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Created attachment 25611 [details]
Minimally reproducing program for Monitor.TryEnter
Process.WaitForExit and Monitor.TryEnter incorrectly return with a failed timeout condition if the system clock is advanced forward by an amount equal to or more than the timeout duration.
This is because Mono is using sem_timedwait under the hood for these. sem_timedwait unfortunately relies on CLOCK_REALTIME which is not a recommended way to express a timeout duration. That's what CLOCK_MONOTONIC is for.
Recently, in Mono 5.2 major improvements were made to use CLOCK_MONOTONIC for the vast majority of timeout based APIs. Anything using mono_os_cond_timedwait seems to now be immune to system clock changes (for platforms that support CLOCK_MONOTONIC). This has been a very welcome and helpful improvement.
But there are still a few APIs that "break" if the system clock is advanced, which seems to be anything using mono_os_sem_timedwait. Fortunately the surface area is much smaller now, but the issue remains nonetheless.
I'm hoping that Mono can completely avoid using sem_timedwait for platforms that support use of CLOCK_MONOTONIC using other means, like pthread_cond_timedwait.
There are also other internal uses of mono_os_sem_timedwait that I'm unable to determine if they exhibit erroneous or detrimental behavior when the system clock is advanced.
Attached are a couple of test programs that demonstrate the issue. I've observed the problem on Debian 8 and 9 (on armhf and x64), and Ubuntu 16.04 (on x64). Run the program in a console and observe a dot '.' output every second. Then from another console advance the system clock forward by more than 3 seconds. The test program will output an error message in response to "timing out" on a blocking wait.
The end result is that if the system clock is moved forward, stuff errors out when it should not.
Created attachment 25612 [details]
Minimally reproducing program for Process.WaitForExit
CORRECTION -- Debian 8 does *not* exhibit the issue for me. Has me scratching my head actually.
I think I've gotten to the bottom of why Debian 8 does not have the issue, whereas Debian 9 and Ubuntu 16.04 do.
It boils down to the glibc version. I think this commit [https://github.com/bminor/glibc/commit/c2f5813ae0a68f6c6d69e66dac2da6e46b9df034] is the critical change. That shipped with glibc 2.22.
Using `ldd --version` you can check your glibc version.
Debian 8: 2.19
Debian 9: 2.24
Ubuntu 16.04: 2.23
The nature of the glibc change was to force use of CLOCK_REALTIME to conform with POSIX requirements. As best I can tell, this resulted in sem_timedwait no longer behaving monotonically. From the point of view of glibc, I think, that was a bug and this change fixed it. So presumably Mono is relying on buggy behavior that got fixed.