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   1  <?php
   2  /*
   3   * Copyright 2012 Google Inc.
   4   *
   5   * Licensed under the Apache License, Version 2.0 (the "License");
   6   * you may not use this file except in compliance with the License.
   7   * You may obtain a copy of the License at
   8   *
   9   *     http://www.apache.org/licenses/LICENSE-2.0
  10   *
  11   * Unless required by applicable law or agreed to in writing, software
  12   * distributed under the License is distributed on an "AS IS" BASIS,
  13   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  14   * See the License for the specific language governing permissions and
  15   * limitations under the License.
  16   */
  17  
  18  require_once 'Google/Http/Request.php';
  19  
  20  /**
  21   * Implement the caching directives specified in rfc2616. This
  22   * implementation is guided by the guidance offered in rfc2616-sec13.
  23   * @author Chirag Shah <[email protected]>
  24   */
  25  class Google_Http_CacheParser
  26  {
  27    public static $CACHEABLE_HTTP_METHODS = array('GET', 'HEAD');
  28    public static $CACHEABLE_STATUS_CODES = array('200', '203', '300', '301');
  29  
  30    /**
  31     * Check if an HTTP request can be cached by a private local cache.
  32     *
  33     * @static
  34     * @param Google_Http_Request $resp
  35     * @return bool True if the request is cacheable.
  36     * False if the request is uncacheable.
  37     */
  38    public static function isRequestCacheable(Google_Http_Request $resp)
  39    {
  40      $method = $resp->getRequestMethod();
  41      if (! in_array($method, self::$CACHEABLE_HTTP_METHODS)) {
  42        return false;
  43      }
  44  
  45      // Don't cache authorized requests/responses.
  46      // [rfc2616-14.8] When a shared cache receives a request containing an
  47      // Authorization field, it MUST NOT return the corresponding response
  48      // as a reply to any other request...
  49      if ($resp->getRequestHeader("authorization")) {
  50        return false;
  51      }
  52  
  53      return true;
  54    }
  55  
  56    /**
  57     * Check if an HTTP response can be cached by a private local cache.
  58     *
  59     * @static
  60     * @param Google_Http_Request $resp
  61     * @return bool True if the response is cacheable.
  62     * False if the response is un-cacheable.
  63     */
  64    public static function isResponseCacheable(Google_Http_Request $resp)
  65    {
  66      // First, check if the HTTP request was cacheable before inspecting the
  67      // HTTP response.
  68      if (false == self::isRequestCacheable($resp)) {
  69        return false;
  70      }
  71  
  72      $code = $resp->getResponseHttpCode();
  73      if (! in_array($code, self::$CACHEABLE_STATUS_CODES)) {
  74        return false;
  75      }
  76  
  77      // The resource is uncacheable if the resource is already expired and
  78      // the resource doesn't have an ETag for revalidation.
  79      $etag = $resp->getResponseHeader("etag");
  80      if (self::isExpired($resp) && $etag == false) {
  81        return false;
  82      }
  83  
  84      // [rfc2616-14.9.2]  If [no-store is] sent in a response, a cache MUST NOT
  85      // store any part of either this response or the request that elicited it.
  86      $cacheControl = $resp->getParsedCacheControl();
  87      if (isset($cacheControl['no-store'])) {
  88        return false;
  89      }
  90  
  91      // Pragma: no-cache is an http request directive, but is occasionally
  92      // used as a response header incorrectly.
  93      $pragma = $resp->getResponseHeader('pragma');
  94      if ($pragma == 'no-cache' || strpos($pragma, 'no-cache') !== false) {
  95        return false;
  96      }
  97  
  98      // [rfc2616-14.44] Vary: * is extremely difficult to cache. "It implies that
  99      // a cache cannot determine from the request headers of a subsequent request
 100      // whether this response is the appropriate representation."
 101      // Given this, we deem responses with the Vary header as uncacheable.
 102      $vary = $resp->getResponseHeader('vary');
 103      if ($vary) {
 104        return false;
 105      }
 106  
 107      return true;
 108    }
 109  
 110    /**
 111     * @static
 112     * @param Google_Http_Request $resp
 113     * @return bool True if the HTTP response is considered to be expired.
 114     * False if it is considered to be fresh.
 115     */
 116    public static function isExpired(Google_Http_Request $resp)
 117    {
 118      // HTTP/1.1 clients and caches MUST treat other invalid date formats,
 119      // especially including the value “0”, as in the past.
 120      $parsedExpires = false;
 121      $responseHeaders = $resp->getResponseHeaders();
 122  
 123      if (isset($responseHeaders['expires'])) {
 124        $rawExpires = $responseHeaders['expires'];
 125        // Check for a malformed expires header first.
 126        if (empty($rawExpires) || (is_numeric($rawExpires) && $rawExpires <= 0)) {
 127          return true;
 128        }
 129  
 130        // See if we can parse the expires header.
 131        $parsedExpires = strtotime($rawExpires);
 132        if (false == $parsedExpires || $parsedExpires <= 0) {
 133          return true;
 134        }
 135      }
 136  
 137      // Calculate the freshness of an http response.
 138      $freshnessLifetime = false;
 139      $cacheControl = $resp->getParsedCacheControl();
 140      if (isset($cacheControl['max-age'])) {
 141        $freshnessLifetime = $cacheControl['max-age'];
 142      }
 143  
 144      $rawDate = $resp->getResponseHeader('date');
 145      $parsedDate = strtotime($rawDate);
 146  
 147      if (empty($rawDate) || false == $parsedDate) {
 148        // We can't default this to now, as that means future cache reads
 149        // will always pass with the logic below, so we will require a
 150        // date be injected if not supplied.
 151        throw new Google_Exception("All cacheable requests must have creation dates.");
 152      }
 153  
 154      if (false == $freshnessLifetime && isset($responseHeaders['expires'])) {
 155        $freshnessLifetime = $parsedExpires - $parsedDate;
 156      }
 157  
 158      if (false == $freshnessLifetime) {
 159        return true;
 160      }
 161  
 162      // Calculate the age of an http response.
 163      $age = max(0, time() - $parsedDate);
 164      if (isset($responseHeaders['age'])) {
 165        $age = max($age, strtotime($responseHeaders['age']));
 166      }
 167  
 168      return $freshnessLifetime <= $age;
 169    }
 170  
 171    /**
 172     * Determine if a cache entry should be revalidated with by the origin.
 173     *
 174     * @param Google_Http_Request $response
 175     * @return bool True if the entry is expired, else return false.
 176     */
 177    public static function mustRevalidate(Google_Http_Request $response)
 178    {
 179      // [13.3] When a cache has a stale entry that it would like to use as a
 180      // response to a client's request, it first has to check with the origin
 181      // server to see if its cached entry is still usable.
 182      return self::isExpired($response);
 183    }
 184  }