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L2 · Infrastructure Deadlines that are guaranteed to be checked, and bus-state monitoring

CanKit.Pro.Reliability

Error/timeout infrastructure for CanKit (arc42 §5.3 / ADR-11; SRS FR-RAW-050/051): a reusable deadline primitive whose expiry is guaranteed to actually be checked and fired, and a bus-state monitor that pushes ICanBus.BusState transitions to a protocol instance — both composed on top of CanKit.Pro.Actor's single-mailbox loop, so there are no free-running timers, no busy loops, and no second background-exception channel.

Status: 1.0.0 – 1.2.3 are withdrawn from nuget.org — they were published as stable before the API had been reviewed. 1.3.0 will be the first release whose API is stable; until it is tagged there is no listed version to install, so the dotnet add package line below resolves nothing and the withdrawn releases come back only on an exact version pin. The public surface can still change until then. See Versioning.

This package depends only on CanKit.Abstractions (for ICanBus/BusState) and CanKit.Pro.Actor (for IProtocolActor). Every protocol instance already runs on a ProtocolActor (FR-RAW-020), so a deadline is not an independent standalone timer — it is scheduled through the actor's own event-driven timer queue, which is exactly why its expiry can never sit as inert, never-checked data (the deep-code-review finding "Deadlines werden gepflegt, aber nie geprüft", Review §1.1 Punkt 10).

using CanKit.Core;
using CanKit.Pro.Actor;
using CanKit.Pro.Reliability;

using var bus = CanBus.Open("virtual://demo/0", cfg => cfg.SetProtocolMode(CanProtocolMode.Can20).Baud(500_000));
using var actor = new ProtocolActor();

// (1) React to bus degradation so a controlled TX can abort/pause and resume (FR-RAW-051).
using var monitor = new BusStateMonitor(bus, actor);
monitor.StateChanged += (_, e) =>
{
    if (e.Current.IsTransmitBlocked())   // BusOff
        AbortActiveTransmission();
    else if (!e.Current.IsDegraded() && e.Previous.IsDegraded())
        ResumeTransmission();            // recovered back to ErrActive
};

// (2) Arm a timeout for a time-bounded transition (FR-RAW-050), e.g. an ISO-TP N_Cr window.
var scheduler = new DeadlineScheduler(actor);
var deadline = scheduler.Arm(TimeSpan.FromMilliseconds(150), () => channel.OnTimeout());

// ... later, when the awaited event arrives in time:
if (deadline.Complete())
{
    // We finished before the deadline fired; onTimeout will not run.
}
// Or refresh it on each consecutive frame instead of letting it expire:
deadline.Rearm(TimeSpan.FromMilliseconds(150));

Deadlines (FR-RAW-050)

  • Guaranteed to be checked, not just stored: onExpired is scheduled via the actor's own Schedule, so it is dispatched and run on the loop rather than sitting as data nobody re-reads.
  • Single, race-free resolution: a deadline is Pending until exactly one of expiry, Complete(), or Dispose() (which is how a deadline is cancelled — there is no separate Cancel()) wins an Interlocked state transition; the others become idempotent no-ops. Complete() returns whether it won — a caller's answer to "did I finish before the deadline fired?".
  • Rearm best-effort semantics: re-arming a still-Pending deadline disposes the old actor-timer handle (best-effort) and arms a new one, using a generation counter so a stale pre-Rearm timer that the actor already dispatched no-ops instead of double-firing. Mirroring the actor's own documented Schedule caveat, a Rearm racing an already-in-flight fire is best-effort, not linearizable.
  • Exceptions: an exception thrown from onExpired propagates out of the actor's Schedule callback and surfaces through the actor's existing BackgroundExceptionOccurred (FR-RAW-023) — there is deliberately no second exception channel.
  • Actor lifetime: disposing the owning actor implicitly stops still-pending deadlines from firing — the actor's FinalDrain discards not-yet-due Schedule callbacks rather than firing them, so a deadline that was Pending when the actor is disposed simply never resolves (neither expires nor errors) and reads exactly like a healthy pending one — none of IsExpired, IsCompleted, IsCancelled will ever become true. Signalling that would need a fourth flag on the public IDeadline, which is a break for implementers, so the rule is instead: resolve outstanding deadlines (Complete()/Dispose()) before disposing the actor they run on. Rearm is the one operation that notices, because it has to talk to the actor: it lets the resulting ObjectDisposedException propagate rather than swallowing it, and forces the deadline to Cancelled so it is not left as an unobservable zombie.

Bus-state monitoring (FR-RAW-051)

  • Self-rearming poll, not a free-running timer: ICanBus.BusState has no change event, and an adapter's ErrorFrameReceived/FaultOccurred may not fire on every transition, so the reliable mechanism is a poll (default 50 ms) driven through the actor's Schedule, staying inside the event-driven-actor model instead of a busy loop.
  • Low-latency hints: ErrorFrameReceived and FaultOccurred are additionally subscribed as hints that Post an immediate out-of-band recheck (so a BusOff is seen near-instantly), without touching the poll timer — the self-rearming poll remains the independent reliability floor. If an adapter refuses these subscriptions (e.g. AllowErrorInfo=false), the monitor degrades cleanly to poll-only.
  • Hints are coalesced: a bus-off or error-passive storm raises ErrorFrameReceived thousands of times per second, so at most one un-run hint recheck is ever outstanding in the mailbox — further hints arriving while it is queued are dropped instead of posted. A recheck is a sample of a level (BusState is a plain getter), not the delivery of a queued event, so N back-to-back samples of an unchanged level report exactly what one reports; what is dropped is mailbox traffic that would otherwise starve the protocol work the state change exists to abort. The gate is released before the sample is taken, so a hint racing an in-flight recheck posts a follow-up and the last hint of a storm is always succeeded by a sample taken after it. Coalescing does not make the monitor miss edges it would otherwise report: as ever, the intermediate levels of a fast ErrWarning → ErrPassive → BusOff cascade are only seen if a sample lands between them — shorten the poll interval if you need finer granularity.
  • Edge-triggered: StateChanged fires only when the newly-read state differs from the last-seen one, for both degrading and recovering transitions (BusOff → ErrActive matters too).
  • Loop-thread cost: each tick reads BusState synchronously on the actor's loop thread; a slow or blocking adapter getter therefore stalls that instance's loop for the duration — a known tradeoff of reusing the actor (which keeps handling single-writer-safe), not a bug fixed here.
  • Lifetime: the poll loop also stops on its own once the owning actor is disposed. Dispose() is still required (and idempotent) to detach the two bus event subscriptions, which are independent of the actor's lifetime.
  • Helpers: BusStateExtensions.IsTransmitBlocked() (true only for BusOff) and IsDegraded() (true for ErrWarning/ErrPassive/BusOff/Unknown). The two treat Unknown differently on purpose: it means "we could not determine the controller state", which is never a basis for reporting health, but is equally never proof that the bus is off — so it degrades, and it does not block transmission on the many adapters that simply never report a state.

Out of scope: FR-RAW-052 (reserved/invalid protocol values)

FR-RAW-052 (a Should: reserved/invalid protocol values in incoming frames — e.g. reserved ISO-TP STmin values 0x80–0xF0/0xFA–0xFF — should be interpreted per-spec, as 127 ms, rather than throwing) is intentionally not implemented in this package. It is protocol-codec-specific: the correct handling lives inside the ISO-TP frame codec, not in a generic reliability primitive, and belongs with the future ISO-TP fix (FR-TP-007, the same review finding as Review §1.1 Punkt 6). Building a generic "reserved value" abstraction here would be speculative over-engineering, so this package deliberately covers only FR-RAW-050 and FR-RAW-051.

Install

dotnet add package CanKit.Pro.Reliability

# plus a CanKit adapter for the hardware you actually talk to, e.g.
dotnet add package CanKit.Adapter.Virtual   # loopback, no hardware
# dotnet add package CanKit.Adapter.PCAN    # Kvaser, Vector, SocketCAN, ZLG, ... likewise

Dependencies: CanKit.Abstractions, CanKit.Pro.Actor.

Part of CanKit.Pro — higher CAN protocol layers built on top of CanKit, which is consumed as a NuGet package rather than forked.

License

MIT — see LICENSE. CanKit itself is a separate project licensed under Apache-2.0; see THIRD-PARTY-NOTICES.md.