[llvm-bugs] [Bug 49721] New: std::binary_semaphore::try_acquire() blocks forever

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=49721

            Bug ID: 49721
           Summary: std::binary_semaphore::try_acquire() blocks forever
           Product: libc++
           Version: unspecified
          Hardware: All
                OS: All
            Status: CONFIRMED
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: raul at tambre.ee
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

Created attachment 24693
  --> https://bugs.llvm.org/attachment.cgi?id=24693&action=edit
repro

[thread.sema.cnt] says for try_acquire():

   Effects: Attempts to atomically decrement counter if it is positive, without
blocking. [...]

libc++'s try_acquire() maps to
try_acquire_for(std::chrono::nanoseconds::zero()) which in the case of
std::binary_semaphore boils down to __libcpp_thread_poll_with_backoff(), which
treats a zero timeout as an infinite timeout, resulting in try_acquire()
blocking forever if it's unable to decrement.

The attached .cpp file demonstrates the problem.

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