[libcxx-commits] [libcxx] [libc++] fix `counting_semaphore` lost wakeups (PR #79265)

[via libcxx-commits]

================
@@ -95,19 +96,22 @@ public:
     _LIBCPP_ASSERT_ARGUMENT_WITHIN_DOMAIN(
         __update <= _LIBCPP_SEMAPHORE_MAX - __old, "update is greater than the expected value");
 
-    if (__old > 0) {
-      // Nothing to do
-    } else if (__update > 1)
+    if (__old == 0) {
----------------
huixie90 wrote:

So we can either do
```
if (__old == 0) {
  __a_.notify_all();
}
```
or as you suggested
```
if (__update > 1)
  __a_.notify_all();
else
  __a_.notify_one();
```

Both of them would solve the original lost wake up described in
https://bugs.llvm.org/show_bug.cgi?id=47013

It seems like that it is a trade off between
1. we don't need to notify anyone unless our counter reaches 0
2. we can just notify one thread if the caller passed `__update == 1`

https://github.com/llvm/llvm-project/pull/79265