[1/2] condition_variable: Report early wakeup of wait_until as no_timeout

As currently implemented, condition_variable always ultimately waits
against std::chrono::system_clock. This clock can be changed in arbitrary
ways by the user which may result in us waking up too early or too late
when measured against the caller-supplied clock.

We can't (yet) do much about waking up too late[1], but
if we wake up too early we must return cv_status::no_timeout to indicate a
spurious wakeup rather than incorrectly returning cv_status::timeout.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=41861
---
 libstdc++-v3/ChangeLog                                            |  7 +++++++
 libstdc++-v3/include/std/condition_variable                       |  8 +++++++-
 libstdc++-v3/testsuite/30_threads/condition_variable/members/2.cc | 52 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 66 insertions(+), 1 deletion(-)


base-commit: 3052e4ec519e4f5456ab63f4954ae098524316ce
--
git-series 0.9.1
BrightSign considers your privacy to be very important. The emails you send to us will be protected and secured. Furthermore, we will only use your email and contact information for the reasons you sent them to us and for tracking how effectively we respond to your requests.

On 20/07/18 17:49 +0100, Mike Crowe wrote:
>As currently implemented, condition_variable always ultimately waits
>against std::chrono::system_clock. This clock can be changed in arbitrary
>ways by the user which may result in us waking up too early or too late
>when measured against the caller-supplied clock.
>
>We can't (yet) do much about waking up too late[1], but
>if we wake up too early we must return cv_status::no_timeout to indicate a
>spurious wakeup rather than incorrectly returning cv_status::timeout.
>
>[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=41861

Committed to trunk, thanks very much.

I reformatted it slightly, to keep the line below 80 columns. The
version I committed is attached.
commit 79a8b4c1d70287ac0e668c4f2335d70d97c3002e
Author: redi <redi@138bc75d-0d04-0410-961f-82ee72b054a4>
Date:   Wed Aug 1 15:39:45 2018 +0000

    Report early wakeup of condition_variable::wait_until as no_timeout
    
    As currently implemented, condition_variable always ultimately waits
    against std::chrono::system_clock. This clock can be changed in arbitrary
    ways by the user which may result in us waking up too early or too late
    when measured against the caller-supplied clock.
    
    We can't (yet) do much about waking up too late (PR 41861), but
    if we wake up too early we must return cv_status::no_timeout to indicate a
    spurious wakeup rather than incorrectly returning cv_status::timeout.
    
    2018-08-01  Mike Crowe  <mac@mcrowe.com>
    
            * include/std/condition_variable (wait_until): Only report timeout
            if we really have timed out when measured against the
            caller-supplied clock.
            * testsuite/30_threads/condition_variable/members/2.cc: Add test
            case to confirm above behaviour.
    
    git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@263224 138bc75d-0d04-0410-961f-82ee72b054a4

diff --git a/libstdc++-v3/include/std/condition_variable b/libstdc++-v3/include/std/condition_variable
index 3f690c81799..c00afa2b7ae 100644
--- a/libstdc++-v3/include/std/condition_variable
+++ b/libstdc++-v3/include/std/condition_variable
@@ -117,7 +117,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
 	const auto __delta = __atime - __c_entry;
 	const auto __s_atime = __s_entry + __delta;
 
-	return __wait_until_impl(__lock, __s_atime);
+	if (__wait_until_impl(__lock, __s_atime) == cv_status::no_timeout)
+	  return cv_status::no_timeout;
+	// We got a timeout when measured against __clock_t but
+	// we need to check against the caller-supplied clock
+	// to tell whether we should return a timeout.
+	if (_Clock::now() < __atime)
+	  return cv_status::no_timeout;
+	return cv_status::timeout;
       }
 
     template<typename _Clock, typename _Duration, typename _Predicate>
diff --git a/libstdc++-v3/testsuite/30_threads/condition_variable/members/2.cc b/libstdc++-v3/testsuite/30_threads/condition_variable/members/2.cc
index 0a44c4fa7cf..09a717801e1 100644
--- a/libstdc++-v3/testsuite/30_threads/condition_variable/members/2.cc
+++ b/libstdc++-v3/testsuite/30_threads/condition_variable/members/2.cc
@@ -51,8 +51,60 @@ void test01()
     }
 }
 
+struct slow_clock
+{
+  using rep = std::chrono::system_clock::rep;
+  using period = std::chrono::system_clock::period;
+  using duration = std::chrono::system_clock::duration;
+  using time_point = std::chrono::time_point<slow_clock, duration>;
+  static constexpr bool is_steady = false;
+
+  static time_point now()
+  {
+    auto real = std::chrono::system_clock::now();
+    return time_point{real.time_since_epoch() / 3};
+  }
+};
+
+
+void test01_alternate_clock()
+{
+  try
+    {
+      std::condition_variable c1;
+      std::mutex m;
+      std::unique_lock<std::mutex> l(m);
+      auto const expire = slow_clock::now() + std::chrono::seconds(1);
+
+      while (slow_clock::now() < expire)
+       {
+         auto const result = c1.wait_until(l, expire);
+
+         // If wait_until returns before the timeout has expired when
+         // measured against the supplied clock, then wait_until must
+         // return no_timeout.
+         if (slow_clock::now() < expire)
+           VERIFY(result == std::cv_status::no_timeout);
+
+         // If wait_until returns timeout then the timeout must have
+         // expired.
+         if (result == std::cv_status::timeout)
+           VERIFY(slow_clock::now() >= expire);
+       }
+    }
+  catch (const std::system_error& e)
+    {
+      VERIFY( false );
+    }
+  catch (...)
+    {
+      VERIFY( false );
+    }
+}
+
 int main()
 {
   test01();
+  test01_alternate_clock();
   return 0;
 }