ap: keep AP beaconing across USB re-enumeration

Starlink's USB ethernet shares a hub with the rtw89 Wi-Fi adapter; when Starlink
flaps, the hub re-enumerates and tears the radio down, so hostapd loses its
interface and exits. Two stock defaults left the AP dark: Restart=on-failure
misses clean exits, and the 5-in-10s start limit makes systemd give up during a
re-enumeration storm.

- hostapd-restart.conf drop-in: Restart=always, RestartSec=5, StartLimitIntervalSec=0
  so hostapd retries forever until the interface returns (verified: SIGKILL -> back
  to state=ENABLED in ~7s).
- van-ap-watchdog daemon: backstop for the case systemd can't see (hostapd running
  but radio wedged). Polls hostapd_cli status; restarts hostapd if the iface is
  present but not ENABLED, waits it out if the iface is mid-re-enumeration.
- hostapd.conf: add ctrl_interface so the watchdog can read the real AP state.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

ap/van-ap-watchdog

@@ -0,0 +1,116 @@
+#!/usr/bin/env python3
+"""van-ap-watchdog — keep the VanLink AP beaconing.
+
+The AP wlan (rtw89, USB) shares a USB hub with the Starlink ethernet (r8152, USB).
+When Starlink's link flaps the hub can re-enumerate and take the Wi-Fi radio down
+with it. The hostapd.service drop-in (restart.conf) handles the common case — hostapd
+exits and systemd restarts it (Restart=always, no start limit) until the interface
+returns. This daemon is the backstop for the case systemd *can't* see: hostapd stays
+running but the radio has wedged and stopped serving (dmesg "timed out to flush queues").
+
+Every `interval` seconds it asks hostapd for its real state via the control socket
+(hostapd_cli status -> state=ENABLED). If the interface is present but the AP is not
+ENABLED for `fail_threshold` checks in a row, it clears any failed state and restarts
+hostapd. If the interface is simply gone (mid re-enumeration) it waits — there is
+nothing to restart onto, and Restart=always reclaims it when it reappears. Stdlib only.
+"""
+
+import re
+import subprocess
+import sys
+import time
+from pathlib import Path
+
+HOSTAPD_CONF = "/etc/hostapd/hostapd.conf"
+CTRL_DIR = "/var/run/hostapd"
+INTERVAL = 15          # seconds between health checks (backstop cadence, not first response)
+FAIL_THRESHOLD = 2     # consecutive bad checks before restarting (debounces re-enum blips)
+
+
+def log(msg, level="info"):
+    # systemd journal severity prefixes (sd-daemon), same convention as van-thermal.
+    pri = {"info": "<6>", "warn": "<4>", "crit": "<2>"}.get(level, "<6>")
+    print(pri + msg, flush=True)
+
+
+def ap_ifname():
+    """The AP interface name, read from hostapd.conf so there's one source of truth."""
+    try:
+        for line in Path(HOSTAPD_CONF).read_text().splitlines():
+            m = re.match(r"\s*interface=(\S+)", line)
+            if m:
+                return m.group(1)
+    except OSError as e:
+        log(f"cannot read {HOSTAPD_CONF} ({e})", "crit")
+    return None
+
+
+def iface_present(ifname):
+    return Path(f"/sys/class/net/{ifname}").exists()
+
+
+def ap_enabled(ifname):
+    """True if hostapd reports the AP as beaconing (state=ENABLED). False if it's
+    running but not enabled; None if the control socket is unreachable (hostapd down)."""
+    try:
+        out = subprocess.run(
+            ["hostapd_cli", "-p", CTRL_DIR, "-i", ifname, "status"],
+            capture_output=True, text=True, timeout=10,
+        ).stdout
+    except (OSError, subprocess.SubprocessError) as e:
+        log(f"hostapd_cli failed: {e}", "warn")
+        return None
+    for line in out.splitlines():
+        if line.startswith("state="):
+            return line.strip() == "state=ENABLED"
+    return None   # no state line -> couldn't talk to hostapd
+
+
+def recover(ifname):
+    log(f"AP {ifname} not beaconing — clearing failed state and restarting hostapd", "warn")
+    # reset-failed first in case some other path parked the unit in a failed state;
+    # harmless when it isn't.
+    subprocess.run(["systemctl", "reset-failed", "hostapd"],
+                   capture_output=True, text=True)
+    r = subprocess.run(["systemctl", "restart", "hostapd"],
+                       capture_output=True, text=True)
+    if r.returncode != 0:
+        log(f"hostapd restart returned {r.returncode}: {r.stderr.strip()}", "warn")
+
+
+def main():
+    ifname = ap_ifname()
+    if not ifname:
+        log("no interface= in hostapd.conf; nothing to watch", "crit")
+        sys.exit(1)
+    log(f"van-ap-watchdog up: watching {ifname} every {INTERVAL}s "
+        f"(restart after {FAIL_THRESHOLD} bad checks)")
+
+    bad = 0
+    waiting = False   # latch so "interface absent" logs once, not every tick
+    while True:
+        if not iface_present(ifname):
+            if not waiting:
+                log(f"{ifname} absent — USB re-enumeration in progress; "
+                    f"waiting for it to return (hostapd Restart=always will reclaim it)", "warn")
+                waiting = True
+            bad = 0
+        else:
+            waiting = False
+            if ap_enabled(ifname):
+                if bad:
+                    log(f"AP {ifname} beaconing again")
+                bad = 0
+            else:
+                bad += 1
+                if bad >= FAIL_THRESHOLD:
+                    recover(ifname)
+                    bad = 0
+        time.sleep(INTERVAL)
+
+
+if __name__ == "__main__":
+    try:
+        main()
+    except KeyboardInterrupt:
+        sys.exit(0)