<?php
/*
* Copyright 2012 Google Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
/**
* Implement the caching directives specified in rfc2616. This
* implementation is guided by the guidance offered in rfc2616-sec13.
* @author Chirag Shah <chirags@google.com>
*/
class Google_CacheParser {
public static $CACHEABLE_HTTP_METHODS = array('GET', 'HEAD');
public static $CACHEABLE_STATUS_CODES = array('200', '203', '300', '301');
private function __construct() {}
/**
* Check if an HTTP request can be cached by a private local cache.
*
* @static
* @param Google_HttpRequest $resp
* @return bool True if the request is cacheable.
* False if the request is uncacheable.
*/
public static function isRequestCacheable (Google_HttpRequest $resp) {
$method = $resp->getRequestMethod();
if (! in_array($method, self::$CACHEABLE_HTTP_METHODS)) {
return false;
}
// Don't cache authorized requests/responses.
// [rfc2616-14.8] When a shared cache receives a request containing an
// Authorization field, it MUST NOT return the corresponding response
// as a reply to any other request...
if ($resp->getRequestHeader("authorization")) {
return false;
}
return true;
}
/**
* Check if an HTTP response can be cached by a private local cache.
*
* @static
* @param Google_HttpRequest $resp
* @return bool True if the response is cacheable.
* False if the response is un-cacheable.
*/
public static function isResponseCacheable (Google_HttpRequest $resp) {
// First, check if the HTTP request was cacheable before inspecting the
// HTTP response.
if (false == self::isRequestCacheable($resp)) {
return false;
}
$code = $resp->getResponseHttpCode();
if (! in_array($code, self::$CACHEABLE_STATUS_CODES)) {
return false;
}
// The resource is uncacheable if the resource is already expired and
// the resource doesn't have an ETag for revalidation.
$etag = $resp->getResponseHeader("etag");
if (self::isExpired($resp) && $etag == false) {
return false;
}
// [rfc2616-14.9.2] If [no-store is] sent in a response, a cache MUST NOT
// store any part of either this response or the request that elicited it.
$cacheControl = $resp->getParsedCacheControl();
if (isset($cacheControl['no-store'])) {
return false;
}
// Pragma: no-cache is an http request directive, but is occasionally
// used as a response header incorrectly.
$pragma = $resp->getResponseHeader('pragma');
if ($pragma == 'no-cache' || strpos($pragma, 'no-cache') !== false) {
return false;
}
// [rfc2616-14.44] Vary: * is extremely difficult to cache. "It implies that
// a cache cannot determine from the request headers of a subsequent request
// whether this response is the appropriate representation."
// Given this, we deem responses with the Vary header as uncacheable.
$vary = $resp->getResponseHeader('vary');
if ($vary) {
return false;
}
return true;
}
/**
* @static
* @param Google_HttpRequest $resp
* @return bool True if the HTTP response is considered to be expired.
* False if it is considered to be fresh.
*/
public static function isExpired(Google_HttpRequest $resp) {
// HTTP/1.1 clients and caches MUST treat other invalid date formats,
// especially including the value “0”, as in the past.
$parsedExpires = false;
$responseHeaders = $resp->getResponseHeaders();
if (isset($responseHeaders['expires'])) {
$rawExpires = $responseHeaders['expires'];
// Check for a malformed expires header first.
if (empty($rawExpires) || (is_numeric($rawExpires) && $rawExpires <= 0)) {
return true;
}
// See if we can parse the expires header.
$parsedExpires = strtotime($rawExpires);
if (false == $parsedExpires || $parsedExpires <= 0) {
return true;
}
}
// Calculate the freshness of an http response.
$freshnessLifetime = false;
$cacheControl = $resp->getParsedCacheControl();
if (isset($cacheControl['max-age'])) {
$freshnessLifetime = $cacheControl['max-age'];
}
$rawDate = $resp->getResponseHeader('date');
$parsedDate = strtotime($rawDate);
if (empty($rawDate) || false == $parsedDate) {
$parsedDate = time();
}
if (false == $freshnessLifetime && isset($responseHeaders['expires'])) {
$freshnessLifetime = $parsedExpires - $parsedDate;
}
if (false == $freshnessLifetime) {
return true;
}
// Calculate the age of an http response.
$age = max(0, time() - $parsedDate);
if (isset($responseHeaders['age'])) {
$age = max($age, strtotime($responseHeaders['age']));
}
return $freshnessLifetime <= $age;
}
/**
* Determine if a cache entry should be revalidated with by the origin.
*
* @param Google_HttpRequest $response
* @return bool True if the entry is expired, else return false.
*/
public static function mustRevalidate(Google_HttpRequest $response) {
// [13.3] When a cache has a stale entry that it would like to use as a
// response to a client's request, it first has to check with the origin
// server to see if its cached entry is still usable.
return self::isExpired($response);
}
}
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