Getting the solver status of compute_control without asserting

[baggepinnen]

Goal

Know whether a compute_control call actually solved its QP when the controller runs inside a simulation/deployment loop, without stopping the loop.

In DiscreteComponents (Dyad components wrapping LinearMPC.jl) each tick calls compute_control from a clocked equation; an infeasible QP must not abort the whole simulation, but the caller wants to log or act on the solver status (hold the previous control, count failures, flag the run). The same holds for a Simulation-style loop written by hand.

What I tried

• compute_control(mpc, x; check = true) (the default) asserts exitflag >= 1 and throws, so the loop dies.
• compute_control(mpc, x; check = false) returns a control on failure (the solver's last iterate, as far as I can tell) but gives no indication that it failed.
• Reaching for form_parameter + solve(mpc, θ) gives the exit flag, but both are internals that are not part of the documented API, so a wrapper built on them breaks silently when they change; compute_control is the only public entry point.
• The generated C (mpc_compute_control) already returns the DAQP exit flag as its return value, so the C side has what the Julia side lacks.

Question

Is there an intended way to get the solver status of the last compute_control call (or the last exit flag / DAQP.flag2status entry) through the public API? If not, would a supported way to obtain it — whatever shape fits the package — be welcome? I'd be happy to help with a PR once the preferred form is clear.

[darnstrom]

Thanks, this is a genuine gap in the public API, and a PR would be welcome.

My preference is to keep compute_control returning only the control vector and add a separate, type-stable entry point, compute_control_result:

result = compute_control_result(mpc, x; r, d, uprev, p)

where result struct could contain the fields control, èxitflag, status, and solver_info.

As an alternative/complement is to introduce a dedicated exception:

struct MPCSolveError <: Exception
     exitflag::Int
    status::Symbol
end

What do you think is the best approach?

[baggepinnen]

I would prefer the compute_control_result since there are several different exit flags that indicate success, and in some cases failures aren't exceptional either but they still need to be handled. I'll open the PR :)