scripts: apply fixes to helpers when using set -eE (#3285)

For an upcoming PR, these changes are required, because the script that
is using the helpers uses `set -eE`. This leads to situations where
errors are not properly handled in our helpers (yet; I plan on changing
that in the future).

target/scripts/helpers/dns.sh

@@ -41,7 +41,7 @@ function _obtain_hostname_and_domainname
   # of this variable or if a more deterministic approach with `cut` should be relied on.
   if [[ $(_get_label_count "${HOSTNAME}") -gt 2 ]]
   then
-    if [[ -n ${OVERRIDE_HOSTNAME} ]]
+    if [[ -n ${OVERRIDE_HOSTNAME:-} ]]
     then
       # Emulates the intended behaviour of `hostname -d`:
       # Assign the HOSTNAME value minus everything up to and including the first `.`

target/scripts/helpers/error.sh

@@ -87,3 +87,26 @@ function _shutdown
   kill -SIGTERM 1               # Trigger graceful DMS shutdown.
   kill -SIGUSR1 "${SCRIPT_PID}" # Stop start-mailserver.sh execution, even when _shutdown() is called from a subshell.
 }
+
+# Calling this function sets up a handler for the `ERR` signal, that occurs when
+# an error is not properly checked (e.g., in an `if`-clause or in an `&&` block).
+#
+# This is mostly useful for debugging. It also helps when using something like `set -eE`,
+# as it shows where the script aborts.
+function _trap_err_signal
+{
+  trap '__log_unexpected_error "${FUNCNAME[0]:-}" "${BASH_COMMAND:-}" "${LINENO:-}" "${?:-}"' ERR
+
+  # shellcheck disable=SC2317
+  function __log_unexpected_error
+  {
+    local MESSAGE="Unexpected error occured :: script = ${SCRIPT:-${0}} "
+    MESSAGE+=" | function = ${1:-none (global)}"
+    MESSAGE+=" | command = ${2:-?}"
+    MESSAGE+=" | line = ${3:-?}"
+    MESSAGE+=" | exit code = ${4:-?}"
+
+    _log 'error' "${MESSAGE}"
+    return 0
+  }
+}

target/scripts/helpers/log.sh

@@ -123,9 +123,9 @@ function _get_log_level_or_default
   if [[ -n ${LOG_LEVEL+set} ]]
   then
     echo "${LOG_LEVEL}"
-  elif [[ -e /etc/dms-settings ]]
+  elif [[ -e /etc/dms-settings ]] && grep -q -E "^LOG_LEVEL='[a-z]+'" /etc/dms-settings
   then
-    grep "^LOG_LEVEL=" /etc/dms-settings | cut -d "'" -f 2
+    grep '^LOG_LEVEL=' /etc/dms-settings | cut -d "'" -f 2
   else
     echo 'info'
   fi

target/scripts/helpers/utils.sh

@@ -16,7 +16,13 @@ function _get_valid_lines_from_file
 # and it will return its value stored in /etc/dms-settings
 function _get_dms_env_value
 {
+  if [[ -f /etc/dms-settings ]]
+  then
     grep "^${1}=" /etc/dms-settings | cut -d "'" -f 2
+  else
+    _log 'warn' "Call to '_get_dms_env_value' but '/etc/dms-settings' is not present"
+    return 1
+  fi
 }
 
 # TODO: `chown -R 5000:5000 /var/mail` has existed since the projects first commit.
@@ -33,6 +39,7 @@ function _chown_var_mail_if_necessary
     _log 'trace' 'Fixing /var/mail permissions'
     chown -R 5000:5000 /var/mail || return 1
   fi
+  return 0
 }
 
 function _require_n_parameters_or_print_usage
@@ -43,6 +50,7 @@ function _require_n_parameters_or_print_usage
 
   [[ ${1:-} == 'help' ]]  && { __usage ; exit 0 ; }
   [[ ${#} -lt ${COUNT} ]] && { __usage ; exit 1 ; }
+  return 0
 }
 
 # NOTE: Postfix commands that read `main.cf` will stall execution,
@@ -56,6 +64,7 @@ function _adjust_mtime_for_postfix_maincf
   then
     touch -d '2 seconds ago' /etc/postfix/main.cf
   fi
+  return 0
 }
 
 function _reload_postfix