lib/dns/resolver.c

	resolver.c revision f26747f494f2e02ffb72b022cc1c54abb58269f4

#include <config.h>

#include <isc/assertions.h>
#include <isc/result.h>
#include <isc/timer.h>
#include <isc/mutex.h>
#include <isc/event.h>
#include <isc/task.h>
#include <isc/stdtime.h>

#include <dns/types.h>
#include <dns/result.h>
#include <dns/name.h>
#include <dns/events.h>
#include <dns/message.h>
#include <dns/dispatch.h>
#include <dns/resolver.h>

#include "../isc/util.h"        /* XXX */

#define DNS_RESOLVER_TRACE
#ifdef DNS_RESOLVER_TRACE
#define RTRACE(m)   printf("res %p: %s\n", res, (m))
#define RRTRACE(r, m)   printf("res %p: %s\n", (r), (m))
#define FCTXTRACE(m)    printf("fctx %p: %s\n", fctx, (m))
#define FTRACE(m)   printf("fetch %p (res %p fctx %p): %s\n", \
                   fetch, fetch->res, fetch->private, (m))
#else
#define RTRACE(m)
#define RRTRACE(r, m)
#define FCTXTRACE(m)
#define FTRACE(m)
#endif

typedef struct query {
    dns_messageid_t         id;
    ISC_LINK(struct query)      link;
    isc_buffer_t            buffer;
    unsigned char           data[512];
} resquery_t;

typedef enum {
    fetchstate_init = 0,
    fetchstate_active,
    fetchstate_exiting,
    fetchstate_done
} fetchstate;

typedef struct fetchctx {
    unsigned int            magic;
    fetchstate          state;
    dns_resolver_t *        res;
    unsigned int            references;
    unsigned int            locknum;
    dns_name_t          name;
    dns_rdatatype_t         type;       /* multiple types??? */
    unsigned int            options;
    /* Locked by lock. */
    isc_timer_t *           timer;
    isc_time_t          expires;
    isc_interval_t          interval;
    ISC_LIST(dns_fetchdoneevent_t)  events;
    isc_event_t         start_event;
    dns_dispatch_t *        dispatcher;
    dns_message_t *         qmessage;
    dns_message_t *         rmessage;
    ISC_LIST(resquery_t)        queries;
    ISC_LINK(struct fetchctx)   link;
} fetchctx_t;

#define FCTX_MAGIC          0x46212121U /* F!!! */
#define VALID_FCTX(fctx)        ((fctx) != NULL && \
                     (fctx)->magic == FCTX_MAGIC)

struct dns_resolver {
    /* Unlocked */
    unsigned int            magic;
    isc_mem_t *         mctx;
    isc_mutex_t         lock;
    dns_rdataclass_t        rdclass;
    isc_timermgr_t *        timermgr;
    /* Locked by lock. */
    unsigned int            references;
    isc_boolean_t           exiting;
    dns_dispatch_t *        shared_dispatcher;
    unsigned int            ntasks;
    unsigned int            next_task;
    isc_task_t **           tasks;
    ISC_LIST(fetchctx_t)        fctxs;
};

#define RES_MAGIC           0x52657321U /* Res! */
#define VALID_RESOLVER(res)     ((res) != NULL && \
                     (res)->magic == RES_MAGIC)


static void destroy(dns_resolver_t *res);

/*
 * Internal fetch routines.  Caller must be holding the proper lock.
 */

static inline dns_result_t
fctx_starttimer(fetchctx_t *fctx) {
    isc_result_t iresult;

    iresult = isc_timer_reset(fctx->timer, isc_timertype_once,
                  &fctx->expires, &fctx->interval,
                  ISC_FALSE);
    if (iresult != ISC_R_SUCCESS) {
        UNEXPECTED_ERROR(__FILE__, __LINE__,
                 "isc_timer_reset(): %s",
                 isc_result_totext(iresult));
        return (DNS_R_UNEXPECTED);
    }

    return (DNS_R_SUCCESS);
}

static inline void
fctx_stoptimer(fetchctx_t *fctx) {
    isc_result_t iresult;

    /*
     * We don't return a result if resetting the timer to inactive fails
     * since there's nothing to be done about it.  Resetting to inactive
     * should never fail anyway, since the code as currently written
     * cannot fail in that case.
     */
    iresult = isc_timer_reset(fctx->timer, isc_timertype_inactive,
                  NULL, NULL, ISC_TRUE);
    if (iresult != ISC_R_SUCCESS) {
        UNEXPECTED_ERROR(__FILE__, __LINE__,
                 "isc_timer_reset(): %s",
                 isc_result_totext(iresult));
    }
}

static void
fctx_done(fetchctx_t *fctx, dns_result_t result) {
    dns_fetchdoneevent_t *event, *next_event;
    isc_task_t *task;
    isc_result_t iresult;

    /*
     * The caller must be holding the proper lock.
     */

    FCTXTRACE("done");

    REQUIRE(fctx->state == fetchstate_active);

    fctx_stoptimer(fctx);

    fctx->state = fetchstate_done;

    for (event = ISC_LIST_HEAD(fctx->events);
         event != NULL;
         event = next_event) {
        next_event = ISC_LIST_NEXT(event, link);
        task = event->sender;
        event->sender = fctx;
        event->result = result;
        iresult = isc_task_send(task, (isc_event_t **)&event);
        if (iresult != ISC_R_SUCCESS) {
            UNEXPECTED_ERROR(__FILE__, __LINE__,
                     "isc_task_send(): %s",
                     isc_result_totext(iresult));
            isc_event_free((isc_event_t **)&event);
        }
        isc_task_detach(&task);
    }
    ISC_LIST_INIT(fctx->events);
}

static dns_result_t
fctx_sendquery(fetchctx_t *fctx) {
    resquery_t *query;
    dns_result_t result;
    dns_rdataset_t qrdataset;

    FCTXTRACE("sendquery");

    result = fctx_starttimer(fctx);
    if (result != DNS_R_SUCCESS)
        return (result);

    dns_message_reset(fctx->rmessage);

    query = isc_mem_get(fctx->res->mctx, sizeof *query);
    if (query == NULL)
        return (DNS_R_NOMEMORY);
    isc_buffer_init(&query->buffer, query->data, sizeof query->data,
            ISC_BUFFERTYPE_BINARY);

    /*
     * XXXRTH get query id from dispatcher...
     */

    fctx->qmessage->opcode = dns_opcode_query;

    /*
     * Set up question.
     */
    dns_rdataset_init(&qrdataset);
    dns_rdataset_makequestion(&qrdataset, fctx->res->rdclass, fctx->type);
    ISC_LIST_INIT(fctx->name.list);
    ISC_LIST_APPEND(fctx->name.list, &qrdataset, link);
    dns_message_addname(fctx->qmessage, &fctx->name, DNS_SECTION_QUESTION);
    if ((fctx->options & DNS_FETCHOPT_RECURSIVE) != 0)
        fctx->qmessage->flags |= DNS_MESSAGEFLAG_RD;
    /*
     * We don't have to set opcode because it defaults to query.
     */

    /*
     * XXXRTH  Add TSIG and/or ENDS0 OPT record tailored to the current
     *         recipient.
     */

    result = dns_message_renderbegin(fctx->qmessage, &query->buffer);
    if (result != DNS_R_SUCCESS)
        goto done;
    result = dns_message_rendersection(fctx->qmessage,
                       DNS_SECTION_QUESTION, 0, 0);
    if (result != DNS_R_SUCCESS)
        goto done;
    result = dns_message_rendersection(fctx->qmessage,
                       DNS_SECTION_ADDITIONAL, 0, 0);
    if (result != DNS_R_SUCCESS)
        goto done;
    result = dns_message_rendersection(fctx->qmessage,
                       DNS_SECTION_TSIG, 0, 0);
    if (result != DNS_R_SUCCESS)
        goto done;
    result = dns_message_renderend(fctx->qmessage);
    if (result != DNS_R_SUCCESS)
        goto done;

    /* XXXRTH do the rest of the work... */

    /*
     * Finally, we've got everything going!
     */
    ISC_LIST_APPEND(fctx->queries, query, link);

    result = DNS_R_SUCCESS;

 done:
    /*
     * It's imperative that we reset the message here, because
     * the rdataset used for the question is on our stack, and won't
     * be valid after we return.
     */
    dns_message_reset(fctx->qmessage);

    if (result != DNS_R_SUCCESS)
        isc_mem_put(fctx->res->mctx, query, sizeof *query);

    return (result);
}

static void
fctx_cancelqueries(fetchctx_t *fctx) {
    resquery_t *query, *next_query;

    FCTXTRACE("cancelqueries");

    for (query = ISC_LIST_HEAD(fctx->queries);
         query != NULL;
         query = next_query) {
        next_query = ISC_LIST_NEXT(query, link);
        /* XXXRTH do the rest of the work... */
        isc_mem_put(fctx->res->mctx, query, sizeof *query);
    }
    ISC_LIST_INIT(fctx->queries);
}

static void
fctx_try(fetchctx_t *fctx) {
    dns_result_t result;

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

    REQUIRE(fctx->state == fetchstate_active);

    FCTXTRACE("try");

    /*
     * XXXRTH  Consult our try strategy routine here, figure out who to
     *         send a query (or queries) to next, and then do it.  If
     *     we've exhaused all our servers for this set of tries,
     *     start again by finding more addresses.
     */

    result = fctx_sendquery(fctx);
    if (result != DNS_R_SUCCESS)
        fctx_done(fctx, result);
}

static void
fctx_destroy(fetchctx_t *fctx) {
    REQUIRE(VALID_FCTX(fctx));
    REQUIRE(fctx->state == fetchstate_done);
    REQUIRE(ISC_LIST_EMPTY(fctx->events));
    REQUIRE(ISC_LIST_EMPTY(fctx->queries));

    FCTXTRACE("destroy");

    isc_timer_detach(&fctx->timer);
    dns_message_destroy(&fctx->rmessage);
    dns_message_destroy(&fctx->qmessage);
    dns_name_free(&fctx->name, fctx->res->mctx);
    isc_mem_put(fctx->res->mctx, fctx, sizeof *fctx);
}

/*
 * Fetch event handlers.
 */

static void
fctx_timeout(isc_task_t *task, isc_event_t *event) {
    fetchctx_t *fctx = event->arg;

    REQUIRE(VALID_FCTX(fctx));

    (void)task; /* Keep compiler quiet. */

    FCTXTRACE("timeout");

    LOCK(&fctx->res->lock);
    INSIST(fctx->state == fetchstate_active);

    if (event->type == ISC_TIMEREVENT_LIFE) {
        fctx_cancelqueries(fctx);
        fctx_done(fctx, DNS_R_TIMEDOUT);
    } else {
        /*
         * We could cancel the running queries here, or we could let
         * them keep going.  Right now we choose the latter...
         */
        fctx_try(fctx);
    }

    UNLOCK(&fctx->res->lock);

    isc_event_free(&event);
}

static void
fctx_start(isc_task_t *task, isc_event_t *event) {
    fetchctx_t *fctx = event->arg;
    isc_boolean_t need_fctx_destroy = ISC_FALSE;
    isc_boolean_t need_resolver_destroy = ISC_FALSE;
    dns_resolver_t *res;

    REQUIRE(VALID_FCTX(fctx));

    res = fctx->res;
    (void)task; /* Keep compiler quiet. */

    FCTXTRACE("start");

    LOCK(&res->lock);

    INSIST(fctx->state == fetchstate_init ||
           fctx->state == fetchstate_exiting);
    if (fctx->state == fetchstate_init) {
        fctx->state = fetchstate_active;
        fctx_try(fctx);
    } else {
        fctx->state = fetchstate_done;
        need_fctx_destroy = ISC_TRUE;
        if (res->exiting && ISC_LIST_EMPTY(res->fctxs))
            need_resolver_destroy = ISC_TRUE;
    }

    UNLOCK(&res->lock);

    if (need_fctx_destroy)
        fctx_destroy(fctx);
    if (need_resolver_destroy)
        destroy(res);
}

/*
 * Fetch Creation, Joining, and Cancelation.
 */

static inline dns_result_t
fctx_join(fetchctx_t *fctx, isc_task_t *task, isc_taskaction_t action,
       void *arg, dns_fetch_t *fetch)
{
    isc_task_t *clone;
    dns_fetchdoneevent_t *event;

    FCTXTRACE("join");

    /*
     * We store the task we're going to send this event to in the
     * sender field.  We'll make the fetch the sender when we actually
     * send the event.
     */
    clone = NULL;
    isc_task_attach(task, &clone);
    event = (dns_fetchdoneevent_t *)
        isc_event_allocate(fctx->res->mctx, clone,
                   DNS_EVENT_FETCHDONE,
                   action, arg, sizeof *event);
    if (event == NULL)
        return (DNS_R_NOMEMORY);
    event->result = DNS_R_SUCCESS;
    event->tag = fetch;
    /*
     * XXX other event initialization here.
     */
    ISC_LIST_APPEND(fctx->events, event, link);

    fctx->references++;

    fetch->magic = DNS_FETCH_MAGIC;
    fetch->res = fctx->res;
    fetch->private = fctx;
    ISC_LINK_INIT(fetch, link);

    return (DNS_R_SUCCESS);
}

static dns_result_t
fctx_create(dns_resolver_t *res, dns_name_t *name, dns_rdatatype_t type,
        unsigned int options,
        isc_task_t *worker, isc_task_t *task, isc_taskaction_t action,
        void *arg, dns_fetch_t *fetch)
{
    fetchctx_t *fctx;
    dns_result_t result = DNS_R_SUCCESS;
    isc_result_t iresult;
    isc_event_t *event;
    isc_interval_t interval;

    fctx = isc_mem_get(res->mctx, sizeof *fctx);
    if (fctx == NULL)
        return (DNS_R_NOMEMORY);
    FCTXTRACE("create");
    dns_name_init(&fctx->name, NULL);
    result = dns_name_dup(name, res->mctx, &fctx->name);
    if (result != DNS_R_SUCCESS)
        goto cleanup_fetch;
    fctx->type = type;
    fctx->options = options;
    fctx->res = res;
    fctx->references = 0;
    fctx->locknum = 0;
    fctx->state = fetchstate_init;
    fctx->dispatcher = NULL;        /* XXX */
    ISC_LIST_INIT(fctx->queries);

    fctx->qmessage = NULL;
    result = dns_message_create(res->mctx, &fctx->qmessage,
                    DNS_MESSAGE_INTENTRENDER);
    if (result != DNS_R_SUCCESS)
        goto cleanup_name;

    fctx->rmessage = NULL;
    result = dns_message_create(res->mctx, &fctx->rmessage,
                    DNS_MESSAGE_INTENTPARSE);
    if (result != DNS_R_SUCCESS)
        goto cleanup_qmessage;

    /*
     * Compute an expiration time for the entire fetch.
     */
    isc_interval_set(&interval, 10, 0);     /* XXXRTH constant */
    iresult = isc_time_nowplusinterval(&fctx->expires, &interval);
    if (iresult != ISC_R_SUCCESS) {
        UNEXPECTED_ERROR(__FILE__, __LINE__,
                 "isc_stdtime_get: %s",
                 isc_result_totext(iresult));
        result = DNS_R_UNEXPECTED;
        goto cleanup_rmessage;
    }

    /*
     * XXX Retry interval initialization.  Should be setup by the
     * transmission strategy routine (when we have one).
     */
    isc_interval_set(&fctx->interval, 1, 500000000);

    /*
     * Create an inactive timer.  It will be made active when the fetch
     * is actually started.
     */
    fctx->timer = NULL;
    iresult = isc_timer_create(res->timermgr, isc_timertype_inactive,
                   NULL, NULL,
                   worker, fctx_timeout,
                   fctx, &fctx->timer);
    if (iresult != ISC_R_SUCCESS) {
        UNEXPECTED_ERROR(__FILE__, __LINE__,
                 "isc_timer_create: %s",
                 isc_result_totext(iresult));
        result = DNS_R_UNEXPECTED;
        goto cleanup_rmessage;
    }

    ISC_LIST_INIT(fctx->events);
    result = fctx_join(fctx, task, action, arg, fetch);
    if (result != DNS_R_SUCCESS)
        goto cleanup_timer;

    ISC_LINK_INIT(fctx, link);
    fctx->magic = FCTX_MAGIC;

    /*
     * The fetch is now ready to go.  We send the start event to its
     * task to get the ball rolling.
     *
     * XXX we should really send this event from dns_resolver_fetch(),
     * after we've unlocked the fetch's lock, otherwise the other task
     * could well block on the lock we're about to release.
     */
    event = &fctx->start_event;
    ISC_EVENT_INIT(event, sizeof *event, 0, NULL, DNS_EVENT_FETCH,
               fctx_start, fctx, (void *)fctx_create, NULL, NULL);
    iresult = isc_task_send(worker, &event);
    if (iresult != ISC_R_SUCCESS) {
        UNEXPECTED_ERROR(__FILE__, __LINE__,
                 "isc_task_send: %s",
                 isc_result_totext(iresult));
        result = DNS_R_UNEXPECTED;
        goto cleanup_timer;
    }

    return (DNS_R_SUCCESS);

 cleanup_timer:
    isc_timer_detach(&fctx->timer);

 cleanup_rmessage:
    dns_message_destroy(&fctx->rmessage);

 cleanup_qmessage:
    dns_message_destroy(&fctx->qmessage);

 cleanup_name:
    dns_name_free(&fctx->name, res->mctx);

 cleanup_fetch:
    isc_mem_put(res->mctx, fctx, sizeof *fctx);

    return (result);
}

static void
fctx_cancel(fetchctx_t *fctx) {
    isc_result_t iresult;

    FCTXTRACE("cancel");

    if (fctx->state != fetchstate_done) {
        fctx_cancelqueries(fctx);

        iresult = isc_timer_reset(fctx->timer, isc_timertype_inactive,
                      NULL, NULL, ISC_TRUE);
        if (iresult != ISC_R_SUCCESS)
            UNEXPECTED_ERROR(__FILE__, __LINE__,
                     "isc_timer_reset(): %s",
                     isc_result_totext(iresult));

        fctx_done(fctx, DNS_R_CANCELED);
    }
}


/*
 * Handle Responses
 */

static inline dns_result_t
same_question(fetchctx_t *fctx) {
    dns_result_t result;
    dns_message_t *message = fctx->rmessage;
    dns_name_t *name;
    dns_rdataset_t *rdataset;

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

    /*
     * XXXRTH  Currently we support only one question.
     */
    if (message->counts[DNS_SECTION_QUESTION] != 1)
        return (DNS_R_FORMERR);

    result = dns_message_firstname(message, DNS_SECTION_QUESTION);
    if (result != DNS_R_SUCCESS)
        return (result);
    name = NULL;
    dns_message_currentname(message, DNS_SECTION_QUESTION, &name);
    rdataset = ISC_LIST_HEAD(name->list);
    INSIST(rdataset != NULL);
    INSIST(ISC_LIST_NEXT(rdataset, link) == NULL);
    if (fctx->type != rdataset->type ||
        fctx->res->rdclass != rdataset->rdclass ||
        !dns_name_equal(&fctx->name, name))
        return (DNS_R_FORMERR);

    return (DNS_R_SUCCESS);
}

static void
fctx_response(isc_task_t *task, isc_event_t *event) {
    dns_result_t result;
    fetchctx_t *fctx = event->arg;
    dns_dispatchevent_t *devent = (dns_dispatchevent_t *)event;
    isc_boolean_t bad_sender = ISC_FALSE;
    dns_message_t *message;

    REQUIRE(VALID_FCTX(fctx));
    REQUIRE(event->type == DNS_EVENT_DISPATCH);

    (void)task;

    LOCK(&fctx->res->lock);
    INSIST(fctx->state == fetchstate_active);

    message = fctx->rmessage;
    result = dns_message_parse(message, &devent->buffer);
    if (result != DNS_R_SUCCESS) {
        switch (result) {
        case DNS_R_FORMERR:
        case DNS_R_UNEXPECTEDEND:
            bad_sender = ISC_TRUE;
            break;
        case DNS_R_MOREDATA:
            result = DNS_R_NOTIMPLEMENTED;
            break;
        }
        goto done;
    }

    INSIST((message->flags & DNS_MESSAGEFLAG_QR) != 0);
    /*
     * INSIST() that the message comes from the place we sent it to,
     * since the dispatch code should ensure this.
     *
     * INSIST() that the message id is correct (this should also be
     * ensured by the dispatch code).
     */

    /*
     * Is it a query response?
     */
    if (message->opcode != dns_opcode_query) {
        /* XXXRTH Log */
        bad_sender = ISC_TRUE;
        goto done;
    }

    /*
     * Is the remote server broken, or does it dislike us?
     */
    if (message->rcode != dns_rcode_noerror &&
        message->rcode != dns_rcode_nxdomain) {
        bad_sender = ISC_TRUE;
        /*
         * XXXRTH If we want to catch a FORMERR caused by an EDNS0
         *        OPT RR, this is the place to do it.
         */
        goto done;
    }

    /*
     * Does the it answer the question we asked?
     */
    result = same_question(fctx);
    if (result != DNS_R_SUCCESS) {
        /* XXXRTH Log */
        if (result == DNS_R_FORMERR)
            bad_sender = ISC_TRUE;
        goto done;
    }

    result = DNS_R_SUCCESS;

 done:
    /*
     * XXXRTH  Record round-trip statistics here.
     */
    if (bad_sender) {
        /*
         * XXXRTH  We will mark the sender as bad here instead
         *         of doing the printf().
         */
        printf("bad sender\n");
        /*
         * Keep trying.
         */
        fctx_try(fctx);
    } else {
        /*
         * All is well, or we got an error fatal to the fetch.
         * In either case, we're done.
         */
        fctx_done(fctx, result);
    }

    UNLOCK(&fctx->res->lock);

    isc_event_free(&event);
}


/***
 *** Resolver Methods
 ***/

static void
destroy(dns_resolver_t *res) {
    unsigned int i;

    REQUIRE(res->exiting);
    REQUIRE(res->references == 0);
    REQUIRE(ISC_LIST_EMPTY(res->fctxs));

    RTRACE("destroy");

    isc_mutex_destroy(&res->lock);
    for (i = 0; i < res->ntasks; i++) {
        isc_task_shutdown(res->tasks[i]);
        isc_task_detach(&res->tasks[i]);
    }
    isc_mem_put(res->mctx, res->tasks,
            res->ntasks * sizeof (isc_task_t *));
    res->magic = 0;
    isc_mem_put(res->mctx, res, sizeof *res);
}

dns_result_t
dns_resolver_create(isc_mem_t *mctx,
            isc_taskmgr_t *taskmgr, unsigned int ntasks,
            isc_timermgr_t *timermgr,
            dns_rdataclass_t rdclass,
            dns_dispatch_t *dispatcher,
            dns_resolver_t **resp)
{
    dns_resolver_t *res;
    dns_result_t result = DNS_R_SUCCESS;
    isc_result_t iresult;
    unsigned int i, tasks_created = 0;

    REQUIRE(resp != NULL && *resp == NULL);
    REQUIRE(ntasks > 0);

    res = isc_mem_get(mctx, sizeof *res);
    if (res == NULL)
        return (DNS_R_NOMEMORY);
    RTRACE("create");
    res->mctx = mctx;
    res->rdclass = rdclass;
    res->timermgr = timermgr;
    res->ntasks = ntasks;
    res->next_task = 0;
    res->shared_dispatcher = dispatcher;        /* XXXRTH: attach! */
    res->tasks = isc_mem_get(mctx, ntasks * sizeof (isc_task_t *));
    if (res->tasks == NULL) {
        result = DNS_R_NOMEMORY;
        goto cleanup_res;
    }
    for (i = 0; i < ntasks; i++) {
        res->tasks[i] = NULL;
        iresult = isc_task_create(taskmgr, mctx, 0, &res->tasks[i]);
        if (iresult != ISC_R_SUCCESS) {
            UNEXPECTED_ERROR(__FILE__, __LINE__,
                     "isc_task_create() failed: %s",
                     isc_result_totext(iresult));
            result = DNS_R_UNEXPECTED;
            goto cleanup_tasks;
        }
        tasks_created++;
    }

    res->references = 1;
    res->exiting = ISC_FALSE;

    iresult = isc_mutex_init(&res->lock);
    if (iresult != ISC_R_SUCCESS) {
        UNEXPECTED_ERROR(__FILE__, __LINE__,
                 "isc_mutex_init() failed: %s",
                 isc_result_totext(iresult));
        result = DNS_R_UNEXPECTED;
        goto cleanup_tasks;
    }

    ISC_LIST_INIT(res->fctxs);

    res->magic = RES_MAGIC;

    *resp = res;

    return (DNS_R_SUCCESS);

 cleanup_tasks:
    for (i = 0; i < tasks_created; i++) {
        isc_task_shutdown(res->tasks[i]);
        isc_task_detach(&res->tasks[i]);
    }
    isc_mem_put(mctx, res->tasks, res->ntasks * sizeof (isc_task_t *));

 cleanup_res:
    isc_mem_put(mctx, res, sizeof *res);

    return (result);
}

void
dns_resolver_attach(dns_resolver_t *source, dns_resolver_t **targetp) {
    REQUIRE(VALID_RESOLVER(source));
    REQUIRE(targetp != NULL && *targetp == NULL);

    RRTRACE(source, "attach");
    LOCK(&source->lock);
    REQUIRE(!source->exiting);
    INSIST(source->references > 0);
    source->references++;
    INSIST(source->references != 0);
    UNLOCK(&source->lock);

    *targetp = source;
}

void
dns_resolver_detach(dns_resolver_t **resp) {
    dns_resolver_t *res;
    isc_boolean_t need_destroy = ISC_FALSE;
    fetchctx_t *fctx;

    REQUIRE(resp != NULL);
    res = *resp;
    REQUIRE(VALID_RESOLVER(res));

    RTRACE("detach");
    LOCK(&res->lock);
    INSIST(res->references > 0);
    res->references--;
    if (res->references == 0) {
        RTRACE("exiting");
        res->exiting = ISC_TRUE;
        for (fctx = ISC_LIST_HEAD(res->fctxs);
             fctx != NULL;
             fctx = ISC_LIST_NEXT(fctx, link))
            fctx_cancel(fctx);
        if (ISC_LIST_EMPTY(res->fctxs))
            need_destroy = ISC_TRUE;
    }
    UNLOCK(&res->lock);

    if (need_destroy)
        destroy(res);

    *resp = NULL;
}

static inline isc_boolean_t
fctx_match(fetchctx_t *fctx, dns_name_t *name, dns_rdatatype_t type,
       unsigned int options)
{
    if (fctx->type != type || fctx->options != options)
        return (ISC_FALSE);
    return (dns_name_equal(&fctx->name, name));
}

dns_result_t
dns_resolver_createfetch(dns_resolver_t *res, dns_name_t *name,
             dns_rdatatype_t type,
             dns_delegation_t *delegation,
             dns_forwarders_t *forwarders,
             unsigned int options, isc_task_t *task,
             isc_taskaction_t action, void *arg,
             dns_fetch_t **fetchp)
{
    dns_fetch_t *fetch;
    fetchctx_t *fctx = NULL;
    dns_result_t result;
    isc_task_t *worker;

    (void)delegation;
    (void)forwarders;

    REQUIRE(VALID_RESOLVER(res));
    REQUIRE(fetchp != NULL && *fetchp == NULL);
    /*
     * We require !res->exiting, since if it were exiting, that would
     * mean that the reference count was wrong!
     */
    REQUIRE(!res->exiting);

    RTRACE("createfetch");

    /* XXXRTH */
    if ((options & DNS_FETCHOPT_TCP) != 0)
        return (DNS_R_NOTIMPLEMENTED);

    fetch = isc_mem_get(res->mctx, sizeof *fetch);
    if (fetch == NULL)
        return (DNS_R_NOMEMORY);

    LOCK(&res->lock);

    /*
     * XXXRTH This is for correctness, and doesn't represent the final
     * way of assigning tasks, or the final form of the fetch table.
     */

    worker = res->tasks[res->next_task];
    res->next_task++;
    if (res->next_task == res->ntasks)
        res->next_task = 0;

    if ((options & DNS_FETCHOPT_UNSHARED) == 0) {
        for (fctx = ISC_LIST_HEAD(res->fctxs);
             fctx != NULL;
             fctx = ISC_LIST_NEXT(fctx, link)) {
            if (fctx_match(fctx, name, type, options))
                break;
        }
    }

    if (fctx == NULL || fctx->state == fetchstate_done) {
        result = fctx_create(res,
                     name, type, options,
                     worker,
                     task, action, arg,
                     fetch);
        if (result == DNS_R_SUCCESS) {
            fctx = fetch->private;
            ISC_LIST_APPEND(res->fctxs, fctx, link);
        }
    } else
        result = fctx_join(fctx, task, action, arg, fetch);

    UNLOCK(&res->lock);

    if (result == DNS_R_SUCCESS) {
        FTRACE("created");
        *fetchp = fetch;
    } else
        isc_mem_put(res->mctx, fetch, sizeof *fetch);

    return (result);
}

void
dns_resolver_destroyfetch(dns_fetch_t **fetchp, isc_task_t *task) {
    dns_fetch_t *fetch;
    dns_fetchdoneevent_t *event, *next_event;
    fetchctx_t *fctx;
    dns_resolver_t *res;
    isc_boolean_t need_fctx_destroy = ISC_FALSE;
    isc_boolean_t need_resolver_destroy = ISC_FALSE;
    isc_task_t *etask;

    /*
     * XXXRTH  We could make it so that even if all the clients detach
     * from the fetch, the fctx keeps going.  Perhaps this should be
     * a resolver option?  Right now if they all go away the fctx will
     * be destroyed too.
     */

    REQUIRE(fetchp != NULL);
    fetch = *fetchp;
    REQUIRE(DNS_FETCH_VALID(fetch));
    res = fetch->res;
    fctx = fetch->private;

    FTRACE("destroyfetch");

    LOCK(&res->lock);

    event = NULL;
    if (fctx->state != fetchstate_done) {
        for (event = ISC_LIST_HEAD(fctx->events);
             event != NULL;
             event = next_event) {
            next_event = ISC_LIST_NEXT(event, link);
            if (event->tag == fetch) {
                ISC_LIST_UNLINK(fctx->events, event, link);
                FTRACE("found");
                break;
            }
        }
    } else if (task != NULL)
        (void)isc_task_purge(task, fctx, DNS_EVENT_FETCHDONE, fetch);

    INSIST(fctx->references > 0);
    fctx->references--;
    if (fctx->references == 0) {
        INSIST(ISC_LIST_EMPTY(fctx->events));
        ISC_LIST_UNLINK(res->fctxs, fctx, link);
        if (fctx->state == fetchstate_init) {
            /*
             * The fctx is still initializing, which means that
             * the start event either hasn't been delivered, or
             * is being processed right now, but is blocked waiting
             * for the lock.
             *
             * Rather than try to purge the event, we simply
             * wait for it to happen, deferring further destruction
             * until it has been processed.
             */
            fctx->state = fetchstate_exiting;
        } else {
            if (fctx->state != fetchstate_done)
                fctx_cancel(fctx);
            need_fctx_destroy = ISC_TRUE;
            if (res->exiting && ISC_LIST_EMPTY(res->fctxs))
                need_resolver_destroy = ISC_TRUE;
        }
    }

    UNLOCK(&res->lock);

    isc_mem_put(res->mctx, fetch, sizeof *fetch);
    *fetchp = NULL;

    if (event != NULL) {
        etask = event->sender;
        isc_task_detach(&etask);
        isc_event_free((isc_event_t **)&event);
    }
    if (need_fctx_destroy)
        fctx_destroy(fctx);
    if (need_resolver_destroy)
        destroy(res);
}