asyn-thread.c

/***************************************************************************
 *                                  _   _ ____  _
 *  Project                     ___| | | |  _ \| |
 *                             / __| | | | |_) | |
 *                            | (__| |_| |  _ <| |___
 *                             \___|\___/|_| \_\_____|
 *
 * Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
 *
 * This software is licensed as described in the file COPYING, which
 * you should have received as part of this distribution. The terms
 * are also available at https://curl.se/docs/copyright.html.
 *
 * You may opt to use, copy, modify, merge, publish, distribute and/or sell
 * copies of the Software, and permit persons to whom the Software is
 * furnished to do so, under the terms of the COPYING file.
 *
 * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
 * KIND, either express or implied.
 *
 * SPDX-License-Identifier: curl
 *
 ***************************************************************************/

#include "curl_setup.h"
#include "socketpair.h"

/***********************************************************************
 * Only for threaded name resolves builds
 **********************************************************************/
#ifdef CURLRES_THREADED

#ifdef HAVE_NETINET_IN_H
#include <netinet/in.h>
#endif
#ifdef HAVE_NETDB_H
#include <netdb.h>
#endif
#ifdef HAVE_ARPA_INET_H
#include <arpa/inet.h>
#endif
#ifdef __VMS
#include <in.h>
#include <inet.h>
#endif

#if defined(USE_THREADS_POSIX) && defined(HAVE_PTHREAD_H)
#  include <pthread.h>
#endif

#ifdef HAVE_GETADDRINFO
#  define RESOLVER_ENOMEM  EAI_MEMORY
#else
#  define RESOLVER_ENOMEM  ENOMEM
#endif

#include "system_win32.h"
#include "urldata.h"
#include "sendf.h"
#include "hostip.h"
#include "hash.h"
#include "share.h"
#include "url.h"
#include "multiif.h"
#include "inet_ntop.h"
#include "curl_threads.h"
#include "connect.h"
/* The last 3 #include files should be in this order */
#include "curl_printf.h"
#include "curl_memory.h"
#include "memdebug.h"

struct resdata {
  struct curltime start;
};

/*
 * Curl_resolver_global_init()
 * Called from curl_global_init() to initialize global resolver environment.
 * Does nothing here.
 */
int Curl_resolver_global_init(void)
{
  return CURLE_OK;
}

/*
 * Curl_resolver_global_cleanup()
 * Called from curl_global_cleanup() to destroy global resolver environment.
 * Does nothing here.
 */
void Curl_resolver_global_cleanup(void)
{
}

/*
 * Curl_resolver_init()
 * Called from curl_easy_init() -> Curl_open() to initialize resolver
 * URL-state specific environment ('resolver' member of the UrlState
 * structure).
 */
CURLcode Curl_resolver_init(struct Curl_easy *easy, void **resolver)
{
  (void)easy;
  *resolver = calloc(1, sizeof(struct resdata));
  if(!*resolver)
    return CURLE_OUT_OF_MEMORY;
  return CURLE_OK;
}

/*
 * Curl_resolver_cleanup()
 * Called from curl_easy_cleanup() -> Curl_close() to cleanup resolver
 * URL-state specific environment ('resolver' member of the UrlState
 * structure).
 */
void Curl_resolver_cleanup(void *resolver)
{
  free(resolver);
}

/*
 * Curl_resolver_duphandle()
 * Called from curl_easy_duphandle() to duplicate resolver URL state-specific
 * environment ('resolver' member of the UrlState structure).
 */
CURLcode Curl_resolver_duphandle(struct Curl_easy *easy, void **to, void *from)
{
  (void)from;
  return Curl_resolver_init(easy, to);
}

static void destroy_async_data(struct Curl_async *);

/*
 * Cancel all possibly still on-going resolves for this connection.
 */
void Curl_resolver_cancel(struct Curl_easy *data)
{
  destroy_async_data(&data->state.async);
}

/* This function is used to init a threaded resolve */
static bool init_resolve_thread(struct Curl_easy *data,
                                const char *hostname, int port,
                                const struct addrinfo *hints);

#ifdef _WIN32
/* Thread sync data used by GetAddrInfoExW for win8+ */
struct thread_sync_data_w8
{
  OVERLAPPED overlapped;
  ADDRINFOEXW_ *res;
  HANDLE cancel_ev;
  ADDRINFOEXW_ hints;
};
#endif

/* Data for synchronization between resolver thread and its parent */
struct thread_sync_data {
#ifdef _WIN32
  struct thread_sync_data_w8 w8;
#endif
  curl_mutex_t *mtx;
  int done;
  int port;
  char *hostname;        /* hostname to resolve, Curl_async.hostname
                            duplicate */
#ifndef CURL_DISABLE_SOCKETPAIR
  struct Curl_easy *data;
  curl_socket_t sock_pair[2]; /* eventfd/pipes/socket pair */
#endif
  int sock_error;
  struct Curl_addrinfo *res;
#ifdef HAVE_GETADDRINFO
  struct addrinfo hints;
#endif
  struct thread_data *td; /* for thread-self cleanup */
};

struct thread_data {
#ifdef _WIN32
  HANDLE complete_ev;
#endif
  curl_thread_t thread_hnd;
  unsigned int poll_interval;
  timediff_t interval_end;
  struct thread_sync_data tsd;
};

static struct thread_sync_data *conn_thread_sync_data(struct Curl_easy *data)
{
  return &(data->state.async.tdata->tsd);
}

/* Destroy resolver thread synchronization data */
static
void destroy_thread_sync_data(struct thread_sync_data *tsd)
{
  if(tsd->mtx) {
    Curl_mutex_destroy(tsd->mtx);
    free(tsd->mtx);
  }

  free(tsd->hostname);

  if(tsd->res)
    Curl_freeaddrinfo(tsd->res);

#ifndef CURL_DISABLE_SOCKETPAIR
  /*
   * close one end of the socket pair (may be done in resolver thread);
   * the other end (for reading) is always closed in the parent thread.
   */
  if(tsd->sock_pair[1] != CURL_SOCKET_BAD) {
    wakeup_close(tsd->sock_pair[1]);
  }
#endif
  memset(tsd, 0, sizeof(*tsd));
}

/* Initialize resolver thread synchronization data */
static
int init_thread_sync_data(struct thread_data *td,
                           const char *hostname,
                           int port,
                           const struct addrinfo *hints)
{
  struct thread_sync_data *tsd = &td->tsd;

  memset(tsd, 0, sizeof(*tsd));

  tsd->td = td;
  tsd->port = port;
  /* Treat the request as done until the thread actually starts so any early
   * cleanup gets done properly.
   */
  tsd->done = 1;
#ifdef HAVE_GETADDRINFO
  DEBUGASSERT(hints);
  tsd->hints = *hints;
#else
  (void) hints;
#endif

  tsd->mtx = malloc(sizeof(curl_mutex_t));
  if(!tsd->mtx)
    goto err_exit;

  Curl_mutex_init(tsd->mtx);

#ifndef CURL_DISABLE_SOCKETPAIR
  /* create socket pair or pipe */
  if(wakeup_create(tsd->sock_pair, FALSE) < 0) {
    tsd->sock_pair[0] = CURL_SOCKET_BAD;
    tsd->sock_pair[1] = CURL_SOCKET_BAD;
    goto err_exit;
  }
#endif
  tsd->sock_error = CURL_ASYNC_SUCCESS;

  /* Copying hostname string because original can be destroyed by parent
   * thread during gethostbyname execution.
   */
  tsd->hostname = strdup(hostname);
  if(!tsd->hostname)
    goto err_exit;

  return 1;

err_exit:
#ifndef CURL_DISABLE_SOCKETPAIR
  if(tsd->sock_pair[0] != CURL_SOCKET_BAD) {
    wakeup_close(tsd->sock_pair[0]);
    tsd->sock_pair[0] = CURL_SOCKET_BAD;
  }
#endif
  destroy_thread_sync_data(tsd);
  return 0;
}

static CURLcode getaddrinfo_complete(struct Curl_easy *data)
{
  struct thread_sync_data *tsd = conn_thread_sync_data(data);
  CURLcode result;

  result = Curl_addrinfo_callback(data, tsd->sock_error, tsd->res);
  /* The tsd->res structure has been copied to async.dns and perhaps the DNS
     cache. Set our copy to NULL so destroy_thread_sync_data does not free it.
  */
  tsd->res = NULL;

  return result;
}

#ifdef _WIN32
static VOID WINAPI
query_complete(DWORD err, DWORD bytes, LPWSAOVERLAPPED overlapped)
{
  size_t ss_size;
  const ADDRINFOEXW_ *ai;
  struct Curl_addrinfo *ca;
  struct Curl_addrinfo *cafirst = NULL;
  struct Curl_addrinfo *calast = NULL;
#ifndef CURL_DISABLE_SOCKETPAIR
#ifdef USE_EVENTFD
  const void *buf;
  const uint64_t val = 1;
#else
  char buf[1];
#endif
#endif
#ifdef __clang__
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wcast-align"
#endif
  struct thread_sync_data *tsd =
    CONTAINING_RECORD(overlapped, struct thread_sync_data, w8.overlapped);
#ifdef __clang__
#pragma clang diagnostic pop
#endif
  struct thread_data *td = tsd->td;
  const ADDRINFOEXW_ *res = tsd->w8.res;
  int error = (int)err;
  (void)bytes;

  if(error == ERROR_SUCCESS) {
    /* traverse the addrinfo list */

    for(ai = res; ai != NULL; ai = ai->ai_next) {
      size_t namelen = ai->ai_canonname ? wcslen(ai->ai_canonname) + 1 : 0;
      /* ignore elements with unsupported address family, */
      /* settle family-specific sockaddr structure size.  */
      if(ai->ai_family == AF_INET)
        ss_size = sizeof(struct sockaddr_in);
#ifdef USE_IPV6
      else if(ai->ai_family == AF_INET6)
        ss_size = sizeof(struct sockaddr_in6);
#endif
      else
        continue;

      /* ignore elements without required address info */
      if(!ai->ai_addr || !(ai->ai_addrlen > 0))
        continue;

      /* ignore elements with bogus address size */
      if((size_t)ai->ai_addrlen < ss_size)
        continue;

      ca = malloc(sizeof(struct Curl_addrinfo) + ss_size + namelen);
      if(!ca) {
        error = EAI_MEMORY;
        break;
      }

      /* copy each structure member individually, member ordering, */
      /* size, or padding might be different for each platform.    */
      ca->ai_flags     = ai->ai_flags;
      ca->ai_family    = ai->ai_family;
      ca->ai_socktype  = ai->ai_socktype;
      ca->ai_protocol  = ai->ai_protocol;
      ca->ai_addrlen   = (curl_socklen_t)ss_size;
      ca->ai_addr      = NULL;
      ca->ai_canonname = NULL;
      ca->ai_next      = NULL;

      ca->ai_addr = (void *)((char *)ca + sizeof(struct Curl_addrinfo));
      memcpy(ca->ai_addr, ai->ai_addr, ss_size);

      if(namelen) {
        size_t i;
        ca->ai_canonname = (void *)((char *)ca->ai_addr + ss_size);
        for(i = 0; i < namelen; ++i) /* convert wide string to ascii */
          ca->ai_canonname[i] = (char)ai->ai_canonname[i];
        ca->ai_canonname[namelen] = '\0';
      }

      /* if the return list is empty, this becomes the first element */
      if(!cafirst)
        cafirst = ca;

      /* add this element last in the return list */
      if(calast)
        calast->ai_next = ca;
      calast = ca;
    }

    /* if we failed, also destroy the Curl_addrinfo list */
    if(error) {
      Curl_freeaddrinfo(cafirst);
      cafirst = NULL;
    }
    else if(!cafirst) {
#ifdef EAI_NONAME
      /* rfc3493 conformant */
      error = EAI_NONAME;
#else
      /* rfc3493 obsoleted */
      error = EAI_NODATA;
#endif
#ifdef USE_WINSOCK
      SET_SOCKERRNO(error);
#endif
    }
    tsd->res = cafirst;
  }

  if(tsd->w8.res) {
    Curl_FreeAddrInfoExW(tsd->w8.res);
    tsd->w8.res = NULL;
  }

  if(error) {
    tsd->sock_error = SOCKERRNO?SOCKERRNO:error;
    if(tsd->sock_error == 0)
      tsd->sock_error = RESOLVER_ENOMEM;
  }
  else {
    Curl_addrinfo_set_port(tsd->res, tsd->port);
  }

  Curl_mutex_acquire(tsd->mtx);
  if(tsd->done) {
    /* too late, gotta clean up the mess */
    Curl_mutex_release(tsd->mtx);
    destroy_thread_sync_data(tsd);
    free(td);
  }
  else {
#ifndef CURL_DISABLE_SOCKETPAIR
    if(tsd->sock_pair[1] != CURL_SOCKET_BAD) {
#ifdef USE_EVENTFD
      buf = &val;
#else
      buf[0] = 1;
#endif
      /* DNS has been resolved, signal client task */
      if(wakeup_write(tsd->sock_pair[1], buf, sizeof(buf)) < 0) {
        /* update sock_erro to errno */
        tsd->sock_error = SOCKERRNO;
      }
    }
#endif
    tsd->done = 1;
    Curl_mutex_release(tsd->mtx);
    if(td->complete_ev)
      SetEvent(td->complete_ev); /* Notify caller that the query completed */
  }
}
#endif

#ifdef HAVE_GETADDRINFO

/*
 * getaddrinfo_thread() resolves a name and then exits.
 *
 * For builds without ARES, but with USE_IPV6, create a resolver thread
 * and wait on it.
 */
static
#if defined(_WIN32_WCE) || defined(CURL_WINDOWS_APP)
DWORD
#else
unsigned int
#endif
CURL_STDCALL getaddrinfo_thread(void *arg)
{
  struct thread_sync_data *tsd = (struct thread_sync_data *)arg;
  struct thread_data *td = tsd->td;
  char service[12];
  int rc;
#ifndef CURL_DISABLE_SOCKETPAIR
#ifdef USE_EVENTFD
  const void *buf;
  const uint64_t val = 1;
#else
  char buf[1];
#endif
#endif

  msnprintf(service, sizeof(service), "%d", tsd->port);

  rc = Curl_getaddrinfo_ex(tsd->hostname, service, &tsd->hints, &tsd->res);

  if(rc) {
    tsd->sock_error = SOCKERRNO?SOCKERRNO:rc;
    if(tsd->sock_error == 0)
      tsd->sock_error = RESOLVER_ENOMEM;
  }
  else {
    Curl_addrinfo_set_port(tsd->res, tsd->port);
  }

  Curl_mutex_acquire(tsd->mtx);
  if(tsd->done) {
    /* too late, gotta clean up the mess */
    Curl_mutex_release(tsd->mtx);
    destroy_thread_sync_data(tsd);
    free(td);
  }
  else {
#ifndef CURL_DISABLE_SOCKETPAIR
    if(tsd->sock_pair[1] != CURL_SOCKET_BAD) {
#ifdef USE_EVENTFD
      buf = &val;
#else
      buf[0] = 1;
#endif
      /* DNS has been resolved, signal client task */
      if(wakeup_write(tsd->sock_pair[1], buf, sizeof(buf)) < 0) {
        /* update sock_erro to errno */
        tsd->sock_error = SOCKERRNO;
      }
    }
#endif
    tsd->done = 1;
    Curl_mutex_release(tsd->mtx);
  }

  return 0;
}

#else /* HAVE_GETADDRINFO */

/*
 * gethostbyname_thread() resolves a name and then exits.
 */
static
#if defined(_WIN32_WCE) || defined(CURL_WINDOWS_APP)
DWORD
#else
unsigned int
#endif
CURL_STDCALL gethostbyname_thread(void *arg)
{
  struct thread_sync_data *tsd = (struct thread_sync_data *)arg;
  struct thread_data *td = tsd->td;

  tsd->res = Curl_ipv4_resolve_r(tsd->hostname, tsd->port);

  if(!tsd->res) {
    tsd->sock_error = SOCKERRNO;
    if(tsd->sock_error == 0)
      tsd->sock_error = RESOLVER_ENOMEM;
  }

  Curl_mutex_acquire(tsd->mtx);
  if(tsd->done) {
    /* too late, gotta clean up the mess */
    Curl_mutex_release(tsd->mtx);
    destroy_thread_sync_data(tsd);
    free(td);
  }
  else {
    tsd->done = 1;
    Curl_mutex_release(tsd->mtx);
  }

  return 0;
}

#endif /* HAVE_GETADDRINFO */

/*
 * destroy_async_data() cleans up async resolver data and thread handle.
 */
static void destroy_async_data(struct Curl_async *async)
{
  if(async->tdata) {
    struct thread_data *td = async->tdata;
    int done;
#ifndef CURL_DISABLE_SOCKETPAIR
    curl_socket_t sock_rd = td->tsd.sock_pair[0];
    struct Curl_easy *data = td->tsd.data;
#endif

    /*
     * if the thread is still blocking in the resolve syscall, detach it and
     * let the thread do the cleanup...
     */
    Curl_mutex_acquire(td->tsd.mtx);
    done = td->tsd.done;
    td->tsd.done = 1;
    Curl_mutex_release(td->tsd.mtx);

    if(!done) {
#ifdef _WIN32
      if(td->complete_ev) {
        CloseHandle(td->complete_ev);
        td->complete_ev = NULL;
      }
#endif
      if(td->thread_hnd != curl_thread_t_null) {
        Curl_thread_destroy(td->thread_hnd);
        td->thread_hnd = curl_thread_t_null;
      }
    }
    else {
#ifdef _WIN32
      if(td->complete_ev) {
        Curl_GetAddrInfoExCancel(&td->tsd.w8.cancel_ev);
        WaitForSingleObject(td->complete_ev, INFINITE);
        CloseHandle(td->complete_ev);
        td->complete_ev = NULL;
      }
#endif
      if(td->thread_hnd != curl_thread_t_null)
        Curl_thread_join(&td->thread_hnd);

      destroy_thread_sync_data(&td->tsd);

      free(async->tdata);
    }
#ifndef CURL_DISABLE_SOCKETPAIR
    /*
     * ensure CURLMOPT_SOCKETFUNCTION fires CURL_POLL_REMOVE
     * before the FD is invalidated to avoid EBADF on EPOLL_CTL_DEL
     */
    Curl_multi_closed(data, sock_rd);
    wakeup_close(sock_rd);
#endif
  }
  async->tdata = NULL;

  free(async->hostname);
  async->hostname = NULL;
}

/*
 * init_resolve_thread() starts a new thread that performs the actual
 * resolve. This function returns before the resolve is done.
 *
 * Returns FALSE in case of failure, otherwise TRUE.
 */
static bool init_resolve_thread(struct Curl_easy *data,
                                const char *hostname, int port,
                                const struct addrinfo *hints)
{
  struct thread_data *td = calloc(1, sizeof(struct thread_data));
  int err = ENOMEM;
  struct Curl_async *asp = &data->state.async;

  data->state.async.tdata = td;
  if(!td)
    goto errno_exit;

  asp->port = port;
  asp->done = FALSE;
  asp->status = 0;
  asp->dns = NULL;
  td->thread_hnd = curl_thread_t_null;
#ifdef _WIN32
  td->complete_ev = NULL;
#endif

  if(!init_thread_sync_data(td, hostname, port, hints)) {
    asp->tdata = NULL;
    free(td);
    goto errno_exit;
  }

  free(asp->hostname);
  asp->hostname = strdup(hostname);
  if(!asp->hostname)
    goto err_exit;

  /* The thread will set this to 1 when complete. */
  td->tsd.done = 0;

#ifdef _WIN32
  if(Curl_isWindows8OrGreater && Curl_FreeAddrInfoExW &&
     Curl_GetAddrInfoExCancel && Curl_GetAddrInfoExW &&
     !Curl_win32_impersonating()) {
#define MAX_NAME_LEN 256 /* max domain name is 253 chars */
#define MAX_PORT_LEN 8
    WCHAR namebuf[MAX_NAME_LEN];
    WCHAR portbuf[MAX_PORT_LEN];
    /* calculate required length */
    int w_len = MultiByteToWideChar(CP_UTF8, MB_ERR_INVALID_CHARS, hostname,
                                    -1, NULL, 0);
    if((w_len > 0) && (w_len < MAX_NAME_LEN)) {
      /* do utf8 conversion */
      w_len = MultiByteToWideChar(CP_UTF8, 0, hostname, -1, namebuf, w_len);
      if((w_len > 0) && (w_len < MAX_NAME_LEN)) {
        swprintf(portbuf, MAX_PORT_LEN, L"%d", port);
        td->tsd.w8.hints.ai_family = hints->ai_family;
        td->tsd.w8.hints.ai_socktype = hints->ai_socktype;
        td->complete_ev = CreateEvent(NULL, TRUE, FALSE, NULL);
        if(!td->complete_ev) {
          /* failed to start, mark it as done here for proper cleanup. */
          td->tsd.done = 1;
          goto err_exit;
        }
        err = Curl_GetAddrInfoExW(namebuf, portbuf, NS_DNS,
                                  NULL, &td->tsd.w8.hints, &td->tsd.w8.res,
                                  NULL, &td->tsd.w8.overlapped,
                                  &query_complete, &td->tsd.w8.cancel_ev);
        if(err != WSA_IO_PENDING)
          query_complete((DWORD)err, 0, &td->tsd.w8.overlapped);
        return TRUE;
      }
    }
  }
#endif

#ifdef HAVE_GETADDRINFO
  td->thread_hnd = Curl_thread_create(getaddrinfo_thread, &td->tsd);
#else
  td->thread_hnd = Curl_thread_create(gethostbyname_thread, &td->tsd);
#endif

  if(td->thread_hnd == curl_thread_t_null) {
    /* The thread never started, so mark it as done here for proper cleanup. */
    td->tsd.done = 1;
    err = errno;
    goto err_exit;
  }

  return TRUE;

err_exit:
  destroy_async_data(asp);

errno_exit:
  errno = err;
  return FALSE;
}

/*
 * 'entry' may be NULL and then no data is returned
 */
static CURLcode thread_wait_resolv(struct Curl_easy *data,
                                   struct Curl_dns_entry **entry,
                                   bool report)
{
  struct thread_data *td;
  CURLcode result = CURLE_OK;

  DEBUGASSERT(data);
  td = data->state.async.tdata;
  DEBUGASSERT(td);
#ifdef _WIN32
  DEBUGASSERT(td->complete_ev || td->thread_hnd != curl_thread_t_null);
#else
  DEBUGASSERT(td->thread_hnd != curl_thread_t_null);
#endif

  /* wait for the thread to resolve the name */
#ifdef _WIN32
  if(td->complete_ev) {
    WaitForSingleObject(td->complete_ev, INFINITE);
    CloseHandle(td->complete_ev);
    td->complete_ev = NULL;
    if(entry)
      result = getaddrinfo_complete(data);
  }
  else
#endif
  if(Curl_thread_join(&td->thread_hnd)) {
    if(entry)
      result = getaddrinfo_complete(data);
  }
  else
    DEBUGASSERT(0);

  data->state.async.done = TRUE;

  if(entry)
    *entry = data->state.async.dns;

  if(!data->state.async.dns && report)
    /* a name was not resolved, report error */
    result = Curl_resolver_error(data);

  destroy_async_data(&data->state.async);

  if(!data->state.async.dns && report)
    connclose(data->conn, "asynch resolve failed");

  return result;
}


/*
 * Until we gain a way to signal the resolver threads to stop early, we must
 * simply wait for them and ignore their results.
 */
void Curl_resolver_kill(struct Curl_easy *data)
{
  struct thread_data *td = data->state.async.tdata;

  /* If we are still resolving, we must wait for the threads to fully clean up,
     unfortunately. Otherwise, we can simply cancel to clean up any resolver
     data. */
#ifdef _WIN32
  if(td && td->complete_ev) {
    Curl_GetAddrInfoExCancel(&td->tsd.w8.cancel_ev);
    (void)thread_wait_resolv(data, NULL, FALSE);
  }
  else
#endif
  if(td && td->thread_hnd != curl_thread_t_null
     && (data->set.quick_exit != 1L))
    (void)thread_wait_resolv(data, NULL, FALSE);
  else
    Curl_resolver_cancel(data);
}

/*
 * Curl_resolver_wait_resolv()
 *
 * Waits for a resolve to finish. This function should be avoided since using
 * this risk getting the multi interface to "hang".
 *
 * If 'entry' is non-NULL, make it point to the resolved dns entry
 *
 * Returns CURLE_COULDNT_RESOLVE_HOST if the host was not resolved,
 * CURLE_OPERATION_TIMEDOUT if a time-out occurred, or other errors.
 *
 * This is the version for resolves-in-a-thread.
 */
CURLcode Curl_resolver_wait_resolv(struct Curl_easy *data,
                                   struct Curl_dns_entry **entry)
{
  return thread_wait_resolv(data, entry, TRUE);
}

/*
 * Curl_resolver_is_resolved() is called repeatedly to check if a previous
 * name resolve request has completed. It should also make sure to time-out if
 * the operation seems to take too long.
 */
CURLcode Curl_resolver_is_resolved(struct Curl_easy *data,
                                   struct Curl_dns_entry **entry)
{
  struct thread_data *td = data->state.async.tdata;
  int done = 0;

  DEBUGASSERT(entry);
  *entry = NULL;

  if(!td) {
    DEBUGASSERT(td);
    return CURLE_COULDNT_RESOLVE_HOST;
  }

  Curl_mutex_acquire(td->tsd.mtx);
  done = td->tsd.done;
  Curl_mutex_release(td->tsd.mtx);

  if(done) {
    getaddrinfo_complete(data);

    if(!data->state.async.dns) {
      CURLcode result = Curl_resolver_error(data);
      destroy_async_data(&data->state.async);
      return result;
    }
    destroy_async_data(&data->state.async);
    *entry = data->state.async.dns;
  }
  else {
    /* poll for name lookup done with exponential backoff up to 250ms */
    /* should be fine even if this converts to 32-bit */
    timediff_t elapsed = Curl_timediff(Curl_now(),
                                       data->progress.t_startsingle);
    if(elapsed < 0)
      elapsed = 0;

    if(td->poll_interval == 0)
      /* Start at 1ms poll interval */
      td->poll_interval = 1;
    else if(elapsed >= td->interval_end)
      /* Back-off exponentially if last interval expired  */
      td->poll_interval *= 2;

    if(td->poll_interval > 250)
      td->poll_interval = 250;

    td->interval_end = elapsed + td->poll_interval;
    Curl_expire(data, td->poll_interval, EXPIRE_ASYNC_NAME);
  }

  return CURLE_OK;
}

int Curl_resolver_getsock(struct Curl_easy *data, curl_socket_t *socks)
{
  int ret_val = 0;
  timediff_t milli;
  timediff_t ms;
  struct resdata *reslv = (struct resdata *)data->state.async.resolver;
#ifndef CURL_DISABLE_SOCKETPAIR
  struct thread_data *td = data->state.async.tdata;
#else
  (void)socks;
#endif

#ifndef CURL_DISABLE_SOCKETPAIR
  if(td) {
    /* return read fd to client for polling the DNS resolution status */
    socks[0] = td->tsd.sock_pair[0];
    td->tsd.data = data;
    ret_val = GETSOCK_READSOCK(0);
  }
  else {
#endif
    ms = Curl_timediff(Curl_now(), reslv->start);
    if(ms < 3)
      milli = 0;
    else if(ms <= 50)
      milli = ms/3;
    else if(ms <= 250)
      milli = 50;
    else
      milli = 200;
    Curl_expire(data, milli, EXPIRE_ASYNC_NAME);
#ifndef CURL_DISABLE_SOCKETPAIR
  }
#endif


  return ret_val;
}

#ifndef HAVE_GETADDRINFO
/*
 * Curl_getaddrinfo() - for platforms without getaddrinfo
 */
struct Curl_addrinfo *Curl_resolver_getaddrinfo(struct Curl_easy *data,
                                                const char *hostname,
                                                int port,
                                                int *waitp)
{
  struct resdata *reslv = (struct resdata *)data->state.async.resolver;

  *waitp = 0; /* default to synchronous response */

  reslv->start = Curl_now();

  /* fire up a new resolver thread! */
  if(init_resolve_thread(data, hostname, port, NULL)) {
    *waitp = 1; /* expect asynchronous response */
    return NULL;
  }

  failf(data, "getaddrinfo() thread failed");

  return NULL;
}

#else /* !HAVE_GETADDRINFO */

/*
 * Curl_resolver_getaddrinfo() - for getaddrinfo
 */
struct Curl_addrinfo *Curl_resolver_getaddrinfo(struct Curl_easy *data,
                                                const char *hostname,
                                                int port,
                                                int *waitp)
{
  struct addrinfo hints;
  int pf = PF_INET;
  struct resdata *reslv = (struct resdata *)data->state.async.resolver;

  *waitp = 0; /* default to synchronous response */

#ifdef CURLRES_IPV6
  if((data->conn->ip_version != CURL_IPRESOLVE_V4) && Curl_ipv6works(data)) {
    /* The stack seems to be IPv6-enabled */
    if(data->conn->ip_version == CURL_IPRESOLVE_V6)
      pf = PF_INET6;
    else
      pf = PF_UNSPEC;
  }
#endif /* CURLRES_IPV6 */

  memset(&hints, 0, sizeof(hints));
  hints.ai_family = pf;
  hints.ai_socktype = (data->conn->transport == TRNSPRT_TCP)?
    SOCK_STREAM : SOCK_DGRAM;

  reslv->start = Curl_now();
  /* fire up a new resolver thread! */
  if(init_resolve_thread(data, hostname, port, &hints)) {
    *waitp = 1; /* expect asynchronous response */
    return NULL;
  }

  failf(data, "getaddrinfo() thread failed to start");
  return NULL;

}

#endif /* !HAVE_GETADDRINFO */

CURLcode Curl_set_dns_servers(struct Curl_easy *data,
                              char *servers)
{
  (void)data;
  (void)servers;
  return CURLE_NOT_BUILT_IN;

}

CURLcode Curl_set_dns_interface(struct Curl_easy *data,
                                const char *interf)
{
  (void)data;
  (void)interf;
  return CURLE_NOT_BUILT_IN;
}

CURLcode Curl_set_dns_local_ip4(struct Curl_easy *data,
                                const char *local_ip4)
{
  (void)data;
  (void)local_ip4;
  return CURLE_NOT_BUILT_IN;
}

CURLcode Curl_set_dns_local_ip6(struct Curl_easy *data,
                                const char *local_ip6)
{
  (void)data;
  (void)local_ip6;
  return CURLE_NOT_BUILT_IN;
}

#endif /* CURLRES_THREADED */