[PATCH] Add c++2a binary_semaphore

[Jonathan Wakely]

On 24/02/20 21:53 -0500, Thomas Rodgers wrote:
>+    bool
>+    _S_futex_wait_until(int* __addr, int __val,
>+			bool __has_timeout = false,
>+			std::chrono::seconds __s = std::chrono::seconds::zero(),
>+			std::chrono::nanoseconds __ns = std::chrono::nanoseconds::zero())
>+    {
>+      if (!__has_timeout)
>+	{
>+	  syscall (SYS_futex, __addr, 0, __val, nullptr);
>+	  // Can't do anything about an error here except abort, so ignore it.
>+	}
>+      else
>+	{
>+	  struct timeval __tv;
>+	  gettimeofday(&__tv, NULL);
>+	  struct timespec __rt;
>+	  __rt.tv_sec = __s.count() - __tv.tv_sec;
>+	  __rt.tv_nsec = __ns.count() - __tv.tv_usec * 1000;
>+	  if (__rt.tv_nsec < 0)
>+	    {
>+	      __rt.tv_nsec += 1000000000;
>+	      --__rt.tv_sec;
>+	    }
>+	  if (__rt.tv_sec < 0)
>+	    return false;
>+
>+	  if (syscall (SYS_futex, __addr, 0, __val, &__rt) == -1)

This syscall has the same problem as https://gcc.gnu.org/PR93421 so we
should avoid introducing a new instance of the bug.