Add script for stripping TinyPilot marker sections (#1720)

Related #1710.

This PR adds a unified script to remove TinyPilot marker sections from
files. We will [use this script in a subsequent
PR](#1722) to simplify the
corresponding instances in the
[unset-static-ip](https://github.com/tiny-pilot/tinypilot/blob/2e211199a21a2a6c285c5d1d2ff60eff6afa77d4/debian-pkg/opt/tinypilot-privileged/scripts/unset-static-ip#L64-L81)
and
[change-hostname](https://github.com/tiny-pilot/tinypilot/blob/8e99d5a666df12d851b14dd4276e8b95ab958493/debian-pkg/opt/tinypilot-privileged/scripts/change-hostname#L62-L79)
privileged scripts.

## Notes

- Unless someone feels strongly about the [originally mentioned
`remove-tinypilot-lines` script
name](https://github.com/tiny-pilot/tinypilot/issues/1710#:~:text=opt/tinypilot/scripts/-,remove%2Dtinypilot%2Dlines,-%3C%20example.conf)
(or any other name), I thought that `strip-marker-sections` sounded most
expressive.
- I added a comment to the `lib/markers.sh` file, to explain the marker
idea in a bit more detail.
- There will be automated bash tests for this in [a subsequent
PR](#1721).


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---------

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

debian-pkg/opt/tinypilot-privileged/scripts/lib/markers.sh

@@ -1,9 +1,13 @@
 #!/bin/bash
+#
+# We use the marker section technique to programmatically add
+# TinyPilot-specific configuration to system config files. Wrapping up such
+# addendums into markers allows us to maintain bits of custom configuration
+# without having to properly parse the entire file.
+#
 # Shellcheck can't detect when variables are used elsewhere so we need to
 # disable the unused variable check for this file.
 # shellcheck disable=SC2034
-#
-# Markers used to denote autogenerated changes to config files..
 
 readonly MARKER_START='# --- AUTOGENERATED BY TINYPILOT - START ---'
 readonly MARKER_END='# --- AUTOGENERATED BY TINYPILOT - END ---'

debian-pkg/opt/tinypilot-privileged/scripts/strip-marker-sections

@@ -0,0 +1,94 @@
+#!/bin/bash
+#
+# Strips TinyPilot marker sections from a file.
+#
+# If the target file contains marker sections, this script removes the markers
+# and any content in between them.
+# If the target file doesn’t contain marker sections, running this script is a
+# noop.
+# If the target file contains unmatched/orphaned markers, this script fails.
+
+# We don’t use `set -x`, because it would output every single iteration of the
+# while loop when iterating through the lines of the target file, and hence
+# generate a lot of noise.
+
+# Exit on first failure.
+set -e
+
+# Exit on unset variable.
+set -u
+
+SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )"
+readonly SCRIPT_DIR
+# shellcheck source=lib/markers.sh
+. "${SCRIPT_DIR}/lib/markers.sh"
+
+print_help() {
+  cat << EOF
+Usage: ${0##*/} [--help] TARGET_FILE
+Strips TinyPilot marker sections from a file.
+  TARGET_FILE   Path to file with marker sections.
+  --help        Display this help and exit.
+EOF
+}
+
+# Parse command-line arguments.
+TARGET_FILE=''
+while [[ "$#" -gt 0 ]]; do
+  case "$1" in
+    --help)
+      print_help
+      exit
+      ;;
+    -*)
+      echo "Illegal option: $1" >&2
+      exit 1
+      ;;
+    *)
+      TARGET_FILE="$1"
+      shift
+      ;;
+  esac
+done
+readonly TARGET_FILE
+
+# Ensure target file is specified.
+if [[ -z "${TARGET_FILE}" ]]; then
+  echo 'Input parameter missing: TARGET_FILE' >&2
+  exit 1
+fi
+
+# Ensure target file exists and is a file.
+if [[ ! -f "${TARGET_FILE}" ]]; then
+  echo "Not a file: ${TARGET_FILE}" >&2
+  exit 1
+fi
+
+# Read the original file line by line, and remove marker sections.
+# This is done by iterating through the lines, and filtering out lines that are
+# either markers themselves, or in between markers.
+preserved_lines=()
+is_in_marker_section='false'
+while IFS='' read -r line; do
+  if [[ "${line}" == "${MARKER_END}" ]]; then
+    if ! "${is_in_marker_section}"; then
+      echo 'Unmatched end marker' >&2
+      exit 1
+    fi
+    is_in_marker_section='false'
+    continue
+  fi
+  if "${is_in_marker_section}" || [[ "${line}" == "${MARKER_START}" ]]; then
+    is_in_marker_section='true'
+    continue
+  fi
+  preserved_lines+=("${line}")
+done < "${TARGET_FILE}"
+
+if "${is_in_marker_section}"; then
+  echo 'Unmatched start marker' >&2
+  exit 1
+fi
+
+# Populate preserved lines to file.
+printf "%s\n" "${preserved_lines[@]}" > "${TARGET_FILE}"