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apache/server/util_time.c
Roy T. Fielding de659cbed0 update license header text 
git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@420983 13f79535-47bb-0310-9956-ffa450edef68
2006-07-11 20:33:53 +00:00

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/* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You 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.
*/
#include "util_time.h"
/* Cache for exploded values of recent timestamps
*/
struct exploded_time_cache_element {
apr_int64_t t;
apr_time_exp_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)
/* Note that AP_TIME_RECENT_THRESHOLD is defined to
* be a power of two minus one in util_time.h, so that
* we can replace a modulo operation with a bitwise AND
* when hashing items into a cache of size
* AP_TIME_RECENT_THRESHOLD+1
*/
#define TIME_CACHE_MASK (AP_TIME_RECENT_THRESHOLD)
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_time_exp_t *xt, apr_time_t t,
struct exploded_time_cache_element *cache,
int use_gmt)
{
apr_int64_t seconds = apr_time_sec(t);
struct exploded_time_cache_element *cache_element =
&(cache[seconds & TIME_CACHE_MASK]);
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_time_exp_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_time_exp_gmt(xt, t);
}
else {
return apr_time_exp_lt(xt, t);
}
}
else {
/* Valid snapshot */
memcpy(xt, &(cache_element_snapshot.xt),
sizeof(apr_time_exp_t));
}
}
else {
apr_status_t r;
if (use_gmt) {
r = apr_time_exp_gmt(xt, t);
}
else {
r = apr_time_exp_lt(xt, t);
}
if (r != APR_SUCCESS) {
return r;
}
cache_element->t = seconds;
memcpy(&(cache_element->xt), xt, sizeof(apr_time_exp_t));
cache_element->t_validate = seconds;
}
xt->tm_usec = (int)apr_time_usec(t);
return APR_SUCCESS;
}
AP_DECLARE(apr_status_t) ap_explode_recent_localtime(apr_time_exp_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_time_exp_t * tm,
apr_time_t t)
{
return cached_explode(tm, t, exploded_cache_gmt, 1);
}
AP_DECLARE(apr_status_t) ap_recent_ctime(char *date_str, apr_time_t t)
{
/* ### This code is a clone of apr_ctime(), except that it
* uses ap_explode_recent_localtime() instead of apr_time_exp_lt().
*/
apr_time_exp_t xt;
const char *s;
int real_year;
/* example: "Wed Jun 30 21:49:08 1993" */
/* 123456789012345678901234 */
ap_explode_recent_localtime(&xt, t);
s = &apr_day_snames[xt.tm_wday][0];
*date_str++ = *s++;
*date_str++ = *s++;
*date_str++ = *s++;
*date_str++ = ' ';
s = &apr_month_snames[xt.tm_mon][0];
*date_str++ = *s++;
*date_str++ = *s++;
*date_str++ = *s++;
*date_str++ = ' ';
*date_str++ = xt.tm_mday / 10 + '0';
*date_str++ = xt.tm_mday % 10 + '0';
*date_str++ = ' ';
*date_str++ = xt.tm_hour / 10 + '0';
*date_str++ = xt.tm_hour % 10 + '0';
*date_str++ = ':';
*date_str++ = xt.tm_min / 10 + '0';
*date_str++ = xt.tm_min % 10 + '0';
*date_str++ = ':';
*date_str++ = xt.tm_sec / 10 + '0';
*date_str++ = xt.tm_sec % 10 + '0';
*date_str++ = ' ';
real_year = 1900 + xt.tm_year;
*date_str++ = real_year / 1000 + '0';
*date_str++ = real_year % 1000 / 100 + '0';
*date_str++ = real_year % 100 / 10 + '0';
*date_str++ = real_year % 10 + '0';
*date_str++ = 0;
return APR_SUCCESS;
}
AP_DECLARE(apr_status_t) ap_recent_rfc822_date(char *date_str, apr_time_t t)
{
/* ### This code is a clone of apr_rfc822_date(), except that it
* uses ap_explode_recent_gmt() instead of apr_time_exp_gmt().
*/
apr_time_exp_t xt;
const char *s;
int real_year;
ap_explode_recent_gmt(&xt, t);
/* example: "Sat, 08 Jan 2000 18:31:41 GMT" */
/* 12345678901234567890123456789 */
s = &apr_day_snames[xt.tm_wday][0];
*date_str++ = *s++;
*date_str++ = *s++;
*date_str++ = *s++;
*date_str++ = ',';
*date_str++ = ' ';
*date_str++ = xt.tm_mday / 10 + '0';
*date_str++ = xt.tm_mday % 10 + '0';
*date_str++ = ' ';
s = &apr_month_snames[xt.tm_mon][0];
*date_str++ = *s++;
*date_str++ = *s++;
*date_str++ = *s++;
*date_str++ = ' ';
real_year = 1900 + xt.tm_year;
/* This routine isn't y10k ready. */
*date_str++ = real_year / 1000 + '0';
*date_str++ = real_year % 1000 / 100 + '0';
*date_str++ = real_year % 100 / 10 + '0';
*date_str++ = real_year % 10 + '0';
*date_str++ = ' ';
*date_str++ = xt.tm_hour / 10 + '0';
*date_str++ = xt.tm_hour % 10 + '0';
*date_str++ = ':';
*date_str++ = xt.tm_min / 10 + '0';
*date_str++ = xt.tm_min % 10 + '0';
*date_str++ = ':';
*date_str++ = xt.tm_sec / 10 + '0';
*date_str++ = xt.tm_sec % 10 + '0';
*date_str++ = ' ';
*date_str++ = 'G';
*date_str++ = 'M';
*date_str++ = 'T';
*date_str++ = 0;
return APR_SUCCESS;
}
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