transmission/libtransmission/announcer-udp.cc

// This file Copyright © 2010-2022 Mnemosyne LLC.
// It may be used under GPLv2 (SPDX: GPL-2.0-only), GPLv3 (SPDX: GPL-3.0-only),
// or any future license endorsed by Mnemosyne LLC.
// License text can be found in the licenses/ folder.

#include <cerrno> /* errno, EAFNOSUPPORT */
#include <cstring> /* memset() */
#include <ctime>
#include <list>
#include <set>
#include <string_view>
#include <vector>

#include <event2/buffer.h>
#include <event2/dns.h>
#include <event2/util.h>

#define LIBTRANSMISSION_ANNOUNCER_MODULE

#include "transmission.h"
#include "announcer.h"
#include "announcer-common.h"
#include "crypto-utils.h" /* tr_rand_buffer() */
#include "log.h"
#include "peer-io.h"
#include "peer-mgr.h" /* tr_peerMgrCompactToPex() */
#include "session.h"
#include "tr-assert.h"
#include "tr-udp.h"
#include "utils.h"

#define dbgmsg(key, ...) tr_logAddDeepNamed(key.c_str(), __VA_ARGS__)

using namespace std::literals;

/****
*****
****/

static void tau_sockaddr_setport(struct sockaddr* sa, tr_port port)
{
    if (sa->sa_family == AF_INET)
    {
        TR_DISCARD_ALIGN(sa, struct sockaddr_in*)->sin_port = htons(port);
    }
    else if (sa->sa_family == AF_INET6)
    {
        TR_DISCARD_ALIGN(sa, struct sockaddr_in6*)->sin6_port = htons(port);
    }
}

static int tau_sendto(tr_session const* session, struct evutil_addrinfo* ai, tr_port port, void const* buf, size_t buflen)
{
    auto sockfd = tr_socket_t{};

    if (ai->ai_addr->sa_family == AF_INET)
    {
        sockfd = session->udp_socket;
    }
    else if (ai->ai_addr->sa_family == AF_INET6)
    {
        sockfd = session->udp6_socket;
    }
    else
    {
        sockfd = TR_BAD_SOCKET;
    }

    if (sockfd == TR_BAD_SOCKET)
    {
        errno = EAFNOSUPPORT;
        return -1;
    }

    tau_sockaddr_setport(ai->ai_addr, port);
    return sendto(sockfd, static_cast<char const*>(buf), buflen, 0, ai->ai_addr, ai->ai_addrlen);
}

/****
*****
****/

static uint32_t evbuffer_read_ntoh_32(struct evbuffer* buf)
{
    auto val = uint32_t{};
    evbuffer_remove(buf, &val, sizeof(uint32_t));
    return ntohl(val);
}

static uint64_t evbuffer_read_ntoh_64(struct evbuffer* buf)
{
    auto val = uint64_t{};
    evbuffer_remove(buf, &val, sizeof(uint64_t));
    return tr_ntohll(val);
}

/****
*****
****/

using tau_connection_t = uint64_t;

static auto constexpr TauConnectionTtlSecs = int{ 60 };

using tau_transaction_t = uint32_t;

static tau_transaction_t tau_transaction_new()
{
    auto tmp = tau_transaction_t{};
    tr_rand_buffer(&tmp, sizeof(tau_transaction_t));
    return tmp;
}

/* used in the "action" field of a request */
enum tau_action_t
{
    TAU_ACTION_CONNECT = 0,
    TAU_ACTION_ANNOUNCE = 1,
    TAU_ACTION_SCRAPE = 2,
    TAU_ACTION_ERROR = 3
};

static bool is_tau_response_message(tau_action_t action, size_t msglen)
{
    if (action == TAU_ACTION_CONNECT)
    {
        return msglen == 16;
    }

    if (action == TAU_ACTION_ANNOUNCE)
    {
        return msglen >= 20;
    }

    if (action == TAU_ACTION_SCRAPE)
    {
        return msglen >= 20;
    }

    if (action == TAU_ACTION_ERROR)
    {
        return msglen >= 8;
    }

    return false;
}

static auto constexpr TauRequestTtl = int{ 60 };

/****
*****
*****  SCRAPE
*****
****/

struct tau_scrape_request
{
    void requestFinished()
    {
        if (callback != nullptr)
        {
            callback(&response, user_data);
        }
    }

    void fail(bool did_connect, bool did_timeout, std::string_view errmsg)
    {
        response.did_connect = did_connect;
        response.did_timeout = did_timeout;
        response.errmsg = errmsg;
        requestFinished();
    }

    void onResponse(tau_action_t action, evbuffer* buf)
    {
        response.did_connect = true;
        response.did_timeout = false;

        if (action == TAU_ACTION_SCRAPE)
        {
            for (int i = 0; i < response.row_count; ++i)
            {
                if (evbuffer_get_length(buf) < sizeof(uint32_t) * 3)
                {
                    break;
                }

                auto& row = response.rows[i];
                row.seeders = evbuffer_read_ntoh_32(buf);
                row.downloads = evbuffer_read_ntoh_32(buf);
                row.leechers = evbuffer_read_ntoh_32(buf);
            }

            requestFinished();
        }
        else
        {
            size_t const buflen = evbuffer_get_length(buf);
            auto const errmsg = action == TAU_ACTION_ERROR && buflen > 0 ?
                std::string_view{ reinterpret_cast<char const*>(evbuffer_pullup(buf, -1)), buflen } :
                _("Unknown error");
            fail(true, false, errmsg);
        }
    }

    std::vector<uint8_t> payload;

    time_t sent_at;
    time_t created_at;
    tau_transaction_t transaction_id;

    tr_scrape_response response;
    tr_scrape_response_func callback;
    void* user_data;
};

static tau_scrape_request make_tau_scrape_request(
    tr_scrape_request const* in,
    tr_scrape_response_func callback,
    void* user_data)
{
    tau_transaction_t const transaction_id = tau_transaction_new();

    /* build the payload */
    auto* buf = evbuffer_new();
    evbuffer_add_hton_32(buf, TAU_ACTION_SCRAPE);
    evbuffer_add_hton_32(buf, transaction_id);
    for (int i = 0; i < in->info_hash_count; ++i)
    {
        evbuffer_add(buf, std::data(in->info_hash[i]), std::size(in->info_hash[i]));
    }
    auto const* const payload_begin = evbuffer_pullup(buf, -1);
    auto const* const payload_end = payload_begin + evbuffer_get_length(buf);

    /* build the tau_scrape_request */

    auto req = tau_scrape_request{};
    req.callback = callback;
    req.created_at = tr_time();
    req.transaction_id = transaction_id;
    req.callback = callback;
    req.user_data = user_data;
    req.response.scrape_url = in->scrape_url;
    req.response.row_count = in->info_hash_count;
    req.payload.assign(payload_begin, payload_end);

    for (int i = 0; i < req.response.row_count; ++i)
    {
        req.response.rows[i].seeders = -1;
        req.response.rows[i].leechers = -1;
        req.response.rows[i].downloads = -1;
        req.response.rows[i].info_hash = in->info_hash[i];
    }

    /* cleanup */
    evbuffer_free(buf);
    return req;
}

/****
*****
*****  ANNOUNCE
*****
****/

struct tau_announce_request
{
    void requestFinished()
    {
        if (this->callback != nullptr)
        {
            this->callback(&this->response, this->user_data);
        }
    }

    void fail(bool did_connect, bool did_timeout, std::string_view errmsg)
    {
        this->response.did_connect = did_connect;
        this->response.did_timeout = did_timeout;
        this->response.errmsg = errmsg;
        this->requestFinished();
    }

    void onResponse(tau_action_t action, struct evbuffer* buf)
    {
        size_t const buflen = evbuffer_get_length(buf);

        this->response.did_connect = true;
        this->response.did_timeout = false;

        if (action == TAU_ACTION_ANNOUNCE && buflen >= 3 * sizeof(uint32_t))
        {
            response.interval = evbuffer_read_ntoh_32(buf);
            response.leechers = evbuffer_read_ntoh_32(buf);
            response.seeders = evbuffer_read_ntoh_32(buf);
            response.pex = tr_peerMgrCompactToPex(evbuffer_pullup(buf, -1), evbuffer_get_length(buf), nullptr, 0);
            requestFinished();
        }
        else
        {
            auto const errmsg = action == TAU_ACTION_ERROR && buflen > 0 ?
                std::string_view{ reinterpret_cast<char const*>(evbuffer_pullup(buf, -1)), buflen } :
                _("Unknown error");
            fail(true, false, errmsg);
        }
    }

    std::vector<uint8_t> payload;

    time_t created_at = 0;
    time_t sent_at = 0;
    tau_transaction_t transaction_id = 0;

    tr_announce_response response = {};

    tr_announce_response_func callback = nullptr;
    void* user_data = nullptr;
};

enum tau_announce_event
{
    /* used in the "event" field of an announce request */
    TAU_ANNOUNCE_EVENT_NONE = 0,
    TAU_ANNOUNCE_EVENT_COMPLETED = 1,
    TAU_ANNOUNCE_EVENT_STARTED = 2,
    TAU_ANNOUNCE_EVENT_STOPPED = 3
};

static tau_announce_event get_tau_announce_event(tr_announce_event e)
{
    switch (e)
    {
    case TR_ANNOUNCE_EVENT_COMPLETED:
        return TAU_ANNOUNCE_EVENT_COMPLETED;

    case TR_ANNOUNCE_EVENT_STARTED:
        return TAU_ANNOUNCE_EVENT_STARTED;

    case TR_ANNOUNCE_EVENT_STOPPED:
        return TAU_ANNOUNCE_EVENT_STOPPED;

    default:
        return TAU_ANNOUNCE_EVENT_NONE;
    }
}

static tau_announce_request make_tau_announce_request(
    tr_announce_request const* in,
    tr_announce_response_func callback,
    void* user_data)
{
    tau_transaction_t const transaction_id = tau_transaction_new();

    /* build the payload */
    auto* buf = evbuffer_new();
    evbuffer_add_hton_32(buf, TAU_ACTION_ANNOUNCE);
    evbuffer_add_hton_32(buf, transaction_id);
    evbuffer_add(buf, std::data(in->info_hash), std::size(in->info_hash));
    evbuffer_add(buf, std::data(in->peer_id), std::size(in->peer_id));
    evbuffer_add_hton_64(buf, in->down);
    evbuffer_add_hton_64(buf, in->leftUntilComplete);
    evbuffer_add_hton_64(buf, in->up);
    evbuffer_add_hton_32(buf, get_tau_announce_event(in->event));
    evbuffer_add_hton_32(buf, 0);
    evbuffer_add_hton_32(buf, in->key);
    evbuffer_add_hton_32(buf, in->numwant);
    evbuffer_add_hton_16(buf, in->port);
    auto const* const payload_begin = evbuffer_pullup(buf, -1);
    auto const* const payload_end = payload_begin + evbuffer_get_length(buf);

    /* build the tau_announce_request */
    auto req = tau_announce_request();
    req.created_at = tr_time();
    req.transaction_id = transaction_id;
    req.callback = callback;
    req.user_data = user_data;
    req.payload.assign(payload_begin, payload_end);
    req.response.seeders = -1;
    req.response.leechers = -1;
    req.response.downloads = -1;
    req.response.info_hash = in->info_hash;

    evbuffer_free(buf);
    return req;
}

/****
*****
*****  TRACKERS
*****
****/

struct tau_tracker
{
    [[nodiscard]] auto isIdle() const
    {
        return std::empty(announces) && std::empty(scrapes) && dns_request == nullptr;
    }

    ~tau_tracker()
    {
        if (this->addr != nullptr)
        {
            evutil_freeaddrinfo(this->addr);
        }
    }

    void failAll(bool did_connect, bool did_timeout, std::string_view errmsg)
    {
        for (auto& req : this->scrapes)
        {
            req.fail(did_connect, did_timeout, errmsg);
        }

        for (auto& req : this->announces)
        {
            req.fail(did_connect, did_timeout, errmsg);
        }

        this->scrapes.clear();
        this->announces.clear();
    }

    tr_session* const session;

    tr_interned_string const key;
    tr_interned_string const host;
    int const port;

    evdns_getaddrinfo_request* dns_request = nullptr;
    evutil_addrinfo* addr = nullptr;
    time_t addr_expiration_time = 0;

    time_t connecting_at = 0;
    time_t connection_expiration_time = 0;
    tau_connection_t connection_id = 0;
    tau_transaction_t connection_transaction_id = 0;

    time_t close_at = 0;

    std::list<tau_announce_request> announces;
    std::list<tau_scrape_request> scrapes;

    tau_tracker(tr_session* session_in, tr_interned_string key_in, tr_interned_string host_in, int port_in)
        : session{ session_in }
        , key{ key_in }
        , host{ host_in }
        , port{ port_in }
    {
    }
};

static void tau_tracker_upkeep(struct tau_tracker* /*tracker*/);

static void tau_tracker_on_dns(int errcode, struct evutil_addrinfo* addr, void* vtracker)
{
    auto* tracker = static_cast<struct tau_tracker*>(vtracker);

    tracker->dns_request = nullptr;
    tracker->addr_expiration_time = tr_time() + 60 * 60; /* one hour */

    if (errcode != 0)
    {
        auto const errmsg = tr_strvJoin("DNS Lookup failed: "sv, evutil_gai_strerror(errcode));
        dbgmsg(tracker->key, "%s", errmsg.c_str());
        tracker->failAll(false, false, errmsg.c_str());
    }
    else
    {
        dbgmsg(tracker->key, "DNS lookup succeeded");
        tracker->addr = addr;
        tau_tracker_upkeep(tracker);
    }
}

static void tau_tracker_send_request(struct tau_tracker* tracker, void const* payload, size_t payload_len)
{
    struct evbuffer* buf = evbuffer_new();
    dbgmsg(tracker->key, "sending request w/connection id %" PRIu64 "\n", tracker->connection_id);
    evbuffer_add_hton_64(buf, tracker->connection_id);
    evbuffer_add_reference(buf, payload, payload_len, nullptr, nullptr);
    (void)tau_sendto(tracker->session, tracker->addr, tracker->port, evbuffer_pullup(buf, -1), evbuffer_get_length(buf));
    evbuffer_free(buf);
}

template<typename T>
static void tau_tracker_send_requests(tau_tracker* tracker, std::list<T>& reqs)
{
    auto const now = tr_time();

    for (auto it = std::begin(reqs); it != std::end(reqs);)
    {
        auto& req = *it;

        if (req.sent_at != 0) // it's already been sent; we're awaiting a response
        {
            ++it;
            continue;
        }

        dbgmsg(tracker->key, "sending req %p", (void*)&req);
        req.sent_at = now;
        tau_tracker_send_request(tracker, std::data(req.payload), std::size(req.payload));

        if (req.callback != nullptr)
        {
            ++it;
            continue;
        }

        // no response needed, so we can remove it now
        it = reqs.erase(it);
    }
}

static void tau_tracker_send_reqs(tau_tracker* tracker)
{
    TR_ASSERT(tracker->dns_request == nullptr);
    TR_ASSERT(tracker->connecting_at == 0);
    TR_ASSERT(tracker->addr != nullptr);
    TR_ASSERT(tracker->connection_expiration_time > tr_time());

    tau_tracker_send_requests(tracker, tracker->announces);
    tau_tracker_send_requests(tracker, tracker->scrapes);
}

static void on_tracker_connection_response(struct tau_tracker* tracker, tau_action_t action, struct evbuffer* buf)
{
    time_t const now = tr_time();

    tracker->connecting_at = 0;
    tracker->connection_transaction_id = 0;

    if (action == TAU_ACTION_CONNECT)
    {
        tracker->connection_id = evbuffer_read_ntoh_64(buf);
        tracker->connection_expiration_time = now + TauConnectionTtlSecs;
        dbgmsg(tracker->key, "Got a new connection ID from tracker: %" PRIu64, tracker->connection_id);
    }
    else
    {
        size_t const buflen = buf != nullptr ? evbuffer_get_length(buf) : 0;

        auto const errmsg = action == TAU_ACTION_ERROR && buflen > 0 ?
            std::string_view{ reinterpret_cast<char const*>(evbuffer_pullup(buf, -1)), buflen } :
            std::string_view{ _("Connection failed") };

        dbgmsg(tracker->key, "%" TR_PRIsv, TR_PRIsv_ARG(errmsg));
        tracker->failAll(true, false, errmsg);
    }

    tau_tracker_upkeep(tracker);
}

static void tau_tracker_timeout_reqs(struct tau_tracker* tracker)
{
    time_t const now = time(nullptr);
    bool const cancel_all = tracker->close_at != 0 && (tracker->close_at <= now);

    if (tracker->connecting_at != 0 && tracker->connecting_at + TauRequestTtl < now)
    {
        on_tracker_connection_response(tracker, TAU_ACTION_ERROR, nullptr);
    }

    if (auto& reqs = tracker->announces; !std::empty(reqs))
    {
        for (auto it = std::begin(reqs); it != std::end(reqs);)
        {
            auto& req = *it;
            if (cancel_all || req.created_at + TauRequestTtl < now)
            {
                dbgmsg(tracker->key, "timeout announce req %p", (void*)&req);
                req.fail(false, true, "");
                it = reqs.erase(it);
            }
            else
            {
                ++it;
            }
        }
    }

    if (auto& reqs = tracker->scrapes; !std::empty(reqs))
    {
        for (auto it = std::begin(reqs); it != std::end(reqs);)
        {
            auto& req = *it;
            if (cancel_all || req.created_at + TauRequestTtl < now)
            {
                dbgmsg(tracker->key, "timeout scrape req %p", &req);
                req.fail(false, true, "");
                it = reqs.erase(it);
            }
            else
            {
                ++it;
            }
        }
    }
}

static void tau_tracker_upkeep_ex(struct tau_tracker* tracker, bool timeout_reqs)
{
    time_t const now = tr_time();
    bool const closing = tracker->close_at != 0;

    /* if the address info is too old, expire it */
    if (tracker->addr != nullptr && (closing || tracker->addr_expiration_time <= now))
    {
        dbgmsg(tracker->host, "Expiring old DNS result");
        evutil_freeaddrinfo(tracker->addr);
        tracker->addr = nullptr;
        tracker->addr_expiration_time = 0;
    }

    /* are there any requests pending? */
    if (tracker->isIdle())
    {
        return;
    }

    // if DNS lookup *recently* failed for this host, do nothing
    if (tracker->addr == nullptr && now < tracker->addr_expiration_time)
    {
        return;
    }

    /* if we don't have an address yet, try & get one now. */
    if (!closing && tracker->addr == nullptr && tracker->dns_request == nullptr)
    {
        struct evutil_addrinfo hints;
        memset(&hints, 0, sizeof(hints));
        hints.ai_family = AF_UNSPEC;
        hints.ai_socktype = SOCK_DGRAM;
        hints.ai_protocol = IPPROTO_UDP;
        dbgmsg(tracker->host, "Trying a new DNS lookup");
        tracker->dns_request = evdns_getaddrinfo(
            tracker->session->evdns_base,
            tr_strlower(tracker->host.sv()).c_str(),
            nullptr,
            &hints,
            tau_tracker_on_dns,
            tracker);
        return;
    }

    dbgmsg(
        tracker->key,
        "addr %p -- connected %d (%zu %zu) -- connecting_at %zu",
        (void*)tracker->addr,
        (int)(tracker->connection_expiration_time > now),
        (size_t)tracker->connection_expiration_time,
        (size_t)now,
        (size_t)tracker->connecting_at);

    /* also need a valid connection ID... */
    if (tracker->addr != nullptr && tracker->connection_expiration_time <= now && tracker->connecting_at == 0)
    {
        struct evbuffer* buf = evbuffer_new();
        tracker->connecting_at = now;
        tracker->connection_transaction_id = tau_transaction_new();
        dbgmsg(tracker->key, "Trying to connect. Transaction ID is %u", tracker->connection_transaction_id);
        evbuffer_add_hton_64(buf, 0x41727101980LL);
        evbuffer_add_hton_32(buf, TAU_ACTION_CONNECT);
        evbuffer_add_hton_32(buf, tracker->connection_transaction_id);
        (void)tau_sendto(tracker->session, tracker->addr, tracker->port, evbuffer_pullup(buf, -1), evbuffer_get_length(buf));
        evbuffer_free(buf);
        return;
    }

    if (timeout_reqs)
    {
        tau_tracker_timeout_reqs(tracker);
    }

    if (tracker->addr != nullptr && tracker->connection_expiration_time > now)
    {
        tau_tracker_send_reqs(tracker);
    }
}

static void tau_tracker_upkeep(struct tau_tracker* tracker)
{
    tau_tracker_upkeep_ex(tracker, true);
}

/****
*****
*****  SESSION
*****
****/

struct tr_announcer_udp
{
    explicit tr_announcer_udp(tr_session* session_in)
        : session{ session_in }
    {
    }

    std::list<tau_tracker> trackers;

    tr_session* const session;
};

static struct tr_announcer_udp* announcer_udp_get(tr_session* session)
{
    if (session->announcer_udp != nullptr)
    {
        return session->announcer_udp;
    }

    auto* const tau = new tr_announcer_udp(session);
    session->announcer_udp = tau;
    return tau;
}

/* Finds the tau_tracker struct that corresponds to this url.
   If it doesn't exist yet, create one. */
static tau_tracker* tau_session_get_tracker(tr_announcer_udp* tau, tr_interned_string announce_url)
{
    // build a lookup key for this tracker
    auto const announce_sv = announce_url.sv();
    auto parsed = tr_urlParseTracker(announce_sv);
    TR_ASSERT(parsed);
    if (!parsed)
    {
        return nullptr;
    }

    // see if we already have it
    auto const key = tr_announcerGetKey(*parsed);
    for (auto& tracker : tau->trackers)
    {
        if (tracker.key == key)
        {
            return &tracker;
        }
    }

    // we don't have it -- build a new one
    tau->trackers.emplace_back(tau->session, key, tr_interned_string(parsed->host), parsed->port);
    auto* const tracker = &tau->trackers.back();
    dbgmsg(tracker->key, "New tau_tracker created");
    return tracker;
}

/****
*****
*****  PUBLIC API
*****
****/

void tr_tracker_udp_upkeep(tr_session* session)
{
    if (auto* const tau = session->announcer_udp; tau != nullptr)
    {
        for (auto& tracker : tau->trackers)
        {
            tau_tracker_upkeep(&tracker);
        }
    }
}

bool tr_tracker_udp_is_idle(tr_session const* session)
{
    auto const* tau = session->announcer_udp;

    return tau == nullptr ||
        std::all_of(std::begin(tau->trackers), std::end(tau->trackers), [](auto const& tracker) { return tracker.isIdle(); });
}

/* drop dead now. */
void tr_tracker_udp_close(tr_session* session)
{
    if (auto* const tau = session->announcer_udp; tau != nullptr)
    {
        session->announcer_udp = nullptr;
        delete tau;
    }
}

/* start shutting down.
   This doesn't destroy everything if there are requests,
   but sets a deadline on how much longer to wait for the remaining ones */
void tr_tracker_udp_start_shutdown(tr_session* session)
{
    time_t const now = time(nullptr);

    if (auto* const tau = session->announcer_udp; tau != nullptr)
    {
        for (auto& tracker : tau->trackers)
        {
            if (tracker.dns_request != nullptr)
            {
                evdns_getaddrinfo_cancel(tracker.dns_request);
            }

            tracker.close_at = now + 3;
            tau_tracker_upkeep(&tracker);
        }
    }
}

/* @brief process an incoming udp message if it's a tracker response.
 * @return true if msg was a tracker response; false otherwise */
bool tau_handle_message(tr_session* session, uint8_t const* msg, size_t msglen)
{
    if (session == nullptr || session->announcer_udp == nullptr)
    {
        return false;
    }

    if (msglen < sizeof(uint32_t) * 2)
    {
        return false;
    }

    /* extract the action_id and see if it makes sense */
    auto* const buf = evbuffer_new();
    evbuffer_add_reference(buf, msg, msglen, nullptr, nullptr);
    auto const action_id = tau_action_t(evbuffer_read_ntoh_32(buf));

    if (!is_tau_response_message(action_id, msglen))
    {
        evbuffer_free(buf);
        return false;
    }

    /* extract the transaction_id and look for a match */
    struct tr_announcer_udp* const tau = session->announcer_udp;
    tau_transaction_t const transaction_id = evbuffer_read_ntoh_32(buf);

    for (auto& tracker : tau->trackers)
    {
        // is it a connection response?
        if (tracker.connecting_at != 0 && transaction_id == tracker.connection_transaction_id)
        {
            dbgmsg(tracker.key, "%" PRIu32 " is my connection request!", transaction_id);
            on_tracker_connection_response(&tracker, action_id, buf);
            evbuffer_free(buf);
            return true;
        }

        // is it a response to one of this tracker's announces?
        if (auto& reqs = tracker.announces; !std::empty(reqs))
        {
            auto it = std::find_if(
                std::begin(reqs),
                std::end(reqs),
                [&transaction_id](auto const& req) { return req.transaction_id == transaction_id; });
            if (it != std::end(reqs))
            {
                dbgmsg(tracker.key, "%" PRIu32 " is an announce request!", transaction_id);
                auto req = *it;
                it = reqs.erase(it);
                req.onResponse(action_id, buf);
                evbuffer_free(buf);
                return true;
            }
        }

        // is it a response to one of this tracker's scrapes?
        if (auto& reqs = tracker.scrapes; !std::empty(reqs))
        {
            auto it = std::find_if(
                std::begin(reqs),
                std::end(reqs),
                [&transaction_id](auto const& req) { return req.transaction_id == transaction_id; });
            if (it != std::end(reqs))
            {
                dbgmsg(tracker.key, "%" PRIu32 " is a scrape request!", transaction_id);
                auto req = *it;
                it = reqs.erase(it);
                req.onResponse(action_id, buf);
                evbuffer_free(buf);
                return true;
            }
        }
    }

    /* no match... */
    evbuffer_free(buf);
    return false;
}

void tr_tracker_udp_announce(
    tr_session* session,
    tr_announce_request const* request,
    tr_announce_response_func response_func,
    void* user_data)
{
    tr_announcer_udp* tau = announcer_udp_get(session);
    tau_tracker* tracker = tau_session_get_tracker(tau, request->announce_url);
    if (tracker == nullptr)
    {
        return;
    }

    tracker->announces.push_back(make_tau_announce_request(request, response_func, user_data));
    tau_tracker_upkeep_ex(tracker, false);
}

void tr_tracker_udp_scrape(
    tr_session* session,
    tr_scrape_request const* request,
    tr_scrape_response_func response_func,
    void* user_data)
{
    tr_announcer_udp* tau = announcer_udp_get(session);
    tau_tracker* tracker = tau_session_get_tracker(tau, request->scrape_url);
    if (tracker == nullptr)
    {
        return;
    }

    tracker->scrapes.push_back(make_tau_scrape_request(request, response_func, user_data));
    tau_tracker_upkeep_ex(tracker, false);
}