introduce NOOP backend and admin notice for unavailable backend (#305)

We should not silently ignore misconfiguration and fall back to
DB caching. We now check for availability and disable caching
via a "no operation" backend. We can also add a notice for APC
now which is no longer supported.

cachify.php

@@ -91,7 +91,7 @@
  * @param string $class_name the class name.
  */
 function cachify_autoload( $class_name ) {
-	if ( in_array( $class_name, array( 'Cachify', 'Cachify_Backend', 'Cachify_CLI', 'Cachify_DB', 'Cachify_HDD', 'Cachify_MEMCACHED', 'Cachify_REDIS' ), true ) ) {
+	if ( in_array( $class_name, array( 'Cachify', 'Cachify_Backend', 'Cachify_CLI', 'Cachify_DB', 'Cachify_HDD', 'Cachify_MEMCACHED', 'Cachify_NOOP', 'Cachify_REDIS' ), true ) ) {
 		require_once sprintf(
 			'%s/inc/class-%s.php',
 			CACHIFY_DIR,

inc/class-cachify-noop.php

@@ -0,0 +1,112 @@
+<?php
+/**
+ * Class for NO-OP caching.
+ *
+ * @package Cachify
+ */
+
+/* Quit */
+defined( 'ABSPATH' ) || exit;
+
+/**
+ * Cachify_NOOP
+ *
+ * No-Op backend which does simply nothing.
+ *
+ * @since 2.4.0
+ */
+final class Cachify_NOOP implements Cachify_Backend {
+
+	/**
+	 * Name of the unavailable caching method, i.e. the reason why we chose no-op.
+	 *
+	 * @var string
+	 */
+	public $unavailable_method;
+
+	/**
+	 * Constructor with name of the unavailable method.
+	 *
+	 * @param string $unavailable_method Name of the unavailable method.
+	 */
+	public function __construct( $unavailable_method = '' ) {
+		$this->unavailable_method = $unavailable_method;
+	}
+
+	/**
+	 * Availability check
+	 *
+	 * @return bool TRUE when installed
+	 */
+	public static function is_available() {
+		return true;
+	}
+
+	/**
+	 * Caching method as string
+	 *
+	 * @return string Caching method
+	 */
+	public static function stringify_method() {
+		return 'NOOP';
+	}
+
+	/**
+	 * Store item in cache
+	 *
+	 * @param string $hash     Hash of the entry.
+	 * @param string $data     Content of the entry.
+	 * @param int    $lifetime Lifetime of the entry.
+	 * @param bool   $sig_detail Show details in signature.
+	 */
+	public static function store_item( $hash, $data, $lifetime, $sig_detail ) {
+		// NOOP.
+	}
+
+	/**
+	 * Read item from cache
+	 *
+	 * @param string $hash Hash of the entry.
+	 *
+	 * @return false No content
+	 */
+	public static function get_item( $hash ) {
+		return false;
+	}
+
+	/**
+	 * Delete item from cache
+	 *
+	 * @param string $hash Hash of the entry.
+	 * @param string $url  URL of the entry [optional].
+	 */
+	public static function delete_item( $hash, $url = '' ) {
+		// NOOP.
+	}
+
+	/**
+	 * Clear the cache
+	 */
+	public static function clear_cache() {
+		// NOOP.
+	}
+
+	/**
+	 * Print the cache
+	 *
+	 * @param bool  $sig_detail Show details in signature.
+	 * @param array $cache      Array of cache values.
+	 */
+	public static function print_cache( $sig_detail, $cache ) {
+		// NOOP.
+	}
+
+	/**
+	 * Get the cache size
+	 *
+	 * @return int Column size
+	 */
+	public static function get_stats() {
+		return 0;
+	}
+}