libstdc++: Fix try_lock_until and try_lock_shared_until on arbitrary clock

This is the equivalent to PR libstdc++/91906, but for shared_mutex.

A non-standard clock may tick more slowly than std::chrono::steady_clock.
This means that we risk returning false early when the specified timeout
may not have expired. This can be avoided by looping until the timeout time
as reported by the non-standard clock has been reached.

Unfortunately, we have no way to tell whether the non-standard clock ticks
more quickly that std::chrono::steady_clock. If it does then we risk
returning later than would be expected, but that is unavoidable without
waking up periodically to check, which would be rather too expensive.

François Dumont pointed out[1] a flaw in an earlier version of this patch
that revealed a hole in the test coverage, so I've added a new test that
try_lock_until acts as try_lock if the timeout has already expired.

[1] https://gcc.gnu.org/ml/libstdc++/2019-10/msg00021.html

2019-12-02  Mike Crowe  <mac@mcrowe.com>

	Fix try_lock_until and try_lock_shared_until on arbitrary clock
	* include/std/shared_mutex (shared_timed_mutex::try_lock_until)
	(shared_timed_mutex::try_lock_shared_until): Loop until the absolute
	timeout time is reached as measured against the appropriate clock.
	* testsuite/30_threads/shared_timed_mutex/try_lock_until/1.cc: New
	file. Test try_lock_until and try_lock_shared_until timeouts against
	various clocks.
	* testsuite/30_threads/shared_timed_mutex/try_lock_until/1.cc: New
	file. Test try_lock_until and try_lock_shared_until timeouts against
	various clocks.

From-SVN: r278904

libstdc++-v3/ChangeLog

@@ -1,5 +1,16 @@
 2019-12-02  Mike Crowe  <mac@mcrowe.com>
 
+	Fix try_lock_until and try_lock_shared_until on arbitrary clock
+	* include/std/shared_mutex (shared_timed_mutex::try_lock_until)
+	(shared_timed_mutex::try_lock_shared_until): Loop until the absolute
+	timeout time is reached as measured against the appropriate clock.
+	* testsuite/30_threads/shared_timed_mutex/try_lock_until/1.cc: New
+	file. Test try_lock_until and try_lock_shared_until timeouts against
+	various clocks.
+	* testsuite/30_threads/shared_timed_mutex/try_lock_until/1.cc: New
+	file. Test try_lock_until and try_lock_shared_until timeouts against
+	various clocks.
+
 	Add full steady_clock support to shared_timed_mutex
 	* acinclude.m4 (GLIBCXX_CHECK_PTHREAD_RWLOCK_CLOCKLOCK): Define
 	to check for the presence of both pthread_rwlock_clockrdlock and

libstdc++-v3/include/std/shared_mutex

@@ -554,9 +554,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       bool
       try_lock_until(const chrono::time_point<_Clock, _Duration>& __atime)
       {
+	// The user-supplied clock may not tick at the same rate as
+	// steady_clock, so we must loop in order to guarantee that
+	// the timeout has expired before returning false.
 	typename _Clock::time_point __now = _Clock::now();
-	auto __rtime = __atime - __now;
+	do {
-	return try_lock_for(__rtime);
+	    auto __rtime = __atime - __now;
+	    if (try_lock_for(__rtime))
+	      return true;
+	    __now = _Clock::now();
+	} while (__atime > __now);
+	return false;
       }
 
     // Shared ownership
@@ -631,9 +639,17 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       try_lock_shared_until(const chrono::time_point<_Clock,
 						     _Duration>& __atime)
       {
+	// The user-supplied clock may not tick at the same rate as
+	// steady_clock, so we must loop in order to guarantee that
+	// the timeout has expired before returning false.
 	typename _Clock::time_point __now = _Clock::now();
-	auto __rtime = __atime - __now;
+	do {
-	return try_lock_shared_for(__rtime);
+	    auto __rtime = __atime - __now;
+	    if (try_lock_shared_for(__rtime))
+	      return true;
+	    __now = _Clock::now();
+	} while (__atime > __now);
+	return false;
       }
 
 #else // ! (_GLIBCXX_USE_PTHREAD_RWLOCK_T && _GTHREAD_USE_MUTEX_TIMEDLOCK)