Introduce GitHub Action to test resolvability of external links [DOC-253] #17

	name: Test external links

	on:
	# Ideally this would be a scheduled action to catch if a website did work, but subsequently went offline
	# But then it would have to notify someone, whereas a PR author notification is easier
	pull_request:

	jobs:
	test-external-links:
	runs-on: ubuntu-latest

	steps:
	- uses: actions/checkout@v4
	- uses: actions/setup-node@v4
	with:
	node-version: 20
	cache: 'npm'

	- name: Install Lynx
	run: \|
	sudo apt-get update
	sudo apt-get install -y lynx

	- name: Build documentation
	run: \|
	npm i
	npm run-script build-local

	- run: \|
	echo "temp_file=$(mktemp)" >> $GITHUB_ENV

	- name: Extract links
	run: \|
	# Extract all unique URLs
	# Faster than potentially checking the same link on multiple pages
	find . -name "*.html" \| while read -r file; do
	lynx -dump -listonly -nonumbers "${file}" \| { grep -E "^http" \|\| test $? = 1; } >> "${temp_file}"
	done

	- name: Check links
	run: \|
	distinct_urls=$(sort -u "${temp_file}")

	while read -r url; do
	if [[ -n "${url}" ]]; then
	echo "::debug::Checking URL '${url}'..."

	status=$(curl --globoff --no-progress-meter --output /dev/null --location --head --write-out "%{http_code}" "${url}" \|\| true)

	if [[ "${status}" -eq 404 ]]; then
	locations=$(grep -rl "${url}")
	echo "::error::❌ URL '${url}' had status ${status} (found in ${locations})" 1>&2
	found_error=1
	else
	echo "::debug::✅ URL '${url}' had status ${status}"
	fi
	fi
	done <<< "${distinct_urls}"

	if [[ "${found_error}" -eq 1 ]]; then
	exit 1
	else
	exit 0
	fi

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