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

target/scripts/helpers/dns.sh

@@ -0,0 +1,62 @@
+#! /bin/bash
+
+# Outputs the DNS label count (delimited by `.`) for the given input string.
+# Useful for determining an FQDN like `mail.example.com` (3), vs `example.com` (2).
+function _get_label_count
+{
+  awk -F '.' '{ print NF }' <<< "${1}"
+}
+
+# Sets HOSTNAME and DOMAINNAME globals used throughout the scripts,
+# and any subprocesses called that intereact with it.
+function _obtain_hostname_and_domainname
+{
+  # Normally this value would match the output of `hostname` which mirrors `/proc/sys/kernel/hostname`,
+  # However for legacy reasons, the system ENV `HOSTNAME` was replaced here with `hostname -f` instead.
+  #
+  # TODO: Consider changing to `DMS_FQDN`; a more accurate name, and removing the `export`, assuming no
+  # subprocess like postconf would be called that would need access to the same value via `$HOSTNAME` ENV.
+  #
+  # TODO: `OVERRIDE_HOSTNAME` was introduced for non-Docker runtimes that could not configure an explicit hostname.
+  # k8s was the particular runtime in 2017. This does not update `/etc/hosts` or other locations, thus risking
+  # inconsistency with expected behaviour. Investigate if it's safe to remove support. (--net=host also uses this as a workaround)
+  export HOSTNAME="${OVERRIDE_HOSTNAME:-$(hostname -f)}"
+
+  # If the container is misconfigured.. `hostname -f` (which derives it's return value from `/etc/hosts` or DNS query),
+  # will result in an error that returns an empty value. This warrants a panic.
+  if [[ -z ${HOSTNAME} ]]
+  then
+    dms_panic__misconfigured 'obtain_hostname' '/etc/hosts'
+  fi
+
+  # If the `HOSTNAME` is more than 2 labels long (eg: mail.example.com),
+  # We take the FQDN from it, minus the 1st label (aka _short hostname_, `hostname -s`).
+  #
+  # TODO: For some reason we're explicitly separating out a domain name from our FQDN,
+  # `hostname -d` was probably not the correct command for this intention either.
+  # Needs further investigation for relevance, and if `/etc/hosts` is important for consumers
+  # 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 ${OVERRIDE_HOSTNAME} ]]
+    then
+      # Emulates the intended behaviour of `hostname -d`:
+      # Assign the HOSTNAME value minus everything up to and including the first `.`
+      DOMAINNAME=${HOSTNAME#*.}
+    else
+      # Operates on the FQDN returned from querying `/etc/hosts` or fallback DNS:
+      #
+      # Note if you want the actual NIS `domainname`, use the `domainname` command,
+      # or `cat /proc/sys/kernel/domainname`.
+      # Our usage of `domainname` is under consideration as legacy, and not advised
+      # going forward. In future our docs should drop any mention of it.
+
+      #shellcheck disable=SC2034
+      DOMAINNAME="$(hostname -d)"
+    fi
+  fi
+
+  # Otherwise we assign the same value (eg: example.com):
+  # Not an else statement in the previous conditional in the event that `hostname -d` fails.
+  DOMAINNAME="${DOMAINNAME:-${HOSTNAME}}"
+}