Delegate to unified strip-marker-sections script (#1722)

Resolves #1710.
Stacked on #1720.

Based on the [simplifications in
`unset-static-ip`](#1719),
this PR removes the code redundancies in the `unset-static-ip` and
`change-hostname` privileged scripts, and calls the new, unified
`strip-marker-sections` script instead.

## Notes

- `change-hostname`:
- Naively speaking, it would seem simpler to *append* the new entry to
the file. I remembered, however, that the prepending was done on
purpose, because in `/etc/hosts` the first matching entry takes
precedence. I captured that as a comment therefore.
- I slightly reordered the code so that all `/etc/hosts`-related logic
is close together now (i.e., clearing the markers, then rewriting the
file).
- `unset-static-ip`
- ~I couldn’t figure out why we have been using `tee` previously to
write to the config file, because we discarded `tee`’s stdout output by
redirecting to `/dev/null`. I’m also not sure why we used `sudo` for
invoking `tee` – [we already execute the script with `root`
privileges](https://github.com/tiny-pilot/tinypilot-pro/blob/cfb99f88a3039d587a9108d427e7ecfa81b7d190/app/static_ip.py#L128).
It’s a bit odd, because I cannot imagine we did that without good
reason~ 🤔 (We [had been using `sudo tee` to avoid permission
issues](#1722 (comment))
when writing the file.)

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---------

Co-authored-by: Jan Heuermann <[email protected]>

debian-pkg/opt/tinypilot-privileged/scripts/change-hostname

@@ -56,36 +56,19 @@ if ! [[ "${NEW_HOSTNAME}" =~ $VALID_HOSTNAME_PATTERN ]] \
   exit 1
 fi
 
-# Retrieve original `/etc/hosts`.
-# - Read file line by line, make sure to preserve all whitespace.
-# - Remove all marker sections.
-etc_hosts_original=()
-is_in_marker_section='false'
-while IFS= read -r line; do
-  if "${is_in_marker_section}" && [[ "${line}" == "${MARKER_END}" ]]; then
-    is_in_marker_section='false'
-    continue
-  fi
-  if "${is_in_marker_section}" || [[ "${line}" == "${MARKER_START}" ]]; then
-    is_in_marker_section='true'
-    continue
-  fi
-  etc_hosts_original+=("${line}")
-done < /etc/hosts
+# Remove any existing marker sections from the `/etc/hosts` file.
+"${SCRIPT_DIR}/strip-marker-sections" /etc/hosts
 
-if "${is_in_marker_section}"; then
-  echo 'Unclosed marker section' >&2
-  exit 1
-fi
+# Populate new entry to `/etc/hosts`.
+# Note that the first matching entry takes precedence, which is why we prepend
+# our new entry.
+OLD_ETC_HOSTS="$(</etc/hosts)"
+readonly OLD_ETC_HOSTS
+printf "%s\n127.0.1.1 %s\n%s\n%s\n" \
+  "${MARKER_START}" "${NEW_HOSTNAME}" "${MARKER_END}" "${OLD_ETC_HOSTS}" \
+  > /etc/hosts
 
 # Populate new hostname to `/etc/hostname`.
 # Note that the value must be newline-terminated, see:
 # https://manpages.debian.org/stretch/systemd/hostname.5.en.html
 printf "%s\n" "${NEW_HOSTNAME}" > /etc/hostname
-
-# Populate corresponding entry to `etc/hosts`.
-OLD_ETC_HOSTS=$(printf "%s\n" "${etc_hosts_original[@]}")
-readonly OLD_ETC_HOSTS
-printf "%s\n127.0.1.1 %s\n%s\n%s\n" \
-  "${MARKER_START}" "${NEW_HOSTNAME}" "${MARKER_END}" "${OLD_ETC_HOSTS}" \
-  > /etc/hosts

debian-pkg/opt/tinypilot-privileged/scripts/unset-static-ip

@@ -17,7 +17,6 @@ EOF
 
 SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )"
 readonly SCRIPT_DIR
-readonly CONFIG_FILE="/etc/dhcpcd.conf"
 # shellcheck source=lib/markers.sh
 . "${SCRIPT_DIR}/lib/markers.sh"
 
@@ -59,33 +58,8 @@ fi
 # Exit on unset variable
 set -u
 
-# Remove any existing auto-generated configuration as populated by the
-# set-static-ip script.
-dhcpcd_original=()
-is_in_marker_section='false'
-while IFS= read -r line; do
-  if "${is_in_marker_section}" && [[ "${line}" == "${MARKER_END}" ]]; then
-    is_in_marker_section='false'
-    continue
-  fi
-  if "${is_in_marker_section}" || [[ "${line}" == "${MARKER_START}" ]]; then
-    is_in_marker_section='true'
-    continue
-  fi
-  dhcpcd_original+=("${line}")
-done < "${CONFIG_FILE}"
-
-if "${is_in_marker_section}"; then
-  echo 'Unclosed marker section' >&2
-  exit 1
-fi
-
-# Convert array of lines to a single string.
-OLD_CONFIG_FILE=$(printf "%s\n" "${dhcpcd_original[@]}")
-readonly OLD_CONFIG_FILE
-
-# Write original file contents back to the file.
-echo "${OLD_CONFIG_FILE}" | sudo tee "${CONFIG_FILE}" > /dev/null
+# Remove any existing marker sections from the config file.
+"${SCRIPT_DIR}/strip-marker-sections" /etc/dhcpcd.conf
 
 # Changes require a reboot so prompt the user.
 if [[ "${QUIET}" == "false" ]] ; then