[libcxx-commits] [libcxx] [libc++] Save duration/timeout locally for condition_variable waits (PR #148330)

[John Sheu via libcxx-commits]

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@@ -188,9 +188,10 @@ public:
   _LIBCPP_HIDE_FROM_ABI cv_status wait_until(_Lock& __lock, const chrono::time_point<_Clock, _Duration>& __t) {
     shared_ptr<mutex> __mut = __mut_;
     unique_lock<mutex> __lk(*__mut);
+    const chrono::time_point<_Clock, _Duration> __t_local = __t;
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johnsheu wrote:

What do you mean by "observable" here?  The specification is for the call to `wait_until()` to take the timeout by-const-ref, so taking it by-value would be nonconforming.  Note though that since `time_point` is also specified to have a copy constructor, changing the specification take it directly by-value would not break existing code.

If you mean taking the timeout by-const-ref and then making a value copy internally: I think that would be conforming, and would be unconditionally possible due to the `time_point` copy constructor.  However, it would make an observable difference from current behavior:

- **Current behavior:** reducing `timeout` while call is blocked may result in "spurious" timeout
- **New behavior:** reducing `timeout` while call is blocked will not change timeout; value at the time the call was made will be used

As far as I can tell, both behaviors are technically permissible under the standard.

https://github.com/llvm/llvm-project/pull/148330