std::condition_variable::wait_for() breakage when system_clock changes C++11 MIPS

[Ward Willats <musl@wardco.com>]

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Hello.

This may be a little too C++ for this list, but I wasn't sure where to turn. Also, I thought I should at least document it for the Google, and see if you folks think I should take this problem elsewhere, or, perhaps more likely, if I have made some stupid error (!)

We are running a C++ 11 app on a MIPS-based Linux/OpenWRT platform and linking against MUSCL 1.1.12 and libstdc++ v3. On cold boot, our system clock is often some random time in the future until ntpd or other software corrects it. This led to odd behavior and this experiment:

If I use the busybox "date --set" to move our system's clock 24 hours into the future, and then call:

std::condition_variable::wait_for() with the pre-canned std::chrono::duration, std::chrono::milliseconds( 120000 ), as a 2 minute wait timeout AND then...

...set the clock BACK 24 hours to the correct/current time with ntpd, wait_for() doesn't return after 2 minutes (assuming no notify() of course). (Presumably, wait_for() is using system_clock, and not steady_clock, and will fire in 24 hours + 2 minutes -- but I haven't waited to find out.)

However, if I repeat this experiment and call std::condition_variable::wait_until() with a timeout time_point calculated as std::chrono::steady_clock::now() + std::chrono::milliseconds( 120000 ), which neatly binds a steady_clock ref into the time_point, it does indeed fire in 2 minutes after the clock is set back, as expected.

In short, the std::condition_variable API that takes a std::chrono:duration does not work, but the one that takes a std::chrono::time_point does.

This is strange to me since sites like cppreference.com <http://cppreference.com/> indicate that both methods use a steady_clock to calculate duration AND imply that wait_for() may even implemented internally in terms of wait_until(). However, in this implementation, both of these assertions seem incorrect. Anyway, it seems like a bug.

Personally, I am fine, as I can implement a wrapper that does our wait_for() in terms of wait_until(), but it took a few days of chasing through our code to pin this down, so figured I share and maybe save someone some pain.

-- Ward






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