--- /dev/null
+/* 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 (!APR_STATUS_IS_SUCCESS(r)) {
+ 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;
+}
Code Review / bottlenecks.git / blobdiff