<?php
/**
 * Single-runner lock (§8.4).
 *
 * Primary: MySQL GET_LOCK on the home connection (auto-released if PHP dies).
 * Fallback: atomic option row with an expiry, taken over by compare-and-swap once expired.
 *
 * @package DatabaseExportGenie
 */

namespace DatabaseExportGenie;

defined( 'ABSPATH' ) || exit;

/**
 * Process-wide lock around ticks.
 */
class Lock {

	const OPTION = 'dbmg_lock';

	/**
	 * How the current lock was taken: 'named' | 'option' | ''.
	 *
	 * @var string
	 */
	private static $held = '';

	/**
	 * Raw option value we wrote (fallback mode).
	 *
	 * @var string
	 */
	private static $value = '';

	/**
	 * @return string GET_LOCK name, unique per install.
	 */
	private static function name() {
		global $wpdb;
		return 'dbmg_' . substr( md5( ABSPATH . '|' . $wpdb->base_prefix . '|' . DB_NAME ), 0, 24 );
	}

	/**
	 * Named locks only when $wpdb is plain wpdb on MySQL: HyperDB/LudicrousDB may send the
	 * SELECT to a replica or another connection than RELEASE_LOCK.
	 *
	 * @return bool
	 */
	private static function use_named() {
		global $wpdb;
		return 'mysql' === Environment::db_engine() && 'wpdb' === get_class( $wpdb ) && apply_filters( 'dbmg_use_named_lock', true );
	}

	/**
	 * @param int $ttl Seconds before a crashed holder's fallback lock can be taken over.
	 * @return bool
	 */
	public static function acquire( $ttl = 60 ) {
		global $wpdb;
		if ( '' !== self::$held ) {
			return true;
		}
		if ( self::use_named() ) {
			$suppress = $wpdb->suppress_errors( true );
			// phpcs:ignore WordPress.DB.DirectDatabaseQuery
			$got = $wpdb->get_var( $wpdb->prepare( 'SELECT GET_LOCK(%s, 0)', self::name() ) );
			$wpdb->suppress_errors( $suppress );
			if ( '1' === (string) $got ) {
				self::$held = 'named';
				return true;
			}
			if ( '0' === (string) $got ) {
				return false;
			}
			// NULL/error → fall through to the option lock.
		}
		$value = wp_generate_password( 12, false ) . '|' . ( time() + $ttl );
		if ( Options::add( self::OPTION, $value ) ) {
			self::$held  = 'option';
			self::$value = $value;
			return true;
		}
		$current = self::raw();
		if ( null !== $current && self::expiry( $current ) < time() && Options::swap( self::OPTION, $current, $value ) ) {
			self::$held  = 'option';
			self::$value = $value;
			return true;
		}
		return false;
	}

	/**
	 * Release whatever we hold.
	 */
	public static function release() {
		global $wpdb;
		if ( 'named' === self::$held ) {
			// phpcs:ignore WordPress.DB.DirectDatabaseQuery
			$wpdb->query( $wpdb->prepare( 'SELECT RELEASE_LOCK(%s)', self::name() ) );
		} elseif ( 'option' === self::$held ) {
			Options::delete( self::OPTION, self::$value );
		}
		self::$held  = '';
		self::$value = '';
	}

	/**
	 * Is someone (else) running a tick right now?
	 *
	 * @return bool
	 */
	public static function is_busy() {
		global $wpdb;
		if ( '' !== self::$held ) {
			return true;
		}
		if ( self::use_named() ) {
			$suppress = $wpdb->suppress_errors( true );
			// phpcs:ignore WordPress.DB.DirectDatabaseQuery
			$free = $wpdb->get_var( $wpdb->prepare( 'SELECT IS_FREE_LOCK(%s)', self::name() ) );
			$wpdb->suppress_errors( $suppress );
			if ( '0' === (string) $free ) {
				return true;
			}
		}
		$current = self::raw();
		return null !== $current && self::expiry( $current ) >= time();
	}

	/**
	 * Clear a stale fallback lock (Resume, §8.5). Named locks free themselves when PHP dies.
	 */
	public static function clear_stale() {
		$current = self::raw();
		if ( null !== $current && self::expiry( $current ) < time() ) {
			Options::delete( self::OPTION, $current );
		}
	}

	/**
	 * Unconditional removal (deactivation / delete-all).
	 */
	public static function force_clear() {
		Options::delete( self::OPTION );
	}

	/**
	 * @return string|null
	 */
	private static function raw() {
		$raw = Options::get( self::OPTION );
		return is_string( $raw ) ? $raw : null;
	}

	/**
	 * @param string $raw token|expiry.
	 * @return int
	 */
	private static function expiry( $raw ) {
		$bits = explode( '|', $raw );
		return isset( $bits[1] ) ? (int) $bits[1] : 0;
	}
}