divide<_Period, nano>;
+    using __ns_rep = nanoseconds::rep;
+    _Rep __result_float = __d.count() * __ratio::num / __ratio::den;
+
+    _Rep __result_max = numeric_limits<__ns_rep>::max();
+    if (__result_float >= __result_max) {
+        return nanoseconds::max();
+    }
+
+    _Rep __result_min = numeric_limits<__ns_rep>::min();
+    if (__result_float <= __result_min) {
+        return nanoseconds::min();
+    }
+
+    return nanoseconds(static_cast<__ns_rep>(__result_float));
+}
+
+template <class _Rep, class _Period>
+inline _LIBCPP_INLINE_VISIBILITY
+typename enable_if
+<
+    !is_floating_point<_Rep>::value,
+    chrono::nanoseconds
+>::type
+__safe_nanosecond_cast(chrono::duration<_Rep, _Period> __d)
+{
+    using namespace chrono;
+    if (__d.count() == 0) {
+        return nanoseconds(0);
+    }
+
+    using __ratio = ratio_divide<_Period, nano>;
+    using __ns_rep = nanoseconds::rep;
+    __ns_rep __result_max = std::numeric_limits<__ns_rep>::max();
+    if (__d.count() > 0 && __d.count() > __result_max / __ratio::num) {
+        return nanoseconds::max();
+    }
+
+    __ns_rep __result_min = std::numeric_limits<__ns_rep>::min();
+    if (__d.count() < 0 && __d.count() < __result_min / __ratio::num) {
+        return nanoseconds::min();
+    }
+
+    __ns_rep __result = __d.count() * __ratio::num / __ratio::den;
+    if (__result == 0) {
+        return nanoseconds(1);
+    }
+
+    return nanoseconds(__result);
 }
 
 #ifndef _LIBCPP_HAS_NO_THREADS
@@ -371,7 +424,15 @@ condition_variable::wait_until(unique_lock<mutex>& __lk,
                                const chrono::time_point<_Clock, _Duration>& __t)
 {
     using namespace chrono;
-    wait_for(__lk, __t - _Clock::now());
+    using __clock_tp_ns = time_point<_Clock, nanoseconds>;
+
+    typename _Clock::time_point __now = _Clock::now();
+    if (__t <= __now)
+        return cv_status::timeout;
+
+    __clock_tp_ns __t_ns = __clock_tp_ns(__safe_nanosecond_cast(__t.time_since_epoch()));
+
+    __do_timed_wait(__lk, __t_ns);
     return _Clock::now() < __t ? cv_status::no_timeout : cv_status::timeout;
 }
 
@@ -397,15 +458,25 @@ condition_variable::wait_for(unique_lock<mutex>& __lk,
     using namespace chrono;
     if (__d <= __d.zero())
         return cv_status::timeout;
-    typedef time_point<system_clock, duration<long double, nano> > __sys_tpf;
-    typedef time_point<system_clock, nanoseconds> __sys_tpi;
-    __sys_tpf _Max = __sys_tpi::max();
+    using __ns_rep = nanoseconds::rep;
     steady_clock::time_point __c_now = steady_clock::now();
-    system_clock::time_point __s_now = system_clock::now();
-    if (_Max - __d > __s_now)
-        __do_timed_wait(__lk, __s_now + __ceil<nanoseconds>(__d));
-    else
-        __do_timed_wait(__lk, __sys_tpi::max());
+
+#if defined(_LIBCPP_HAS_COND_CLOCKWAIT)
+    using __clock_tp_ns = time_point<steady_clock, nanoseconds>;
+    __ns_rep __now_count_ns = __safe_nanosecond_cast(__c_now.time_since_epoch()).count();
+#else
+    using __clock_tp_ns = time_point<system_clock, nanoseconds>;
+    __ns_rep __now_count_ns = __safe_nanosecond_cast(system_clock::now().time_since_epoch()).count();
+#endif
+
+    __ns_rep __d_ns_count = __safe_nanosecond_cast(__d).count();
+
+    if (__now_count_ns > numeric_limits<__ns_rep>::max() - __d_ns_count) {
+        __do_timed_wait(__lk, __clock_tp_ns::max());
+    } else {
+        __do_timed_wait(__lk, __clock_tp_ns(nanoseconds(__now_count_ns + __d_ns_count)));
+    }
+
     return steady_clock::now() - __c_now < __d ? cv_status::no_timeout :
                                                  cv_status::timeout;
 }
@@ -421,6 +492,46 @@ condition_variable::wait_for(unique_lock<mutex>& __lk,
                       _VSTD::move(__pred));
 }
 
+#if defined(_LIBCPP_HAS_COND_CLOCKWAIT)
+inline
+void
+condition_variable::__do_timed_wait(unique_lock<mutex>& __lk,
+     chrono::time_point<chrono::steady_clock, chrono::nanoseconds> __tp) _NOEXCEPT
+{
+    using namespace chrono;
+    if (!__lk.owns_lock())
+        __throw_system_error(EPERM,
+                            "condition_variable::timed wait: mutex not locked");
+    nanoseconds __d = __tp.time_since_epoch();
+    timespec __ts;
+    seconds __s = duration_cast<seconds>(__d);
+    using __ts_sec = decltype(__ts.tv_sec);
+    const __ts_sec __ts_sec_max = numeric_limits<__ts_sec>::max();
+    if (__s.count() < __ts_sec_max)
+    {
+        __ts.tv_sec = static_cast<__ts_sec>(__s.count());
+        __ts.tv_nsec = (__d - __s).count();
+    }
+    else
+    {
+        __ts.tv_sec = __ts_sec_max;
+        __ts.tv_nsec = giga::num - 1;
+    }
+    int __ec = pthread_cond_clockwait(&__cv_, __lk.mutex()->native_handle(), CLOCK_MONOTONIC, &__ts);
+    if (__ec != 0 && __ec != ETIMEDOUT)
+        __throw_system_error(__ec, "condition_variable timed_wait failed");
+}
+#endif // _LIBCPP_HAS_COND_CLOCKWAIT
+
+template <class _Clock>
+inline
+void
+condition_variable::__do_timed_wait(unique_lock<mutex>& __lk,
+     chrono::time_point<_Clock, chrono::nanoseconds> __tp) _NOEXCEPT
+{
+    wait_for(__lk, __tp - _Clock::now());
+}
+
 #endif // !_LIBCPP_HAS_NO_THREADS
 
 _LIBCPP_END_NAMESPACE_STD

libcxx/test/std/thread/thread.condition/thread.condition.condvar/wait_until.pass.cpp

@@ -25,12 +25,12 @@
 
 #include "test_macros.h"
 
-struct Clock
+struct TestClock
 {
     typedef std::chrono::milliseconds duration;
     typedef duration::rep             rep;
     typedef duration::period          period;
-    typedef std::chrono::time_point<Clock> time_point;
+    typedef std::chrono::time_point<TestClock> time_point;
     static const bool is_steady =  true;
 
     static time_point now()
@@ -50,35 +50,40 @@ int test2 = 0;
 
 int runs = 0;
 
+template <typename Clock>
 void f()
 {
     std::unique_lock<std::mutex> lk(mut);
     assert(test2 == 0);
     test1 = 1;
     cv.notify_one();
-    Clock::time_point t0 = Clock::now();
-    Clock::time_point t = t0 + Clock::duration(250);
+    typename Clock::time_point t0 = Clock::now();
+    typename Clock::time_point t = t0 + std::chrono::milliseconds(250);
     while (test2 == 0 && cv.wait_until(lk, t) == std::cv_status::no_timeout)
         ;
-    Clock::time_point t1 = Clock::now();
+    typename Clock::time_point t1 = Clock::now();
     if (runs == 0)
     {
-        assert(t1 - t0 < Clock::duration(250));
+        assert(t1 - t0 < std::chrono::milliseconds(250));
         assert(test2 != 0);
     }
     else
     {
-        assert(t1 - t0 - Clock::duration(250) < Clock::duration(50));
+        assert(t1 - t0 - std::chrono::milliseconds(250) < std::chrono::milliseconds(50));
         assert(test2 == 0);
     }
     ++runs;
 }
 
-int main(int, char**)
+template <typename Clock>
+void run_test()
 {
+    runs = 0;
+    test1 = 0;
+    test2 = 0;
     {
         std::unique_lock<std::mutex>lk(mut);
-        std::thread t(f);
+        std::thread t(f<Clock>);
         assert(test1 == 0);
         while (test1 == 0)
             cv.wait(lk);
@@ -92,14 +97,20 @@ int main(int, char**)
     test2 = 0;
     {
         std::unique_lock<std::mutex>lk(mut);
-        std::thread t(f);
+        std::thread t(f<Clock>);
         assert(test1 == 0);
         while (test1 == 0)
             cv.wait(lk);
         assert(test1 != 0);
         lk.unlock();
         t.join();
     }
+}
 
-  return 0;
+int main(int, char**)
+{
+    run_test<TestClock>();
+    run_test<std::chrono::steady_clock>();
+    run_test<std::chrono::system_clock>();
+    return 0;
 }