build--sync: lock transaction with paclock(1)

An alternative to this is using paclock --run:

  paclock --run -- aur build--sync

Locking inside aur-build--sync keeps this implementation detail hidden,
e.g. compare the sudoers rules

  NOPASSWD: /usr/lib/aurutils/aur-build--sync

and

  NOPASSWD: /usr/bin/paclock --run aur build--sync

where in the latter the path is defined by AUR_LIB_DIR and aur(1)
instead of defined by system policy.

lib/aur-build--sync

@@ -10,6 +10,12 @@ if (( ! $# )); then
     exit 1
 fi
 
+# lock alpm database
+lock_file=$(paclock --print)
+lock_key=$argv0:$(hostname):$$
+paclock --lockfile="$lock_file" --key="$lock_key"
+
+# sync repository to pacman
 arg_repo=$1
 pacsync "$arg_repo"
 pacsync "$arg_repo" --dbext=.files
@@ -19,9 +25,7 @@ while IFS='/' read -r repo name; do
     if [[ $repo == "$arg_repo" ]]; then
         targets+=("$name")
     fi
-done < <(
-    pacman -Sup --print-format '%r/%n'
-)
+done < <(pacman -Sup --print-format '%r/%n')
 wait $!
 
 if (( ${#targets[@]} )); then
@@ -29,4 +33,7 @@ if (( ${#targets[@]} )); then
     pacman -S --noconfirm "${targets[@]}"
 fi
 
+# release lock
+paclock --lockfile="$lock_file" --key="$lock_key" --unlock
+
 # vim: set et sw=4 sts=4 ft=sh: