path: root/contrib/libs/curl/lib/asyn-thread.c

/***************************************************************************
 *                                  _   _ ____  _
 *  Project                     ___| | | |  _ \| |
 *                             / __| | | | |_) | |
 *                            | (__| |_| |  _ <| |___
 *                             \___|\___/|_| \_\_____|
 *
 * Copyright (C) 1998 - 2022, 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)
#  ifdef HAVE_PTHREAD_H
#    include <pthread.h>
#  endif
#elif defined(USE_THREADS_WIN32)
#  ifdef HAVE_PROCESS_H
#    include <process.h>
#  endif
#endif

#if (defined(NETWARE) && defined(__NOVELL_LIBC__))
#undef in_addr_t
#define in_addr_t unsigned long
#endif

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

#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"
#include "socketpair.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;
};

/* Doubly linked list of orphaned thread handles. */
struct thread_list {
  curl_thread_t handle;

  /* 'exiting' is set true right before an orphaned thread exits.
     it should only be set by the orphaned thread from
     signal_orphan_is_exiting(). */
  bool exiting;

  struct thread_list *prev, *next;
};

/* Orphaned threads: A global list of resolver threads that could not be
 * completed in time and so they were abandoned by their parent. The list is
 * culled periodically by soon-to-be exiting orphans to wait on and destroy
 * those that are in the process of or have since exited, which is fast. On
 * global cleanup we wait on and destroy any remaining threads, which may be
 * slow but at that point we cannot defer it any longer.
 */
struct orphaned_threads {
  /* Mutex to lock this. To avoid deadlock the thread-specific thread_sync_data
     mutex cannot be used as an inner lock when orphaned_threads is locked. */
  curl_mutex_t mutex;

  /* List of orphaned threads. */
  struct thread_list *first, *last;

  /* Count of threads in the list that are in the process of or have exited.
     (ie .exiting member of the thread_list item is set true) */
  size_t exiting_count;
};

static struct orphaned_threads orphaned_threads;

/* Flags for wait_and_destroy_orphaned_threads().
   They're documented above the function definition. */
#define WAIT_DESTROY_ALL                   (1<<0)
#define WAIT_DESTROY_EXITING_THREADS_ONLY  (1<<1)

static void wait_and_destroy_orphaned_threads(int flags);
static void signal_orphan_is_exiting(struct thread_list *orphan);


/*
 * Curl_resolver_global_init()
 * Called from curl_global_init() to initialize global resolver environment.
 */
int Curl_resolver_global_init(void)
{
  memset(&orphaned_threads, 0, sizeof(orphaned_threads));

  if(Curl_mutex_init(&orphaned_threads.mutex))
    return CURLE_FAILED_INIT;

  return CURLE_OK;
}

/*
 * Curl_resolver_global_cleanup()
 * Called from curl_global_cleanup() to destroy global resolver environment.
 */
void Curl_resolver_global_cleanup(void)
{
  /* Take ownership of all orphaned resolver threads and wait for them to exit.
     This is necessary because the user may choose to unload the shared library
     that is/contains libcurl. */
  wait_and_destroy_orphaned_threads(WAIT_DESTROY_ALL);
  Curl_mutex_destroy(&orphaned_threads.mutex);
}

/*
 * 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);


/* Data for synchronization between resolver thread and its parent */
struct thread_sync_data {
  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]; /* 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 {
  curl_thread_t thread_hnd;
  unsigned int poll_interval;
  timediff_t interval_end;
  struct thread_sync_data tsd;
  /* 'reserved' memory must be available in case the thread is orphaned */
  void *reserved;
};

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) {
    sclose(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;

  if(Curl_mutex_init(tsd->mtx)) {
    free(tsd->mtx);
    tsd->mtx = NULL;
    goto err_exit;
  }

#ifndef CURL_DISABLE_SOCKETPAIR
  /* create socket pair, avoid AF_LOCAL since it doesn't build on Solaris */
  if(Curl_socketpair(AF_UNIX, SOCK_STREAM, 0, &tsd->sock_pair[0]) < 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) {
    sclose(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 doesn't free it.
  */
  tsd->res = NULL;

  return result;
}


#ifdef HAVE_GETADDRINFO

/*
 * getaddrinfo_thread() resolves a name and then exits.
 *
 * For builds without ARES, but with ENABLE_IPV6, create a resolver thread
 * and wait on it.
 */
static unsigned int CURL_STDCALL getaddrinfo_thread(void *arg)
{
  struct thread_sync_data *tsd = (struct thread_sync_data *)arg;
  struct thread_data *td = tsd->td;
  struct thread_list *orphan = NULL;
  char service[12];
  int rc;
#ifndef CURL_DISABLE_SOCKETPAIR
  char buf[1];
#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);
    orphan = (struct thread_list *)td->reserved;
    free(td);
  }
  else {
#ifndef CURL_DISABLE_SOCKETPAIR
    if(tsd->sock_pair[1] != CURL_SOCKET_BAD) {
      /* DNS has been resolved, signal client task */
      buf[0] = 1;
      if(swrite(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(orphan)
    signal_orphan_is_exiting(orphan);

  return 0;
}

#else /* HAVE_GETADDRINFO */

/*
 * gethostbyname_thread() resolves a name and then exits.
 */
static unsigned int CURL_STDCALL gethostbyname_thread(void *arg)
{
  struct thread_sync_data *tsd = (struct thread_sync_data *)arg;
  struct thread_data *td = tsd->td;
  struct thread_list *orphan = NULL;

  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);
    orphan = (struct thread_list *)td->reserved;
    free(td);
  }
  else {
    tsd->done = 1;
    Curl_mutex_release(tsd->mtx);
  }

  if(orphan)
    signal_orphan_is_exiting(orphan);

  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

    /* We can't wait any longer for the resolver thread so if it's not done
     * then it must be orphaned.
     *
     * 1) add thread to orphaned threads list
     * 2) set thread done (this signals to thread it has been orphaned)
     *
     * An orphaned thread does most of its own cleanup, and any remaining
     * cleanup is handled during global cleanup.
     */

    Curl_mutex_acquire(td->tsd.mtx);

    if(!td->tsd.done && td->thread_hnd != curl_thread_t_null) {
      struct thread_list *orphan = (struct thread_list *)td->reserved;

      Curl_mutex_acquire(&orphaned_threads.mutex);

#ifdef DEBUGBUILD
      {
        struct thread_list empty;
        memset(&empty, 0, sizeof(empty));
        DEBUGASSERT(!memcmp(&empty, orphan, sizeof(empty)));
      }
#endif

      orphan->handle = td->thread_hnd;
      orphan->exiting = false;

      if(orphaned_threads.last) {
        orphaned_threads.last->next = orphan;
        orphan->prev = orphaned_threads.last;
      }
      else {
        orphaned_threads.first = orphan;
        orphan->prev = NULL;
      }
      orphaned_threads.last = orphan;
      orphan->next = NULL;

      Curl_mutex_release(&orphaned_threads.mutex);
    }

    done = td->tsd.done;
    td->tsd.done = 1;

    Curl_mutex_release(td->tsd.mtx);

    if(done) {
      if(td->thread_hnd != curl_thread_t_null)
        Curl_thread_join(&td->thread_hnd);

      destroy_thread_sync_data(&td->tsd);
      free(td->reserved);
      free(td);
    }
#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);
    sclose(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;
  td->reserved = calloc(1, sizeof(struct thread_list));

  if(!td->reserved || !init_thread_sync_data(td, hostname, port, hints)) {
    asp->tdata = NULL;
    free(td->reserved);
    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 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);
  DEBUGASSERT(td->thread_hnd != curl_thread_t_null);

  /* wait for the thread to resolve the name */
  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're still resolving, we must wait for the threads to fully clean up,
     unfortunately.  Otherwise, we can simply cancel to clean up any resolver
     data. */
  if(td && td->thread_hnd != curl_thread_t_null)
    (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 */
    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;
}

/* Helper function to wait and destroy some or all orphaned threads.
 *
 * WAIT_DESTROY_ALL:
 * Wait and destroy all orphaned threads. This operation is not safe to specify
 * in code that could run in any thread that may be orphaned (ie any resolver
 * thread). Waiting on all orphaned threads may take some time. This operation
 * must be specified in the call from global cleanup, and ideally nowhere else.
 *
 * WAIT_DESTROY_EXITING_THREADS_ONLY:
 * Wait and destroy only orphaned threads that are in the process of or have
 * since exited (ie those with .exiting set true). This is fast.
 *
 * When the calling thread owns orphaned_threads.mutex it must not call this
 * function or deadlock my occur.
 */
static void wait_and_destroy_orphaned_threads(int flags)
{
  struct thread_list *thread = NULL;

  Curl_mutex_acquire(&orphaned_threads.mutex);

  if((flags & WAIT_DESTROY_EXITING_THREADS_ONLY)) {
    struct thread_list *p, *next;
    struct thread_list *first = NULL, *last = NULL;

    if(!orphaned_threads.exiting_count) {
      Curl_mutex_release(&orphaned_threads.mutex);
      return;
    }

    for(p = orphaned_threads.first; p; p = next) {
      next = p->next;

      if(!p->exiting)
        continue;

      /* remove thread list item from orphaned_threads */
      if(p->prev)
        p->prev->next = p->next;
      if(p->next)
        p->next->prev = p->prev;
      if(orphaned_threads.first == p)
        orphaned_threads.first = p->next;
      if(orphaned_threads.last == p)
        orphaned_threads.last = p->prev;

      /* add thread list item to new thread list */
      if(last) {
        last->next = p;
        p->prev = last;
      }
      else {
        first = p;
        p->prev = NULL;
      }
      last = p;
      p->next = NULL;
    }

    thread = first;
    orphaned_threads.exiting_count = 0;
  }
  else if((flags & WAIT_DESTROY_ALL)) {
    thread = orphaned_threads.first;
    orphaned_threads.first = NULL;
    orphaned_threads.last = NULL;
    orphaned_threads.exiting_count = 0;
  }

  Curl_mutex_release(&orphaned_threads.mutex);

  /* Wait and free. Must be done unlocked or there could be deadlock. */
  while(thread) {
    struct thread_list *next = thread->next;
    Curl_thread_join(&thread->handle);
    free(thread);
    thread = next;
  }
}

/* Helper function that must be called from an orphaned thread right before it
   exits. */
static void signal_orphan_is_exiting(struct thread_list *orphan)
{
  DEBUGASSERT(orphan->handle && !orphan->exiting);

  wait_and_destroy_orphaned_threads(WAIT_DESTROY_EXITING_THREADS_ONLY);

  Curl_mutex_acquire(&orphaned_threads.mutex);

  orphan->exiting = true;
  orphaned_threads.exiting_count++;

  Curl_mutex_release(&orphaned_threads.mutex);
}

#endif /* CURLRES_THREADED */