add utils/lock.sh

a Bash implementation of lock using `flock(2)`.

Library/Homebrew/utils/lock.sh

+# create a lock using `flock(2)`. A name is required as first argument.
+# the lock will be automatically unlocked when the shell process quits.
+# Noted due to the fixed FD, a shell process can only create one lock.
+lock() {
+  local name="$1"
+  local lock_file="/tmp/homebrew${HOMEBREW_REPOSITORY//\//-}-${name}.lock"
+  # 200 is the file descriptor used in the lock.
+  # This FD should be used exclusively for lock purpose.
+  # Any value except 0(stdin), 1(stdout) and 2(stderr) can do the job.
+  # Noted, FD is unique per process but it will be shared to subprocess.
+  # It is recommended to choose a large number to avoid conflicting with
+  # other FD opened by the script.
+  #
+  # close FD first, this is required if parent process holds a different lock.
+  exec 200>&-
+  # open the lock file to FD, so the shell process can hold the lock.
+  exec 200>"$lock_file"
+  if ! _create_lock 200
+  then
+    odie <<EOS
+Another active Homebrew process is already in progress.
+Please wait for it to finish or terminate it to continue.
+EOS
+  fi
+}
+
+_create_lock() {
+  local lock_fd="$1"
+  if [[ -n "$(which ruby)" ]]
+  then
+    ruby -e "File.new($lock_fd).flock(File::LOCK_EX | File::LOCK_NB) || exit(1)"
+  elif [[ -n "$(which flock)" ]]
+  then
+    flock -n "$lock_fd"
+  else
+    onoe "Cannot create lock, please avoid running brew in parallel."
+  fi
+}