fetch: use per-package locks for destructive operations

lib/aur-fetch

@@ -119,6 +119,7 @@ if (( ! $# )); then
     exit 1
 fi
 
+# XXX: race with concurrent processes
 if [[ -v results_file ]]; then
     : >"$results_file" || exit 1 # truncate file
 fi
@@ -160,6 +161,10 @@ for pkg in "${packages[@]}"; do
         continue
 
     elif [[ -d $pkg/.git ]]; then
+        # Per-package lock
+        exec {fd}< "$pkg"/.git
+        flock --wait 5 "$fd" || exit 1
+
         # Avoid issues with filesystem boundaries (#274)
         git() { command git -C "$pkg" "$@"; }
 
@@ -213,6 +218,7 @@ for pkg in "${packages[@]}"; do
         if [[ -v results_file ]]; then
             results "$sync_pkg" "$orig_head" "$head" "$PWD/$pkg" "$results_file"
         fi
+        exec {fd}<&- # release lock
 
     # Otherwise, try to clone anew
     else