#!/bin/bash
# FIDO key presence detection daemon.
# Checks whether a FIDO2/U2F security key is plugged in, at an adaptive
# interval (20s under light load, 120s under moderate load, 600s under heavy
# load), and shares caffeine's systemd-inhibit idle lock while a key is
# present, so hypridle never fires during an active session.
#
# Detection: `fido2-token -L` (libfido2) enumerates connected FIDO CTAP
# devices by USB HID usage page — no touch/tap required, so this is a pure
# presence check, not an authentication. Non-empty output = a key is present.
#
# This replaced an earlier webcam motion-detection design: simpler, no camera
# contention with video calls/howdy, and no OpenCV dependency.

# Shared with caffeine.sh: both the manual toggle and this daemon drive the
# same systemd-inhibit lock, so caffeine-status.sh reflects either source.
PID_FILE="/tmp/caffeine-inhibit.pid"
# Marks that *this daemon* (not the manual caffeine toggle) currently holds
# the lock, so a "key removed" tick never releases a manually-started session.
OWNED_FLAG="/tmp/presence-inhibit-owned"
# Records whether a FIDO key is present on the last tick. presence-status.sh
# reads this so the Eww widget can show presence as a distinct signal from
# the idle lock. During a MANUAL caffeine session the daemon still checks (the
# check is cheap and doesn't touch any device other apps might want), so this
# stays live even then — only the inhibit ownership differs.
PRESENCE_FLAG="/tmp/presence-detected"

INTERVAL_LIGHT=20    # seconds between checks when CPU-or-RAM usage < LOAD_LIGHT
INTERVAL_MID=120     # seconds between checks when usage is between the two thresholds
INTERVAL_HEAVY=600   # seconds between checks when usage >= LOAD_HEAVY
LOAD_LIGHT=0.20       # CPU-or-RAM fraction below which we poll at INTERVAL_LIGHT
LOAD_HEAVY=0.50       # CPU-or-RAM fraction at/above which we back off to INTERVAL_HEAVY
NPROC="$(nproc)"

# True if any FIDO2/U2F authenticator is currently enumerable over USB.
# fido2-token -L lists one line per connected device and needs no PIN, touch,
# or tap — it's a pure USB HID enumeration, so this never prompts the key.
_fido_key_present() {
    [[ -n "$(fido2-token -L 2>/dev/null)" ]]
}

# Prints current CPU-or-RAM usage as a fraction (0..1): the higher of the
# core-count-normalized 1-minute load average and the used-RAM fraction. Load
# average is a cheap, sampling-free proxy for "CPU busy" — no extra measurement
# delay per tick.
_resource_usage() {
    local load1 cpu_frac mem_frac
    load1="$(awk '{print $1}' /proc/loadavg)"
    cpu_frac="$(awk -v l="$load1" -v n="$NPROC" 'BEGIN{print l/n}')"
    mem_frac="$(free | awk '/^Mem:/ {print $3/$2}')"
    awk -v c="$cpu_frac" -v m="$mem_frac" 'BEGIN{print (c>m)?c:m}'
}

# Three-tier poll interval based on current resource usage:
#   >= LOAD_HEAVY  -> INTERVAL_HEAVY (heavy load: back off, stay out of the way)
#   <  LOAD_LIGHT  -> INTERVAL_LIGHT (near-idle: poll fast, cheap and responsive)
#   otherwise      -> INTERVAL_MID   (moderate load)
_next_interval() {
    local u; u="$(_resource_usage)"
    awk -v u="$u" -v heavy="$LOAD_HEAVY" -v light="$LOAD_LIGHT" \
        -v fh="$INTERVAL_HEAVY" -v fl="$INTERVAL_LIGHT" -v fm="$INTERVAL_MID" \
        'BEGIN{ if (u >= heavy) print fh; else if (u < light) print fl; else print fm }'
}

# Returns true if the inhibitor sentinel process is still alive.
_inhibit_running() {
    [[ -f "$PID_FILE" ]] && kill -0 "$(cat "$PID_FILE")" 2>/dev/null
}

_start_inhibit() {
    # Guard: don't start a second inhibitor if one is already active — whether
    # it's ours from a previous tick or a manually-started caffeine session.
    _inhibit_running && return
    # --what=idle: target the logind idle-inhibit lock that hypridle polls.
    # "sleep infinity" is the sentinel; its PID is saved so we can kill it later.
    systemd-inhibit --what=idle --who="presence-detect" \
        --why="FIDO key present" --mode=block \
        sleep infinity &
    echo $! > "$PID_FILE"
    touch "$OWNED_FLAG"
    # logger writes to the system journal — visible via `journalctl -t presence-detect`.
    logger -t presence-detect "FIDO key present — idle inhibited"
}

_stop_inhibit() {
    _inhibit_running || return
    # Never release a lock we didn't start — that would be a manual caffeine
    # session, which must persist regardless of presence.
    [[ -f "$OWNED_FLAG" ]] || return
    # Killing the sleep process releases the systemd-inhibit lock automatically.
    kill "$(cat "$PID_FILE")" 2>/dev/null
    rm -f "$PID_FILE" "$OWNED_FLAG"
    logger -t presence-detect "FIDO key removed — idle inhibit released"
}

_cleanup() {
    # On daemon stop (systemd unit stop, user logout, etc.), release the idle
    # lock (only if we're the one holding it) and clear the presence flag —
    # with the daemon gone, nothing is watching.
    _stop_inhibit
    rm -f "$PRESENCE_FLAG"
    exit 0
}
# Intercept termination signals to ensure the inhibitor PID is never orphaned.
trap _cleanup SIGTERM SIGINT SIGHUP

while true; do
    # When idle is inhibited MANUALLY (a caffeine toggle, not this daemon), the
    # lock is already held regardless of presence. Detected the same way
    # caffeine-manual-status.sh does it: the shared lock is alive but we don't
    # own it. We still update PRESENCE_FLAG below since checking is cheap and
    # doesn't touch a device other apps care about — only inhibit ownership
    # differs during a manual session.
    if _fido_key_present; then
        touch "$PRESENCE_FLAG"
        # No-op if a manual caffeine session already holds the lock — only
        # _stop_inhibit needs the ownership check; starting is always safe
        # since _start_inhibit itself no-ops when a lock is already held.
        _start_inhibit
    else
        rm -f "$PRESENCE_FLAG"
        _stop_inhibit
    fi

    sleep "$(_next_interval)"
done