#!/bin/sh

set -eu

# Share template paths with the Nginx image's envsubst entrypoint.
readonly TEMPLATE_DIR="${NGINX_ENVSUBST_TEMPLATE_DIR:-/etc/nginx/templates}"
readonly TEMPLATE_SUFFIX="${NGINX_ENVSUBST_TEMPLATE_SUFFIX:-.template}"
readonly OUTPUT_DIR="${NGINX_ENVSUBST_OUTPUT_DIR:-/etc/nginx}"
readonly ENVSUBST_SCRIPT="${NGINX_RELOAD_ENVSUBST_SCRIPT:-/docker-entrypoint.d/20-envsubst-on-templates.sh}"
readonly MANAGED_DIRS="conf.d snippets"
readonly LOCK_FILE="${NGINX_RELOAD_LOCK_FILE:-/tmp/nginx-reload.lock}"

acquire_lock() {
	if ! command -v flock >/dev/null 2>&1; then
		echo "Error: flock is required to serialize configuration reloads." >&2
		return 1
	fi

	# Keep FD 9 open through rollback and cleanup; never unlink the lock file.
	exec 9>"$LOCK_FILE"
	if ! flock -n 9; then
		echo "Error: cannot acquire reload lock; another reload may be running: $LOCK_FILE" >&2
		return 1
	fi
}

is_managed_template() {
	for managed_dir in $MANAGED_DIRS; do
		case "$1" in
		"$managed_dir"/*) return 0 ;;
		esac
	done
	return 1
}

validate_templates() {
	if [ ! -d "$TEMPLATE_DIR" ]; then
		echo "Error: template directory does not exist: $TEMPLATE_DIR" >&2
		return 1
	fi

	if [ ! -x "$ENVSUBST_SCRIPT" ]; then
		echo "Error: envsubst script is not executable: $ENVSUBST_SCRIPT" >&2
		return 1
	fi

	# Collect first so a failed find cannot be masked by a pipeline.
	templates="$(find "$TEMPLATE_DIR" -follow -type f -name "*$TEMPLATE_SUFFIX" -print)"
	printf '%s\n' "$templates" | while IFS= read -r template; do
		[ -n "$template" ] || continue
		relative="${template#"$TEMPLATE_DIR"/}"
		if ! is_managed_template "$relative"; then
			echo "Error: unmanaged template path: $relative" >&2
			exit 1
		fi
	done
}

backup_config() {
	for dir in $MANAGED_DIRS; do
		if [ -d "$OUTPUT_DIR/$dir" ]; then
			cp -a "$OUTPUT_DIR/$dir" "$1/$dir"
		fi
	done
}

generate_config() {
	for dir in $MANAGED_DIRS; do
		if [ -d "$OUTPUT_DIR/$dir" ]; then
			outputs="$(find "$OUTPUT_DIR/$dir" -type f -print)"
			printf '%s\n' "$outputs" | while IFS= read -r output; do
				[ -n "$output" ] || continue
				relative="${output#"$OUTPUT_DIR"/}"
				if [ ! -f "$TEMPLATE_DIR/$relative$TEMPLATE_SUFFIX" ]; then
					echo "Removing stale generated file: $relative"
				fi
			done
		fi

		rm -rf "$OUTPUT_DIR/$dir"
		mkdir -p "$OUTPUT_DIR/$dir"
	done
	NGINX_ENVSUBST_TEMPLATE_DIR="$TEMPLATE_DIR" \
		NGINX_ENVSUBST_TEMPLATE_SUFFIX="$TEMPLATE_SUFFIX" \
		NGINX_ENVSUBST_OUTPUT_DIR="$OUTPUT_DIR" \
		"$ENVSUBST_SCRIPT"
}

restore_config() {
	restore_failed=0
	for dir in $MANAGED_DIRS; do
		if ! rm -rf "$OUTPUT_DIR/$dir"; then
			restore_failed=1
			continue
		fi
		if [ -d "$1/$dir" ]; then
			cp -a "$1/$dir" "$OUTPUT_DIR/$dir" || restore_failed=1
		fi
	done
	[ "$restore_failed" -eq 0 ]
}

finish() {
	finish_status="$1"
	trap - EXIT
	trap '' HUP INT TERM

	if [ "$2" = modifying ]; then
		echo "Reload failed. Restoring previous generated configuration." >&2
		[ "$finish_status" -ne 0 ] || finish_status=1
		if ! restore_config "$3"; then
			echo "Error: configuration rollback incomplete. Backup preserved at: $3" >&2
			exit "$finish_status"
		fi
	fi

	if ! rm -rf "$3"; then
		echo "Error: cannot remove configuration backup: $3" >&2
		[ "$finish_status" -ne 0 ] || finish_status=1
	fi
	exit "$finish_status"
}

main() {
	acquire_lock
	validate_templates
	backup_dir="$(mktemp -d /tmp/nginx-reload.XXXXXX)"
	transaction_state=backing_up
	trap 'finish "$?" "$transaction_state" "$backup_dir"' EXIT
	trap 'exit 129' HUP
	trap 'exit 130' INT
	trap 'exit 143' TERM

	backup_config "$backup_dir"
	# Only a complete backup permits mutation and subsequent restoration.
	transaction_state=modifying
	generate_config
	nginx -t
	nginx -s reload
	transaction_state=committed
}

main "$@"