Add BIND 9.2.4rc7.

[dragonfly.git] / contrib / bind-9.2.4rc7 / lib / isc / task.c

/*
 * Copyright (C) 2004  Internet Systems Consortium, Inc. ("ISC")
 * Copyright (C) 1998-2002  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 ISC DISCLAIMS ALL WARRANTIES WITH
 * REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
 * AND FITNESS.  IN NO EVENT SHALL ISC 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: task.c,v 1.85.2.5 2004/03/09 06:11:52 marka Exp $ */

/*
 * Principal Author: Bob Halley
 */

/*
 * XXXRTH  Need to document the states a task can be in, and the rules
 * for changing states.
 */

#include <config.h>

#include <isc/condition.h>
#include <isc/event.h>
#include <isc/magic.h>
#include <isc/mem.h>
#include <isc/msgs.h>
#include <isc/platform.h>
#include <isc/string.h>
#include <isc/task.h>
#include <isc/thread.h>
#include <isc/util.h>

#ifndef ISC_PLATFORM_USETHREADS
#include "task_p.h"
#endif /* ISC_PLATFORM_USETHREADS */

#define ISC_TASK_NAMES 1

#ifdef ISC_TASK_TRACE
#define XTRACE(m)               fprintf(stderr, "task %p thread %lu: %s\n", \
                                       task, isc_thread_self(), (m))
#define XTTRACE(t, m)           fprintf(stderr, "task %p thread %lu: %s\n", \
                                       (t), isc_thread_self(), (m))
#define XTHREADTRACE(m)         fprintf(stderr, "thread %lu: %s\n", \
                                       isc_thread_self(), (m))
#else
#define XTRACE(m)
#define XTTRACE(t, m)
#define XTHREADTRACE(m)
#endif

/***
 *** Types.
 ***/

typedef enum {
        task_state_idle, task_state_ready, task_state_running,
        task_state_done
} task_state_t;

#define TASK_MAGIC                      ISC_MAGIC('T', 'A', 'S', 'K')
#define VALID_TASK(t)                   ISC_MAGIC_VALID(t, TASK_MAGIC)

struct isc_task {
        /* Not locked. */
        unsigned int                    magic;
        isc_taskmgr_t *                 manager;
        isc_mutex_t                     lock;
        /* Locked by task lock. */
        task_state_t                    state;
        unsigned int                    references;
        isc_eventlist_t                 events;
        isc_eventlist_t                 on_shutdown;
        unsigned int                    quantum;
        unsigned int                    flags;
#ifdef ISC_TASK_NAMES
        char                            name[16];
        void *                          tag;
#endif
        /* Locked by task manager lock. */
        LINK(isc_task_t)                link;
        LINK(isc_task_t)                ready_link;
};

#define TASK_F_SHUTTINGDOWN             0x01

#define TASK_SHUTTINGDOWN(t)            (((t)->flags & TASK_F_SHUTTINGDOWN) \
                                         != 0)

#define TASK_MANAGER_MAGIC              ISC_MAGIC('T', 'S', 'K', 'M')
#define VALID_MANAGER(m)                ISC_MAGIC_VALID(m, TASK_MANAGER_MAGIC)

struct isc_taskmgr {
        /* Not locked. */
        unsigned int                    magic;
        isc_mem_t *                     mctx;
        isc_mutex_t                     lock;
        unsigned int                    workers;
#ifdef ISC_PLATFORM_USETHREADS
        isc_thread_t *                  threads;
#endif /* ISC_PLATFORM_USETHREADS */
        /* Locked by task manager lock. */
        unsigned int                    default_quantum;
        LIST(isc_task_t)                tasks;
        isc_tasklist_t                  ready_tasks;
#ifdef ISC_PLATFORM_USETHREADS
        isc_condition_t                 work_available;
        isc_condition_t                 exclusive_granted;
#endif /* ISC_PLATFORM_USETHREADS */
        unsigned int                    tasks_running;
        isc_boolean_t                   exclusive_requested;
        isc_boolean_t                   exiting;
#ifndef ISC_PLATFORM_USETHREADS
        unsigned int                    refs;
#endif /* ISC_PLATFORM_USETHREADS */
};

#define DEFAULT_TASKMGR_QUANTUM         10
#define DEFAULT_DEFAULT_QUANTUM         5
#define FINISHED(m)                     ((m)->exiting && EMPTY((m)->tasks))

#ifndef ISC_PLATFORM_USETHREADS
static isc_taskmgr_t *taskmgr = NULL;
#endif /* ISC_PLATFORM_USETHREADS */

/***
 *** Tasks.
 ***/

static void
task_finished(isc_task_t *task) {
        isc_taskmgr_t *manager = task->manager;

        REQUIRE(EMPTY(task->events));
        REQUIRE(EMPTY(task->on_shutdown));
        REQUIRE(task->references == 0);
        REQUIRE(task->state == task_state_done);

        XTRACE("task_finished");

        LOCK(&manager->lock);
        UNLINK(manager->tasks, task, link);
#ifdef ISC_PLATFORM_USETHREADS
        if (FINISHED(manager)) {
                /*
                 * All tasks have completed and the
                 * task manager is exiting.  Wake up
                 * any idle worker threads so they
                 * can exit.
                 */
                BROADCAST(&manager->work_available);
        }
#endif /* ISC_PLATFORM_USETHREADS */
        UNLOCK(&manager->lock);

        DESTROYLOCK(&task->lock);
        task->magic = 0;
        isc_mem_put(manager->mctx, task, sizeof *task);
}

isc_result_t
isc_task_create(isc_taskmgr_t *manager, unsigned int quantum,
                isc_task_t **taskp)
{
        isc_task_t *task;
        isc_boolean_t exiting;

        REQUIRE(VALID_MANAGER(manager));
        REQUIRE(taskp != NULL && *taskp == NULL);

        task = isc_mem_get(manager->mctx, sizeof *task);
        if (task == NULL)
                return (ISC_R_NOMEMORY);
        XTRACE("isc_task_create");
        task->manager = manager;
        if (isc_mutex_init(&task->lock) != ISC_R_SUCCESS) {
                isc_mem_put(manager->mctx, task, sizeof *task);
                UNEXPECTED_ERROR(__FILE__, __LINE__,
                                 "isc_mutex_init() %s",
                                 isc_msgcat_get(isc_msgcat, ISC_MSGSET_GENERAL,
                                                ISC_MSG_FAILED, "failed"));
                return (ISC_R_UNEXPECTED);
        }
        task->state = task_state_idle;
        task->references = 1;
        INIT_LIST(task->events);
        INIT_LIST(task->on_shutdown);
        task->quantum = quantum;
        task->flags = 0;
#ifdef ISC_TASK_NAMES
        memset(task->name, 0, sizeof task->name);
        task->tag = NULL;
#endif
        INIT_LINK(task, link);
        INIT_LINK(task, ready_link);

        exiting = ISC_FALSE;
        LOCK(&manager->lock);
        if (!manager->exiting) {
                if (task->quantum == 0)
                        task->quantum = manager->default_quantum;
                APPEND(manager->tasks, task, link);
        } else
                exiting = ISC_TRUE;
        UNLOCK(&manager->lock);

        if (exiting) {
                DESTROYLOCK(&task->lock);
                isc_mem_put(manager->mctx, task, sizeof *task);
                return (ISC_R_SHUTTINGDOWN);
        }

        task->magic = TASK_MAGIC;
        *taskp = task;

        return (ISC_R_SUCCESS);
}

void
isc_task_attach(isc_task_t *source, isc_task_t **targetp) {

        /*
         * Attach *targetp to source.
         */

        REQUIRE(VALID_TASK(source));
        REQUIRE(targetp != NULL && *targetp == NULL);

        XTTRACE(source, "isc_task_attach");

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

        *targetp = source;
}

static inline isc_boolean_t
task_shutdown(isc_task_t *task) {
        isc_boolean_t was_idle = ISC_FALSE;
        isc_event_t *event, *prev;

        /*
         * Caller must be holding the task's lock.
         */

        XTRACE("task_shutdown");

        if (! TASK_SHUTTINGDOWN(task)) {
                XTRACE(isc_msgcat_get(isc_msgcat, ISC_MSGSET_GENERAL,
                                      ISC_MSG_SHUTTINGDOWN, "shutting down"));
                task->flags |= TASK_F_SHUTTINGDOWN;
                if (task->state == task_state_idle) {
                        INSIST(EMPTY(task->events));
                        task->state = task_state_ready;
                        was_idle = ISC_TRUE;
                }
                INSIST(task->state == task_state_ready ||
                       task->state == task_state_running);
                /*
                 * Note that we post shutdown events LIFO.
                 */
                for (event = TAIL(task->on_shutdown);
                     event != NULL;
                     event = prev) {
                        prev = PREV(event, ev_link);
                        DEQUEUE(task->on_shutdown, event, ev_link);
                        ENQUEUE(task->events, event, ev_link);
                }
        }

        return (was_idle);
}

static inline void
task_ready(isc_task_t *task) {
        isc_taskmgr_t *manager = task->manager;

        REQUIRE(VALID_MANAGER(manager));
        REQUIRE(task->state == task_state_ready);

        XTRACE("task_ready");

        LOCK(&manager->lock);

        ENQUEUE(manager->ready_tasks, task, ready_link);
#ifdef ISC_PLATFORM_USETHREADS
        SIGNAL(&manager->work_available);
#endif /* ISC_PLATFORM_USETHREADS */

        UNLOCK(&manager->lock);
}

static inline isc_boolean_t
task_detach(isc_task_t *task) {

        /*
         * Caller must be holding the task lock.
         */

        REQUIRE(task->references > 0);

        XTRACE("detach");

        task->references--;
        if (task->references == 0 && task->state == task_state_idle) {
                INSIST(EMPTY(task->events));
                /*
                 * There are no references to this task, and no
                 * pending events.  We could try to optimize and
                 * either initiate shutdown or clean up the task,
                 * depending on its state, but it's easier to just
                 * make the task ready and allow run() or the event
                 * loop to deal with shutting down and termination.
                 */
                task->state = task_state_ready;
                return (ISC_TRUE);
        }

        return (ISC_FALSE);
}

void
isc_task_detach(isc_task_t **taskp) {
        isc_task_t *task;
        isc_boolean_t was_idle;

        /*
         * Detach *taskp from its task.
         */

        REQUIRE(taskp != NULL);
        task = *taskp;
        REQUIRE(VALID_TASK(task));

        XTRACE("isc_task_detach");

        LOCK(&task->lock);
        was_idle = task_detach(task);
        UNLOCK(&task->lock);

        if (was_idle)
                task_ready(task);

        *taskp = NULL;
}

static inline isc_boolean_t
task_send(isc_task_t *task, isc_event_t **eventp) {
        isc_boolean_t was_idle = ISC_FALSE;
        isc_event_t *event;

        /*
         * Caller must be holding the task lock.
         */

        REQUIRE(eventp != NULL);
        event = *eventp;
        REQUIRE(event != NULL);
        REQUIRE(event->ev_type > 0);
        REQUIRE(task->state != task_state_done);

        XTRACE("task_send");

        if (task->state == task_state_idle) {
                was_idle = ISC_TRUE;
                INSIST(EMPTY(task->events));
                task->state = task_state_ready;
        }
        INSIST(task->state == task_state_ready ||
               task->state == task_state_running);
        ENQUEUE(task->events, event, ev_link);
        *eventp = NULL;

        return (was_idle);
}

void
isc_task_send(isc_task_t *task, isc_event_t **eventp) {
        isc_boolean_t was_idle;

        /*
         * Send '*event' to 'task'.
         */

        REQUIRE(VALID_TASK(task));

        XTRACE("isc_task_send");

        /*
         * We're trying hard to hold locks for as short a time as possible.
         * We're also trying to hold as few locks as possible.  This is why
         * some processing is deferred until after the lock is released.
         */
        LOCK(&task->lock);
        was_idle = task_send(task, eventp);
        UNLOCK(&task->lock);

        if (was_idle) {
                /*
                 * We need to add this task to the ready queue.
                 *
                 * We've waited until now to do it because making a task
                 * ready requires locking the manager.  If we tried to do
                 * this while holding the task lock, we could deadlock.
                 *
                 * We've changed the state to ready, so no one else will
                 * be trying to add this task to the ready queue.  The
                 * only way to leave the ready state is by executing the
                 * task.  It thus doesn't matter if events are added,
                 * removed, or a shutdown is started in the interval
                 * between the time we released the task lock, and the time
                 * we add the task to the ready queue.
                 */
                task_ready(task);
        }
}

void
isc_task_sendanddetach(isc_task_t **taskp, isc_event_t **eventp) {
        isc_boolean_t idle1, idle2;
        isc_task_t *task;

        /*
         * Send '*event' to '*taskp' and then detach '*taskp' from its
         * task.
         */

        REQUIRE(taskp != NULL);
        task = *taskp;
        REQUIRE(VALID_TASK(task));

        XTRACE("isc_task_sendanddetach");

        LOCK(&task->lock);
        idle1 = task_send(task, eventp);
        idle2 = task_detach(task);
        UNLOCK(&task->lock);

        /*
         * If idle1, then idle2 shouldn't be true as well since we're holding
         * the task lock, and thus the task cannot switch from ready back to
         * idle.
         */
        INSIST(!(idle1 && idle2));

        if (idle1 || idle2)
                task_ready(task);

        *taskp = NULL;
}

#define PURGE_OK(event) (((event)->ev_attributes & ISC_EVENTATTR_NOPURGE) == 0)

static unsigned int
dequeue_events(isc_task_t *task, void *sender, isc_eventtype_t first,
               isc_eventtype_t last, void *tag,
               isc_eventlist_t *events, isc_boolean_t purging)
{
        isc_event_t *event, *next_event;
        unsigned int count = 0;

        REQUIRE(VALID_TASK(task));
        REQUIRE(last >= first);

        XTRACE("dequeue_events");

        /*
         * Events matching 'sender', whose type is >= first and <= last, and
         * whose tag is 'tag' will be dequeued.  If 'purging', matching events
         * which are marked as unpurgable will not be dequeued.
         *
         * sender == NULL means "any sender", and tag == NULL means "any tag".
         */

        LOCK(&task->lock);

        for (event = HEAD(task->events); event != NULL; event = next_event) {
                next_event = NEXT(event, ev_link);
                if (event->ev_type >= first && event->ev_type <= last &&
                    (sender == NULL || event->ev_sender == sender) &&
                    (tag == NULL || event->ev_tag == tag) &&
                    (!purging || PURGE_OK(event))) {
                        DEQUEUE(task->events, event, ev_link);
                        ENQUEUE(*events, event, ev_link);
                        count++;
                }
        }

        UNLOCK(&task->lock);

        return (count);
}

unsigned int
isc_task_purgerange(isc_task_t *task, void *sender, isc_eventtype_t first,
                    isc_eventtype_t last, void *tag)
{
        unsigned int count;
        isc_eventlist_t events;
        isc_event_t *event, *next_event;

        /*
         * Purge events from a task's event queue.
         */

        XTRACE("isc_task_purgerange");

        ISC_LIST_INIT(events);

        count = dequeue_events(task, sender, first, last, tag, &events,
                               ISC_TRUE);

        for (event = HEAD(events); event != NULL; event = next_event) {
                next_event = NEXT(event, ev_link);
                isc_event_free(&event);
        }

        /*
         * Note that purging never changes the state of the task.
         */

        return (count);
}

unsigned int
isc_task_purge(isc_task_t *task, void *sender, isc_eventtype_t type,
               void *tag)
{
        /*
         * Purge events from a task's event queue.
         */

        XTRACE("isc_task_purge");

        return (isc_task_purgerange(task, sender, type, type, tag));
}

isc_boolean_t
isc_task_purgeevent(isc_task_t *task, isc_event_t *event) {
        isc_event_t *curr_event, *next_event;

        /*
         * Purge 'event' from a task's event queue.
         *
         * XXXRTH:  WARNING:  This method may be removed before beta.
         */

        REQUIRE(VALID_TASK(task));

        /*
         * If 'event' is on the task's event queue, it will be purged,
         * unless it is marked as unpurgeable.  'event' does not have to be
         * on the task's event queue; in fact, it can even be an invalid
         * pointer.  Purging only occurs if the event is actually on the task's
         * event queue.
         *
         * Purging never changes the state of the task.
         */

        LOCK(&task->lock);
        for (curr_event = HEAD(task->events);
             curr_event != NULL;
             curr_event = next_event) {
                next_event = NEXT(curr_event, ev_link);
                if (curr_event == event && PURGE_OK(event)) {
                        DEQUEUE(task->events, curr_event, ev_link);
                        break;
                }
        }
        UNLOCK(&task->lock);

        if (curr_event == NULL)
                return (ISC_FALSE);

        isc_event_free(&curr_event);

        return (ISC_TRUE);
}

unsigned int
isc_task_unsendrange(isc_task_t *task, void *sender, isc_eventtype_t first,
                     isc_eventtype_t last, void *tag,
                     isc_eventlist_t *events)
{
        /*
         * Remove events from a task's event queue.
         */

        XTRACE("isc_task_unsendrange");

        return (dequeue_events(task, sender, first, last, tag, events,
                               ISC_FALSE));
}

unsigned int
isc_task_unsend(isc_task_t *task, void *sender, isc_eventtype_t type,
                void *tag, isc_eventlist_t *events)
{
        /*
         * Remove events from a task's event queue.
         */

        XTRACE("isc_task_unsend");

        return (dequeue_events(task, sender, type, type, tag, events,
                               ISC_FALSE));
}

isc_result_t
isc_task_onshutdown(isc_task_t *task, isc_taskaction_t action, const void *arg)
{
        isc_boolean_t disallowed = ISC_FALSE;
        isc_result_t result = ISC_R_SUCCESS;
        isc_event_t *event;

        /*
         * Send a shutdown event with action 'action' and argument 'arg' when
         * 'task' is shutdown.
         */

        REQUIRE(VALID_TASK(task));
        REQUIRE(action != NULL);

        event = isc_event_allocate(task->manager->mctx,
                                   NULL,
                                   ISC_TASKEVENT_SHUTDOWN,
                                   action,
                                   arg,
                                   sizeof *event);
        if (event == NULL)
                return (ISC_R_NOMEMORY);

        LOCK(&task->lock);
        if (TASK_SHUTTINGDOWN(task)) {
                disallowed = ISC_TRUE;
                result = ISC_R_SHUTTINGDOWN;
        } else
                ENQUEUE(task->on_shutdown, event, ev_link);
        UNLOCK(&task->lock);

        if (disallowed)
                isc_mem_put(task->manager->mctx, event, sizeof *event);

        return (result);
}

void
isc_task_shutdown(isc_task_t *task) {
        isc_boolean_t was_idle;

        /*
         * Shutdown 'task'.
         */

        REQUIRE(VALID_TASK(task));

        LOCK(&task->lock);
        was_idle = task_shutdown(task);
        UNLOCK(&task->lock);

        if (was_idle)
                task_ready(task);
}

void
isc_task_destroy(isc_task_t **taskp) {

        /*
         * Destroy '*taskp'.
         */

        REQUIRE(taskp != NULL);

        isc_task_shutdown(*taskp);
        isc_task_detach(taskp);
}

void
isc_task_setname(isc_task_t *task, const char *name, void *tag) {

        /*
         * Name 'task'.
         */

        REQUIRE(VALID_TASK(task));

#ifdef ISC_TASK_NAMES
        LOCK(&task->lock);
        memset(task->name, 0, sizeof(task->name));
        strncpy(task->name, name, sizeof(task->name) - 1);
        task->tag = tag;
        UNLOCK(&task->lock);
#else
        UNUSED(name);
        UNUSED(tag);
#endif

}

const char *
isc_task_getname(isc_task_t *task) {
        return (task->name);
}

void *
isc_task_gettag(isc_task_t *task) {
        return (task->tag);
}


/***
 *** Task Manager.
 ***/
static void
dispatch(isc_taskmgr_t *manager) {
        isc_task_t *task;
#ifndef ISC_PLATFORM_USETHREADS
        unsigned int total_dispatch_count = 0;
        isc_tasklist_t ready_tasks;
#endif /* ISC_PLATFORM_USETHREADS */

        REQUIRE(VALID_MANAGER(manager));

        /*
         * Again we're trying to hold the lock for as short a time as possible
         * and to do as little locking and unlocking as possible.
         *
         * In both while loops, the appropriate lock must be held before the
         * while body starts.  Code which acquired the lock at the top of
         * the loop would be more readable, but would result in a lot of
         * extra locking.  Compare:
         *
         * Straightforward:
         *
         *      LOCK();
         *      ...
         *      UNLOCK();
         *      while (expression) {
         *              LOCK();
         *              ...
         *              UNLOCK();
         *
         *              Unlocked part here...
         *
         *              LOCK();
         *              ...
         *              UNLOCK();
         *      }
         *
         * Note how if the loop continues we unlock and then immediately lock.
         * For N iterations of the loop, this code does 2N+1 locks and 2N+1
         * unlocks.  Also note that the lock is not held when the while
         * condition is tested, which may or may not be important, depending
         * on the expression.
         *
         * As written:
         *
         *      LOCK();
         *      while (expression) {
         *              ...
         *              UNLOCK();
         *
         *              Unlocked part here...
         *
         *              LOCK();
         *              ...
         *      }
         *      UNLOCK();
         *
         * For N iterations of the loop, this code does N+1 locks and N+1
         * unlocks.  The while expression is always protected by the lock.
         */

#ifndef ISC_PLATFORM_USETHREADS
        ISC_LIST_INIT(ready_tasks);
#endif
        LOCK(&manager->lock);
        while (!FINISHED(manager)) {
#ifdef ISC_PLATFORM_USETHREADS
                /*
                 * For reasons similar to those given in the comment in
                 * isc_task_send() above, it is safe for us to dequeue
                 * the task while only holding the manager lock, and then
                 * change the task to running state while only holding the
                 * task lock.
                 */
                while ((EMPTY(manager->ready_tasks) ||
                        manager->exclusive_requested) &&
                        !FINISHED(manager)) 
                {
                        XTHREADTRACE(isc_msgcat_get(isc_msgcat,
                                                    ISC_MSGSET_GENERAL,
                                                    ISC_MSG_WAIT, "wait"));
                        WAIT(&manager->work_available, &manager->lock);
                        XTHREADTRACE(isc_msgcat_get(isc_msgcat,
                                                    ISC_MSGSET_TASK,
                                                    ISC_MSG_AWAKE, "awake"));
                }
#else /* ISC_PLATFORM_USETHREADS */
                if (total_dispatch_count >= DEFAULT_TASKMGR_QUANTUM ||
                    EMPTY(manager->ready_tasks))
                        break;
#endif /* ISC_PLATFORM_USETHREADS */
                XTHREADTRACE(isc_msgcat_get(isc_msgcat, ISC_MSGSET_TASK,
                                            ISC_MSG_WORKING, "working"));

                task = HEAD(manager->ready_tasks);
                if (task != NULL) {
                        unsigned int dispatch_count = 0;
                        isc_boolean_t done = ISC_FALSE;
                        isc_boolean_t requeue = ISC_FALSE;
                        isc_boolean_t finished = ISC_FALSE;
                        isc_event_t *event;

                        INSIST(VALID_TASK(task));

                        /*
                         * Note we only unlock the manager lock if we actually
                         * have a task to do.  We must reacquire the manager
                         * lock before exiting the 'if (task != NULL)' block.
                         */
                        DEQUEUE(manager->ready_tasks, task, ready_link);
                        manager->tasks_running++;
                        UNLOCK(&manager->lock);

                        LOCK(&task->lock);
                        INSIST(task->state == task_state_ready);
                        task->state = task_state_running;
                        XTRACE(isc_msgcat_get(isc_msgcat, ISC_MSGSET_GENERAL,
                                              ISC_MSG_RUNNING, "running"));
                        do {
                                if (!EMPTY(task->events)) {
                                        event = HEAD(task->events);
                                        DEQUEUE(task->events, event, ev_link);

                                        /*
                                         * Execute the event action.
                                         */
                                        XTRACE(isc_msgcat_get(isc_msgcat,
                                                            ISC_MSGSET_TASK,
                                                            ISC_MSG_EXECUTE,
                                                            "execute action"));
                                        if (event->ev_action != NULL) {
                                                UNLOCK(&task->lock);
                                                (event->ev_action)(task,event);
                                                LOCK(&task->lock);
                                        }
                                        dispatch_count++;
#ifndef ISC_PLATFORM_USETHREADS
                                        total_dispatch_count++;
#endif /* ISC_PLATFORM_USETHREADS */
                                }

                                if (task->references == 0 &&
                                    EMPTY(task->events) &&
                                    !TASK_SHUTTINGDOWN(task)) {
                                        isc_boolean_t was_idle;

                                        /*
                                         * There are no references and no
                                         * pending events for this task,
                                         * which means it will not become
                                         * runnable again via an external
                                         * action (such as sending an event
                                         * or detaching).
                                         *
                                         * We initiate shutdown to prevent
                                         * it from becoming a zombie.
                                         *
                                         * We do this here instead of in
                                         * the "if EMPTY(task->events)" block
                                         * below because:
                                         *
                                         *      If we post no shutdown events,
                                         *      we want the task to finish.
                                         *
                                         *      If we did post shutdown events,
                                         *      will still want the task's
                                         *      quantum to be applied.
                                         */
                                        was_idle = task_shutdown(task);
                                        INSIST(!was_idle);
                                }

                                if (EMPTY(task->events)) {
                                        /*
                                         * Nothing else to do for this task
                                         * right now.
                                         */
                                        XTRACE(isc_msgcat_get(isc_msgcat,
                                                              ISC_MSGSET_TASK,
                                                              ISC_MSG_EMPTY,
                                                              "empty"));
                                        if (task->references == 0 &&
                                            TASK_SHUTTINGDOWN(task)) {
                                                /*
                                                 * The task is done.
                                                 */
                                                XTRACE(isc_msgcat_get(
                                                               isc_msgcat,
                                                               ISC_MSGSET_TASK,
                                                               ISC_MSG_DONE,
                                                               "done"));
                                                finished = ISC_TRUE;
                                                task->state = task_state_done;
                                        } else
                                                task->state = task_state_idle;
                                        done = ISC_TRUE;
                                } else if (dispatch_count >= task->quantum) {
                                        /*
                                         * Our quantum has expired, but
                                         * there is more work to be done.
                                         * We'll requeue it to the ready
                                         * queue later.
                                         *
                                         * We don't check quantum until
                                         * dispatching at least one event,
                                         * so the minimum quantum is one.
                                         */
                                        XTRACE(isc_msgcat_get(isc_msgcat,
                                                              ISC_MSGSET_TASK,
                                                              ISC_MSG_QUANTUM,
                                                              "quantum"));
                                        task->state = task_state_ready;
                                        requeue = ISC_TRUE;
                                        done = ISC_TRUE;
                                }
                        } while (!done);
                        UNLOCK(&task->lock);

                        if (finished)
                                task_finished(task);

                        LOCK(&manager->lock);
                        manager->tasks_running--;
#ifdef ISC_PLATFORM_USETHREADS
                        if (manager->exclusive_requested &&
                            manager->tasks_running == 1) {
                                SIGNAL(&manager->exclusive_granted);
                        }
#endif /* ISC_PLATFORM_USETHREADS */
                        if (requeue) {
                                /*
                                 * We know we're awake, so we don't have
                                 * to wakeup any sleeping threads if the
                                 * ready queue is empty before we requeue.
                                 *
                                 * A possible optimization if the queue is
                                 * empty is to 'goto' the 'if (task != NULL)'
                                 * block, avoiding the ENQUEUE of the task
                                 * and the subsequent immediate DEQUEUE
                                 * (since it is the only executable task).
                                 * We don't do this because then we'd be
                                 * skipping the exit_requested check.  The
                                 * cost of ENQUEUE is low anyway, especially
                                 * when you consider that we'd have to do
                                 * an extra EMPTY check to see if we could
                                 * do the optimization.  If the ready queue
                                 * were usually nonempty, the 'optimization'
                                 * might even hurt rather than help.
                                 */
#ifdef ISC_PLATFORM_USETHREADS
                                ENQUEUE(manager->ready_tasks, task,
                                        ready_link);
#else
                                ENQUEUE(ready_tasks, task, ready_link);
#endif
                        }
                }
        }
#ifndef ISC_PLATFORM_USETHREADS
        ISC_LIST_APPENDLIST(manager->ready_tasks, ready_tasks, ready_link);
#endif
        UNLOCK(&manager->lock);
}

#ifdef ISC_PLATFORM_USETHREADS
static isc_threadresult_t
#ifdef _WIN32
WINAPI
#endif
run(void *uap) {
        isc_taskmgr_t *manager = uap;

        XTHREADTRACE(isc_msgcat_get(isc_msgcat, ISC_MSGSET_GENERAL,
                                    ISC_MSG_STARTING, "starting"));

        dispatch(manager);

        XTHREADTRACE(isc_msgcat_get(isc_msgcat, ISC_MSGSET_GENERAL,
                                    ISC_MSG_EXITING, "exiting"));

        return ((isc_threadresult_t)0);
}
#endif /* ISC_PLATFORM_USETHREADS */

static void
manager_free(isc_taskmgr_t *manager) {
        isc_mem_t *mctx;

#ifdef ISC_PLATFORM_USETHREADS
        (void)isc_condition_destroy(&manager->exclusive_granted);       
        (void)isc_condition_destroy(&manager->work_available);
        isc_mem_put(manager->mctx, manager->threads,
                    manager->workers * sizeof (isc_thread_t));
#endif /* ISC_PLATFORM_USETHREADS */
        DESTROYLOCK(&manager->lock);
        manager->magic = 0;
        mctx = manager->mctx;
        isc_mem_put(mctx, manager, sizeof *manager);
        isc_mem_detach(&mctx);
}

isc_result_t
isc_taskmgr_create(isc_mem_t *mctx, unsigned int workers,
                   unsigned int default_quantum, isc_taskmgr_t **managerp)
{
        isc_result_t result;
        unsigned int i, started = 0;
        isc_taskmgr_t *manager;

        /*
         * Create a new task manager.
         */

        REQUIRE(workers > 0);
        REQUIRE(managerp != NULL && *managerp == NULL);

#ifndef ISC_PLATFORM_USETHREADS
        UNUSED(i);
        UNUSED(started);
        UNUSED(workers);

        if (taskmgr != NULL) {
                taskmgr->refs++;
                *managerp = taskmgr;
                return (ISC_R_SUCCESS);
        }
#endif /* ISC_PLATFORM_USETHREADS */

        manager = isc_mem_get(mctx, sizeof *manager);
        if (manager == NULL)
                return (ISC_R_NOMEMORY);
        manager->magic = TASK_MANAGER_MAGIC;
        manager->mctx = NULL;
        manager->workers = 0;
        if (isc_mutex_init(&manager->lock) != ISC_R_SUCCESS) {
                UNEXPECTED_ERROR(__FILE__, __LINE__,
                                 "isc_mutex_init() %s",
                                 isc_msgcat_get(isc_msgcat, ISC_MSGSET_GENERAL,
                                                ISC_MSG_FAILED, "failed"));
                result = ISC_R_UNEXPECTED;
                goto cleanup_mgr;
        }
#ifdef ISC_PLATFORM_USETHREADS
        manager->threads = isc_mem_get(mctx, workers * sizeof (isc_thread_t));
        if (manager->threads == NULL) {
                result = ISC_R_NOMEMORY;
                goto cleanup_lock;
        }
        if (isc_condition_init(&manager->work_available) != ISC_R_SUCCESS) {
                UNEXPECTED_ERROR(__FILE__, __LINE__,
                                 "isc_condition_init() %s",
                                 isc_msgcat_get(isc_msgcat, ISC_MSGSET_GENERAL,
                                                ISC_MSG_FAILED, "failed"));
                result = ISC_R_UNEXPECTED;
                goto cleanup_threads;
        }
        if (isc_condition_init(&manager->exclusive_granted) != ISC_R_SUCCESS) {
                UNEXPECTED_ERROR(__FILE__, __LINE__,
                                 "isc_condition_init() %s",
                                 isc_msgcat_get(isc_msgcat, ISC_MSGSET_GENERAL,
                                                ISC_MSG_FAILED, "failed"));
                result = ISC_R_UNEXPECTED;
                goto cleanup_workavailable;
        }
#endif /* ISC_PLATFORM_USETHREADS */
        if (default_quantum == 0)
                default_quantum = DEFAULT_DEFAULT_QUANTUM;
        manager->default_quantum = default_quantum;
        INIT_LIST(manager->tasks);
        INIT_LIST(manager->ready_tasks);
        manager->tasks_running = 0;
        manager->exclusive_requested = ISC_FALSE;
        manager->exiting = ISC_FALSE;
        manager->workers = 0;

        isc_mem_attach(mctx, &manager->mctx);

#ifdef ISC_PLATFORM_USETHREADS
        LOCK(&manager->lock);
        /*
         * Start workers.
         */
        for (i = 0; i < workers; i++) {
                if (isc_thread_create(run, manager,
                                      &manager->threads[manager->workers]) ==
                    ISC_R_SUCCESS) {
                        manager->workers++;
                        started++;
                }
        }
        UNLOCK(&manager->lock);

        if (started == 0) {
                manager_free(manager);
                return (ISC_R_NOTHREADS);
        }
        isc_thread_setconcurrency(workers);
#else /* ISC_PLATFORM_USETHREADS */
        manager->refs = 1;
        taskmgr = manager;
#endif /* ISC_PLATFORM_USETHREADS */

        *managerp = manager;

        return (ISC_R_SUCCESS);

#ifdef ISC_PLATFORM_USETHREADS
 cleanup_workavailable:
        (void)isc_condition_destroy(&manager->work_available);
 cleanup_threads:
        isc_mem_put(mctx, manager->threads, workers * sizeof (isc_thread_t));
 cleanup_lock:
        DESTROYLOCK(&manager->lock);
#endif
 cleanup_mgr:
        isc_mem_put(mctx, manager, sizeof *manager);
        return (result);
}

void
isc_taskmgr_destroy(isc_taskmgr_t **managerp) {
        isc_taskmgr_t *manager;
        isc_task_t *task;
        unsigned int i;

        /*
         * Destroy '*managerp'.
         */

        REQUIRE(managerp != NULL);
        manager = *managerp;
        REQUIRE(VALID_MANAGER(manager));

#ifndef ISC_PLATFORM_USETHREADS
        UNUSED(i);

        if (manager->refs > 1) {
                manager->refs--;
                *managerp = NULL;
                return;
        }
#endif /* ISC_PLATFORM_USETHREADS */

        XTHREADTRACE("isc_taskmgr_destroy");
        /*
         * Only one non-worker thread may ever call this routine.
         * If a worker thread wants to initiate shutdown of the
         * task manager, it should ask some non-worker thread to call
         * isc_taskmgr_destroy(), e.g. by signalling a condition variable
         * that the startup thread is sleeping on.
         */

        /*
         * Unlike elsewhere, we're going to hold this lock a long time.
         * We need to do so, because otherwise the list of tasks could
         * change while we were traversing it.
         *
         * This is also the only function where we will hold both the
         * task manager lock and a task lock at the same time.
         */

        LOCK(&manager->lock);

        /*
         * Make sure we only get called once.
         */
        INSIST(!manager->exiting);
        manager->exiting = ISC_TRUE;

        /*
         * Post shutdown event(s) to every task (if they haven't already been
         * posted).
         */
        for (task = HEAD(manager->tasks);
             task != NULL;
             task = NEXT(task, link)) {
                LOCK(&task->lock);
                if (task_shutdown(task))
                        ENQUEUE(manager->ready_tasks, task, ready_link);
                UNLOCK(&task->lock);
        }
#ifdef ISC_PLATFORM_USETHREADS
        /*
         * Wake up any sleeping workers.  This ensures we get work done if
         * there's work left to do, and if there are already no tasks left
         * it will cause the workers to see manager->exiting.
         */
        BROADCAST(&manager->work_available);
        UNLOCK(&manager->lock);

        /*
         * Wait for all the worker threads to exit.
         */
        for (i = 0; i < manager->workers; i++)
                (void)isc_thread_join(manager->threads[i], NULL);
#else /* ISC_PLATFORM_USETHREADS */
        /*
         * Dispatch the shutdown events.
         */
        UNLOCK(&manager->lock);
        while (isc__taskmgr_ready())
                (void)isc__taskmgr_dispatch();
        INSIST(ISC_LIST_EMPTY(manager->tasks));
#endif /* ISC_PLATFORM_USETHREADS */

        manager_free(manager);

        *managerp = NULL;
}

#ifndef ISC_PLATFORM_USETHREADS
isc_boolean_t
isc__taskmgr_ready(void) {
        if (taskmgr == NULL)
                return (ISC_FALSE);
        return (ISC_TF(!ISC_LIST_EMPTY(taskmgr->ready_tasks)));
}

isc_result_t
isc__taskmgr_dispatch(void) {
        isc_taskmgr_t *manager = taskmgr;

        if (taskmgr == NULL)
                return (ISC_R_NOTFOUND);

        dispatch(manager);

        return (ISC_R_SUCCESS);
}

#endif /* ISC_PLATFORM_USETHREADS */

isc_result_t
isc_task_beginexclusive(isc_task_t *task) {
#ifdef ISC_PLATFORM_USETHREADS  
        isc_taskmgr_t *manager = task->manager;
        REQUIRE(task->state == task_state_running);
        LOCK(&manager->lock);
        if (manager->exclusive_requested) {
                UNLOCK(&manager->lock);                 
                return (ISC_R_LOCKBUSY);
        }
        manager->exclusive_requested = ISC_TRUE;
        while (manager->tasks_running > 1) {
                WAIT(&manager->exclusive_granted, &manager->lock);
        }
        UNLOCK(&manager->lock); 
#else
        UNUSED(task);
#endif
        return (ISC_R_SUCCESS);
}

void
isc_task_endexclusive(isc_task_t *task) {
#ifdef ISC_PLATFORM_USETHREADS  
        isc_taskmgr_t *manager = task->manager;
        REQUIRE(task->state == task_state_running);
        LOCK(&manager->lock);
        REQUIRE(manager->exclusive_requested);
        manager->exclusive_requested = ISC_FALSE;
        BROADCAST(&manager->work_available);
        UNLOCK(&manager->lock);
#else
        UNUSED(task);
#endif
}