link_manager.h

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// Copyright (C) 2026 Douglas Quigg (dstroy0) <dquigg123@gmail.com>
// SPDX-License-Identifier: AGPL-3.0-or-later
 
/**
 * @file link_manager.h
 * @brief Multi-interface egress selection + graceful escalation/failover (DETWS_ENABLE_LINK_MANAGER).
 *
 * Once a device has more than one network interface (a wired Ethernet PHY brought up alongside WiFi STA,
 * plus maybe a softAP), something has to decide which one carries traffic and when to switch: escalate to
 * the wired link when it comes up (usually faster / more reliable), and fail over to WiFi when it drops.
 * The stack owns the routes and `det_net_egress()` reports the live one; this is the *policy* that drives
 * it - a small table of interfaces (each a kind + priority + up/down) with a deterministic "best link
 * that is up" selection, plus change detection so the app only reconfigures on an actual transition.
 *
 * Pure, no heap, no stdlib, host-testable. The real PHY bring-up (esp_eth) and the netif reconfigure are
 * the app's; this just says which interface should be active.
 */
 
#ifndef DETERMINISTICESPASYNCWEBSERVER_LINK_MANAGER_H
#define DETERMINISTICESPASYNCWEBSERVER_LINK_MANAGER_H
 
#include "ServerConfig.h"
#include <stddef.h>
#include <stdint.h>
 
#if DETWS_ENABLE_LINK_MANAGER
 
/** @brief Interface kind (informational; selection is by priority). Stored in a uint8_t field and
 *  compared, so integer constants in a namespacing struct - cast-free. */
struct LinkKind
{
    static constexpr uint8_t LINK_KIND_ETH = 0;      ///< wired Ethernet PHY.
    static constexpr uint8_t LINK_KIND_WIFI_STA = 1; ///< WiFi station.
    static constexpr uint8_t LINK_KIND_WIFI_AP = 2;  ///< WiFi softAP.
    static constexpr uint8_t LINK_KIND_OTHER = 3;
};
 
/** @brief One managed interface. */
struct LinkIface
{
    uint8_t kind;     ///< LINK_KIND_*.
    uint8_t priority; ///< higher wins when up (ties break to the lower index).
    bool up;          ///< link currently up.
};
 
/** @brief The link-manager state over a caller-owned interface table. */
struct LinkManager
{
    LinkIface *ifaces;
    size_t n;
    int active; ///< index of the active egress, or -1 if none is up.
};
 
/** @brief Initialize over caller storage and compute the initial active egress. */
void detws_link_init(LinkManager *m, LinkIface *ifaces, size_t n);
 
/** @brief Best interface that is up (highest priority, lower index breaks ties). @return index or -1. */
int detws_link_select(const LinkManager *m);
 
/** @brief The current active egress index (-1 if none). */
int detws_link_active(const LinkManager *m);
 
/**
 * @brief Set an interface's up/down state and recompute the active egress.
 * @param from (may be null) the previous active index.
 * @param to   (may be null) the new active index.
 * @return true if the active egress changed (escalation or failover happened).
 */
bool detws_link_set(LinkManager *m, size_t idx, bool up, int *from, int *to);
 
#endif // DETWS_ENABLE_LINK_MANAGER
#endif // DETERMINISTICESPASYNCWEBSERVER_LINK_MANAGER_H