Set the timezone from the public IP on NetworkManager connect

arch/etc/NetworkManager/dispatcher.d/90-timezone

@@ -0,0 +1,58 @@
+#!/bin/sh
+set -eu
+
+PATH=/usr/bin:/bin
+
+IFACE="${1:-}"
+ACTION="${2:-}"
+
+case "$ACTION" in
+    up|connectivity-change) ;;
+    *) exit 0 ;;
+esac
+
+if [ "$IFACE" = "lo" ]; then
+    exit 0
+fi
+
+STAMP=/var/lib/nm-timezone/last-attempt
+MIN_INTERVAL=600
+
+# Rate limit on attempts, not successes, so a flapping link cannot hammer the API.
+if [ -f "$STAMP" ]; then
+    age=$(( $(date +%s) - $(stat -c %Y "$STAMP") ))
+    if [ "$age" -lt "$MIN_INTERVAL" ]; then
+        exit 0
+    fi
+fi
+mkdir -p "$(dirname "$STAMP")"
+: > "$STAMP"
+
+lookup() {
+    curl -fsS --max-time 5 "$1" 2>/dev/null | tr -d '[:space:]'
+}
+
+tz=$(lookup https://ipapi.co/timezone) || tz=""
+if [ -z "$tz" ]; then
+    tz=$(lookup https://ipinfo.io/timezone) || tz=""
+fi
+
+# Reject anything that is not a bare Region/City before it reaches the filesystem.
+case "$tz" in
+    */*) ;;
+    *) exit 0 ;;
+esac
+case "$tz" in
+    *[!A-Za-z0-9/_+-]* | /* | */ | */../*) exit 0 ;;
+esac
+if [ ! -f "/usr/share/zoneinfo/$tz" ]; then
+    exit 0
+fi
+
+current=$(timedatectl show -p Timezone --value)
+if [ "$tz" = "$current" ]; then
+    exit 0
+fi
+
+timedatectl set-timezone "$tz"
+logger -t nm-timezone "timezone $current -> $tz"

arch/install-arch.sh

@@ -54,6 +54,11 @@ install_system_configs() {
         "$DOTFILES/arch/etc/udev/rules.d/90-backlight-on-power.rules" \
         /etc/udev/rules.d/90-backlight-on-power.rules
     sudo udevadm control --reload-rules
+    # Follows the laptop across continents: resolves the public IP to a timezone
+    # on every NetworkManager connect and sets it only when it differs.
+    sudo install -D -m 0755 -o root -g root \
+        "$DOTFILES/arch/etc/NetworkManager/dispatcher.d/90-timezone" \
+        /etc/NetworkManager/dispatcher.d/90-timezone
     sudo install -D -m 0644 -o root -g root \
         "$DOTFILES/arch/etc/tmpfiles.d/mglru.conf" \
         /etc/tmpfiles.d/mglru.conf