[libcxx-dev] system_error in shared_future::wait_for

[Louis Dionne via libcxx-dev]

> On May 14, 2019, at 17:57, Louis Dionne via libcxx-dev <libcxx-dev at lists.llvm.org> wrote:
> 
> 
> 
>> On May 14, 2019, at 11:49, b via libcxx-dev <libcxx-dev at lists.llvm.org <mailto:libcxx-dev at lists.llvm.org>> wrote:
>> 
>> Hello everyone,
>> 
>> I am new to this list. I am writing because I think I might have caught a bug in libc++. What I hope to get is a quick assessment of my issue so that I know if I should file this as a bug or some hints in how I should proceed to gather more information or even rectify the issue on my end.
>> 
>> The issue I am seeing popped up in a macOS project (Xcode 10.2.1) I am working on but I was able to condense it into a very small test program, see below for the code.
>> 
>> The issue presents itself as a crash in shared_future::wait_for due to an uncatchable exception. The crash does not occur immediately but only after a (huge) number of calls. Console output in Xcode reads  "libc++abi.dylib: terminating with uncaught exception of type std::__1::system_error: condition_variable timed_wait failed: Invalid argument“. See backtrace further down for details. Unfortunately I cannot step into code of the method where the exception originates which is condition_variable::__do_timed_wait. I browsed the code in the libcxx repository and found that the error must be the result code EINVAL from a call to pthread_cond_timedwait.
>> 
>> The interesting bit is that the issue can be reproduced in each test run within (mostly) less than 10 minutes on an iMac18,3/i7 and on a MacBookPro15,1/i7 but never on a MacBookPro15,2/i5. I only have one machine of each of those available so I cannot be sure how these results hold up.
>> 
>> 1. Is there something fundamentally wrong with my code, i.e. how I use the shared_future?
>> 
>> 2. Is this likely to be a bug in libc++ or is it more likely to be an issue with the BSD level API and/or the hardware? In case of the latter option where should I seek contact?
>> 
>> 3. I would like to be able to step into the code for condition_variable::__do_timed_wait and get the debugger info for the local variables. Would this be simply a matter of pulling the libcxx repository, building and linking it? I am not a command line compilation guy so I was hoping for some good documentation on how to do this with and for Xcode. However, if must be, I am willing to accept my fate.
> 
> In order to do that with 100% reproduceability, you'd have to build libc++ exactly the same way it was built for the platform you're running on, which is not easily achievable at the moment. I'll take a look.
> 
> I'll try running this program overnight to reproduce:
> 
>    cat <<EOF | clang++ -xc++ - -std=c++17 && ./a.out
>    #include <future>
>    #include <thread>
> 
>    using namespace std;
> 
>    int main(int argc, const char * argv[]) {
>       promise<void> thePromise = promise<void> {};
>       shared_future<void> sharedFuture = thePromise.get_future().share();
> 
>       thread anotherThread = thread( [sharedFuture]
>       {
>          int debugCount = 0;
>          while (sharedFuture.wait_for(100ns) == future_status::timeout)
>          {
>             debugCount++;
>          }
>       });
> 
>       sharedFuture.wait();
> 
>       return 0;
>    }
>    EOF
> 
> However, it would be useful for me to know the OSes you've been able to reproduce it on.
> 
> Thanks,
> Louis

Just to follow up on this, I managed to reproduce without libc++ concurrency primitives (but still using std::chrono), and I've followed up with our OS folks to help me figure out what's going on. I just wanted to let you know I've acknowledged the problem and we're working on it -- I don't think it's just a misusage of the facilities.

Louis

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