src/core/ext/filters/client_channel/client_channel.c

/*
 *
 * Copyright 2015 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 "src/core/ext/filters/client_channel/client_channel.h"

#include <stdbool.h>
#include <stdio.h>
#include <string.h>

#include <grpc/support/alloc.h>
#include <grpc/support/log.h>
#include <grpc/support/string_util.h>
#include <grpc/support/sync.h>
#include <grpc/support/useful.h>

#include "src/core/ext/filters/client_channel/http_connect_handshaker.h"
#include "src/core/ext/filters/client_channel/lb_policy_registry.h"
#include "src/core/ext/filters/client_channel/proxy_mapper_registry.h"
#include "src/core/ext/filters/client_channel/resolver_registry.h"
#include "src/core/ext/filters/client_channel/retry_throttle.h"
#include "src/core/ext/filters/client_channel/subchannel.h"
#include "src/core/ext/filters/deadline/deadline_filter.h"
#include "src/core/lib/channel/channel_args.h"
#include "src/core/lib/channel/connected_channel.h"
#include "src/core/lib/iomgr/combiner.h"
#include "src/core/lib/iomgr/iomgr.h"
#include "src/core/lib/iomgr/polling_entity.h"
#include "src/core/lib/profiling/timers.h"
#include "src/core/lib/slice/slice_internal.h"
#include "src/core/lib/support/string.h"
#include "src/core/lib/surface/channel.h"
#include "src/core/lib/transport/connectivity_state.h"
#include "src/core/lib/transport/metadata.h"
#include "src/core/lib/transport/metadata_batch.h"
#include "src/core/lib/transport/service_config.h"
#include "src/core/lib/transport/static_metadata.h"

/* Client channel implementation */

grpc_tracer_flag grpc_client_channel_trace =
    GRPC_TRACER_INITIALIZER(false, "client_channel");

/*************************************************************************
 * METHOD-CONFIG TABLE
 */

typedef enum {
  /* zero so it can be default initialized */
  WAIT_FOR_READY_UNSET = 0,
  WAIT_FOR_READY_FALSE,
  WAIT_FOR_READY_TRUE
} wait_for_ready_value;

typedef struct {
  gpr_refcount refs;
  gpr_timespec timeout;
  wait_for_ready_value wait_for_ready;
} method_parameters;

static method_parameters *method_parameters_ref(
    method_parameters *method_params) {
  gpr_ref(&method_params->refs);
  return method_params;
}

static void method_parameters_unref(method_parameters *method_params) {
  if (gpr_unref(&method_params->refs)) {
    gpr_free(method_params);
  }
}

static void method_parameters_free(grpc_exec_ctx *exec_ctx, void *value) {
  method_parameters_unref(value);
}

static bool parse_wait_for_ready(grpc_json *field,
                                 wait_for_ready_value *wait_for_ready) {
  if (field->type != GRPC_JSON_TRUE && field->type != GRPC_JSON_FALSE) {
    return false;
  }
  *wait_for_ready = field->type == GRPC_JSON_TRUE ? WAIT_FOR_READY_TRUE
                                                  : WAIT_FOR_READY_FALSE;
  return true;
}

static bool parse_timeout(grpc_json *field, gpr_timespec *timeout) {
  if (field->type != GRPC_JSON_STRING) return false;
  size_t len = strlen(field->value);
  if (field->value[len - 1] != 's') return false;
  char *buf = gpr_strdup(field->value);
  buf[len - 1] = '\0';  // Remove trailing 's'.
  char *decimal_point = strchr(buf, '.');
  if (decimal_point != NULL) {
    *decimal_point = '\0';
    timeout->tv_nsec = gpr_parse_nonnegative_int(decimal_point + 1);
    if (timeout->tv_nsec == -1) {
      gpr_free(buf);
      return false;
    }
    // There should always be exactly 3, 6, or 9 fractional digits.
    int multiplier = 1;
    switch (strlen(decimal_point + 1)) {
      case 9:
        break;
      case 6:
        multiplier *= 1000;
        break;
      case 3:
        multiplier *= 1000000;
        break;
      default:  // Unsupported number of digits.
        gpr_free(buf);
        return false;
    }
    timeout->tv_nsec *= multiplier;
  }
  timeout->tv_sec = gpr_parse_nonnegative_int(buf);
  gpr_free(buf);
  if (timeout->tv_sec == -1) return false;
  return true;
}

static void *method_parameters_create_from_json(const grpc_json *json) {
  wait_for_ready_value wait_for_ready = WAIT_FOR_READY_UNSET;
  gpr_timespec timeout = {0, 0, GPR_TIMESPAN};
  for (grpc_json *field = json->child; field != NULL; field = field->next) {
    if (field->key == NULL) continue;
    if (strcmp(field->key, "waitForReady") == 0) {
      if (wait_for_ready != WAIT_FOR_READY_UNSET) return NULL;  // Duplicate.
      if (!parse_wait_for_ready(field, &wait_for_ready)) return NULL;
    } else if (strcmp(field->key, "timeout") == 0) {
      if (timeout.tv_sec > 0 || timeout.tv_nsec > 0) return NULL;  // Duplicate.
      if (!parse_timeout(field, &timeout)) return NULL;
    }
  }
  method_parameters *value = gpr_malloc(sizeof(method_parameters));
  gpr_ref_init(&value->refs, 1);
  value->timeout = timeout;
  value->wait_for_ready = wait_for_ready;
  return value;
}

struct external_connectivity_watcher;

/*************************************************************************
 * CHANNEL-WIDE FUNCTIONS
 */

typedef struct client_channel_channel_data {
  /** resolver for this channel */
  grpc_resolver *resolver;
  /** have we started resolving this channel */
  bool started_resolving;
  /** is deadline checking enabled? */
  bool deadline_checking_enabled;
  /** client channel factory */
  grpc_client_channel_factory *client_channel_factory;

  /** combiner protecting all variables below in this data structure */
  grpc_combiner *combiner;
  /** currently active load balancer */
  grpc_lb_policy *lb_policy;
  /** retry throttle data */
  grpc_server_retry_throttle_data *retry_throttle_data;
  /** maps method names to method_parameters structs */
  grpc_slice_hash_table *method_params_table;
  /** incoming resolver result - set by resolver.next() */
  grpc_channel_args *resolver_result;
  /** a list of closures that are all waiting for resolver result to come in */
  grpc_closure_list waiting_for_resolver_result_closures;
  /** resolver callback */
  grpc_closure on_resolver_result_changed;
  /** connectivity state being tracked */
  grpc_connectivity_state_tracker state_tracker;
  /** when an lb_policy arrives, should we try to exit idle */
  bool exit_idle_when_lb_policy_arrives;
  /** owning stack */
  grpc_channel_stack *owning_stack;
  /** interested parties (owned) */
  grpc_pollset_set *interested_parties;

  /* external_connectivity_watcher_list head is guarded by its own mutex, since
   * counts need to be grabbed immediately without polling on a cq */
  gpr_mu external_connectivity_watcher_list_mu;
  struct external_connectivity_watcher *external_connectivity_watcher_list_head;

  /* the following properties are guarded by a mutex since API's require them
     to be instantaneously available */
  gpr_mu info_mu;
  char *info_lb_policy_name;
  /** service config in JSON form */
  char *info_service_config_json;
} channel_data;

/** We create one watcher for each new lb_policy that is returned from a
    resolver, to watch for state changes from the lb_policy. When a state
    change is seen, we update the channel, and create a new watcher. */
typedef struct {
  channel_data *chand;
  grpc_closure on_changed;
  grpc_connectivity_state state;
  grpc_lb_policy *lb_policy;
} lb_policy_connectivity_watcher;

static void watch_lb_policy_locked(grpc_exec_ctx *exec_ctx, channel_data *chand,
                                   grpc_lb_policy *lb_policy,
                                   grpc_connectivity_state current_state);

static void set_channel_connectivity_state_locked(grpc_exec_ctx *exec_ctx,
                                                  channel_data *chand,
                                                  grpc_connectivity_state state,
                                                  grpc_error *error,
                                                  const char *reason) {
  /* TODO: Improve failure handling:
   * - Make it possible for policies to return GRPC_CHANNEL_TRANSIENT_FAILURE.
   * - Hand over pending picks from old policies during the switch that happens
   *   when resolver provides an update. */
  if (chand->lb_policy != NULL) {
    if (state == GRPC_CHANNEL_TRANSIENT_FAILURE) {
      /* cancel picks with wait_for_ready=false */
      grpc_lb_policy_cancel_picks_locked(
          exec_ctx, chand->lb_policy,
          /* mask= */ GRPC_INITIAL_METADATA_WAIT_FOR_READY,
          /* check= */ 0, GRPC_ERROR_REF(error));
    } else if (state == GRPC_CHANNEL_SHUTDOWN) {
      /* cancel all picks */
      grpc_lb_policy_cancel_picks_locked(exec_ctx, chand->lb_policy,
                                         /* mask= */ 0, /* check= */ 0,
                                         GRPC_ERROR_REF(error));
    }
  }
  if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
    gpr_log(GPR_DEBUG, "chand=%p: setting connectivity state to %s", chand,
            grpc_connectivity_state_name(state));
  }
  grpc_connectivity_state_set(exec_ctx, &chand->state_tracker, state, error,
                              reason);
}

static void on_lb_policy_state_changed_locked(grpc_exec_ctx *exec_ctx,
                                              void *arg, grpc_error *error) {
  lb_policy_connectivity_watcher *w = arg;
  grpc_connectivity_state publish_state = w->state;
  /* check if the notification is for the latest policy */
  if (w->lb_policy == w->chand->lb_policy) {
    if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
      gpr_log(GPR_DEBUG, "chand=%p: lb_policy=%p state changed to %s", w->chand,
              w->lb_policy, grpc_connectivity_state_name(w->state));
    }
    if (publish_state == GRPC_CHANNEL_SHUTDOWN && w->chand->resolver != NULL) {
      publish_state = GRPC_CHANNEL_TRANSIENT_FAILURE;
      grpc_resolver_channel_saw_error_locked(exec_ctx, w->chand->resolver);
      GRPC_LB_POLICY_UNREF(exec_ctx, w->chand->lb_policy, "channel");
      w->chand->lb_policy = NULL;
    }
    set_channel_connectivity_state_locked(exec_ctx, w->chand, publish_state,
                                          GRPC_ERROR_REF(error), "lb_changed");
    if (w->state != GRPC_CHANNEL_SHUTDOWN) {
      watch_lb_policy_locked(exec_ctx, w->chand, w->lb_policy, w->state);
    }
  }
  GRPC_CHANNEL_STACK_UNREF(exec_ctx, w->chand->owning_stack, "watch_lb_policy");
  gpr_free(w);
}

static void watch_lb_policy_locked(grpc_exec_ctx *exec_ctx, channel_data *chand,
                                   grpc_lb_policy *lb_policy,
                                   grpc_connectivity_state current_state) {
  lb_policy_connectivity_watcher *w = gpr_malloc(sizeof(*w));
  GRPC_CHANNEL_STACK_REF(chand->owning_stack, "watch_lb_policy");
  w->chand = chand;
  GRPC_CLOSURE_INIT(&w->on_changed, on_lb_policy_state_changed_locked, w,
                    grpc_combiner_scheduler(chand->combiner));
  w->state = current_state;
  w->lb_policy = lb_policy;
  grpc_lb_policy_notify_on_state_change_locked(exec_ctx, lb_policy, &w->state,
                                               &w->on_changed);
}

static void start_resolving_locked(grpc_exec_ctx *exec_ctx,
                                   channel_data *chand) {
  if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
    gpr_log(GPR_DEBUG, "chand=%p: starting name resolution", chand);
  }
  GPR_ASSERT(!chand->started_resolving);
  chand->started_resolving = true;
  GRPC_CHANNEL_STACK_REF(chand->owning_stack, "resolver");
  grpc_resolver_next_locked(exec_ctx, chand->resolver, &chand->resolver_result,
                            &chand->on_resolver_result_changed);
}

typedef struct {
  char *server_name;
  grpc_server_retry_throttle_data *retry_throttle_data;
} service_config_parsing_state;

static void parse_retry_throttle_params(const grpc_json *field, void *arg) {
  service_config_parsing_state *parsing_state = arg;
  if (strcmp(field->key, "retryThrottling") == 0) {
    if (parsing_state->retry_throttle_data != NULL) return;  // Duplicate.
    if (field->type != GRPC_JSON_OBJECT) return;
    int max_milli_tokens = 0;
    int milli_token_ratio = 0;
    for (grpc_json *sub_field = field->child; sub_field != NULL;
         sub_field = sub_field->next) {
      if (sub_field->key == NULL) return;
      if (strcmp(sub_field->key, "maxTokens") == 0) {
        if (max_milli_tokens != 0) return;  // Duplicate.
        if (sub_field->type != GRPC_JSON_NUMBER) return;
        max_milli_tokens = gpr_parse_nonnegative_int(sub_field->value);
        if (max_milli_tokens == -1) return;
        max_milli_tokens *= 1000;
      } else if (strcmp(sub_field->key, "tokenRatio") == 0) {
        if (milli_token_ratio != 0) return;  // Duplicate.
        if (sub_field->type != GRPC_JSON_NUMBER) return;
        // We support up to 3 decimal digits.
        size_t whole_len = strlen(sub_field->value);
        uint32_t multiplier = 1;
        uint32_t decimal_value = 0;
        const char *decimal_point = strchr(sub_field->value, '.');
        if (decimal_point != NULL) {
          whole_len = (size_t)(decimal_point - sub_field->value);
          multiplier = 1000;
          size_t decimal_len = strlen(decimal_point + 1);
          if (decimal_len > 3) decimal_len = 3;
          if (!gpr_parse_bytes_to_uint32(decimal_point + 1, decimal_len,
                                         &decimal_value)) {
            return;
          }
          uint32_t decimal_multiplier = 1;
          for (size_t i = 0; i < (3 - decimal_len); ++i) {
            decimal_multiplier *= 10;
          }
          decimal_value *= decimal_multiplier;
        }
        uint32_t whole_value;
        if (!gpr_parse_bytes_to_uint32(sub_field->value, whole_len,
                                       &whole_value)) {
          return;
        }
        milli_token_ratio = (int)((whole_value * multiplier) + decimal_value);
        if (milli_token_ratio <= 0) return;
      }
    }
    parsing_state->retry_throttle_data =
        grpc_retry_throttle_map_get_data_for_server(
            parsing_state->server_name, max_milli_tokens, milli_token_ratio);
  }
}

static void on_resolver_result_changed_locked(grpc_exec_ctx *exec_ctx,
                                              void *arg, grpc_error *error) {
  channel_data *chand = arg;
  if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
    gpr_log(GPR_DEBUG, "chand=%p: got resolver result: error=%s", chand,
            grpc_error_string(error));
  }
  // Extract the following fields from the resolver result, if non-NULL.
  bool lb_policy_updated = false;
  char *lb_policy_name = NULL;
  bool lb_policy_name_changed = false;
  grpc_lb_policy *new_lb_policy = NULL;
  char *service_config_json = NULL;
  grpc_server_retry_throttle_data *retry_throttle_data = NULL;
  grpc_slice_hash_table *method_params_table = NULL;
  if (chand->resolver_result != NULL) {
    // Find LB policy name.
    const grpc_arg *channel_arg =
        grpc_channel_args_find(chand->resolver_result, GRPC_ARG_LB_POLICY_NAME);
    if (channel_arg != NULL) {
      GPR_ASSERT(channel_arg->type == GRPC_ARG_STRING);
      lb_policy_name = channel_arg->value.string;
    }
    // Special case: If at least one balancer address is present, we use
    // the grpclb policy, regardless of what the resolver actually specified.
    channel_arg =
        grpc_channel_args_find(chand->resolver_result, GRPC_ARG_LB_ADDRESSES);
    if (channel_arg != NULL && channel_arg->type == GRPC_ARG_POINTER) {
      grpc_lb_addresses *addresses = channel_arg->value.pointer.p;
      bool found_balancer_address = false;
      for (size_t i = 0; i < addresses->num_addresses; ++i) {
        if (addresses->addresses[i].is_balancer) {
          found_balancer_address = true;
          break;
        }
      }
      if (found_balancer_address) {
        if (lb_policy_name != NULL && strcmp(lb_policy_name, "grpclb") != 0) {
          gpr_log(GPR_INFO,
                  "resolver requested LB policy %s but provided at least one "
                  "balancer address -- forcing use of grpclb LB policy",
                  lb_policy_name);
        }
        lb_policy_name = "grpclb";
      }
    }
    // Use pick_first if nothing was specified and we didn't select grpclb
    // above.
    if (lb_policy_name == NULL) lb_policy_name = "pick_first";
    grpc_lb_policy_args lb_policy_args;
    lb_policy_args.args = chand->resolver_result;
    lb_policy_args.client_channel_factory = chand->client_channel_factory;
    lb_policy_args.combiner = chand->combiner;
    // Check to see if we're already using the right LB policy.
    // Note: It's safe to use chand->info_lb_policy_name here without
    // taking a lock on chand->info_mu, because this function is the
    // only thing that modifies its value, and it can only be invoked
    // once at any given time.
    lb_policy_name_changed =
        chand->info_lb_policy_name == NULL ||
        strcmp(chand->info_lb_policy_name, lb_policy_name) != 0;
    if (chand->lb_policy != NULL && !lb_policy_name_changed) {
      // Continue using the same LB policy.  Update with new addresses.
      lb_policy_updated = true;
      grpc_lb_policy_update_locked(exec_ctx, chand->lb_policy, &lb_policy_args);
    } else {
      // Instantiate new LB policy.
      new_lb_policy =
          grpc_lb_policy_create(exec_ctx, lb_policy_name, &lb_policy_args);
      if (new_lb_policy == NULL) {
        gpr_log(GPR_ERROR, "could not create LB policy \"%s\"", lb_policy_name);
      }
    }
    // Find service config.
    channel_arg =
        grpc_channel_args_find(chand->resolver_result, GRPC_ARG_SERVICE_CONFIG);
    if (channel_arg != NULL) {
      GPR_ASSERT(channel_arg->type == GRPC_ARG_STRING);
      service_config_json = gpr_strdup(channel_arg->value.string);
      grpc_service_config *service_config =
          grpc_service_config_create(service_config_json);
      if (service_config != NULL) {
        channel_arg =
            grpc_channel_args_find(chand->resolver_result, GRPC_ARG_SERVER_URI);
        GPR_ASSERT(channel_arg != NULL);
        GPR_ASSERT(channel_arg->type == GRPC_ARG_STRING);
        grpc_uri *uri =
            grpc_uri_parse(exec_ctx, channel_arg->value.string, true);
        GPR_ASSERT(uri->path[0] != '\0');
        service_config_parsing_state parsing_state;
        memset(&parsing_state, 0, sizeof(parsing_state));
        parsing_state.server_name =
            uri->path[0] == '/' ? uri->path + 1 : uri->path;
        grpc_service_config_parse_global_params(
            service_config, parse_retry_throttle_params, &parsing_state);
        grpc_uri_destroy(uri);
        retry_throttle_data = parsing_state.retry_throttle_data;
        method_params_table = grpc_service_config_create_method_config_table(
            exec_ctx, service_config, method_parameters_create_from_json,
            method_parameters_free);
        grpc_service_config_destroy(service_config);
      }
    }
    // Before we clean up, save a copy of lb_policy_name, since it might
    // be pointing to data inside chand->resolver_result.
    // The copy will be saved in chand->lb_policy_name below.
    lb_policy_name = gpr_strdup(lb_policy_name);
    grpc_channel_args_destroy(exec_ctx, chand->resolver_result);
    chand->resolver_result = NULL;
  }
  if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
    gpr_log(GPR_DEBUG,
            "chand=%p: resolver result: lb_policy_name=\"%s\"%s, "
            "service_config=\"%s\"",
            chand, lb_policy_name, lb_policy_name_changed ? " (changed)" : "",
            service_config_json);
  }
  // Now swap out fields in chand.  Note that the new values may still
  // be NULL if (e.g.) the resolver failed to return results or the
  // results did not contain the necessary data.
  //
  // First, swap out the data used by cc_get_channel_info().
  gpr_mu_lock(&chand->info_mu);
  if (lb_policy_name != NULL) {
    gpr_free(chand->info_lb_policy_name);
    chand->info_lb_policy_name = lb_policy_name;
  }
  if (service_config_json != NULL) {
    gpr_free(chand->info_service_config_json);
    chand->info_service_config_json = service_config_json;
  }
  gpr_mu_unlock(&chand->info_mu);
  // Swap out the retry throttle data.
  if (chand->retry_throttle_data != NULL) {
    grpc_server_retry_throttle_data_unref(chand->retry_throttle_data);
  }
  chand->retry_throttle_data = retry_throttle_data;
  // Swap out the method params table.
  if (chand->method_params_table != NULL) {
    grpc_slice_hash_table_unref(exec_ctx, chand->method_params_table);
  }
  chand->method_params_table = method_params_table;
  // If we have a new LB policy or are shutting down (in which case
  // new_lb_policy will be NULL), swap out the LB policy, unreffing the
  // old one and removing its fds from chand->interested_parties.
  // Note that we do NOT do this if either (a) we updated the existing
  // LB policy above or (b) we failed to create the new LB policy (in
  // which case we want to continue using the most recent one we had).
  if (new_lb_policy != NULL || error != GRPC_ERROR_NONE ||
      chand->resolver == NULL) {
    if (chand->lb_policy != NULL) {
      if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
        gpr_log(GPR_DEBUG, "chand=%p: unreffing lb_policy=%p", chand,
                chand->lb_policy);
      }
      grpc_pollset_set_del_pollset_set(exec_ctx,
                                       chand->lb_policy->interested_parties,
                                       chand->interested_parties);
      GRPC_LB_POLICY_UNREF(exec_ctx, chand->lb_policy, "channel");
    }
    chand->lb_policy = new_lb_policy;
  }
  // Now that we've swapped out the relevant fields of chand, check for
  // error or shutdown.
  if (error != GRPC_ERROR_NONE || chand->resolver == NULL) {
    if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
      gpr_log(GPR_DEBUG, "chand=%p: shutting down", chand);
    }
    if (chand->resolver != NULL) {
      if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
        gpr_log(GPR_DEBUG, "chand=%p: shutting down resolver", chand);
      }
      grpc_resolver_shutdown_locked(exec_ctx, chand->resolver);
      GRPC_RESOLVER_UNREF(exec_ctx, chand->resolver, "channel");
      chand->resolver = NULL;
    }
    set_channel_connectivity_state_locked(
        exec_ctx, chand, GRPC_CHANNEL_SHUTDOWN,
        GRPC_ERROR_CREATE_REFERENCING_FROM_STATIC_STRING(
            "Got resolver result after disconnection", &error, 1),
        "resolver_gone");
    GRPC_CHANNEL_STACK_UNREF(exec_ctx, chand->owning_stack, "resolver");
    grpc_closure_list_fail_all(&chand->waiting_for_resolver_result_closures,
                               GRPC_ERROR_CREATE_REFERENCING_FROM_STATIC_STRING(
                                   "Channel disconnected", &error, 1));
    GRPC_CLOSURE_LIST_SCHED(exec_ctx,
                            &chand->waiting_for_resolver_result_closures);
  } else {  // Not shutting down.
    grpc_connectivity_state state = GRPC_CHANNEL_TRANSIENT_FAILURE;
    grpc_error *state_error =
        GRPC_ERROR_CREATE_FROM_STATIC_STRING("No load balancing policy");
    if (new_lb_policy != NULL) {
      if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
        gpr_log(GPR_DEBUG, "chand=%p: initializing new LB policy", chand);
      }
      GRPC_ERROR_UNREF(state_error);
      state = grpc_lb_policy_check_connectivity_locked(exec_ctx, new_lb_policy,
                                                       &state_error);
      grpc_pollset_set_add_pollset_set(exec_ctx,
                                       new_lb_policy->interested_parties,
                                       chand->interested_parties);
      GRPC_CLOSURE_LIST_SCHED(exec_ctx,
                              &chand->waiting_for_resolver_result_closures);
      if (chand->exit_idle_when_lb_policy_arrives) {
        grpc_lb_policy_exit_idle_locked(exec_ctx, new_lb_policy);
        chand->exit_idle_when_lb_policy_arrives = false;
      }
      watch_lb_policy_locked(exec_ctx, chand, new_lb_policy, state);
    }
    if (!lb_policy_updated) {
      set_channel_connectivity_state_locked(exec_ctx, chand, state,
                                            GRPC_ERROR_REF(state_error),
                                            "new_lb+resolver");
    }
    grpc_resolver_next_locked(exec_ctx, chand->resolver,
                              &chand->resolver_result,
                              &chand->on_resolver_result_changed);
    GRPC_ERROR_UNREF(state_error);
  }
}

static void start_transport_op_locked(grpc_exec_ctx *exec_ctx, void *arg,
                                      grpc_error *error_ignored) {
  grpc_transport_op *op = arg;
  grpc_channel_element *elem = op->handler_private.extra_arg;
  channel_data *chand = elem->channel_data;

  if (op->on_connectivity_state_change != NULL) {
    grpc_connectivity_state_notify_on_state_change(
        exec_ctx, &chand->state_tracker, op->connectivity_state,
        op->on_connectivity_state_change);
    op->on_connectivity_state_change = NULL;
    op->connectivity_state = NULL;
  }

  if (op->send_ping != NULL) {
    if (chand->lb_policy == NULL) {
      GRPC_CLOSURE_SCHED(
          exec_ctx, op->send_ping,
          GRPC_ERROR_CREATE_FROM_STATIC_STRING("Ping with no load balancing"));
    } else {
      grpc_lb_policy_ping_one_locked(exec_ctx, chand->lb_policy, op->send_ping);
      op->bind_pollset = NULL;
    }
    op->send_ping = NULL;
  }

  if (op->disconnect_with_error != GRPC_ERROR_NONE) {
    if (chand->resolver != NULL) {
      set_channel_connectivity_state_locked(
          exec_ctx, chand, GRPC_CHANNEL_SHUTDOWN,
          GRPC_ERROR_REF(op->disconnect_with_error), "disconnect");
      grpc_resolver_shutdown_locked(exec_ctx, chand->resolver);
      GRPC_RESOLVER_UNREF(exec_ctx, chand->resolver, "channel");
      chand->resolver = NULL;
      if (!chand->started_resolving) {
        grpc_closure_list_fail_all(&chand->waiting_for_resolver_result_closures,
                                   GRPC_ERROR_REF(op->disconnect_with_error));
        GRPC_CLOSURE_LIST_SCHED(exec_ctx,
                                &chand->waiting_for_resolver_result_closures);
      }
      if (chand->lb_policy != NULL) {
        grpc_pollset_set_del_pollset_set(exec_ctx,
                                         chand->lb_policy->interested_parties,
                                         chand->interested_parties);
        GRPC_LB_POLICY_UNREF(exec_ctx, chand->lb_policy, "channel");
        chand->lb_policy = NULL;
      }
    }
    GRPC_ERROR_UNREF(op->disconnect_with_error);
  }
  GRPC_CHANNEL_STACK_UNREF(exec_ctx, chand->owning_stack, "start_transport_op");

  GRPC_CLOSURE_SCHED(exec_ctx, op->on_consumed, GRPC_ERROR_NONE);
}

static void cc_start_transport_op(grpc_exec_ctx *exec_ctx,
                                  grpc_channel_element *elem,
                                  grpc_transport_op *op) {
  channel_data *chand = elem->channel_data;

  GPR_ASSERT(op->set_accept_stream == false);
  if (op->bind_pollset != NULL) {
    grpc_pollset_set_add_pollset(exec_ctx, chand->interested_parties,
                                 op->bind_pollset);
  }

  op->handler_private.extra_arg = elem;
  GRPC_CHANNEL_STACK_REF(chand->owning_stack, "start_transport_op");
  GRPC_CLOSURE_SCHED(
      exec_ctx,
      GRPC_CLOSURE_INIT(&op->handler_private.closure, start_transport_op_locked,
                        op, grpc_combiner_scheduler(chand->combiner)),
      GRPC_ERROR_NONE);
}

static void cc_get_channel_info(grpc_exec_ctx *exec_ctx,
                                grpc_channel_element *elem,
                                const grpc_channel_info *info) {
  channel_data *chand = elem->channel_data;
  gpr_mu_lock(&chand->info_mu);
  if (info->lb_policy_name != NULL) {
    *info->lb_policy_name = chand->info_lb_policy_name == NULL
                                ? NULL
                                : gpr_strdup(chand->info_lb_policy_name);
  }
  if (info->service_config_json != NULL) {
    *info->service_config_json =
        chand->info_service_config_json == NULL
            ? NULL
            : gpr_strdup(chand->info_service_config_json);
  }
  gpr_mu_unlock(&chand->info_mu);
}

/* Constructor for channel_data */
static grpc_error *cc_init_channel_elem(grpc_exec_ctx *exec_ctx,
                                        grpc_channel_element *elem,
                                        grpc_channel_element_args *args) {
  channel_data *chand = elem->channel_data;
  GPR_ASSERT(args->is_last);
  GPR_ASSERT(elem->filter == &grpc_client_channel_filter);
  // Initialize data members.
  chand->combiner = grpc_combiner_create();
  gpr_mu_init(&chand->info_mu);
  gpr_mu_init(&chand->external_connectivity_watcher_list_mu);

  gpr_mu_lock(&chand->external_connectivity_watcher_list_mu);
  chand->external_connectivity_watcher_list_head = NULL;
  gpr_mu_unlock(&chand->external_connectivity_watcher_list_mu);

  chand->owning_stack = args->channel_stack;
  GRPC_CLOSURE_INIT(&chand->on_resolver_result_changed,
                    on_resolver_result_changed_locked, chand,
                    grpc_combiner_scheduler(chand->combiner));
  chand->interested_parties = grpc_pollset_set_create();
  grpc_connectivity_state_init(&chand->state_tracker, GRPC_CHANNEL_IDLE,
                               "client_channel");
  // Record client channel factory.
  const grpc_arg *arg = grpc_channel_args_find(args->channel_args,
                                               GRPC_ARG_CLIENT_CHANNEL_FACTORY);
  if (arg == NULL) {
    return GRPC_ERROR_CREATE_FROM_STATIC_STRING(
        "Missing client channel factory in args for client channel filter");
  }
  if (arg->type != GRPC_ARG_POINTER) {
    return GRPC_ERROR_CREATE_FROM_STATIC_STRING(
        "client channel factory arg must be a pointer");
  }
  grpc_client_channel_factory_ref(arg->value.pointer.p);
  chand->client_channel_factory = arg->value.pointer.p;
  // Get server name to resolve, using proxy mapper if needed.
  arg = grpc_channel_args_find(args->channel_args, GRPC_ARG_SERVER_URI);
  if (arg == NULL) {
    return GRPC_ERROR_CREATE_FROM_STATIC_STRING(
        "Missing server uri in args for client channel filter");
  }
  if (arg->type != GRPC_ARG_STRING) {
    return GRPC_ERROR_CREATE_FROM_STATIC_STRING(
        "server uri arg must be a string");
  }
  char *proxy_name = NULL;
  grpc_channel_args *new_args = NULL;
  grpc_proxy_mappers_map_name(exec_ctx, arg->value.string, args->channel_args,
                              &proxy_name, &new_args);
  // Instantiate resolver.
  chand->resolver = grpc_resolver_create(
      exec_ctx, proxy_name != NULL ? proxy_name : arg->value.string,
      new_args != NULL ? new_args : args->channel_args,
      chand->interested_parties, chand->combiner);
  if (proxy_name != NULL) gpr_free(proxy_name);
  if (new_args != NULL) grpc_channel_args_destroy(exec_ctx, new_args);
  if (chand->resolver == NULL) {
    return GRPC_ERROR_CREATE_FROM_STATIC_STRING("resolver creation failed");
  }
  chand->deadline_checking_enabled =
      grpc_deadline_checking_enabled(args->channel_args);
  return GRPC_ERROR_NONE;
}

static void shutdown_resolver_locked(grpc_exec_ctx *exec_ctx, void *arg,
                                     grpc_error *error) {
  grpc_resolver *resolver = arg;
  grpc_resolver_shutdown_locked(exec_ctx, resolver);
  GRPC_RESOLVER_UNREF(exec_ctx, resolver, "channel");
}

/* Destructor for channel_data */
static void cc_destroy_channel_elem(grpc_exec_ctx *exec_ctx,
                                    grpc_channel_element *elem) {
  channel_data *chand = elem->channel_data;
  if (chand->resolver != NULL) {
    GRPC_CLOSURE_SCHED(
        exec_ctx, GRPC_CLOSURE_CREATE(shutdown_resolver_locked, chand->resolver,
                                      grpc_combiner_scheduler(chand->combiner)),
        GRPC_ERROR_NONE);
  }
  if (chand->client_channel_factory != NULL) {
    grpc_client_channel_factory_unref(exec_ctx, chand->client_channel_factory);
  }
  if (chand->lb_policy != NULL) {
    grpc_pollset_set_del_pollset_set(exec_ctx,
                                     chand->lb_policy->interested_parties,
                                     chand->interested_parties);
    GRPC_LB_POLICY_UNREF(exec_ctx, chand->lb_policy, "channel");
  }
  gpr_free(chand->info_lb_policy_name);
  gpr_free(chand->info_service_config_json);
  if (chand->retry_throttle_data != NULL) {
    grpc_server_retry_throttle_data_unref(chand->retry_throttle_data);
  }
  if (chand->method_params_table != NULL) {
    grpc_slice_hash_table_unref(exec_ctx, chand->method_params_table);
  }
  grpc_connectivity_state_destroy(exec_ctx, &chand->state_tracker);
  grpc_pollset_set_destroy(exec_ctx, chand->interested_parties);
  GRPC_COMBINER_UNREF(exec_ctx, chand->combiner, "client_channel");
  gpr_mu_destroy(&chand->info_mu);
  gpr_mu_destroy(&chand->external_connectivity_watcher_list_mu);
}

/*************************************************************************
 * PER-CALL FUNCTIONS
 */

// Max number of batches that can be pending on a call at any given
// time.  This includes:
//   recv_initial_metadata
//   send_initial_metadata
//   recv_message
//   send_message
//   recv_trailing_metadata
//   send_trailing_metadata
// We also add room for a single cancel_stream batch.
#define MAX_WAITING_BATCHES 7

/** Call data.  Holds a pointer to grpc_subchannel_call and the
    associated machinery to create such a pointer.
    Handles queueing of stream ops until a call object is ready, waiting
    for initial metadata before trying to create a call object,
    and handling cancellation gracefully. */
typedef struct client_channel_call_data {
  // State for handling deadlines.
  // The code in deadline_filter.c requires this to be the first field.
  // TODO(roth): This is slightly sub-optimal in that grpc_deadline_state
  // and this struct both independently store pointers to the call stack
  // and call combiner.  If/when we have time, find a way to avoid this
  // without breaking the grpc_deadline_state abstraction.
  grpc_deadline_state deadline_state;

  grpc_slice path;  // Request path.
  gpr_timespec call_start_time;
  gpr_timespec deadline;
  gpr_arena *arena;
  grpc_call_stack *owning_call;
  grpc_call_combiner *call_combiner;

  grpc_server_retry_throttle_data *retry_throttle_data;
  method_parameters *method_params;

  grpc_subchannel_call *subchannel_call;
  grpc_error *error;

  grpc_lb_policy *lb_policy;  // Holds ref while LB pick is pending.
  grpc_closure lb_pick_closure;
  grpc_closure lb_pick_cancel_closure;

  grpc_connected_subchannel *connected_subchannel;
  grpc_call_context_element subchannel_call_context[GRPC_CONTEXT_COUNT];
  grpc_polling_entity *pollent;

  grpc_transport_stream_op_batch *waiting_for_pick_batches[MAX_WAITING_BATCHES];
  size_t waiting_for_pick_batches_count;
  grpc_closure handle_pending_batch_in_call_combiner[MAX_WAITING_BATCHES];

  grpc_transport_stream_op_batch *initial_metadata_batch;

  grpc_linked_mdelem lb_token_mdelem;

  grpc_closure on_complete;
  grpc_closure *original_on_complete;
} call_data;

grpc_subchannel_call *grpc_client_channel_get_subchannel_call(
    grpc_call_element *elem) {
  call_data *calld = elem->call_data;
  return calld->subchannel_call;
}

// This is called via the call combiner, so access to calld is synchronized.
static void waiting_for_pick_batches_add(
    call_data *calld, grpc_transport_stream_op_batch *batch) {
  if (batch->send_initial_metadata) {
    GPR_ASSERT(calld->initial_metadata_batch == NULL);
    calld->initial_metadata_batch = batch;
  } else {
    GPR_ASSERT(calld->waiting_for_pick_batches_count < MAX_WAITING_BATCHES);
    calld->waiting_for_pick_batches[calld->waiting_for_pick_batches_count++] =
        batch;
  }
}

// This is called via the call combiner, so access to calld is synchronized.
static void fail_pending_batch_in_call_combiner(grpc_exec_ctx *exec_ctx,
                                                void *arg, grpc_error *error) {
  call_data *calld = arg;
  if (calld->waiting_for_pick_batches_count > 0) {
    --calld->waiting_for_pick_batches_count;
    grpc_transport_stream_op_batch_finish_with_failure(
        exec_ctx,
        calld->waiting_for_pick_batches[calld->waiting_for_pick_batches_count],
        GRPC_ERROR_REF(error), calld->call_combiner);
  }
}

// This is called via the call combiner, so access to calld is synchronized.
static void waiting_for_pick_batches_fail(grpc_exec_ctx *exec_ctx,
                                          grpc_call_element *elem,
                                          grpc_error *error) {
  call_data *calld = elem->call_data;
  if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
    gpr_log(GPR_DEBUG,
            "chand=%p calld=%p: failing %" PRIdPTR " pending batches: %s",
            elem->channel_data, calld, calld->waiting_for_pick_batches_count,
            grpc_error_string(error));
  }
  for (size_t i = 0; i < calld->waiting_for_pick_batches_count; ++i) {
    GRPC_CLOSURE_INIT(&calld->handle_pending_batch_in_call_combiner[i],
                      fail_pending_batch_in_call_combiner, calld,
                      grpc_schedule_on_exec_ctx);
    GRPC_CALL_COMBINER_START(exec_ctx, calld->call_combiner,
                             &calld->handle_pending_batch_in_call_combiner[i],
                             GRPC_ERROR_REF(error),
                             "waiting_for_pick_batches_fail");
  }
  if (calld->initial_metadata_batch != NULL) {
    grpc_transport_stream_op_batch_finish_with_failure(
        exec_ctx, calld->initial_metadata_batch, GRPC_ERROR_REF(error),
        calld->call_combiner);
  } else {
    GRPC_CALL_COMBINER_STOP(exec_ctx, calld->call_combiner,
                            "waiting_for_pick_batches_fail");
  }
  GRPC_ERROR_UNREF(error);
}

// This is called via the call combiner, so access to calld is synchronized.
static void run_pending_batch_in_call_combiner(grpc_exec_ctx *exec_ctx,
                                               void *arg, grpc_error *ignored) {
  call_data *calld = arg;
  if (calld->waiting_for_pick_batches_count > 0) {
    --calld->waiting_for_pick_batches_count;
    grpc_subchannel_call_process_op(
        exec_ctx, calld->subchannel_call,
        calld->waiting_for_pick_batches[calld->waiting_for_pick_batches_count]);
  }
}

// This is called via the call combiner, so access to calld is synchronized.
static void waiting_for_pick_batches_resume(grpc_exec_ctx *exec_ctx,
                                            grpc_call_element *elem) {
  channel_data *chand = elem->channel_data;
  call_data *calld = elem->call_data;
  if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
    gpr_log(GPR_DEBUG, "chand=%p calld=%p: sending %" PRIdPTR
                       " pending batches to subchannel_call=%p",
            chand, calld, calld->waiting_for_pick_batches_count,
            calld->subchannel_call);
  }
  for (size_t i = 0; i < calld->waiting_for_pick_batches_count; ++i) {
    GRPC_CLOSURE_INIT(&calld->handle_pending_batch_in_call_combiner[i],
                      run_pending_batch_in_call_combiner, calld,
                      grpc_schedule_on_exec_ctx);
    GRPC_CALL_COMBINER_START(exec_ctx, calld->call_combiner,
                             &calld->handle_pending_batch_in_call_combiner[i],
                             GRPC_ERROR_NONE,
                             "waiting_for_pick_batches_resume");
  }
  GPR_ASSERT(calld->initial_metadata_batch != NULL);
  grpc_subchannel_call_process_op(exec_ctx, calld->subchannel_call,
                                  calld->initial_metadata_batch);
}

// Applies service config to the call.  Must be invoked once we know
// that the resolver has returned results to the channel.
static void apply_service_config_to_call_locked(grpc_exec_ctx *exec_ctx,
                                                grpc_call_element *elem) {
  channel_data *chand = elem->channel_data;
  call_data *calld = elem->call_data;
  if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
    gpr_log(GPR_DEBUG, "chand=%p calld=%p: applying service config to call",
            chand, calld);
  }
  if (chand->retry_throttle_data != NULL) {
    calld->retry_throttle_data =
        grpc_server_retry_throttle_data_ref(chand->retry_throttle_data);
  }
  if (chand->method_params_table != NULL) {
    calld->method_params = grpc_method_config_table_get(
        exec_ctx, chand->method_params_table, calld->path);
    if (calld->method_params != NULL) {
      method_parameters_ref(calld->method_params);
      // If the deadline from the service config is shorter than the one
      // from the client API, reset the deadline timer.
      if (chand->deadline_checking_enabled &&
          gpr_time_cmp(calld->method_params->timeout,
                       gpr_time_0(GPR_TIMESPAN)) != 0) {
        const gpr_timespec per_method_deadline =
            gpr_time_add(calld->call_start_time, calld->method_params->timeout);
        if (gpr_time_cmp(per_method_deadline, calld->deadline) < 0) {
          calld->deadline = per_method_deadline;
          grpc_deadline_state_reset(exec_ctx, elem, calld->deadline);
        }
      }
    }
  }
}

static void create_subchannel_call_locked(grpc_exec_ctx *exec_ctx,
                                          grpc_call_element *elem,
                                          grpc_error *error) {
  channel_data *chand = elem->channel_data;
  call_data *calld = elem->call_data;
  const grpc_connected_subchannel_call_args call_args = {
      .pollent = calld->pollent,
      .path = calld->path,
      .start_time = calld->call_start_time,
      .deadline = calld->deadline,
      .arena = calld->arena,
      .context = calld->subchannel_call_context,
      .call_combiner = calld->call_combiner};
  grpc_error *new_error = grpc_connected_subchannel_create_call(
      exec_ctx, calld->connected_subchannel, &call_args,
      &calld->subchannel_call);
  if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
    gpr_log(GPR_DEBUG, "chand=%p calld=%p: create subchannel_call=%p: error=%s",
            chand, calld, calld->subchannel_call, grpc_error_string(new_error));
  }
  if (new_error != GRPC_ERROR_NONE) {
    new_error = grpc_error_add_child(new_error, error);
    waiting_for_pick_batches_fail(exec_ctx, elem, new_error);
  } else {
    waiting_for_pick_batches_resume(exec_ctx, elem);
  }
  GRPC_ERROR_UNREF(error);
}

static void subchannel_ready_locked(grpc_exec_ctx *exec_ctx,
                                    grpc_call_element *elem,
                                    grpc_error *error) {
  call_data *calld = elem->call_data;
  channel_data *chand = elem->channel_data;
  grpc_polling_entity_del_from_pollset_set(exec_ctx, calld->pollent,
                                           chand->interested_parties);
  if (calld->connected_subchannel == NULL) {
    // Failed to create subchannel.
    GRPC_ERROR_UNREF(calld->error);
    calld->error = error == GRPC_ERROR_NONE
                       ? GRPC_ERROR_CREATE_FROM_STATIC_STRING(
                             "Call dropped by load balancing policy")
                       : GRPC_ERROR_CREATE_REFERENCING_FROM_STATIC_STRING(
                             "Failed to create subchannel", &error, 1);
    if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
      gpr_log(GPR_DEBUG,
              "chand=%p calld=%p: failed to create subchannel: error=%s", chand,
              calld, grpc_error_string(calld->error));
    }
    waiting_for_pick_batches_fail(exec_ctx, elem, GRPC_ERROR_REF(calld->error));
  } else {
    /* Create call on subchannel. */
    create_subchannel_call_locked(exec_ctx, elem, GRPC_ERROR_REF(error));
  }
  GRPC_ERROR_UNREF(error);
}

/** Return true if subchannel is available immediately (in which case
    subchannel_ready_locked() should not be called), or false otherwise (in
    which case subchannel_ready_locked() should be called when the subchannel
    is available). */
static bool pick_subchannel_locked(grpc_exec_ctx *exec_ctx,
                                   grpc_call_element *elem);

typedef struct {
  grpc_call_element *elem;
  bool finished;
  grpc_closure closure;
  grpc_closure cancel_closure;
} pick_after_resolver_result_args;

// Note: This runs under the client_channel combiner, but will NOT be
// holding the call combiner.
static void pick_after_resolver_result_cancel_locked(grpc_exec_ctx *exec_ctx,
                                                     void *arg,
                                                     grpc_error *error) {
  pick_after_resolver_result_args *args = arg;
  if (args->finished) {
    gpr_free(args);
    return;
  }
  args->finished = true;
  grpc_call_element *elem = args->elem;
  channel_data *chand = elem->channel_data;
  call_data *calld = elem->call_data;
  // If we don't yet have a resolver result, then a closure for
  // pick_after_resolver_result_done_locked() will have been added to
  // chand->waiting_for_resolver_result_closures, and it may not be invoked
  // until after this call has been destroyed.  We mark the operation as
  // finished, so that when pick_after_resolver_result_done_locked()
  // is called, it will be a no-op.  We also immediately invoke
  // subchannel_ready_locked() to propagate the error back to the caller.
  if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
    gpr_log(GPR_DEBUG,
            "chand=%p calld=%p: cancelling pick waiting for resolver result",
            chand, calld);
  }
  // Note: Although we are not in the call combiner here, we are
  // basically stealing the call combiner from the pending pick, so
  // it's safe to call subchannel_ready_locked() here -- we are
  // essentially calling it here instead of calling it in
  // pick_after_resolver_result_done_locked().
  subchannel_ready_locked(
      exec_ctx, elem,
      GRPC_ERROR_CREATE_REFERENCING_FROM_STATIC_STRING("Pick cancelled",
                                                       &error, 1));
}

static void pick_after_resolver_result_done_locked(grpc_exec_ctx *exec_ctx,
                                                   void *arg,
                                                   grpc_error *error) {
  pick_after_resolver_result_args *args = arg;
  if (args->finished) {
    /* cancelled, do nothing */
    if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
      gpr_log(GPR_DEBUG, "call cancelled before resolver result");
    }
    gpr_free(args);
    return;
  }
  args->finished = true;
  grpc_call_element *elem = args->elem;
  channel_data *chand = elem->channel_data;
  call_data *calld = elem->call_data;
  grpc_call_combiner_set_notify_on_cancel(exec_ctx, calld->call_combiner,
                                          NULL);
  if (error != GRPC_ERROR_NONE) {
    if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
      gpr_log(GPR_DEBUG, "chand=%p calld=%p: resolver failed to return data",
              chand, calld);
    }
    subchannel_ready_locked(exec_ctx, elem, GRPC_ERROR_REF(error));
  } else {
    if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
      gpr_log(GPR_DEBUG, "chand=%p calld=%p: resolver returned, doing pick",
              chand, calld);
    }
    if (pick_subchannel_locked(exec_ctx, elem)) {
      subchannel_ready_locked(exec_ctx, elem, GRPC_ERROR_NONE);
    }
  }
}

static void pick_after_resolver_result_start_locked(grpc_exec_ctx *exec_ctx,
                                                    grpc_call_element *elem) {
  channel_data *chand = elem->channel_data;
  call_data *calld = elem->call_data;
  if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
    gpr_log(GPR_DEBUG,
            "chand=%p calld=%p: deferring pick pending resolver result", chand,
            calld);
  }
  pick_after_resolver_result_args *args =
      (pick_after_resolver_result_args *)gpr_zalloc(sizeof(*args));
  args->elem = elem;
  GRPC_CLOSURE_INIT(&args->closure, pick_after_resolver_result_done_locked,
                    args, grpc_combiner_scheduler(chand->combiner));
  grpc_closure_list_append(&chand->waiting_for_resolver_result_closures,
                           &args->closure, GRPC_ERROR_NONE);
  grpc_call_combiner_set_notify_on_cancel(
      exec_ctx, calld->call_combiner,
      GRPC_CLOSURE_INIT(&args->cancel_closure,
                        pick_after_resolver_result_cancel_locked, args,
                        grpc_combiner_scheduler(chand->combiner)));
}

// Note: This runs under the client_channel combiner, but will NOT be
// holding the call combiner.
static void pick_callback_cancel_locked(grpc_exec_ctx *exec_ctx, void *arg,
                                        grpc_error *error) {
  grpc_call_element *elem = arg;
  channel_data *chand = elem->channel_data;
  call_data *calld = elem->call_data;
  if (error != GRPC_ERROR_NONE && calld->lb_policy != NULL) {
    if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
      gpr_log(GPR_DEBUG, "chand=%p calld=%p: cancelling pick from LB policy %p",
              chand, calld, calld->lb_policy);
    }
    grpc_lb_policy_cancel_pick_locked(exec_ctx, calld->lb_policy,
                                      &calld->connected_subchannel,
                                      GRPC_ERROR_REF(error));
  }
  GRPC_CALL_STACK_UNREF(exec_ctx, calld->owning_call, "pick_callback_cancel");
}

// Callback invoked by grpc_lb_policy_pick_locked() for async picks.
// Unrefs the LB policy and invokes subchannel_ready_locked().
static void pick_callback_done_locked(grpc_exec_ctx *exec_ctx, void *arg,
                                      grpc_error *error) {
  grpc_call_element *elem = arg;
  channel_data *chand = elem->channel_data;
  call_data *calld = elem->call_data;
  if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
    gpr_log(GPR_DEBUG, "chand=%p calld=%p: pick completed asynchronously",
            chand, calld);
  }
  grpc_call_combiner_set_notify_on_cancel(exec_ctx, calld->call_combiner, NULL);
  GPR_ASSERT(calld->lb_policy != NULL);
  GRPC_LB_POLICY_UNREF(exec_ctx, calld->lb_policy, "pick_subchannel");
  calld->lb_policy = NULL;
  subchannel_ready_locked(exec_ctx, elem, GRPC_ERROR_REF(error));
}

// Takes a ref to chand->lb_policy and calls grpc_lb_policy_pick_locked().
// If the pick was completed synchronously, unrefs the LB policy and
// returns true.
static bool pick_callback_start_locked(grpc_exec_ctx *exec_ctx,
                                       grpc_call_element *elem,
                                       const grpc_lb_policy_pick_args *inputs) {
  channel_data *chand = elem->channel_data;
  call_data *calld = elem->call_data;
  if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
    gpr_log(GPR_DEBUG, "chand=%p calld=%p: starting pick on lb_policy=%p",
            chand, calld, chand->lb_policy);
  }
  // Keep a ref to the LB policy in calld while the pick is pending.
  GRPC_LB_POLICY_REF(chand->lb_policy, "pick_subchannel");
  calld->lb_policy = chand->lb_policy;
  GRPC_CLOSURE_INIT(&calld->lb_pick_closure, pick_callback_done_locked, elem,
                    grpc_combiner_scheduler(chand->combiner));
  const bool pick_done = grpc_lb_policy_pick_locked(
      exec_ctx, chand->lb_policy, inputs, &calld->connected_subchannel,
      calld->subchannel_call_context, NULL, &calld->lb_pick_closure);
  if (pick_done) {
    /* synchronous grpc_lb_policy_pick call. Unref the LB policy. */
    if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
      gpr_log(GPR_DEBUG, "chand=%p calld=%p: pick completed synchronously",
              chand, calld);
    }
    GRPC_LB_POLICY_UNREF(exec_ctx, calld->lb_policy, "pick_subchannel");
    calld->lb_policy = NULL;
  } else {
    GRPC_CALL_STACK_REF(calld->owning_call, "pick_callback_cancel");
    grpc_call_combiner_set_notify_on_cancel(
        exec_ctx, calld->call_combiner,
        GRPC_CLOSURE_INIT(&calld->lb_pick_cancel_closure,
                          pick_callback_cancel_locked, elem,
                          grpc_combiner_scheduler(chand->combiner)));
  }
  return pick_done;
}

static bool pick_subchannel_locked(grpc_exec_ctx *exec_ctx,
                                   grpc_call_element *elem) {
  GPR_TIMER_BEGIN("pick_subchannel", 0);
  channel_data *chand = elem->channel_data;
  call_data *calld = elem->call_data;
  bool pick_done = false;
  if (chand->lb_policy != NULL) {
    apply_service_config_to_call_locked(exec_ctx, elem);
    // If the application explicitly set wait_for_ready, use that.
    // Otherwise, if the service config specified a value for this
    // method, use that.
    uint32_t initial_metadata_flags =
        calld->initial_metadata_batch->payload->send_initial_metadata
            .send_initial_metadata_flags;
    const bool wait_for_ready_set_from_api =
        initial_metadata_flags &
        GRPC_INITIAL_METADATA_WAIT_FOR_READY_EXPLICITLY_SET;
    const bool wait_for_ready_set_from_service_config =
        calld->method_params != NULL &&
        calld->method_params->wait_for_ready != WAIT_FOR_READY_UNSET;
    if (!wait_for_ready_set_from_api &&
        wait_for_ready_set_from_service_config) {
      if (calld->method_params->wait_for_ready == WAIT_FOR_READY_TRUE) {
        initial_metadata_flags |= GRPC_INITIAL_METADATA_WAIT_FOR_READY;
      } else {
        initial_metadata_flags &= ~GRPC_INITIAL_METADATA_WAIT_FOR_READY;
      }
    }
    const grpc_lb_policy_pick_args inputs = {
        calld->initial_metadata_batch->payload->send_initial_metadata
            .send_initial_metadata,
        initial_metadata_flags, &calld->lb_token_mdelem};
    pick_done = pick_callback_start_locked(exec_ctx, elem, &inputs);
  } else if (chand->resolver != NULL) {
    if (!chand->started_resolving) {
      start_resolving_locked(exec_ctx, chand);
    }
    pick_after_resolver_result_start_locked(exec_ctx, elem);
  } else {
    subchannel_ready_locked(
        exec_ctx, elem, GRPC_ERROR_CREATE_FROM_STATIC_STRING("Disconnected"));
  }
  GPR_TIMER_END("pick_subchannel", 0);
  return pick_done;
}

static void start_pick_locked(grpc_exec_ctx *exec_ctx, void *arg,
                              grpc_error *error_ignored) {
  GPR_TIMER_BEGIN("start_pick_locked", 0);
  grpc_call_element *elem = (grpc_call_element *)arg;
  call_data *calld = (call_data *)elem->call_data;
  channel_data *chand = (channel_data *)elem->channel_data;
  GPR_ASSERT(calld->connected_subchannel == NULL);
  if (pick_subchannel_locked(exec_ctx, elem)) {
    // Pick was returned synchronously.
    if (calld->connected_subchannel == NULL) {
      GRPC_ERROR_UNREF(calld->error);
      calld->error = GRPC_ERROR_CREATE_FROM_STATIC_STRING(
          "Call dropped by load balancing policy");
      waiting_for_pick_batches_fail(exec_ctx, elem,
                                    GRPC_ERROR_REF(calld->error));
    } else {
      // Create subchannel call.
      create_subchannel_call_locked(exec_ctx, elem, GRPC_ERROR_NONE);
    }
  } else {
    // Pick will be done asynchronously.  Add the call's polling entity to
    // the channel's interested_parties, so that I/O for the resolver
    // and LB policy can be done under it.
    grpc_polling_entity_add_to_pollset_set(exec_ctx, calld->pollent,
                                           chand->interested_parties);
  }
  GPR_TIMER_END("start_pick_locked", 0);
}

static void on_complete(grpc_exec_ctx *exec_ctx, void *arg, grpc_error *error) {
  grpc_call_element *elem = arg;
  call_data *calld = elem->call_data;
  if (calld->retry_throttle_data != NULL) {
    if (error == GRPC_ERROR_NONE) {
      grpc_server_retry_throttle_data_record_success(
          calld->retry_throttle_data);
    } else {
      // TODO(roth): In a subsequent PR, check the return value here and
      // decide whether or not to retry.  Note that we should only
      // record failures whose statuses match the configured retryable
      // or non-fatal status codes.
      grpc_server_retry_throttle_data_record_failure(
          calld->retry_throttle_data);
    }
  }
  GRPC_CLOSURE_RUN(exec_ctx, calld->original_on_complete,
                   GRPC_ERROR_REF(error));
}

static void cc_start_transport_stream_op_batch(
    grpc_exec_ctx *exec_ctx, grpc_call_element *elem,
    grpc_transport_stream_op_batch *batch) {
  call_data *calld = elem->call_data;
  channel_data *chand = elem->channel_data;
  if (chand->deadline_checking_enabled) {
    grpc_deadline_state_client_start_transport_stream_op_batch(exec_ctx, elem,
                                                               batch);
  }
  GPR_TIMER_BEGIN("cc_start_transport_stream_op_batch", 0);
  // If we've previously been cancelled, immediately fail any new batches.
  if (calld->error != GRPC_ERROR_NONE) {
    if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
      gpr_log(GPR_DEBUG, "chand=%p calld=%p: failing batch with error: %s",
              chand, calld, grpc_error_string(calld->error));
    }
    grpc_transport_stream_op_batch_finish_with_failure(
        exec_ctx, batch, GRPC_ERROR_REF(calld->error), calld->call_combiner);
    goto done;
  }
  if (batch->cancel_stream) {
    // Stash a copy of cancel_error in our call data, so that we can use
    // it for subsequent operations.  This ensures that if the call is
    // cancelled before any batches are passed down (e.g., if the deadline
    // is in the past when the call starts), we can return the right
    // error to the caller when the first batch does get passed down.
    GRPC_ERROR_UNREF(calld->error);
    calld->error = GRPC_ERROR_REF(batch->payload->cancel_stream.cancel_error);
    if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
      gpr_log(GPR_DEBUG, "chand=%p calld=%p: recording cancel_error=%s", chand,
              calld, grpc_error_string(calld->error));
    }
    // If we have a subchannel call, send the cancellation batch down.
    // Otherwise, fail all pending batches.
    if (calld->subchannel_call != NULL) {
      grpc_subchannel_call_process_op(exec_ctx, calld->subchannel_call, batch);
    } else {
      waiting_for_pick_batches_add(calld, batch);
      waiting_for_pick_batches_fail(exec_ctx, elem,
                                    GRPC_ERROR_REF(calld->error));
    }
    goto done;
  }
  // Intercept on_complete for recv_trailing_metadata so that we can
  // check retry throttle status.
  if (batch->recv_trailing_metadata) {
    GPR_ASSERT(batch->on_complete != NULL);
    calld->original_on_complete = batch->on_complete;
    GRPC_CLOSURE_INIT(&calld->on_complete, on_complete, elem,
                      grpc_schedule_on_exec_ctx);
    batch->on_complete = &calld->on_complete;
  }
  // Check if we've already gotten a subchannel call.
  // Note that once we have completed the pick, we do not need to enter
  // the channel combiner, which is more efficient (especially for
  // streaming calls).
  if (calld->subchannel_call != NULL) {
    if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
      gpr_log(GPR_DEBUG,
              "chand=%p calld=%p: sending batch to subchannel_call=%p", chand,
              calld, calld->subchannel_call);
    }
    grpc_subchannel_call_process_op(exec_ctx, calld->subchannel_call, batch);
    goto done;
  }
  // We do not yet have a subchannel call.
  // Add the batch to the waiting-for-pick list.
  waiting_for_pick_batches_add(calld, batch);
  // For batches containing a send_initial_metadata op, enter the channel
  // combiner to start a pick.
  if (batch->send_initial_metadata) {
    if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
      gpr_log(GPR_DEBUG, "chand=%p calld=%p: entering combiner", chand, calld);
    }
    GRPC_CLOSURE_SCHED(
        exec_ctx,
        GRPC_CLOSURE_INIT(&batch->handler_private.closure, start_pick_locked,
                          elem, grpc_combiner_scheduler(chand->combiner)),
        GRPC_ERROR_NONE);
  } else {
    // For all other batches, release the call combiner.
    if (GRPC_TRACER_ON(grpc_client_channel_trace)) {
      gpr_log(GPR_DEBUG,
              "chand=%p calld=%p: saved batch, yeilding call combiner", chand,
              calld);
    }
    GRPC_CALL_COMBINER_STOP(exec_ctx, calld->call_combiner,
                            "batch does not include send_initial_metadata");
  }
done:
  GPR_TIMER_END("cc_start_transport_stream_op_batch", 0);
}

/* Constructor for call_data */
static grpc_error *cc_init_call_elem(grpc_exec_ctx *exec_ctx,
                                     grpc_call_element *elem,
                                     const grpc_call_element_args *args) {
  call_data *calld = elem->call_data;
  channel_data *chand = elem->channel_data;
  // Initialize data members.
  calld->path = grpc_slice_ref_internal(args->path);
  calld->call_start_time = args->start_time;
  calld->deadline = gpr_convert_clock_type(args->deadline, GPR_CLOCK_MONOTONIC);
  calld->arena = args->arena;
  calld->owning_call = args->call_stack;
  calld->call_combiner = args->call_combiner;
  if (chand->deadline_checking_enabled) {
    grpc_deadline_state_init(exec_ctx, elem, args->call_stack,
                             args->call_combiner, calld->deadline);
  }
  return GRPC_ERROR_NONE;
}

/* Destructor for call_data */
static void cc_destroy_call_elem(grpc_exec_ctx *exec_ctx,
                                 grpc_call_element *elem,
                                 const grpc_call_final_info *final_info,
                                 grpc_closure *then_schedule_closure) {
  call_data *calld = elem->call_data;
  channel_data *chand = elem->channel_data;
  if (chand->deadline_checking_enabled) {
    grpc_deadline_state_destroy(exec_ctx, elem);
  }
  grpc_slice_unref_internal(exec_ctx, calld->path);
  if (calld->method_params != NULL) {
    method_parameters_unref(calld->method_params);
  }
  GRPC_ERROR_UNREF(calld->error);
  if (calld->subchannel_call != NULL) {
    grpc_subchannel_call_set_cleanup_closure(calld->subchannel_call,
                                             then_schedule_closure);
    then_schedule_closure = NULL;
    GRPC_SUBCHANNEL_CALL_UNREF(exec_ctx, calld->subchannel_call,
                               "client_channel_destroy_call");
  }
  GPR_ASSERT(calld->lb_policy == NULL);
  GPR_ASSERT(calld->waiting_for_pick_batches_count == 0);
  if (calld->connected_subchannel != NULL) {
    GRPC_CONNECTED_SUBCHANNEL_UNREF(exec_ctx, calld->connected_subchannel,
                                    "picked");
  }
  for (size_t i = 0; i < GRPC_CONTEXT_COUNT; ++i) {
    if (calld->subchannel_call_context[i].value != NULL) {
      calld->subchannel_call_context[i].destroy(
          calld->subchannel_call_context[i].value);
    }
  }
  GRPC_CLOSURE_SCHED(exec_ctx, then_schedule_closure, GRPC_ERROR_NONE);
}

static void cc_set_pollset_or_pollset_set(grpc_exec_ctx *exec_ctx,
                                          grpc_call_element *elem,
                                          grpc_polling_entity *pollent) {
  call_data *calld = elem->call_data;
  calld->pollent = pollent;
}

/*************************************************************************
 * EXPORTED SYMBOLS
 */

const grpc_channel_filter grpc_client_channel_filter = {
    cc_start_transport_stream_op_batch,
    cc_start_transport_op,
    sizeof(call_data),
    cc_init_call_elem,
    cc_set_pollset_or_pollset_set,
    cc_destroy_call_elem,
    sizeof(channel_data),
    cc_init_channel_elem,
    cc_destroy_channel_elem,
    cc_get_channel_info,
    "client-channel",
};

static void try_to_connect_locked(grpc_exec_ctx *exec_ctx, void *arg,
                                  grpc_error *error_ignored) {
  channel_data *chand = arg;
  if (chand->lb_policy != NULL) {
    grpc_lb_policy_exit_idle_locked(exec_ctx, chand->lb_policy);
  } else {
    chand->exit_idle_when_lb_policy_arrives = true;
    if (!chand->started_resolving && chand->resolver != NULL) {
      start_resolving_locked(exec_ctx, chand);
    }
  }
  GRPC_CHANNEL_STACK_UNREF(exec_ctx, chand->owning_stack, "try_to_connect");
}

grpc_connectivity_state grpc_client_channel_check_connectivity_state(
    grpc_exec_ctx *exec_ctx, grpc_channel_element *elem, int try_to_connect) {
  channel_data *chand = elem->channel_data;
  grpc_connectivity_state out =
      grpc_connectivity_state_check(&chand->state_tracker);
  if (out == GRPC_CHANNEL_IDLE && try_to_connect) {
    GRPC_CHANNEL_STACK_REF(chand->owning_stack, "try_to_connect");
    GRPC_CLOSURE_SCHED(
        exec_ctx, GRPC_CLOSURE_CREATE(try_to_connect_locked, chand,
                                      grpc_combiner_scheduler(chand->combiner)),
        GRPC_ERROR_NONE);
  }
  return out;
}

typedef struct external_connectivity_watcher {
  channel_data *chand;
  grpc_polling_entity pollent;
  grpc_closure *on_complete;
  grpc_closure *watcher_timer_init;
  grpc_connectivity_state *state;
  grpc_closure my_closure;
  struct external_connectivity_watcher *next;
} external_connectivity_watcher;

static external_connectivity_watcher *lookup_external_connectivity_watcher(
    channel_data *chand, grpc_closure *on_complete) {
  gpr_mu_lock(&chand->external_connectivity_watcher_list_mu);
  external_connectivity_watcher *w =
      chand->external_connectivity_watcher_list_head;
  while (w != NULL && w->on_complete != on_complete) {
    w = w->next;
  }
  gpr_mu_unlock(&chand->external_connectivity_watcher_list_mu);
  return w;
}

static void external_connectivity_watcher_list_append(
    channel_data *chand, external_connectivity_watcher *w) {
  GPR_ASSERT(!lookup_external_connectivity_watcher(chand, w->on_complete));

  gpr_mu_lock(&w->chand->external_connectivity_watcher_list_mu);
  GPR_ASSERT(!w->next);
  w->next = chand->external_connectivity_watcher_list_head;
  chand->external_connectivity_watcher_list_head = w;
  gpr_mu_unlock(&w->chand->external_connectivity_watcher_list_mu);
}

static void external_connectivity_watcher_list_remove(
    channel_data *chand, external_connectivity_watcher *too_remove) {
  GPR_ASSERT(
      lookup_external_connectivity_watcher(chand, too_remove->on_complete));
  gpr_mu_lock(&chand->external_connectivity_watcher_list_mu);
  if (too_remove == chand->external_connectivity_watcher_list_head) {
    chand->external_connectivity_watcher_list_head = too_remove->next;
    gpr_mu_unlock(&chand->external_connectivity_watcher_list_mu);
    return;
  }
  external_connectivity_watcher *w =
      chand->external_connectivity_watcher_list_head;
  while (w != NULL) {
    if (w->next == too_remove) {
      w->next = w->next->next;
      gpr_mu_unlock(&chand->external_connectivity_watcher_list_mu);
      return;
    }
    w = w->next;
  }
  GPR_UNREACHABLE_CODE(return );
}

int grpc_client_channel_num_external_connectivity_watchers(
    grpc_channel_element *elem) {
  channel_data *chand = elem->channel_data;
  int count = 0;

  gpr_mu_lock(&chand->external_connectivity_watcher_list_mu);
  external_connectivity_watcher *w =
      chand->external_connectivity_watcher_list_head;
  while (w != NULL) {
    count++;
    w = w->next;
  }
  gpr_mu_unlock(&chand->external_connectivity_watcher_list_mu);

  return count;
}

static void on_external_watch_complete(grpc_exec_ctx *exec_ctx, void *arg,
                                       grpc_error *error) {
  external_connectivity_watcher *w = arg;
  grpc_closure *follow_up = w->on_complete;
  grpc_polling_entity_del_from_pollset_set(exec_ctx, &w->pollent,
                                           w->chand->interested_parties);
  GRPC_CHANNEL_STACK_UNREF(exec_ctx, w->chand->owning_stack,
                           "external_connectivity_watcher");
  external_connectivity_watcher_list_remove(w->chand, w);
  gpr_free(w);
  GRPC_CLOSURE_RUN(exec_ctx, follow_up, GRPC_ERROR_REF(error));
}

static void watch_connectivity_state_locked(grpc_exec_ctx *exec_ctx, void *arg,
                                            grpc_error *error_ignored) {
  external_connectivity_watcher *w = arg;
  external_connectivity_watcher *found = NULL;
  if (w->state != NULL) {
    external_connectivity_watcher_list_append(w->chand, w);
    GRPC_CLOSURE_RUN(exec_ctx, w->watcher_timer_init, GRPC_ERROR_NONE);
    GRPC_CLOSURE_INIT(&w->my_closure, on_external_watch_complete, w,
                      grpc_schedule_on_exec_ctx);
    grpc_connectivity_state_notify_on_state_change(
        exec_ctx, &w->chand->state_tracker, w->state, &w->my_closure);
  } else {
    GPR_ASSERT(w->watcher_timer_init == NULL);
    found = lookup_external_connectivity_watcher(w->chand, w->on_complete);
    if (found) {
      GPR_ASSERT(found->on_complete == w->on_complete);
      grpc_connectivity_state_notify_on_state_change(
          exec_ctx, &found->chand->state_tracker, NULL, &found->my_closure);
    }
    grpc_polling_entity_del_from_pollset_set(exec_ctx, &w->pollent,
                                             w->chand->interested_parties);
    GRPC_CHANNEL_STACK_UNREF(exec_ctx, w->chand->owning_stack,
                             "external_connectivity_watcher");
    gpr_free(w);
  }
}

void grpc_client_channel_watch_connectivity_state(
    grpc_exec_ctx *exec_ctx, grpc_channel_element *elem,
    grpc_polling_entity pollent, grpc_connectivity_state *state,
    grpc_closure *closure, grpc_closure *watcher_timer_init) {
  channel_data *chand = elem->channel_data;
  external_connectivity_watcher *w = gpr_zalloc(sizeof(*w));
  w->chand = chand;
  w->pollent = pollent;
  w->on_complete = closure;
  w->state = state;
  w->watcher_timer_init = watcher_timer_init;
  grpc_polling_entity_add_to_pollset_set(exec_ctx, &w->pollent,
                                         chand->interested_parties);
  GRPC_CHANNEL_STACK_REF(w->chand->owning_stack,
                         "external_connectivity_watcher");
  GRPC_CLOSURE_SCHED(
      exec_ctx,
      GRPC_CLOSURE_INIT(&w->my_closure, watch_connectivity_state_locked, w,
                        grpc_combiner_scheduler(chand->combiner)),
      GRPC_ERROR_NONE);
}