Good morning,
I am facing a crash in my application. When the user tries to start it, he waits like a minute and then a std::exception is raised. Really I could not reproduce the bug by myself, but it seems quite a common problem.
The only thing I could track is the following line in syslog:
BUG in libdispatch client: kevent[EVFILT_MACHPORT] monitored resource vanished before the source cancel handler was invoked
Then, I start to google it and I can not find much more...I can only "suppose" that is some problem with GCD (that I do not use afaik, or at least not directly...). What I saw in Internet is that it is related with MacOSX Sierra. But the majority of forum have no answer, just a lot of tries without a unique result. Maybe the only web page that seems a bit clear about a workaround (that I have not tested, and anyway I do not want to use) is this.
So...:
This libdispatch log message is not fatal, and is almost certainly not related to your crash, which sounds like an abort due to an uncaught C++ exception (without a crashreport/backtrace it is difficult to say anything more). libdispatch does not itself generate any C++ exceptions FWIW.
As to the meaning of that specific log message, it relates to the following section in the dispatch_source_create(3) manpage:
CANCELLATION: Important: a cancellation handler is required for file descriptor and mach port based sources in order to safely close the descriptor or destroy the port. Closing the descriptor or port before the cancellation handler has run may result in a race condition: if a new descriptor is allocated with the same value as the recently closed descriptor while the source's event handler is still running, the event handler may read/write data to the wrong descriptor.
If you see the EVFILT_MACHPORT "vanished" log message, somebody in your process has violated that API contract and deallocated a machport while a dispatch source was still monitoring it (causing the kernel to generate an EV_VANISHED kevent), see the source code.