refactoring: split helper functions into smaller scripts (#2420)

target/scripts/helpers/lock.sh

@@ -0,0 +1,39 @@
+#! /bin/bash
+
+# This becomes the sourcing script name
+# (example: check-for-changes.sh)
+SCRIPT_NAME="$(basename "$0")"
+# Used inside of lock files to identify them and
+# prevent removal by other instances of docker-mailserver
+LOCK_ID="$(uuid)"
+
+function _create_lock
+{
+  LOCK_FILE="/tmp/docker-mailserver/${SCRIPT_NAME}.lock"
+  while [[ -e "${LOCK_FILE}" ]]
+  do
+    _notify 'warn' "Lock file ${LOCK_FILE} exists. Another ${SCRIPT_NAME} execution is happening. Trying again shortly..."
+    # Handle stale lock files left behind on crashes
+    # or premature/non-graceful exits of containers while they're making changes
+    if [[ -n "$(find "${LOCK_FILE}" -mmin +1 2>/dev/null)" ]]
+    then
+      _notify 'warn' "Lock file older than 1 minute. Removing stale lock file."
+      rm -f "${LOCK_FILE}"
+      _notify 'inf' "Removed stale lock ${LOCK_FILE}."
+    fi
+    sleep 5
+  done
+  trap _remove_lock EXIT
+  echo "${LOCK_ID}" > "${LOCK_FILE}"
+}
+
+function _remove_lock
+{
+  LOCK_FILE="${LOCK_FILE:-"/tmp/docker-mailserver/${SCRIPT_NAME}.lock"}"
+  [[ -z "${LOCK_ID}" ]] && _errex "Cannot remove ${LOCK_FILE} as there is no LOCK_ID set"
+  if [[ -e "${LOCK_FILE}" ]] && grep -q "${LOCK_ID}" "${LOCK_FILE}" # Ensure we don't delete a lock that's not ours
+  then
+    rm -f "${LOCK_FILE}"
+    _notify 'inf' "Removed lock ${LOCK_FILE}."
+  fi
+}