//
//
// Copyright 2016 gRPC authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
//

#include <grpc/impl/channel_arg_names.h>
#include <grpc/support/port_platform.h>

#include <algorithm>
#include <vector>

#include "absl/strings/string_view.h"
#include "src/core/lib/iomgr/sockaddr.h"
#include "src/core/util/status_helper.h"

// IWYU pragma: no_include <arpa/inet.h>
// IWYU pragma: no_include <arpa/nameser.h>
// IWYU pragma: no_include <inttypes.h>
// IWYU pragma: no_include <netdb.h>
// IWYU pragma: no_include <netinet/in.h>
// IWYU pragma: no_include <stdlib.h>
// IWYU pragma: no_include <sys/socket.h>

#if GRPC_ARES == 1

#include <address_sorting/address_sorting.h>
#include <ares.h>
#include <grpc/support/alloc.h>
#include <grpc/support/string_util.h>
#include <grpc/support/sync.h>
#include <string.h>
#include <sys/types.h>  // IWYU pragma: keep

#include <string>
#include <utility>

#include "absl/log/check.h"
#include "absl/log/log.h"
#include "absl/status/status.h"
#include "absl/status/statusor.h"
#include "absl/strings/str_cat.h"
#include "absl/strings/str_format.h"
#include "src/core/lib/address_utils/parse_address.h"
#include "src/core/lib/address_utils/sockaddr_utils.h"
#include "src/core/lib/channel/channel_args.h"
#include "src/core/lib/iomgr/error.h"
#include "src/core/lib/iomgr/exec_ctx.h"
#include "src/core/lib/iomgr/nameser.h"  // IWYU pragma: keep
#include "src/core/lib/iomgr/resolved_address.h"
#include "src/core/lib/iomgr/timer.h"
#include "src/core/resolver/dns/c_ares/grpc_ares_ev_driver.h"
#include "src/core/resolver/dns/c_ares/grpc_ares_wrapper.h"
#include "src/core/util/debug_location.h"
#include "src/core/util/host_port.h"
#include "src/core/util/string.h"
#include "src/core/util/time.h"

using grpc_core::EndpointAddresses;
using grpc_core::EndpointAddressesList;

typedef struct fd_node {
  // default constructor exists only for linked list manipulation
  fd_node() : ev_driver(nullptr) {}

  explicit fd_node(grpc_ares_ev_driver* ev_driver) : ev_driver(ev_driver) {}

  /// the owner of this fd node
  grpc_ares_ev_driver* const ev_driver;
  /// a closure wrapping on_readable_locked, which should be
  /// invoked when the grpc_fd in this node becomes readable.
  grpc_closure read_closure ABSL_GUARDED_BY(&grpc_ares_request::mu);
  /// a closure wrapping on_writable_locked, which should be
  /// invoked when the grpc_fd in this node becomes writable.
  grpc_closure write_closure ABSL_GUARDED_BY(&grpc_ares_request::mu);
  /// next fd node in the list
  struct fd_node* next ABSL_GUARDED_BY(&grpc_ares_request::mu);

  /// wrapped fd that's polled by grpc's poller for the current platform
  grpc_core::GrpcPolledFd* grpc_polled_fd
      ABSL_GUARDED_BY(&grpc_ares_request::mu);
  /// if the readable closure has been registered
  bool readable_registered ABSL_GUARDED_BY(&grpc_ares_request::mu);
  /// if the writable closure has been registered
  bool writable_registered ABSL_GUARDED_BY(&grpc_ares_request::mu);
  /// if the fd has been shutdown yet from grpc iomgr perspective
  bool already_shutdown ABSL_GUARDED_BY(&grpc_ares_request::mu);
} fd_node;

struct grpc_ares_ev_driver {
  explicit grpc_ares_ev_driver(grpc_ares_request* request) : request(request) {}

  /// the ares_channel owned by this event driver
  ares_channel channel ABSL_GUARDED_BY(&grpc_ares_request::mu);
  /// pollset set for driving the IO events of the channel
  grpc_pollset_set* pollset_set ABSL_GUARDED_BY(&grpc_ares_request::mu);
  /// refcount of the event driver
  gpr_refcount refs;

  /// a list of grpc_fd that this event driver is currently using.
  fd_node* fds ABSL_GUARDED_BY(&grpc_ares_request::mu);
  /// is this event driver being shut down
  bool shutting_down ABSL_GUARDED_BY(&grpc_ares_request::mu);
  /// request object that's using this ev driver
  grpc_ares_request* const request;
  /// Owned by the ev_driver. Creates new GrpcPolledFd's
  std::unique_ptr<grpc_core::GrpcPolledFdFactory> polled_fd_factory
      ABSL_GUARDED_BY(&grpc_ares_request::mu);
  /// query timeout in milliseconds
  int query_timeout_ms ABSL_GUARDED_BY(&grpc_ares_request::mu);
  /// alarm to cancel active queries
  grpc_timer query_timeout ABSL_GUARDED_BY(&grpc_ares_request::mu);
  /// cancels queries on a timeout
  grpc_closure on_timeout_locked ABSL_GUARDED_BY(&grpc_ares_request::mu);
  /// alarm to poll ares_process on in case fd events don't happen
  grpc_timer ares_backup_poll_alarm ABSL_GUARDED_BY(&grpc_ares_request::mu);
  /// polls ares_process on a periodic timer
  grpc_closure on_ares_backup_poll_alarm_locked
      ABSL_GUARDED_BY(&grpc_ares_request::mu);
};

// TODO(apolcyn): make grpc_ares_hostbyname_request a sub-class
// of GrpcAresQuery.
typedef struct grpc_ares_hostbyname_request {
  /// following members are set in create_hostbyname_request_locked
  ///
  /// the top-level request instance
  grpc_ares_request* parent_request;
  /// host to resolve, parsed from the name to resolve
  char* host;
  /// port to fill in sockaddr_in, parsed from the name to resolve
  uint16_t port;
  /// is it a grpclb address
  bool is_balancer;
  /// for logging and errors: the query type ("A" or "AAAA")
  const char* qtype;
} grpc_ares_hostbyname_request;

static void grpc_ares_request_ref_locked(grpc_ares_request* r)
    ABSL_EXCLUSIVE_LOCKS_REQUIRED(r->mu);
static void grpc_ares_request_unref_locked(grpc_ares_request* r)
    ABSL_EXCLUSIVE_LOCKS_REQUIRED(r->mu);

// TODO(apolcyn): as a part of C++-ification, find a way to
// organize per-query and per-resolution information in such a way
// that doesn't involve allocating a number of different data
// structures.
class GrpcAresQuery final {
 public:
  explicit GrpcAresQuery(grpc_ares_request* r, const std::string& name)
      : r_(r), name_(name) {
    grpc_ares_request_ref_locked(r_);
  }

  ~GrpcAresQuery() { grpc_ares_request_unref_locked(r_); }

  grpc_ares_request* parent_request() { return r_; }

  const std::string& name() { return name_; }

 private:
  // the top level request instance
  grpc_ares_request* r_;
  /// for logging and errors
  const std::string name_;
};

static absl::Status AresStatusToAbslStatus(int status,
                                           absl::string_view error_msg) {
  switch (status) {
    case ARES_ECANCELLED:
      return absl::CancelledError(error_msg);
    case ARES_ENOTIMP:
      return absl::UnimplementedError(error_msg);
    case ARES_ENOTFOUND:
      return absl::NotFoundError(error_msg);
    case ARES_ECONNREFUSED:
      return absl::UnavailableError(error_msg);
    default:
      return absl::UnknownError(error_msg);
  }
}

static grpc_ares_ev_driver* grpc_ares_ev_driver_ref(
    grpc_ares_ev_driver* ev_driver)
    ABSL_EXCLUSIVE_LOCKS_REQUIRED(&grpc_ares_request::mu) {
  GRPC_TRACE_VLOG(cares_resolver, 2)
      << "(c-ares resolver) request:" << ev_driver->request << " Ref ev_driver "
      << ev_driver;
  gpr_ref(&ev_driver->refs);
  return ev_driver;
}

static void grpc_ares_complete_request_locked(grpc_ares_request* r)
    ABSL_EXCLUSIVE_LOCKS_REQUIRED(r->mu);

static void grpc_ares_ev_driver_unref(grpc_ares_ev_driver* ev_driver)
    ABSL_EXCLUSIVE_LOCKS_REQUIRED(&grpc_ares_request::mu) {
  GRPC_TRACE_VLOG(cares_resolver, 2)
      << "(c-ares resolver) request:" << ev_driver->request
      << " Unref ev_driver " << ev_driver;
  if (gpr_unref(&ev_driver->refs)) {
    GRPC_TRACE_VLOG(cares_resolver, 2)
        << "(c-ares resolver) request:" << ev_driver->request
        << " destroy ev_driver " << ev_driver;
    CHECK_EQ(ev_driver->fds, nullptr);
    ares_destroy(ev_driver->channel);
    grpc_ares_complete_request_locked(ev_driver->request);
    delete ev_driver;
  }
}

static void fd_node_destroy_locked(fd_node* fdn)
    ABSL_EXCLUSIVE_LOCKS_REQUIRED(&grpc_ares_request::mu) {
  GRPC_TRACE_VLOG(cares_resolver, 2)
      << "(c-ares resolver) request:" << fdn->ev_driver->request
      << " delete fd: " << fdn->grpc_polled_fd->GetName();
  CHECK(!fdn->readable_registered);
  CHECK(!fdn->writable_registered);
  CHECK(fdn->already_shutdown);
  delete fdn->grpc_polled_fd;
  delete fdn;
}

static void fd_node_shutdown_locked(fd_node* fdn, const char* reason)
    ABSL_EXCLUSIVE_LOCKS_REQUIRED(&grpc_ares_request::mu) {
  if (!fdn->already_shutdown) {
    fdn->already_shutdown = true;
    fdn->grpc_polled_fd->ShutdownLocked(GRPC_ERROR_CREATE(reason));
  }
}

void grpc_ares_ev_driver_on_queries_complete_locked(
    grpc_ares_ev_driver* ev_driver)
    ABSL_EXCLUSIVE_LOCKS_REQUIRED(&grpc_ares_request::mu) {
  // We mark the event driver as being shut down.
  // grpc_ares_notify_on_event_locked will shut down any remaining
  // fds.
  ev_driver->shutting_down = true;
  grpc_timer_cancel(&ev_driver->query_timeout);
  grpc_timer_cancel(&ev_driver->ares_backup_poll_alarm);
  grpc_ares_ev_driver_unref(ev_driver);
}

void grpc_ares_ev_driver_shutdown_locked(grpc_ares_ev_driver* ev_driver)
    ABSL_EXCLUSIVE_LOCKS_REQUIRED(&grpc_ares_request::mu) {
  ev_driver->shutting_down = true;
  fd_node* fn = ev_driver->fds;
  while (fn != nullptr) {
    fd_node_shutdown_locked(fn, "grpc_ares_ev_driver_shutdown");
    fn = fn->next;
  }
}

// Search fd in the fd_node list head. This is an O(n) search, the max possible
// value of n is ARES_GETSOCK_MAXNUM (16). n is typically 1 - 2 in our tests.
static fd_node* pop_fd_node_locked(fd_node** head, ares_socket_t as)
    ABSL_EXCLUSIVE_LOCKS_REQUIRED(&grpc_ares_request::mu) {
  fd_node phony_head;
  phony_head.next = *head;
  fd_node* node = &phony_head;
  while (node->next != nullptr) {
    if (node->next->grpc_polled_fd->GetWrappedAresSocketLocked() == as) {
      fd_node* ret = node->next;
      node->next = node->next->next;
      *head = phony_head.next;
      return ret;
    }
    node = node->next;
  }
  return nullptr;
}

static grpc_core::Timestamp calculate_next_ares_backup_poll_alarm(
    grpc_ares_ev_driver* driver)
    ABSL_EXCLUSIVE_LOCKS_REQUIRED(&grpc_ares_request::mu) {
  // An alternative here could be to use ares_timeout to try to be more
  // accurate, but that would require using "struct timeval"'s, which just makes
  // things a bit more complicated. So just poll every second, as suggested
  // by the c-ares code comments.
  grpc_core::Duration until_next_ares_backup_poll_alarm =
      grpc_core::Duration::Seconds(1);
  GRPC_TRACE_VLOG(cares_resolver, 2)
      << "(c-ares resolver) request:" << driver->request
      << " ev_driver=" << driver << ". next ares process poll time in "
      << until_next_ares_backup_poll_alarm.millis() << " ms";
  return grpc_core::Timestamp::Now() + until_next_ares_backup_poll_alarm;
}

static void on_timeout(void* arg, grpc_error_handle error) {
  grpc_ares_ev_driver* driver = static_cast<grpc_ares_ev_driver*>(arg);
  grpc_core::MutexLock lock(&driver->request->mu);
  GRPC_TRACE_VLOG(cares_resolver, 2)
      << "(c-ares resolver) request:" << driver->request
      << " ev_driver=" << driver
      << " on_timeout_locked. driver->shutting_down=" << driver->shutting_down
      << ". err=" << grpc_core::StatusToString(error);
  if (!driver->shutting_down && error.ok()) {
    grpc_ares_ev_driver_shutdown_locked(driver);
  }
  grpc_ares_ev_driver_unref(driver);
}

static void grpc_ares_notify_on_event_locked(grpc_ares_ev_driver* ev_driver)
    ABSL_EXCLUSIVE_LOCKS_REQUIRED(&grpc_ares_request::mu);

// In case of non-responsive DNS servers, dropped packets, etc., c-ares has
// intelligent timeout and retry logic, which we can take advantage of by
// polling ares_process_fd on time intervals. Overall, the c-ares library is
// meant to be called into and given a chance to proceed name resolution:
//   a) when fd events happen
//   b) when some time has passed without fd events having happened
// For the latter, we use this backup poller. Also see
// https://github.com/grpc/grpc/pull/17688 description for more details.
static void on_ares_backup_poll_alarm(void* arg, grpc_error_handle error) {
  grpc_ares_ev_driver* driver = static_cast<grpc_ares_ev_driver*>(arg);
  grpc_core::MutexLock lock(&driver->request->mu);
  GRPC_TRACE_VLOG(cares_resolver, 2)
      << "(c-ares resolver) request:" << driver->request
      << " ev_driver=" << driver
      << " on_ares_backup_poll_alarm_locked. driver->shutting_down="
      << driver->shutting_down << ". err=" << grpc_core::StatusToString(error);
  if (!driver->shutting_down && error.ok()) {
    fd_node* fdn = driver->fds;
    while (fdn != nullptr) {
      if (!fdn->already_shutdown) {
        GRPC_TRACE_VLOG(cares_resolver, 2)
            << "(c-ares resolver) request:" << driver->request
            << " ev_driver=" << driver
            << " on_ares_backup_poll_alarm_locked; ares_process_fd. fd="
            << fdn->grpc_polled_fd->GetName();
        ares_socket_t as = fdn->grpc_polled_fd->GetWrappedAresSocketLocked();
        ares_process_fd(driver->channel, as, as);
      }
      fdn = fdn->next;
    }
    if (!driver->shutting_down) {
      // InvalidateNow to avoid getting stuck re-initializing this timer
      // in a loop while draining the currently-held WorkSerializer.
      // Also see https://github.com/grpc/grpc/issues/26079.
      grpc_core::ExecCtx::Get()->InvalidateNow();
      grpc_core::Timestamp next_ares_backup_poll_alarm =
          calculate_next_ares_backup_poll_alarm(driver);
      grpc_ares_ev_driver_ref(driver);
      GRPC_CLOSURE_INIT(&driver->on_ares_backup_poll_alarm_locked,
                        on_ares_backup_poll_alarm, driver,
                        grpc_schedule_on_exec_ctx);
      grpc_timer_init(&driver->ares_backup_poll_alarm,
                      next_ares_backup_poll_alarm,
                      &driver->on_ares_backup_poll_alarm_locked);
    }
    grpc_ares_notify_on_event_locked(driver);
  }
  grpc_ares_ev_driver_unref(driver);
}

static void on_readable(void* arg, grpc_error_handle error) {
  fd_node* fdn = static_cast<fd_node*>(arg);
  grpc_core::MutexLock lock(&fdn->ev_driver->request->mu);
  CHECK(fdn->readable_registered);
  grpc_ares_ev_driver* ev_driver = fdn->ev_driver;
  const ares_socket_t as = fdn->grpc_polled_fd->GetWrappedAresSocketLocked();
  fdn->readable_registered = false;
  GRPC_TRACE_VLOG(cares_resolver, 2)
      << "(c-ares resolver) request:" << fdn->ev_driver->request
      << " readable on " << fdn->grpc_polled_fd->GetName();
  if (error.ok() && !ev_driver->shutting_down) {
    ares_process_fd(ev_driver->channel, as, ARES_SOCKET_BAD);
  } else {
    // If error is not absl::OkStatus() or the resolution was cancelled, it
    // means the fd has been shutdown or timed out. The pending lookups made on
    // this ev_driver will be cancelled by the following ares_cancel() and the
    // on_done callbacks will be invoked with a status of ARES_ECANCELLED. The
    // remaining file descriptors in this ev_driver will be cleaned up in the
    // following grpc_ares_notify_on_event_locked().
    ares_cancel(ev_driver->channel);
  }
  grpc_ares_notify_on_event_locked(ev_driver);
  grpc_ares_ev_driver_unref(ev_driver);
}

static void on_writable(void* arg, grpc_error_handle error) {
  fd_node* fdn = static_cast<fd_node*>(arg);
  grpc_core::MutexLock lock(&fdn->ev_driver->request->mu);
  CHECK(fdn->writable_registered);
  grpc_ares_ev_driver* ev_driver = fdn->ev_driver;
  const ares_socket_t as = fdn->grpc_polled_fd->GetWrappedAresSocketLocked();
  fdn->writable_registered = false;
  GRPC_TRACE_VLOG(cares_resolver, 2)
      << "(c-ares resolver) request:" << ev_driver->request << " writable on "
      << fdn->grpc_polled_fd->GetName();
  if (error.ok() && !ev_driver->shutting_down) {
    ares_process_fd(ev_driver->channel, ARES_SOCKET_BAD, as);
  } else {
    // If error is not absl::OkStatus() or the resolution was cancelled, it
    // means the fd has been shutdown or timed out. The pending lookups made on
    // this ev_driver will be cancelled by the following ares_cancel() and the
    // on_done callbacks will be invoked with a status of ARES_ECANCELLED. The
    // remaining file descriptors in this ev_driver will be cleaned up in the
    // following grpc_ares_notify_on_event_locked().
    ares_cancel(ev_driver->channel);
  }
  grpc_ares_notify_on_event_locked(ev_driver);
  grpc_ares_ev_driver_unref(ev_driver);
}

// Get the file descriptors used by the ev_driver's ares channel, register
// driver_closure with these filedescriptors.
static void grpc_ares_notify_on_event_locked(grpc_ares_ev_driver* ev_driver)
    ABSL_EXCLUSIVE_LOCKS_REQUIRED(&grpc_ares_request::mu) {
  fd_node* new_list = nullptr;
  if (!ev_driver->shutting_down) {
    ares_socket_t socks[ARES_GETSOCK_MAXNUM];
    int socks_bitmask =
        ares_getsock(ev_driver->channel, socks, ARES_GETSOCK_MAXNUM);
    for (size_t i = 0; i < ARES_GETSOCK_MAXNUM; i++) {
      if (ARES_GETSOCK_READABLE(socks_bitmask, i) ||
          ARES_GETSOCK_WRITABLE(socks_bitmask, i)) {
        fd_node* fdn = pop_fd_node_locked(&ev_driver->fds, socks[i]);
        // Create a new fd_node if sock[i] is not in the fd_node list.
        if (fdn == nullptr) {
          fdn = new fd_node(ev_driver);
          fdn->grpc_polled_fd =
              ev_driver->polled_fd_factory->NewGrpcPolledFdLocked(
                  socks[i], ev_driver->pollset_set);
          GRPC_TRACE_VLOG(cares_resolver, 2)
              << "(c-ares resolver) request:" << ev_driver->request
              << " new fd: " << fdn->grpc_polled_fd->GetName();
          fdn->readable_registered = false;
          fdn->writable_registered = false;
          fdn->already_shutdown = false;
        }
        fdn->next = new_list;
        new_list = fdn;
        // Register read_closure if the socket is readable and read_closure has
        // not been registered with this socket.
        if (ARES_GETSOCK_READABLE(socks_bitmask, i) &&
            !fdn->readable_registered) {
          grpc_ares_ev_driver_ref(ev_driver);
          GRPC_CLOSURE_INIT(&fdn->read_closure, on_readable, fdn,
                            grpc_schedule_on_exec_ctx);
          if (fdn->grpc_polled_fd->IsFdStillReadableLocked()) {
            GRPC_TRACE_VLOG(cares_resolver, 2)
                << "(c-ares resolver) request:" << ev_driver->request
                << " schedule direct read on: "
                << fdn->grpc_polled_fd->GetName();
            grpc_core::ExecCtx::Run(DEBUG_LOCATION, &fdn->read_closure,
                                    absl::OkStatus());
          } else {
            GRPC_TRACE_VLOG(cares_resolver, 2)
                << "(c-ares resolver) request:" << ev_driver->request
                << " notify read on: " << fdn->grpc_polled_fd->GetName();
            fdn->grpc_polled_fd->RegisterForOnReadableLocked(
                &fdn->read_closure);
          }
          fdn->readable_registered = true;
        }
        // Register write_closure if the socket is writable and write_closure
        // has not been registered with this socket.
        if (ARES_GETSOCK_WRITABLE(socks_bitmask, i) &&
            !fdn->writable_registered) {
          GRPC_TRACE_VLOG(cares_resolver, 2)
              << "(c-ares resolver) request:" << ev_driver->request
              << " notify write on: " << fdn->grpc_polled_fd->GetName();
          grpc_ares_ev_driver_ref(ev_driver);
          GRPC_CLOSURE_INIT(&fdn->write_closure, on_writable, fdn,
                            grpc_schedule_on_exec_ctx);
          fdn->grpc_polled_fd->RegisterForOnWriteableLocked(
              &fdn->write_closure);
          fdn->writable_registered = true;
        }
      }
    }
  }
  // Any remaining fds in ev_driver->fds were not returned by ares_getsock() and
  // are therefore no longer in use, so they can be shut down and removed from
  // the list.
  while (ev_driver->fds != nullptr) {
    fd_node* cur = ev_driver->fds;
    ev_driver->fds = ev_driver->fds->next;
    fd_node_shutdown_locked(cur, "c-ares fd shutdown");
    if (!cur->readable_registered && !cur->writable_registered) {
      fd_node_destroy_locked(cur);
    } else {
      cur->next = new_list;
      new_list = cur;
    }
  }
  ev_driver->fds = new_list;
}

void grpc_ares_ev_driver_start_locked(grpc_ares_ev_driver* ev_driver)
    ABSL_EXCLUSIVE_LOCKS_REQUIRED(&grpc_ares_request::mu) {
  grpc_ares_notify_on_event_locked(ev_driver);
  // Initialize overall DNS resolution timeout alarm
  grpc_core::Duration timeout =
      ev_driver->query_timeout_ms == 0
          ? grpc_core::Duration::Infinity()
          : grpc_core::Duration::Milliseconds(ev_driver->query_timeout_ms);
  GRPC_TRACE_VLOG(cares_resolver, 2)
      << "(c-ares resolver) request:" << ev_driver->request
      << " ev_driver=" << ev_driver
      << " grpc_ares_ev_driver_start_locked. timeout in " << timeout.millis()
      << " ms";
  grpc_ares_ev_driver_ref(ev_driver);
  GRPC_CLOSURE_INIT(&ev_driver->on_timeout_locked, on_timeout, ev_driver,
                    grpc_schedule_on_exec_ctx);
  grpc_timer_init(&ev_driver->query_timeout,
                  grpc_core::Timestamp::Now() + timeout,
                  &ev_driver->on_timeout_locked);
  // Initialize the backup poll alarm
  grpc_core::Timestamp next_ares_backup_poll_alarm =
      calculate_next_ares_backup_poll_alarm(ev_driver);
  grpc_ares_ev_driver_ref(ev_driver);
  GRPC_CLOSURE_INIT(&ev_driver->on_ares_backup_poll_alarm_locked,
                    on_ares_backup_poll_alarm, ev_driver,
                    grpc_schedule_on_exec_ctx);
  grpc_timer_init(&ev_driver->ares_backup_poll_alarm,
                  next_ares_backup_poll_alarm,
                  &ev_driver->on_ares_backup_poll_alarm_locked);
}

static void noop_inject_channel_config(ares_channel* /*channel*/) {}

void (*grpc_ares_test_only_inject_config)(ares_channel* channel) =
    noop_inject_channel_config;

bool g_grpc_ares_test_only_force_tcp = false;

grpc_error_handle grpc_ares_ev_driver_create_locked(
    grpc_ares_ev_driver** ev_driver, grpc_pollset_set* pollset_set,
    int query_timeout_ms, grpc_ares_request* request)
    ABSL_EXCLUSIVE_LOCKS_REQUIRED(request->mu) {
  *ev_driver = new grpc_ares_ev_driver(request);
  ares_options opts;
  memset(&opts, 0, sizeof(opts));
  opts.flags |= ARES_FLAG_STAYOPEN;
  if (g_grpc_ares_test_only_force_tcp) {
    opts.flags |= ARES_FLAG_USEVC;
  }
  int status = ares_init_options(&(*ev_driver)->channel, &opts, ARES_OPT_FLAGS);
  grpc_ares_test_only_inject_config(&(*ev_driver)->channel);
  GRPC_TRACE_VLOG(cares_resolver, 2) << "(c-ares resolver) request:" << request
                                     << " grpc_ares_ev_driver_create_locked";
  if (status != ARES_SUCCESS) {
    grpc_error_handle err = GRPC_ERROR_CREATE(absl::StrCat(
        "Failed to init ares channel. C-ares error: ", ares_strerror(status)));
    delete *ev_driver;
    return err;
  }
  gpr_ref_init(&(*ev_driver)->refs, 1);
  (*ev_driver)->pollset_set = pollset_set;
  (*ev_driver)->fds = nullptr;
  (*ev_driver)->shutting_down = false;
  (*ev_driver)->polled_fd_factory =
      grpc_core::NewGrpcPolledFdFactory(&(*ev_driver)->request->mu);
  (*ev_driver)
      ->polled_fd_factory->ConfigureAresChannelLocked((*ev_driver)->channel);
  (*ev_driver)->query_timeout_ms = query_timeout_ms;
  return absl::OkStatus();
}

static void log_address_sorting_list(const grpc_ares_request* r,
                                     const EndpointAddressesList& addresses,
                                     const char* input_output_str) {
  for (size_t i = 0; i < addresses.size(); i++) {
    auto addr_str = grpc_sockaddr_to_string(&addresses[i].address(), true);
    LOG(INFO) << "(c-ares resolver) request:" << r
              << " c-ares address sorting: " << input_output_str << "[" << i
              << "]="
              << (addr_str.ok() ? addr_str->c_str()
                                : addr_str.status().ToString().c_str());
  }
}

void grpc_cares_wrapper_address_sorting_sort(const grpc_ares_request* r,
                                             EndpointAddressesList* addresses) {
  if (GRPC_TRACE_FLAG_ENABLED(cares_address_sorting)) {
    log_address_sorting_list(r, *addresses, "input");
  }
  address_sorting_sortable* sortables = static_cast<address_sorting_sortable*>(
      gpr_zalloc(sizeof(address_sorting_sortable) * addresses->size()));
  for (size_t i = 0; i < addresses->size(); ++i) {
    sortables[i].user_data = &(*addresses)[i];
    memcpy(&sortables[i].dest_addr.addr, &(*addresses)[i].address().addr,
           (*addresses)[i].address().len);
    sortables[i].dest_addr.len = (*addresses)[i].address().len;
  }
  address_sorting_rfc_6724_sort(sortables, addresses->size());
  EndpointAddressesList sorted;
  sorted.reserve(addresses->size());
  for (size_t i = 0; i < addresses->size(); ++i) {
    sorted.emplace_back(
        *static_cast<EndpointAddresses*>(sortables[i].user_data));
  }
  gpr_free(sortables);
  *addresses = std::move(sorted);
  if (GRPC_TRACE_FLAG_ENABLED(cares_address_sorting)) {
    log_address_sorting_list(r, *addresses, "output");
  }
}

static void grpc_ares_request_ref_locked(grpc_ares_request* r)
    ABSL_EXCLUSIVE_LOCKS_REQUIRED(r->mu) {
  r->pending_queries++;
}

static void grpc_ares_request_unref_locked(grpc_ares_request* r)
    ABSL_EXCLUSIVE_LOCKS_REQUIRED(r->mu) {
  r->pending_queries--;
  if (r->pending_queries == 0u) {
    grpc_ares_ev_driver_on_queries_complete_locked(r->ev_driver);
  }
}

void grpc_ares_complete_request_locked(grpc_ares_request* r)
    ABSL_EXCLUSIVE_LOCKS_REQUIRED(r->mu) {
  // Invoke on_done callback and destroy the request
  r->ev_driver = nullptr;
  if (r->addresses_out != nullptr && *r->addresses_out != nullptr) {
    grpc_cares_wrapper_address_sorting_sort(r, r->addresses_out->get());
    r->error = absl::OkStatus();
    // TODO(apolcyn): allow c-ares to return a service config
    // with no addresses along side it
  }
  if (r->balancer_addresses_out != nullptr) {
    EndpointAddressesList* balancer_addresses =
        r->balancer_addresses_out->get();
    if (balancer_addresses != nullptr) {
      grpc_cares_wrapper_address_sorting_sort(r, balancer_addresses);
    }
  }
  grpc_core::ExecCtx::Run(DEBUG_LOCATION, r->on_done, r->error);
}

// Note that the returned object takes a reference to qtype, so
// qtype must outlive it.
static grpc_ares_hostbyname_request* create_hostbyname_request_locked(
    grpc_ares_request* parent_request, const char* host, uint16_t port,
    bool is_balancer, const char* qtype)
    ABSL_EXCLUSIVE_LOCKS_REQUIRED(parent_request->mu) {
  GRPC_TRACE_VLOG(cares_resolver, 2)
      << "(c-ares resolver) request:" << parent_request
      << " create_hostbyname_request_locked host:" << host << " port:" << port
      << " is_balancer:" << is_balancer << " qtype:" << qtype;
  grpc_ares_hostbyname_request* hr = new grpc_ares_hostbyname_request();
  hr->parent_request = parent_request;
  hr->host = gpr_strdup(host);
  hr->port = port;
  hr->is_balancer = is_balancer;
  hr->qtype = qtype;
  grpc_ares_request_ref_locked(parent_request);
  return hr;
}

static void destroy_hostbyname_request_locked(grpc_ares_hostbyname_request* hr)
    ABSL_EXCLUSIVE_LOCKS_REQUIRED(hr->parent_request->mu) {
  grpc_ares_request_unref_locked(hr->parent_request);
  gpr_free(hr->host);
  delete hr;
}

static void on_hostbyname_done_locked(void* arg, int status, int /*timeouts*/,
                                      struct hostent* hostent)
    ABSL_NO_THREAD_SAFETY_ANALYSIS {
  // This callback is invoked from the c-ares library, so disable thread safety
  // analysis. Note that we are guaranteed to be holding r->mu, though.
  grpc_ares_hostbyname_request* hr =
      static_cast<grpc_ares_hostbyname_request*>(arg);
  grpc_ares_request* r = hr->parent_request;
  if (status == ARES_SUCCESS) {
    GRPC_TRACE_VLOG(cares_resolver, 2)
        << "(c-ares resolver) request:" << r
        << " on_hostbyname_done_locked qtype=" << hr->qtype
        << " host=" << hr->host << " ARES_SUCCESS";
    std::unique_ptr<EndpointAddressesList>* address_list_ptr =
        hr->is_balancer ? r->balancer_addresses_out : r->addresses_out;
    if (*address_list_ptr == nullptr) {
      *address_list_ptr = std::make_unique<EndpointAddressesList>();
    }
    EndpointAddressesList& addresses = **address_list_ptr;
    for (size_t i = 0; hostent->h_addr_list[i] != nullptr; ++i) {
      grpc_core::ChannelArgs args;
      if (hr->is_balancer) {
        args = args.Set(GRPC_ARG_DEFAULT_AUTHORITY, hr->host);
      }
      grpc_resolved_address address;
      memset(&address, 0, sizeof(address));
      switch (hostent->h_addrtype) {
        case AF_INET6: {
          address.len = sizeof(struct sockaddr_in6);
          auto* addr = reinterpret_cast<struct sockaddr_in6*>(&address.addr);
          memcpy(&addr->sin6_addr, hostent->h_addr_list[i],
                 sizeof(struct in6_addr));
          addr->sin6_family = static_cast<unsigned char>(hostent->h_addrtype);
          addr->sin6_port = hr->port;
          char output[INET6_ADDRSTRLEN];
          ares_inet_ntop(AF_INET6, &addr->sin6_addr, output, INET6_ADDRSTRLEN);
          GRPC_TRACE_VLOG(cares_resolver, 2)
              << "(c-ares resolver) request:" << r
              << " c-ares resolver gets a AF_INET6 result: \n"
              << "  addr: " << output << "\n  port: " << ntohs(hr->port)
              << "\n  sin6_scope_id: " << addr->sin6_scope_id << "\n";
          break;
        }
        case AF_INET: {
          address.len = sizeof(struct sockaddr_in);
          auto* addr = reinterpret_cast<struct sockaddr_in*>(&address.addr);
          memcpy(&addr->sin_addr, hostent->h_addr_list[i],
                 sizeof(struct in_addr));
          addr->sin_family = static_cast<unsigned char>(hostent->h_addrtype);
          addr->sin_port = hr->port;
          char output[INET_ADDRSTRLEN];
          ares_inet_ntop(AF_INET, &addr->sin_addr, output, INET_ADDRSTRLEN);
          GRPC_TRACE_VLOG(cares_resolver, 2)
              << "(c-ares resolver) request:" << r
              << " c-ares resolver gets a AF_INET result: \n  addr: " << output
              << "\n  port: " << ntohs(hr->port) << "\n";
          break;
        }
      }
      addresses.emplace_back(address, args);
    }
  } else {
    std::string error_msg = absl::StrFormat(
        "C-ares status is not ARES_SUCCESS qtype=%s name=%s is_balancer=%d: %s",
        hr->qtype, hr->host, hr->is_balancer, ares_strerror(status));
    GRPC_TRACE_VLOG(cares_resolver, 2)
        << "(c-ares resolver) request:" << r
        << " on_hostbyname_done_locked: " << error_msg;
    r->error = grpc_error_add_child(AresStatusToAbslStatus(status, error_msg),
                                    r->error);
  }
  destroy_hostbyname_request_locked(hr);
}

static void on_srv_query_done_locked(void* arg, int status, int /*timeouts*/,
                                     unsigned char* abuf,
                                     int alen) ABSL_NO_THREAD_SAFETY_ANALYSIS {
  // This callback is invoked from the c-ares library, so disable thread safety
  // analysis. Note that we are guaranteed to be holding r->mu, though.
  GrpcAresQuery* q = static_cast<GrpcAresQuery*>(arg);
  grpc_ares_request* r = q->parent_request();
  if (status == ARES_SUCCESS) {
    GRPC_TRACE_VLOG(cares_resolver, 2)
        << "(c-ares resolver) request:" << r
        << " on_srv_query_done_locked name=" << q->name() << " ARES_SUCCESS";
    struct ares_srv_reply* reply;
    const int parse_status = ares_parse_srv_reply(abuf, alen, &reply);
    GRPC_TRACE_VLOG(cares_resolver, 2)
        << "(c-ares resolver) request:" << r
        << " ares_parse_srv_reply: " << parse_status;
    if (parse_status == ARES_SUCCESS) {
      for (struct ares_srv_reply* srv_it = reply; srv_it != nullptr;
           srv_it = srv_it->next) {
        if (grpc_ares_query_ipv6()) {
          grpc_ares_hostbyname_request* hr = create_hostbyname_request_locked(
              r, srv_it->host, htons(srv_it->port), true /* is_balancer */,
              "AAAA");
          ares_gethostbyname(r->ev_driver->channel, hr->host, AF_INET6,
                             on_hostbyname_done_locked, hr);
        }
        grpc_ares_hostbyname_request* hr = create_hostbyname_request_locked(
            r, srv_it->host, htons(srv_it->port), true /* is_balancer */, "A");
        ares_gethostbyname(r->ev_driver->channel, hr->host, AF_INET,
                           on_hostbyname_done_locked, hr);
      }
    }
    if (reply != nullptr) {
      ares_free_data(reply);
    }
  } else {
    std::string error_msg = absl::StrFormat(
        "C-ares status is not ARES_SUCCESS qtype=SRV name=%s: %s", q->name(),
        ares_strerror(status));
    GRPC_TRACE_VLOG(cares_resolver, 2)
        << "(c-ares resolver) request:" << r
        << " on_srv_query_done_locked: " << error_msg;
    r->error = grpc_error_add_child(AresStatusToAbslStatus(status, error_msg),
                                    r->error);
  }
  delete q;
}

static const char g_service_config_attribute_prefix[] = "grpc_config=";

static void on_txt_done_locked(void* arg, int status, int /*timeouts*/,
                               unsigned char* buf,
                               int len) ABSL_NO_THREAD_SAFETY_ANALYSIS {
  // This callback is invoked from the c-ares library, so disable thread safety
  // analysis. Note that we are guaranteed to be holding r->mu, though.
  GrpcAresQuery* q = static_cast<GrpcAresQuery*>(arg);
  std::unique_ptr<GrpcAresQuery> query_deleter(q);
  grpc_ares_request* r = q->parent_request();
  const size_t prefix_len = sizeof(g_service_config_attribute_prefix) - 1;
  struct ares_txt_ext* result = nullptr;
  struct ares_txt_ext* reply = nullptr;
  if (status != ARES_SUCCESS) goto fail;
  GRPC_TRACE_VLOG(cares_resolver, 2)
      << "(c-ares resolver) request:" << r
      << " on_txt_done_locked name=" << q->name() << " ARES_SUCCESS";
  status = ares_parse_txt_reply_ext(buf, len, &reply);
  if (status != ARES_SUCCESS) goto fail;
  // Find service config in TXT record.
  for (result = reply; result != nullptr; result = result->next) {
    if (result->record_start &&
        memcmp(result->txt, g_service_config_attribute_prefix, prefix_len) ==
            0) {
      break;
    }
  }
  // Found a service config record.
  if (result != nullptr) {
    size_t service_config_len = result->length - prefix_len;
    *r->service_config_json_out =
        static_cast<char*>(gpr_malloc(service_config_len + 1));
    memcpy(*r->service_config_json_out, result->txt + prefix_len,
           service_config_len);
    for (result = result->next; result != nullptr && !result->record_start;
         result = result->next) {
      *r->service_config_json_out = static_cast<char*>(
          gpr_realloc(*r->service_config_json_out,
                      service_config_len + result->length + 1));
      memcpy(*r->service_config_json_out + service_config_len, result->txt,
             result->length);
      service_config_len += result->length;
    }
    (*r->service_config_json_out)[service_config_len] = '\0';
    GRPC_TRACE_VLOG(cares_resolver, 2)
        << "(c-ares resolver) request:" << r
        << " found service config: " << *r->service_config_json_out;
  }
  // Clean up.
  ares_free_data(reply);
  grpc_ares_request_unref_locked(r);
  return;
fail:
  std::string error_msg =
      absl::StrFormat("C-ares status is not ARES_SUCCESS qtype=TXT name=%s: %s",
                      q->name(), ares_strerror(status));
  GRPC_TRACE_VLOG(cares_resolver, 2) << "(c-ares resolver) request:" << r
                                     << " on_txt_done_locked " << error_msg;
  r->error =
      grpc_error_add_child(AresStatusToAbslStatus(status, error_msg), r->error);
}

grpc_error_handle set_request_dns_server(grpc_ares_request* r,
                                         absl::string_view dns_server)
    ABSL_EXCLUSIVE_LOCKS_REQUIRED(r->mu) {
  if (!dns_server.empty()) {
    GRPC_TRACE_VLOG(cares_resolver, 2)
        << "(c-ares resolver) request:" << r << " Using DNS server "
        << dns_server.data();
    grpc_resolved_address addr;
    if (grpc_parse_ipv4_hostport(dns_server, &addr, /*log_errors=*/false)) {
      r->dns_server_addr.family = AF_INET;
      struct sockaddr_in* in = reinterpret_cast<struct sockaddr_in*>(addr.addr);
      memcpy(&r->dns_server_addr.addr.addr4, &in->sin_addr,
             sizeof(struct in_addr));
      r->dns_server_addr.tcp_port = grpc_sockaddr_get_port(&addr);
      r->dns_server_addr.udp_port = grpc_sockaddr_get_port(&addr);
    } else if (grpc_parse_ipv6_hostport(dns_server, &addr,
                                        /*log_errors=*/false)) {
      r->dns_server_addr.family = AF_INET6;
      struct sockaddr_in6* in6 =
          reinterpret_cast<struct sockaddr_in6*>(addr.addr);
      memcpy(&r->dns_server_addr.addr.addr6, &in6->sin6_addr,
             sizeof(struct in6_addr));
      r->dns_server_addr.tcp_port = grpc_sockaddr_get_port(&addr);
      r->dns_server_addr.udp_port = grpc_sockaddr_get_port(&addr);
    } else {
      return GRPC_ERROR_CREATE(
          absl::StrCat("cannot parse authority ", dns_server));
    }
    int status =
        ares_set_servers_ports(r->ev_driver->channel, &r->dns_server_addr);
    if (status != ARES_SUCCESS) {
      return GRPC_ERROR_CREATE(absl::StrCat(
          "C-ares status is not ARES_SUCCESS: ", ares_strerror(status)));
    }
  }
  return absl::OkStatus();
}

// Common logic for all lookup methods.
// If an error occurs, callers must run the client callback.
grpc_error_handle grpc_dns_lookup_ares_continued(
    grpc_ares_request* r, const char* dns_server, const char* name,
    const char* default_port, grpc_pollset_set* interested_parties,
    int query_timeout_ms, std::string* host, std::string* port, bool check_port)
    ABSL_EXCLUSIVE_LOCKS_REQUIRED(r->mu) {
  grpc_error_handle error;
  // parse name, splitting it into host and port parts
  grpc_core::SplitHostPort(name, host, port);
  if (host->empty()) {
    error =
        GRPC_ERROR_CREATE(absl::StrCat("unparsable host:port \"", name, "\""));
    return error;
  } else if (check_port && port->empty()) {
    if (default_port == nullptr || strlen(default_port) == 0) {
      error = GRPC_ERROR_CREATE(absl::StrCat("no port in name \"", name, "\""));
      return error;
    }
    *port = default_port;
  }
  error = grpc_ares_ev_driver_create_locked(&r->ev_driver, interested_parties,
                                            query_timeout_ms, r);
  if (!error.ok()) return error;
  // If dns_server is specified, use it.
  error = set_request_dns_server(r, dns_server);
  return error;
}

static bool inner_resolve_as_ip_literal_locked(
    const char* name, const char* default_port,
    std::unique_ptr<grpc_core::EndpointAddressesList>* addrs, std::string* host,
    std::string* port, std::string* hostport) {
  if (!grpc_core::SplitHostPort(name, host, port)) {
    LOG(ERROR) << "Failed to parse " << name
               << " to host:port while attempting to resolve as ip literal.";
    return false;
  }
  if (port->empty()) {
    if (default_port == nullptr || strlen(default_port) == 0) {
      LOG(ERROR) << "No port or default port for " << name
                 << " while attempting to resolve as ip literal.";
      return false;
    }
    *port = default_port;
  }
  grpc_resolved_address addr;
  *hostport = grpc_core::JoinHostPort(*host, atoi(port->c_str()));
  if (grpc_parse_ipv4_hostport(hostport->c_str(), &addr,
                               false /* log errors */) ||
      grpc_parse_ipv6_hostport(hostport->c_str(), &addr,
                               false /* log errors */)) {
    CHECK(*addrs == nullptr);
    *addrs = std::make_unique<EndpointAddressesList>();
    (*addrs)->emplace_back(addr, grpc_core::ChannelArgs());
    return true;
  }
  return false;
}

static bool resolve_as_ip_literal_locked(
    const char* name, const char* default_port,
    std::unique_ptr<grpc_core::EndpointAddressesList>* addrs) {
  std::string host;
  std::string port;
  std::string hostport;
  bool out = inner_resolve_as_ip_literal_locked(name, default_port, addrs,
                                                &host, &port, &hostport);
  return out;
}

static bool target_matches_localhost_inner(const char* name, std::string* host,
                                           std::string* port) {
  if (!grpc_core::SplitHostPort(name, host, port)) {
    LOG(ERROR) << "Unable to split host and port for name: " << name;
    return false;
  }
  return gpr_stricmp(host->c_str(), "localhost") == 0;
}

static bool target_matches_localhost(const char* name) {
  std::string host;
  std::string port;
  return target_matches_localhost_inner(name, &host, &port);
}

#ifdef GRPC_ARES_RESOLVE_LOCALHOST_MANUALLY
static bool inner_maybe_resolve_localhost_manually_locked(
    const grpc_ares_request* r, const char* name, const char* default_port,
    std::unique_ptr<grpc_core::EndpointAddressesList>* addrs, std::string* host,
    std::string* port) {
  grpc_core::SplitHostPort(name, host, port);
  if (host->empty()) {
    LOG(ERROR) << "Failed to parse " << name
               << " into host:port during manual localhost resolution check.";
    return false;
  }
  if (port->empty()) {
    if (default_port == nullptr || strlen(default_port) == 0) {
      LOG(ERROR) << "No port or default port for " << name
                 << " during manual localhost resolution check.";
      return false;
    }
    *port = default_port;
  }
  if (gpr_stricmp(host->c_str(), "localhost") == 0) {
    CHECK(*addrs == nullptr);
    *addrs = std::make_unique<grpc_core::EndpointAddressesList>();
    uint16_t numeric_port = grpc_strhtons(port->c_str());
    grpc_resolved_address address;
    // Append the ipv6 loopback address.
    memset(&address, 0, sizeof(address));
    auto* ipv6_loopback_addr =
        reinterpret_cast<struct sockaddr_in6*>(&address.addr);
    ((char*)&ipv6_loopback_addr->sin6_addr)[15] = 1;
    ipv6_loopback_addr->sin6_family = AF_INET6;
    ipv6_loopback_addr->sin6_port = numeric_port;
    address.len = sizeof(struct sockaddr_in6);
    (*addrs)->emplace_back(address, grpc_core::ChannelArgs());
    // Append the ipv4 loopback address.
    memset(&address, 0, sizeof(address));
    auto* ipv4_loopback_addr =
        reinterpret_cast<struct sockaddr_in*>(&address.addr);
    ((char*)&ipv4_loopback_addr->sin_addr)[0] = 0x7f;
    ((char*)&ipv4_loopback_addr->sin_addr)[3] = 0x01;
    ipv4_loopback_addr->sin_family = AF_INET;
    ipv4_loopback_addr->sin_port = numeric_port;
    address.len = sizeof(struct sockaddr_in);
    (*addrs)->emplace_back(address, grpc_core::ChannelArgs());
    // Let the address sorter figure out which one should be tried first.
    grpc_cares_wrapper_address_sorting_sort(r, addrs->get());
    return true;
  }
  return false;
}

static bool grpc_ares_maybe_resolve_localhost_manually_locked(
    const grpc_ares_request* r, const char* name, const char* default_port,
    std::unique_ptr<grpc_core::EndpointAddressesList>* addrs) {
  std::string host;
  std::string port;
  return inner_maybe_resolve_localhost_manually_locked(r, name, default_port,
                                                       addrs, &host, &port);
}
#else   // GRPC_ARES_RESOLVE_LOCALHOST_MANUALLY
static bool grpc_ares_maybe_resolve_localhost_manually_locked(
    const grpc_ares_request* /*r*/, const char* /*name*/,
    const char* /*default_port*/,
    std::unique_ptr<grpc_core::EndpointAddressesList>* /*addrs*/) {
  return false;
}
#endif  // GRPC_ARES_RESOLVE_LOCALHOST_MANUALLY

static grpc_ares_request* grpc_dns_lookup_hostname_ares_impl(
    const char* dns_server, const char* name, const char* default_port,
    grpc_pollset_set* interested_parties, grpc_closure* on_done,
    std::unique_ptr<grpc_core::EndpointAddressesList>* addrs,
    int query_timeout_ms) {
  grpc_ares_request* r = new grpc_ares_request();
  grpc_core::MutexLock lock(&r->mu);
  r->ev_driver = nullptr;
  r->on_done = on_done;
  r->addresses_out = addrs;
  GRPC_TRACE_VLOG(cares_resolver, 2)
      << "(c-ares resolver) request:" << r
      << " c-ares grpc_dns_lookup_hostname_ares_impl name=" << name
      << ", default_port=" << default_port;
  // Early out if the target is an ipv4 or ipv6 literal.
  if (resolve_as_ip_literal_locked(name, default_port, addrs)) {
    grpc_ares_complete_request_locked(r);
    return r;
  }
  // Early out if the target is localhost and we're on Windows.
  if (grpc_ares_maybe_resolve_localhost_manually_locked(r, name, default_port,
                                                        addrs)) {
    grpc_ares_complete_request_locked(r);
    return r;
  }
  // Look up name using c-ares lib.
  std::string host;
  std::string port;
  grpc_error_handle error = grpc_dns_lookup_ares_continued(
      r, dns_server, name, default_port, interested_parties, query_timeout_ms,
      &host, &port, true);
  if (!error.ok()) {
    grpc_core::ExecCtx::Run(DEBUG_LOCATION, r->on_done, error);
    return r;
  }
  r->pending_queries = 1;
  grpc_ares_hostbyname_request* hr = nullptr;
  if (grpc_ares_query_ipv6()) {
    hr = create_hostbyname_request_locked(r, host.c_str(),
                                          grpc_strhtons(port.c_str()),
                                          /*is_balancer=*/false, "AAAA");
    ares_gethostbyname(r->ev_driver->channel, hr->host, AF_INET6,
                       on_hostbyname_done_locked, hr);
  }
  hr = create_hostbyname_request_locked(r, host.c_str(),
                                        grpc_strhtons(port.c_str()),
                                        /*is_balancer=*/false, "A");
  ares_gethostbyname(r->ev_driver->channel, hr->host, AF_INET,
                     on_hostbyname_done_locked, hr);
  grpc_ares_ev_driver_start_locked(r->ev_driver);
  grpc_ares_request_unref_locked(r);
  return r;
}

grpc_ares_request* grpc_dns_lookup_srv_ares_impl(
    const char* dns_server, const char* name,
    grpc_pollset_set* interested_parties, grpc_closure* on_done,
    std::unique_ptr<grpc_core::EndpointAddressesList>* balancer_addresses,
    int query_timeout_ms) {
  grpc_ares_request* r = new grpc_ares_request();
  grpc_core::MutexLock lock(&r->mu);
  r->ev_driver = nullptr;
  r->on_done = on_done;
  r->balancer_addresses_out = balancer_addresses;
  GRPC_TRACE_VLOG(cares_resolver, 2)
      << "(c-ares resolver) request:" << r
      << " c-ares grpc_dns_lookup_srv_ares_impl name=" << name;
  grpc_error_handle error;
  // Don't query for SRV records if the target is "localhost"
  if (target_matches_localhost(name)) {
    grpc_core::ExecCtx::Run(DEBUG_LOCATION, r->on_done, error);
    return r;
  }
  // Look up name using c-ares lib.
  std::string host;
  std::string port;
  error = grpc_dns_lookup_ares_continued(r, dns_server, name, nullptr,
                                         interested_parties, query_timeout_ms,
                                         &host, &port, false);
  if (!error.ok()) {
    grpc_core::ExecCtx::Run(DEBUG_LOCATION, r->on_done, error);
    return r;
  }
  r->pending_queries = 1;
  // Query the SRV record
  std::string service_name = absl::StrCat("_grpclb._tcp.", host);
  GrpcAresQuery* srv_query = new GrpcAresQuery(r, service_name);
  ares_query(r->ev_driver->channel, service_name.c_str(), ns_c_in, ns_t_srv,
             on_srv_query_done_locked, srv_query);
  grpc_ares_ev_driver_start_locked(r->ev_driver);
  grpc_ares_request_unref_locked(r);
  return r;
}

grpc_ares_request* grpc_dns_lookup_txt_ares_impl(
    const char* dns_server, const char* name,
    grpc_pollset_set* interested_parties, grpc_closure* on_done,
    char** service_config_json, int query_timeout_ms) {
  grpc_ares_request* r = new grpc_ares_request();
  grpc_core::MutexLock lock(&r->mu);
  r->ev_driver = nullptr;
  r->on_done = on_done;
  r->service_config_json_out = service_config_json;
  GRPC_TRACE_VLOG(cares_resolver, 2)
      << "(c-ares resolver) request:" << r
      << " c-ares grpc_dns_lookup_txt_ares_impl name=" << name;
  grpc_error_handle error;
  // Don't query for TXT records if the target is "localhost"
  if (target_matches_localhost(name)) {
    grpc_core::ExecCtx::Run(DEBUG_LOCATION, r->on_done, error);
    return r;
  }
  // Look up name using c-ares lib.
  std::string host;
  std::string port;
  error = grpc_dns_lookup_ares_continued(r, dns_server, name, nullptr,
                                         interested_parties, query_timeout_ms,
                                         &host, &port, false);
  if (!error.ok()) {
    grpc_core::ExecCtx::Run(DEBUG_LOCATION, r->on_done, error);
    return r;
  }
  r->pending_queries = 1;
  // Query the TXT record
  std::string config_name = absl::StrCat("_grpc_config.", host);
  GrpcAresQuery* txt_query = new GrpcAresQuery(r, config_name);
  ares_search(r->ev_driver->channel, config_name.c_str(), ns_c_in, ns_t_txt,
              on_txt_done_locked, txt_query);
  grpc_ares_ev_driver_start_locked(r->ev_driver);
  grpc_ares_request_unref_locked(r);
  return r;
}

grpc_ares_request* (*grpc_dns_lookup_hostname_ares)(
    const char* dns_server, const char* name, const char* default_port,
    grpc_pollset_set* interested_parties, grpc_closure* on_done,
    std::unique_ptr<grpc_core::EndpointAddressesList>* addrs,
    int query_timeout_ms) = grpc_dns_lookup_hostname_ares_impl;

grpc_ares_request* (*grpc_dns_lookup_srv_ares)(
    const char* dns_server, const char* name,
    grpc_pollset_set* interested_parties, grpc_closure* on_done,
    std::unique_ptr<grpc_core::EndpointAddressesList>* balancer_addresses,
    int query_timeout_ms) = grpc_dns_lookup_srv_ares_impl;

grpc_ares_request* (*grpc_dns_lookup_txt_ares)(
    const char* dns_server, const char* name,
    grpc_pollset_set* interested_parties, grpc_closure* on_done,
    char** service_config_json,
    int query_timeout_ms) = grpc_dns_lookup_txt_ares_impl;

static void grpc_cancel_ares_request_impl(grpc_ares_request* r) {
  CHECK_NE(r, nullptr);
  grpc_core::MutexLock lock(&r->mu);
  GRPC_TRACE_VLOG(cares_resolver, 2)
      << "(c-ares resolver) request:" << r
      << " grpc_cancel_ares_request ev_driver:" << r->ev_driver;
  if (r->ev_driver != nullptr) {
    grpc_ares_ev_driver_shutdown_locked(r->ev_driver);
  }
}

void (*grpc_cancel_ares_request)(grpc_ares_request* r) =
    grpc_cancel_ares_request_impl;

// ares_library_init and ares_library_cleanup are currently no-op except under
// Windows. Calling them may cause race conditions when other parts of the
// binary calls these functions concurrently.
#ifdef GPR_WINDOWS
grpc_error_handle grpc_ares_init(void) {
  int status = ares_library_init(ARES_LIB_INIT_ALL);
  if (status != ARES_SUCCESS) {
    return GRPC_ERROR_CREATE(
        absl::StrCat("ares_library_init failed: ", ares_strerror(status)));
  }
  return absl::OkStatus();
}

void grpc_ares_cleanup(void) { ares_library_cleanup(); }
#else
grpc_error_handle grpc_ares_init(void) { return absl::OkStatus(); }
void grpc_ares_cleanup(void) {}
#endif  // GPR_WINDOWS

#endif  // GRPC_ARES == 1