| /* ==================================================================== |
| * The Apache Software License, Version 1.1 |
| * |
| * Copyright (c) 2001 The Apache Software Foundation. All rights |
| * reserved. |
| * |
| * Redistribution and use in source and binary forms, with or without |
| * modification, are permitted provided that the following conditions |
| * are met: |
| * |
| * 1. Redistributions of source code must retain the above copyright |
| * notice, this list of conditions and the following disclaimer. |
| * |
| * 2. Redistributions in binary form must reproduce the above copyright |
| * notice, this list of conditions and the following disclaimer in |
| * the documentation and/or other materials provided with the |
| * distribution. |
| * |
| * 3. The end-user documentation included with the redistribution, |
| * if any, must include the following acknowledgment: |
| * "This product includes software developed by the |
| * Apache Software Foundation (http://www.apache.org/)." |
| * Alternately, this acknowledgment may appear in the software itself, |
| * if and wherever such third-party acknowledgments normally appear. |
| * |
| * 4. The names "Apache" and "Apache Software Foundation" must |
| * not be used to endorse or promote products derived from this |
| * software without prior written permission. For written |
| * permission, please contact apache@apache.org. |
| * |
| * 5. Products derived from this software may not be called "Apache", |
| * nor may "Apache" appear in their name, without prior written |
| * permission of the Apache Software Foundation. |
| * |
| * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESSED OR IMPLIED |
| * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES |
| * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE |
| * DISCLAIMED. IN NO EVENT SHALL THE APACHE SOFTWARE FOUNDATION OR |
| * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, |
| * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT |
| * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF |
| * USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND |
| * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, |
| * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT |
| * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF |
| * SUCH DAMAGE. |
| * ==================================================================== |
| * |
| * This software consists of voluntary contributions made by many |
| * individuals on behalf of the Apache Software Foundation. For more |
| * information on the Apache Software Foundation, please see |
| * <http://www.apache.org/>. |
| * |
| */ |
| |
| #include "util_time.h" |
| |
| /* Cache for exploded values of recent timestamps |
| */ |
| |
| struct exploded_time_cache_element { |
| apr_int64_t t; |
| apr_exploded_time_t xt; |
| apr_int64_t t_validate; /* please see comments in cached_explode() */ |
| }; |
| |
| /* the "+ 1" is for the current second: */ |
| #define TIME_CACHE_SIZE (AP_TIME_RECENT_THRESHOLD + 1) |
| |
| static struct exploded_time_cache_element exploded_cache_localtime[TIME_CACHE_SIZE]; |
| static struct exploded_time_cache_element exploded_cache_gmt[TIME_CACHE_SIZE]; |
| |
| |
| static apr_status_t cached_explode(apr_exploded_time_t *xt, apr_time_t t, |
| struct exploded_time_cache_element *cache, |
| int use_gmt) |
| { |
| apr_int64_t seconds = t / APR_USEC_PER_SEC; |
| struct exploded_time_cache_element *cache_element = |
| &(cache[seconds % TIME_CACHE_SIZE]); |
| struct exploded_time_cache_element cache_element_snapshot; |
| |
| /* The cache is implemented as a ring buffer. Each second, |
| * it uses a different element in the buffer. The timestamp |
| * in the element indicates whether the element contains the |
| * exploded time for the current second (vs the time |
| * 'now - AP_TIME_RECENT_THRESHOLD' seconds ago). If the |
| * cached value is for the current time, we use it. Otherwise, |
| * we compute the apr_exploded_time_t and store it in this |
| * cache element. Note that the timestamp in the cache |
| * element is updated only after the exploded time. Thus |
| * if two threads hit this cache element simultaneously |
| * at the start of a new second, they'll both explode the |
| * time and store it. I.e., the writers will collide, but |
| * they'll be writing the same value. |
| */ |
| if (cache_element->t >= seconds) { |
| /* There is an intentional race condition in this design: |
| * in a multithreaded app, one thread might be reading |
| * from this cache_element to resolve a timestamp from |
| * TIME_CACHE_SIZE seconds ago at the same time that |
| * another thread is copying the exploded form of the |
| * current time into the same cache_element. (I.e., the |
| * first thread might hit this element of the ring buffer |
| * just as the element is being recycled.) This can |
| * also happen at the start of a new second, if a |
| * reader accesses the cache_element after a writer |
| * has updated cache_element.t but before the writer |
| * has finished updating the whole cache_element. |
| * |
| * Rather than trying to prevent this race condition |
| * with locks, we allow it to happen and then detect |
| * and correct it. The detection works like this: |
| * Step 1: Take a "snapshot" of the cache element by |
| * copying it into a temporary buffer. |
| * Step 2: Check whether the snapshot contains consistent |
| * data: the timestamps at the start and end of |
| * the cache_element should both match the 'seconds' |
| * value that we computed from the input time. |
| * If these three don't match, then the snapshot |
| * shows the cache_element in the middle of an |
| * update, and its contents are invalid. |
| * Step 3: If the snapshot is valid, use it. Otherwise, |
| * just give up on the cache and explode the |
| * input time. |
| */ |
| memcpy(&cache_element_snapshot, cache_element, |
| sizeof(struct exploded_time_cache_element)); |
| if ((seconds != cache_element_snapshot.t) || |
| (seconds != cache_element_snapshot.t_validate)) { |
| /* Invalid snapshot */ |
| if (use_gmt) { |
| return apr_explode_gmt(xt, t); |
| } |
| else { |
| return apr_explode_localtime(xt, t); |
| } |
| } |
| else { |
| /* Valid snapshot */ |
| memcpy(xt, &(cache_element_snapshot.xt), |
| sizeof(apr_exploded_time_t)); |
| } |
| } |
| else { |
| apr_status_t r; |
| if (use_gmt) { |
| r = apr_explode_gmt(xt, t); |
| } |
| else { |
| r = apr_explode_localtime(xt, t); |
| } |
| if (!APR_STATUS_IS_SUCCESS(r)) { |
| return r; |
| } |
| cache_element->t = seconds; |
| memcpy(&(cache_element->xt), xt, sizeof(apr_exploded_time_t)); |
| cache_element->t_validate = seconds; |
| } |
| xt->tm_usec = t % APR_USEC_PER_SEC; |
| return APR_SUCCESS; |
| } |
| |
| |
| AP_DECLARE(apr_status_t) ap_explode_recent_localtime(apr_exploded_time_t * tm, |
| apr_time_t t) |
| { |
| return cached_explode(tm, t, exploded_cache_localtime, 0); |
| } |
| |
| AP_DECLARE(apr_status_t) ap_explode_recent_gmt(apr_exploded_time_t * tm, |
| apr_time_t t) |
| { |
| return cached_explode(tm, t, exploded_cache_gmt, 1); |
| } |
