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

1 files changed, 22 insertions, 11 deletions

diff --git a/libcxx/test/std/thread/thread.condition/thread.condition.condvar/wait_until.pass.cpp b/libcxx/test/std/thread/thread.condition/thread.condition.condvar/wait_until.pass.cpp
index 12ccf3f1c06..e5c77f28eb8 100644
--- a/libcxx/test/std/thread/thread.condition/thread.condition.condvar/wait_until.pass.cpp
+++ b/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);
@@ -92,7 +97,7 @@ 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);
@@ -100,6 +105,12 @@ int main(int, char**)
         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;
 }