chrono clock should be defined in config.hpp

[thirtytwobits]

See https://en.cppreference.com/w/cpp/named_req/TrivialClock

This is a design change, not a bug.

Using chrono becomes tricky when implementing the platform layer because C++ chrono treats system time as a global resource. Furthermore, on some systems chrono clocks are implemented by lower-level (read -> C) clocks also used by the platform implementation. We may want to remove ITimeProvider in favor of simply designating a chrono clock all parts of libcyphal will use as a compile-time configuration (with defaults) allowing a given platform to ensure the same timebase by providing a custom chrono clock backed by the low-level implementation for said platform.

If libcyphal always uses a compile-time-configured clock then unit testing is just as easy to handle as a dummy clock can be configured for tests when control over time is needed.

There doesn't seem to be another other benefit to supporting polymorphism in our timekeeping mechanism, unless I'm missing something.

[serges147]

Scott, sorry but I don't get what is the problem currently. Could you please elaborate and give concrete example where currently implemented approach doesn't fit.

If libcyphal always uses a compile-time-configured clock then unit testing is just as easy to handle as a dummy clock...

In my numerous unit tests I have everywhere artificial control over time very easy (see VirtualTimeScheduler and its scheduleAt method), so I don't quite understand your unit testing concern either.

ITimeProvider interface and its now() method were introduced naturally (extracted from initial IExecutor) when it became obvious that there are presentation level entities (server and subscriber) which don't need whole executor functionality but only its time providing part.

[pavel-kirienko]

As we discussed at the call, one way forward is to define our own MonotonicClock as described in the design doc which by default simply wraps std::steady_clock. There will be an ifdef configuration parameter that allows the user to provide their own libcyphal::MonotonicClock defined elsewhere; this could be done via a compile time macro that works like this:

#ifndef LIBCYPHAL_CUSTOM_CLOCK_HEADER
// the default implementation that proxies to std::steady_clock
#else
#   include LIBCYPHAL_CUSTOM_CLOCK_HEADER
#endif

[pavel-kirienko]

An alternative is to define our own MonotonicClock that declares but does not define static MonotonicClock::now(), so that the user provides their own definition somewhere in any translation unit. This approach does not require preprocessor configs.

[thirtytwobits]

I've just had the best idea (no really, don't argue, this is the best idea ;-) )

We should define the clock type in config.hpp (see #419) so we can use the application's native timepoint representation and avoid unnecessary division operations. I'm running into this now where the ratio and representation for time on FreeRTOS is different so every use of time by libcyphal has to translate even before it knows the value is needed or that the native format is (would have been) okay.

[thirtytwobits]

I changed the name of this issue because, as I implement FreeRTOS support, I'm realizing this problem is worse than I originally thought. libcyphal dictates a chrono::time_point which includes the clock type itself. In the FreeRTOS port I'm developing there are two clock domains so the clock type in a time_point is a key indicator that a clock domain boundary is being crossed (thar be dragons), however, because libcyphal is defining it's own clock I'm forced to synthesize time_point values. I'm going to get it wrong at some point because I'm purposefully usurping the type system to implement ITimeProvider and to interact with libcyphal.

[thirtytwobits]

Making this high-priority because the change will have a large blast radius. The more we use this library the harder it's going to be to change this.

[serges147]

Fixed by pr #441