failsafe.h

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// Copyright (C) 2026 Douglas Quigg (dstroy0) <dquigg123@gmail.com>
// SPDX-License-Identifier: AGPL-3.0-or-later
 
/**
 * @file failsafe.h
 * @brief Software watchdog: deadlock detection + fail-safe safe-state (DETWS_ENABLE_FAILSAFE).
 *
 * A fixed registry of "lifelines" - a task, worker, or control loop that must check in
 * (`detws_failsafe_feed`) at least every `deadline_ms`. If one stops checking in (a hang, a
 * deadlock, a wedged loop), `detws_failsafe_check()` detects it and fires a breach callback exactly
 * once per stuck episode, so the app can drive its outputs to a known-safe state (motors off, valves
 * closed), log, and optionally reset. It complements the hardware task watchdog: this one is
 * app-defined, per-lifeline, and knows *which* subsystem wedged.
 *
 * Zero heap (a static registry), no stdlib. The overdue test is a wrap-safe unsigned time delta, so it
 * is correct across a `millis()` rollover. The evaluation core takes an explicit `now`, so it is fully
 * host-testable with a synthetic clock; the no-`now` wrappers read the pluggable `detws_millis()`.
 */
 
#ifndef DETERMINISTICESPASYNCWEBSERVER_FAILSAFE_H
#define DETERMINISTICESPASYNCWEBSERVER_FAILSAFE_H
 
#include "ServerConfig.h"
#include <stddef.h>
#include <stdint.h>
 
#if DETWS_ENABLE_FAILSAFE
 
/** @brief One monitored lifeline. */
struct DetwsLifeline
{
    const char *name;      ///< label (for the breach callback + JSON); not copied.
    uint32_t deadline_ms;  ///< max interval between feeds before it is considered stuck.
    uint32_t last_feed_ms; ///< time of the last check-in (detws_millis units).
    bool armed;            ///< slot in use.
    bool breached;         ///< currently in breach (so the callback fires once per episode).
};
 
/** @brief Breach callback: invoked once when @p id (named @p name) misses its deadline. */
typedef void (*DetwsFailsafeCb)(int id, const char *name, void *arg);
 
// ---------------------------------------------------------------------------
// Host-testable core
// ---------------------------------------------------------------------------
 
/**
 * @brief Is a lifeline overdue at @p now?
 *
 * Wrap-safe: the unsigned delta `now - last_feed` is correct across a millis() rollover as long as the
 * true gap is under 2^32 ms (~49 days), which any real deadline is.
 */
static inline bool detws_lifeline_overdue(uint32_t now, uint32_t last_feed_ms, uint32_t deadline_ms)
{
    return (uint32_t)(now - last_feed_ms) > deadline_ms;
}
 
// ---------------------------------------------------------------------------
// Registry API
// ---------------------------------------------------------------------------
 
/** @brief Clear the whole registry and the breach callback (mainly for tests / re-init). */
void detws_failsafe_reset(void);
 
/**
 * @brief Register a lifeline that must check in at least every @p deadline_ms.
 * @param name        label used in the breach callback + JSON (borrowed, not copied).
 * @param deadline_ms max allowed interval between feeds.
 * @return the lifeline id (>= 0), or -1 if the registry (DETWS_FAILSAFE_MAX_LIFELINES) is full.
 *
 * The lifeline starts fed at the registration time, so it is not instantly overdue.
 */
int detws_failsafe_register(const char *name, uint32_t deadline_ms);
int detws_failsafe_register_at(const char *name, uint32_t deadline_ms, uint32_t now);
 
/** @brief Check in lifeline @p id (clears any breach). Reads detws_millis(). */
bool detws_failsafe_feed(int id);
bool detws_failsafe_feed_at(int id, uint32_t now);
 
/** @brief Install the breach callback (fired once per stuck episode). */
void detws_failsafe_on_breach(DetwsFailsafeCb cb, void *arg);
 
/**
 * @brief Evaluate every armed lifeline; fire the callback for each newly-overdue one.
 * @return a bitmask of the lifeline ids currently in breach (bit i set = id i breached).
 *
 * A lifeline that is overdue and not already flagged fires the callback once and is marked breached
 * until it is fed again; already-breached lifelines stay in the returned mask but do not re-fire.
 */
uint32_t detws_failsafe_check_at(uint32_t now);
uint32_t detws_failsafe_check(void);
 
/** @brief Serialize the registry as JSON for a /health-style endpoint. @return bytes written (excl NUL). */
int detws_failsafe_json_at(uint32_t now, char *out, size_t cap);
 
#endif // DETWS_ENABLE_FAILSAFE
#endif // DETERMINISTICESPASYNCWEBSERVER_FAILSAFE_H