lib/dns/cache.c

	cache.c revision 03609d0b8f6e458d6bd4b460f4eb3f7270f487ef
/*
 * Copyright (C) 1999-2001  Internet Software Consortium.
 *
 * Permission to use, copy, modify, and distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND INTERNET SOFTWARE CONSORTIUM
 * DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL
 * INTERNET SOFTWARE CONSORTIUM BE LIABLE FOR ANY SPECIAL, DIRECT,
 * INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING
 * FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT,
 * NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION
 * WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */

/* $Id: cache.c,v 1.35 2001/03/08 01:22:48 tale Exp $ */

#include <config.h>

#include <isc/mem.h>
#include <isc/task.h>
#include <isc/time.h>
#include <isc/timer.h>
#include <isc/util.h>

#include <dns/cache.h>
#include <dns/db.h>
#include <dns/dbiterator.h>
#include <dns/events.h>
#include <dns/log.h>
#include <dns/masterdump.h>
#include <dns/result.h>

#define CACHE_MAGIC     0x24242424U     /* $$$$. */
#define VALID_CACHE(cache)  ISC_MAGIC_VALID(cache, CACHE_MAGIC)

/***
 ***    Types
 ***/

/*
 * A cache_cleaner_t encapsulsates the state of the periodic
 * cache cleaning.
 */

typedef struct cache_cleaner cache_cleaner_t;

typedef enum {
    cleaner_s_idle, /* Waiting for cleaning-interval to expire. */
    cleaner_s_busy, /* Currently cleaning. */
    cleaner_s_done  /* Freed enough memory after being overmem. */
} cleaner_state_t;

/*
 * Convenience macros for comprehensive assertion checking.
 */
#define CLEANER_IDLE(c) ((c)->state == cleaner_s_idle && \
             (c)->iterator == NULL && \
             (c)->resched_event != NULL)
#define CLEANER_BUSY(c) ((c)->state == cleaner_s_busy && \
             (c)->iterator != NULL && \
             (c)->resched_event == NULL)

struct cache_cleaner {
    dns_cache_t *cache;
    isc_task_t  *task;
    unsigned int    cleaning_interval; /* The cleaning-interval from
                          named.conf, in seconds. */
    isc_timer_t     *cleaning_timer;
    isc_event_t *resched_event; /* Sent by cleaner task to
                       itself to reschedule */
    isc_event_t *overmem_event;

    dns_dbiterator_t *iterator;
    int          increment; /* Number of names to
                       clean in one increment */
    cleaner_state_t  state;     /* Idle/Busy. */
    isc_boolean_t    overmem;   /* The cache is in an overmem state. */
};

/*
 * The actual cache object.
 */

struct dns_cache {
    /* Unlocked */
    unsigned int        magic;
    isc_mutex_t     lock;
    isc_mutex_t     filelock;
    isc_mem_t       *mctx;

    /* Locked by 'lock'. */
    int         references;
    int         live_tasks;
    dns_rdataclass_t    rdclass;
    dns_db_t        *db;
    cache_cleaner_t     cleaner;

    /* Locked by 'filelock'. */
    char *          filename;
    /* Access to the on-disk cache file is also locked by 'filelock'. */
};

/***
 ***    Functions
 ***/

static isc_result_t
cache_cleaner_init(dns_cache_t *cache,
           isc_taskmgr_t *taskmgr, isc_timermgr_t *timermgr,
           cache_cleaner_t *cleaner);

static void
cleaning_timer_action(isc_task_t *task, isc_event_t *event);

static void
incremental_cleaning_action(isc_task_t *task, isc_event_t *event);

static void
cleaner_shutdown_action(isc_task_t *task, isc_event_t *event);

static void
overmem_cleaning_action(isc_task_t *task, isc_event_t *event);

isc_result_t
dns_cache_create(isc_mem_t *mctx, isc_taskmgr_t *taskmgr,
         isc_timermgr_t *timermgr, dns_rdataclass_t rdclass,
         const char *db_type, unsigned int db_argc, char **db_argv,
         dns_cache_t **cachep)
{
    isc_result_t result;
    dns_cache_t *cache;

    REQUIRE(cachep != NULL);
    REQUIRE(*cachep == NULL);
    REQUIRE(mctx != NULL);

    cache = isc_mem_get(mctx, sizeof *cache);
    if (cache == NULL)
        return (ISC_R_NOMEMORY);

    cache->mctx = NULL;
    isc_mem_attach(mctx, &cache->mctx);

    result = isc_mutex_init(&cache->lock);
    if (result != ISC_R_SUCCESS) {
        UNEXPECTED_ERROR(__FILE__, __LINE__,
                 "isc_mutex_init() failed: %s",
                 dns_result_totext(result));
        result = ISC_R_UNEXPECTED;
        goto cleanup_mem;
    }

    result = isc_mutex_init(&cache->filelock);
    if (result != ISC_R_SUCCESS) {
        UNEXPECTED_ERROR(__FILE__, __LINE__,
                 "isc_mutex_init() failed: %s",
                 dns_result_totext(result));
        result = ISC_R_UNEXPECTED;
        goto cleanup_lock;
    }

    cache->references = 1;
    cache->live_tasks = 0;
    cache->rdclass = rdclass;

    cache->db = NULL;
    result = dns_db_create(cache->mctx, db_type, dns_rootname,
                   dns_dbtype_cache, rdclass, db_argc, db_argv,
                   &cache->db);
    if (result != ISC_R_SUCCESS)
        goto cleanup_filelock;

    cache->filename = NULL;

    cache->magic = CACHE_MAGIC;

    result = cache_cleaner_init(cache, taskmgr, timermgr,
                    &cache->cleaner);
    if (result != ISC_R_SUCCESS)
        goto cleanup_db;

    *cachep = cache;
    return (ISC_R_SUCCESS);

 cleanup_db:
    dns_db_detach(&cache->db);
 cleanup_filelock:
    DESTROYLOCK(&cache->filelock);
 cleanup_lock:
    DESTROYLOCK(&cache->lock);
 cleanup_mem:
    isc_mem_put(mctx, cache, sizeof *cache);
    isc_mem_detach(&mctx);
    return (result);
}

static void
cache_free(dns_cache_t *cache) {
    isc_mem_t *mctx;

    REQUIRE(VALID_CACHE(cache));
    REQUIRE(cache->references == 0);

    isc_mem_setwater(cache->mctx, NULL, NULL, 0, 0);

    if (cache->cleaner.task != NULL)
        isc_task_detach(&cache->cleaner.task);

    if (cache->cleaner.overmem_event != NULL)
        isc_event_free(&cache->cleaner.overmem_event);

    if (cache->cleaner.resched_event != NULL)
        isc_event_free(&cache->cleaner.resched_event);

    if (cache->cleaner.iterator != NULL)
        dns_dbiterator_destroy(&cache->cleaner.iterator);

    if (cache->filename) {
        isc_mem_free(cache->mctx, cache->filename);
        cache->filename = NULL;
    }

    if (cache->db)
        dns_db_detach(&cache->db);

    DESTROYLOCK(&cache->lock);
    DESTROYLOCK(&cache->filelock);
    cache->magic = 0;
    mctx = cache->mctx;
    isc_mem_put(cache->mctx, cache, sizeof *cache);
    isc_mem_detach(&mctx);
}


void
dns_cache_attach(dns_cache_t *cache, dns_cache_t **targetp) {

    REQUIRE(VALID_CACHE(cache));
    REQUIRE(targetp != NULL && *targetp == NULL);

    LOCK(&cache->lock);
    cache->references++;
    UNLOCK(&cache->lock);

    *targetp = cache;
}

void
dns_cache_detach(dns_cache_t **cachep) {
    dns_cache_t *cache;
    isc_boolean_t free_cache = ISC_FALSE;

    REQUIRE(cachep != NULL);
    cache = *cachep;
    REQUIRE(VALID_CACHE(cache));

    LOCK(&cache->lock);
    REQUIRE(cache->references > 0);
    cache->references--;
    if (cache->references == 0) {
        cache->cleaner.overmem = ISC_FALSE;
        free_cache = ISC_TRUE;
    }
    UNLOCK(&cache->lock);
    *cachep = NULL;
    if (free_cache) {
        /*
         * When the cache is shut down, dump it to a file if one is
         * specified.
         */
        dns_cache_dump(cache);

        /* XXXRTH  This is not locked! */
        if (cache->live_tasks > 0)
            isc_task_shutdown(cache->cleaner.task);
        else
            cache_free(cache);
    }
}

void
dns_cache_attachdb(dns_cache_t *cache, dns_db_t **dbp) {
    REQUIRE(VALID_CACHE(cache));
    REQUIRE(dbp != NULL && *dbp == NULL);
    REQUIRE(cache->db != NULL);
    LOCK(&cache->lock);
    dns_db_attach(cache->db, dbp);
    UNLOCK(&cache->lock);
}

isc_result_t
dns_cache_setfilename(dns_cache_t *cache, char *filename) {
    char *newname;

    REQUIRE(VALID_CACHE(cache));
    REQUIRE(filename != NULL);

    newname = isc_mem_strdup(cache->mctx, filename);
    if (newname == NULL)
        return (ISC_R_NOMEMORY);
    LOCK(&cache->filelock);
    if (cache->filename)
        isc_mem_free(cache->mctx, cache->filename);
    cache->filename = newname;
    UNLOCK(&cache->filelock);
    return (ISC_R_SUCCESS);
}

isc_result_t
dns_cache_load(dns_cache_t *cache) {
    isc_result_t result;

    REQUIRE(VALID_CACHE(cache));

    if (cache->filename == NULL)
        return (ISC_R_SUCCESS);
    LOCK(&cache->filelock);
    result = dns_db_load(cache->db, cache->filename);
    UNLOCK(&cache->filelock);
    return (result);
}

isc_result_t
dns_cache_dump(dns_cache_t *cache) {
    isc_result_t result;

    REQUIRE(VALID_CACHE(cache));

    if (cache->filename == NULL)
        return (ISC_R_SUCCESS);
    LOCK(&cache->filelock);
    result = dns_master_dump(cache->mctx, cache->db, NULL,
                 &dns_master_style_cache, cache->filename);
    UNLOCK(&cache->filelock);
    return (result);
}

void
dns_cache_setcleaninginterval(dns_cache_t *cache, unsigned int t) {
    LOCK(&cache->lock);

    /*
     * It may be the case that the cache has already shut down.
     * If so, it has no timer.
     */
    if (cache->cleaner.cleaning_timer == NULL)
        goto unlock;

    cache->cleaner.cleaning_interval = t;
    if (t == 0) {
        isc_timer_reset(cache->cleaner.cleaning_timer,
                isc_timertype_inactive, NULL, NULL, ISC_TRUE);
    } else {
        isc_interval_t interval;
        isc_interval_set(&interval, cache->cleaner.cleaning_interval,
                 0);
        isc_timer_reset(cache->cleaner.cleaning_timer,
                isc_timertype_ticker,
                NULL, &interval, ISC_FALSE);
    }
 unlock:
    UNLOCK(&cache->lock);
}

/*
 * Initialize the cache cleaner object at *cleaner.
 * Space for the object must be allocated by the caller.
 */

static isc_result_t
cache_cleaner_init(dns_cache_t *cache, isc_taskmgr_t *taskmgr,
           isc_timermgr_t *timermgr, cache_cleaner_t *cleaner)
{
    isc_result_t result;

    cleaner->increment = 100;
    cleaner->state = cleaner_s_idle;
    cleaner->cache = cache;
    cleaner->iterator = NULL;
    cleaner->overmem = ISC_FALSE;

    cleaner->task = NULL;
    cleaner->cleaning_timer = NULL;
    cleaner->resched_event = NULL;
    cleaner->overmem_event = NULL;

    if (taskmgr != NULL && timermgr != NULL) {
        result = isc_task_create(taskmgr, 1, &cleaner->task);
        if (result != ISC_R_SUCCESS) {
            UNEXPECTED_ERROR(__FILE__, __LINE__,
                     "isc_task_create() failed: %s",
                     dns_result_totext(result));
            result = ISC_R_UNEXPECTED;
            goto cleanup;
        }
        cleaner->cache->live_tasks++;
        isc_task_setname(cleaner->task, "cachecleaner", cleaner);

        result = isc_task_onshutdown(cleaner->task,
                         cleaner_shutdown_action, cache);
        if (result != ISC_R_SUCCESS) {
            UNEXPECTED_ERROR(__FILE__, __LINE__,
                     "cache cleaner: "
                     "isc_task_onshutdown() failed: %s",
                     dns_result_totext(result));
            goto cleanup;
        }

        cleaner->cleaning_interval = 0; /* Initially turned off. */
        result = isc_timer_create(timermgr, isc_timertype_inactive,
                       NULL, NULL,
                       cleaner->task,
                       cleaning_timer_action, cleaner,
                       &cleaner->cleaning_timer);
        if (result != ISC_R_SUCCESS) {
            UNEXPECTED_ERROR(__FILE__, __LINE__,
                     "isc_timer_create() failed: %s",
                     dns_result_totext(result));
            result = ISC_R_UNEXPECTED;
            goto cleanup;
        }

        cleaner->resched_event =
            isc_event_allocate(cache->mctx, cleaner,
                       DNS_EVENT_CACHECLEAN,
                       incremental_cleaning_action,
                       cleaner, sizeof(isc_event_t));
        if (cleaner->resched_event == NULL) {
            result = ISC_R_NOMEMORY;
            goto cleanup;
        }

        cleaner->overmem_event =
            isc_event_allocate(cache->mctx, cleaner,
                       DNS_EVENT_CACHEOVERMEM,
                       overmem_cleaning_action,
                       cleaner, sizeof(isc_event_t));
        if (cleaner->overmem_event == NULL) {
            result = ISC_R_NOMEMORY;
            goto cleanup;
        }
    }

    return (ISC_R_SUCCESS);

 cleanup:
    if (cleaner->resched_event != NULL)
        isc_event_free(&cleaner->resched_event);
    if (cleaner->cleaning_timer != NULL)
        isc_timer_detach(&cleaner->cleaning_timer);
    if (cleaner->task != NULL)
        isc_task_detach(&cleaner->task);
    return (result);
}

static void
begin_cleaning(cache_cleaner_t *cleaner) {
    isc_result_t result;

    REQUIRE(CLEANER_IDLE(cleaner));

    /*
     * Create an iterator and position it at the beginning of the cache.
     */
    result = dns_db_createiterator(cleaner->cache->db, ISC_FALSE,
                       &cleaner->iterator);
    if (result != ISC_R_SUCCESS)
        isc_log_write(dns_lctx, DNS_LOGCATEGORY_DATABASE,
                  DNS_LOGMODULE_CACHE, ISC_LOG_WARNING,
                  "cache cleaner could not create "
                  "iterator: %s", isc_result_totext(result));
    else {
        dns_dbiterator_setcleanmode(cleaner->iterator, ISC_TRUE);
        result = dns_dbiterator_first(cleaner->iterator);
    }

    if (result != ISC_R_SUCCESS) {
        /*
         * If the result is ISC_R_NOMORE, the database is empty,
         * so there is nothing to be cleaned.
         */
        if (result != ISC_R_NOMORE)
            UNEXPECTED_ERROR(__FILE__, __LINE__,
                     "cache cleaner: "
                     "dns_dbiterator_first() failed: %s",
                     dns_result_totext(result));

        if (cleaner->iterator != NULL)
            dns_dbiterator_destroy(&cleaner->iterator);
    } else {
        /*
         * Pause the iterator to free its lock.
         */
        result = dns_dbiterator_pause(cleaner->iterator);
        RUNTIME_CHECK(result == ISC_R_SUCCESS);

        isc_log_write(dns_lctx, DNS_LOGCATEGORY_DATABASE,
                  DNS_LOGMODULE_CACHE, ISC_LOG_DEBUG(1),
                  "begin cache cleaning, mem inuse %d",
                  isc_mem_inuse(cleaner->cache->mctx));
        cleaner->state = cleaner_s_busy;
        isc_task_send(cleaner->task, &cleaner->resched_event);
    }

    return;
}

static void
end_cleaning(cache_cleaner_t *cleaner, isc_event_t *event) {
    REQUIRE(CLEANER_BUSY(cleaner));
    REQUIRE(event != NULL);

    dns_dbiterator_destroy(&cleaner->iterator);

    isc_log_write(dns_lctx, DNS_LOGCATEGORY_DATABASE, DNS_LOGMODULE_CACHE,
              ISC_LOG_DEBUG(1), "end cache cleaning, mem inuse %d",
              isc_mem_inuse(cleaner->cache->mctx));

    cleaner->state = cleaner_s_idle;
    cleaner->resched_event = event;
}

/*
 * This is run once for every cache-cleaning-interval as defined in named.conf.
 */
static void
cleaning_timer_action(isc_task_t *task, isc_event_t *event) {
    cache_cleaner_t *cleaner = event->ev_arg;

    UNUSED(task);

    INSIST(task == cleaner->task);
    INSIST(event->ev_type == ISC_TIMEREVENT_TICK);

    if (cleaner->state == cleaner_s_idle)
        begin_cleaning(cleaner);
    else
        isc_log_write(dns_lctx, DNS_LOGCATEGORY_DATABASE,
                  DNS_LOGMODULE_CACHE, ISC_LOG_WARNING,
                  "cache cleaner did not finish "
                  "in one cleaning-interval");
    isc_event_free(&event);
}

static void
overmem_cleaning_action(isc_task_t *task, isc_event_t *event) {
    cache_cleaner_t *cleaner = event->ev_arg;

    UNUSED(task);

    INSIST(task == cleaner->task);
    INSIST(event->ev_type == DNS_EVENT_CACHEOVERMEM);
    INSIST(cleaner->overmem_event == NULL);

    if (cleaner->overmem) {
        if (cleaner->state == cleaner_s_idle)
            begin_cleaning(cleaner);
    } else {
        if (cleaner->state == cleaner_s_busy)
            cleaner->state = cleaner_s_done;
    }

    cleaner->overmem_event = event;
}

/*
 * Do incremental cleaning.
 */
static void
incremental_cleaning_action(isc_task_t *task, isc_event_t *event) {
    isc_result_t result;
    cache_cleaner_t *cleaner = event->ev_arg;
    isc_stdtime_t now;
    int n_names;

    INSIST(event->ev_type == DNS_EVENT_CACHECLEAN);

    if (cleaner->state == cleaner_s_done) {
        end_cleaning(cleaner, event);
        return;
    }

    INSIST(CLEANER_BUSY(cleaner));

    n_names = cleaner->increment;
    isc_stdtime_get(&now);

    REQUIRE(DNS_DBITERATOR_VALID(cleaner->iterator));

    while (n_names-- > 0) {
        dns_dbnode_t *node = NULL;

        result = dns_dbiterator_current(cleaner->iterator, &node,
                        NULL);
        if (result != ISC_R_SUCCESS) {
            UNEXPECTED_ERROR(__FILE__, __LINE__,
                 "cache cleaner: dns_dbiterator_current() "
                 "failed: %s", dns_result_totext(result));

            end_cleaning(cleaner, event);
            return;
        }

        /*
         * Step to the next node.
         */
        result = dns_dbiterator_next(cleaner->iterator);
        if (result != ISC_R_SUCCESS) {
            /*
             * Either the end was reached (ISC_R_NOMORE) or
             * some error was signaled.  If the cache is still
             * overmem and no error was encountered,
             * keep trying to clean it, otherwise stop cleanng.
             */
            if (result != ISC_R_NOMORE)
                UNEXPECTED_ERROR(__FILE__, __LINE__,
                         "cache cleaner: "
                         "dns_dbiterator_next() "
                         "failed: %s",
                         dns_result_totext(result));
            else if (cleaner->overmem) {
                result = dns_dbiterator_first(cleaner->
                                  iterator);
                if (result == ISC_R_SUCCESS) {
                    isc_log_write(dns_lctx,
                              DNS_LOGCATEGORY_DATABASE,
                              DNS_LOGMODULE_CACHE,
                              ISC_LOG_DEBUG(1),
                              "cache cleaner: "
                              "still overmem, "
                              "reset and try again");
                    continue;
                }
            }

            end_cleaning(cleaner, event);
            return;
        }
    }

    /*
     * We have successfully performed a cleaning increment but have
     * not gone through the entire cache.  Free the iterator locks
     * and reschedule another batch.  If it fails, just try to continue
     * anyway.
     */
    result = dns_dbiterator_pause(cleaner->iterator);
    RUNTIME_CHECK(result == ISC_R_SUCCESS);

    isc_task_send(task, &event);
    INSIST(CLEANER_BUSY(cleaner));

    return;
}

/*
 * Do immediate cleaning.
 */
isc_result_t
dns_cache_clean(dns_cache_t *cache, isc_stdtime_t now) {
    isc_result_t result;
    dns_dbiterator_t *iterator = NULL;

    REQUIRE(VALID_CACHE(cache));

    result = dns_db_createiterator(cache->db, ISC_FALSE, &iterator);
    if (result != ISC_R_SUCCESS)
        return result;

    result = dns_dbiterator_first(iterator);

    while (result == ISC_R_SUCCESS) {
        dns_dbnode_t *node = NULL;
        result = dns_dbiterator_current(iterator, &node,
                        (dns_name_t *)NULL);
        if (result != ISC_R_SUCCESS)
            break;

        /*
         * Check TTLs, mark expired rdatasets stale.
         */
        result = dns_db_expirenode(cache->db, node, now);
        if (result != ISC_R_SUCCESS) {
            UNEXPECTED_ERROR(__FILE__, __LINE__,
                     "cache cleaner: dns_db_expirenode() "
                     "failed: %s",
                     dns_result_totext(result));
            /*
             * Continue anyway.
             */
        }

        /*
         * This is where the actual freeing takes place.
         */
        dns_db_detachnode(cache->db, &node);

        result = dns_dbiterator_next(iterator);
    }

    dns_dbiterator_destroy(&iterator);

    if (result == ISC_R_NOMORE)
        result = ISC_R_SUCCESS;

    return result;
}

static void
water(void *arg, int mark) {
    dns_cache_t *cache = arg;
    isc_boolean_t overmem = ISC_TF(mark == ISC_MEM_HIWATER);

    REQUIRE(VALID_CACHE(cache));
    dns_db_overmem(cache->db, overmem);
    cache->cleaner.overmem = overmem;

    if (overmem && cache->cleaner.overmem_event != NULL) {
        isc_task_send(cache->cleaner.task,
                  &cache->cleaner.overmem_event);
    }
}

void
dns_cache_setcachesize(dns_cache_t *cache, isc_uint32_t size) {
    isc_uint32_t lowater;
    isc_uint32_t hiwater;

    REQUIRE(VALID_CACHE(cache));

    /*
     * Impose a minumum cache size -- this should perhaps be larger,
     * but for now is just meant ensure that hiwater and lowater are
     * less than the size.
     */
    if (size != 0 && size < 8)
        size = 8;

    hiwater = size - (size >> 3);   /* ~(7/8) */
    lowater = size - (size >> 2);   /* ~(3/4) */

    /*
     * This resets the overmem status if already in an overmem condition,
     * even though the cache is perhaps still overmem even with new values.
     * The overmem condition will be resignaled.
     * XXXDCL i am not sure this is entirely right.  what happens to
     * the active cleaner?
     */
    dns_db_overmem(cache->db, ISC_FALSE);
    cache->cleaner.overmem = ISC_FALSE;

    if (size == 0 || hiwater == 0 || lowater == 0)
        /*
         * Disable cache memory limiting.
         */
        isc_mem_setwater(cache->mctx, NULL, 0, 0, 0);
    else
        /*
         * Establish new cache memory limits (either for the first
         * time, or replacing other limits).
         */
        isc_mem_setwater(cache->mctx, water, cache, hiwater, lowater);
}

/*
 * The cleaner task is shutting down; do the necessary cleanup.
 */
static void
cleaner_shutdown_action(isc_task_t *task, isc_event_t *event) {
    dns_cache_t *cache = event->ev_arg;
    isc_boolean_t should_free = ISC_FALSE;

    UNUSED(task);

    LOCK(&cache->lock);

    INSIST(event->ev_type == ISC_TASKEVENT_SHUTDOWN);
    isc_event_free(&event);

    cache->live_tasks--;
    INSIST(cache->live_tasks == 0);

    if (cache->references == 0)
        should_free = ISC_TRUE;

    /*
     * By detaching the timer in the context of its task,
     * we are guaranteed that there will be no further timer
     * events.
     */
    if (cache->cleaner.cleaning_timer != NULL)
        isc_timer_detach(&cache->cleaner.cleaning_timer);

    /* Make sure we don't reschedule anymore. */
    isc_task_purge(task, NULL, DNS_EVENT_CACHECLEAN, NULL);

    UNLOCK(&cache->lock);

    if (should_free)
        cache_free(cache);
}