improve version and mirror updating

utils.sh

@@ -199,6 +199,29 @@ initramfs_build_failed() {
   (( succeeded == 0 || succeeded < built ))
 }
 
+# pacman downloads from the first mirror in this list (system/mirrors.sh keeps
+# it fresh). Overridable for the tests.
+MIRRORLIST="${MIRRORLIST:-/etc/pacman.d/mirrorlist}"
+# A mirror further behind Arch than this is stale; most sync every few hours
+MIRROR_STALE_HOURS=24
+
+# The first mirror in MIRRORLIST, without its /$repo/os/$arch
+first_mirror() {
+  sed -nE 's/^[[:space:]]*Server[[:space:]]*=[[:space:]]*//p' "$MIRRORLIST" 2> /dev/null \
+    | head -n 1 | sed -E 's#/\$repo.*##'
+}
+
+# Print how many hours ago the first mirror last synced with Arch, from the
+# timestamp mirrors publish in lastsync. Fails if it couldn't tell.
+mirror_age_hours() {
+  local mirror lastsync
+  mirror="$(first_mirror)"
+  [ -n "$mirror" ] || return 1
+  lastsync="$(curl -sf --max-time 10 "$mirror/lastsync" 2> /dev/null)" || return 1
+  [[ "$lastsync" =~ ^[0-9]+$ ]] || return 1
+  echo $(( ($(date +%s) - lastsync) / 3600 ))
+}
+
 # Install a script atomically (copy to a temp file, then rename over the target)
 # so a copy of it that is running right now keeps reading the old file
 install_script() {