[PATCH] libstdc++: Fix future.wait_until when given a negative time_point

[William Tsai]

The template `future.wait_until` will expand to
`_M_load_and_test_until_impl` where it will call
`_M_load_and_test_until*` with given time_point casted into second and
nanosecond. The callee expects the caller to provide the values
correctly from caller while the caller did not make check with those
values. One possible error is that if `future.wait_until` is given with
a negative time_point, the underlying system call will raise an error as
the system call does not accept second < 0 and nanosecond < 1.

Following is a simple testcase:
```
#include <chrono>
#include <future>
#include <iostream>

using namespace std;

int main() {
    promise<void> p;
    future<void> f = p.get_future();
    chrono::steady_clock::time_point tp(chrono::milliseconds{-10});
    future_status status = f.wait_until(tp);
    if (status == future_status::timeout) {
        cout << "Timed out" << endl;
    }
    return 0;
}
```

libstdc++-v3/ChangeLog:

        * include/bits/atomic_futex.h: Check if __s and __ns is valid before
        calling before calling _M_load_and_test_until*

Signed-off-by: William Tsai <william.t...@sifive.com>
---
 libstdc++-v3/include/bits/atomic_futex.h | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/libstdc++-v3/include/bits/atomic_futex.h 
b/libstdc++-v3/include/bits/atomic_futex.h
index dd654174873..4c31946a97f 100644
--- a/libstdc++-v3/include/bits/atomic_futex.h
+++ b/libstdc++-v3/include/bits/atomic_futex.h
@@ -173,6 +173,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
       auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
       // XXX correct?
+      if ((__s.time_since_epoch().count() < 0) || (__ns.count() < 0))
+       return false;
       return _M_load_and_test_until(__assumed, __operand, __equal, __mo,
          true, __s.time_since_epoch(), __ns);
     }
@@ -186,6 +188,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
       auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
       // XXX correct?
+      if ((__s.time_since_epoch().count() < 0) || (__ns.count() < 0))
+       return false;
       return _M_load_and_test_until_steady(__assumed, __operand, __equal, __mo,
          true, __s.time_since_epoch(), __ns);
     }
-- 
2.37.1