src/core/security/credentials.c

/*
 *
 * Copyright 2015, Google Inc.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met:
 *
 *     * Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 *     * Redistributions in binary form must reproduce the above
 * copyright notice, this list of conditions and the following disclaimer
 * in the documentation and/or other materials provided with the
 * distribution.
 *     * Neither the name of Google Inc. nor the names of its
 * contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *
 */

#include "src/core/security/credentials.h"

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

#include "src/core/channel/channel_args.h"
#include "src/core/channel/http_client_filter.h"
#include "src/core/json/json.h"
#include "src/core/httpcli/httpcli.h"
#include "src/core/iomgr/iomgr.h"
#include "src/core/support/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/thd.h>
#include <grpc/support/time.h>

/* -- Common. -- */

struct grpc_credentials_metadata_request
{
  grpc_credentials *creds;
  grpc_credentials_metadata_cb cb;
  void *user_data;
};

static grpc_credentials_metadata_request *
grpc_credentials_metadata_request_create (grpc_credentials * creds, grpc_credentials_metadata_cb cb, void *user_data)
{
  grpc_credentials_metadata_request *r = gpr_malloc (sizeof (grpc_credentials_metadata_request));
  r->creds = grpc_credentials_ref (creds);
  r->cb = cb;
  r->user_data = user_data;
  return r;
}

static void
grpc_credentials_metadata_request_destroy (grpc_credentials_metadata_request * r)
{
  grpc_credentials_unref (r->creds);
  gpr_free (r);
}

grpc_credentials *
grpc_credentials_ref (grpc_credentials * creds)
{
  if (creds == NULL)
    return NULL;
  gpr_ref (&creds->refcount);
  return creds;
}

void
grpc_credentials_unref (grpc_credentials * creds)
{
  if (creds == NULL)
    return;
  if (gpr_unref (&creds->refcount))
    {
      creds->vtable->destruct (creds);
      gpr_free (creds);
    }
}

void
grpc_credentials_release (grpc_credentials * creds)
{
  grpc_credentials_unref (creds);
}

int
grpc_credentials_has_request_metadata (grpc_credentials * creds)
{
  if (creds == NULL)
    return 0;
  return creds->vtable->has_request_metadata (creds);
}

int
grpc_credentials_has_request_metadata_only (grpc_credentials * creds)
{
  if (creds == NULL)
    return 0;
  return creds->vtable->has_request_metadata_only (creds);
}

void
grpc_credentials_get_request_metadata (grpc_exec_ctx * exec_ctx, grpc_credentials * creds, grpc_pollset * pollset, const char *service_url, grpc_credentials_metadata_cb cb, void *user_data)
{
  if (creds == NULL || !grpc_credentials_has_request_metadata (creds) || creds->vtable->get_request_metadata == NULL)
    {
      if (cb != NULL)
	{
	  cb (exec_ctx, user_data, NULL, 0, GRPC_CREDENTIALS_OK);
	}
      return;
    }
  creds->vtable->get_request_metadata (exec_ctx, creds, pollset, service_url, cb, user_data);
}

grpc_security_status
grpc_credentials_create_security_connector (grpc_credentials * creds, const char *target, const grpc_channel_args * args, grpc_credentials * request_metadata_creds, grpc_channel_security_connector ** sc, grpc_channel_args ** new_args)
{
  *new_args = NULL;
  if (creds == NULL || creds->vtable->create_security_connector == NULL || grpc_credentials_has_request_metadata_only (creds))
    {
      gpr_log (GPR_ERROR, "Invalid credentials for creating a security connector.");
      return GRPC_SECURITY_ERROR;
    }
  return creds->vtable->create_security_connector (creds, target, args, request_metadata_creds, sc, new_args);
}

grpc_server_credentials *
grpc_server_credentials_ref (grpc_server_credentials * creds)
{
  if (creds == NULL)
    return NULL;
  gpr_ref (&creds->refcount);
  return creds;
}

void
grpc_server_credentials_unref (grpc_server_credentials * creds)
{
  if (creds == NULL)
    return;
  if (gpr_unref (&creds->refcount))
    {
      creds->vtable->destruct (creds);
      if (creds->processor.destroy != NULL && creds->processor.state != NULL)
	{
	  creds->processor.destroy (creds->processor.state);
	}
      gpr_free (creds);
    }
}

void
grpc_server_credentials_release (grpc_server_credentials * creds)
{
  grpc_server_credentials_unref (creds);
}

grpc_security_status
grpc_server_credentials_create_security_connector (grpc_server_credentials * creds, grpc_security_connector ** sc)
{
  if (creds == NULL || creds->vtable->create_security_connector == NULL)
    {
      gpr_log (GPR_ERROR, "Server credentials cannot create security context.");
      return GRPC_SECURITY_ERROR;
    }
  return creds->vtable->create_security_connector (creds, sc);
}

void
grpc_server_credentials_set_auth_metadata_processor (grpc_server_credentials * creds, grpc_auth_metadata_processor processor)
{
  if (creds == NULL)
    return;
  if (creds->processor.destroy != NULL && creds->processor.state != NULL)
    {
      creds->processor.destroy (creds->processor.state);
    }
  creds->processor = processor;
}

/* -- Ssl credentials. -- */

static void
ssl_destruct (grpc_credentials * creds)
{
  grpc_ssl_credentials *c = (grpc_ssl_credentials *) creds;
  if (c->config.pem_root_certs != NULL)
    gpr_free (c->config.pem_root_certs);
  if (c->config.pem_private_key != NULL)
    gpr_free (c->config.pem_private_key);
  if (c->config.pem_cert_chain != NULL)
    gpr_free (c->config.pem_cert_chain);
}

static void
ssl_server_destruct (grpc_server_credentials * creds)
{
  grpc_ssl_server_credentials *c = (grpc_ssl_server_credentials *) creds;
  size_t i;
  for (i = 0; i < c->config.num_key_cert_pairs; i++)
    {
      if (c->config.pem_private_keys[i] != NULL)
	{
	  gpr_free (c->config.pem_private_keys[i]);
	}
      if (c->config.pem_cert_chains[i] != NULL)
	{
	  gpr_free (c->config.pem_cert_chains[i]);
	}
    }
  if (c->config.pem_private_keys != NULL)
    gpr_free (c->config.pem_private_keys);
  if (c->config.pem_private_keys_sizes != NULL)
    {
      gpr_free (c->config.pem_private_keys_sizes);
    }
  if (c->config.pem_cert_chains != NULL)
    gpr_free (c->config.pem_cert_chains);
  if (c->config.pem_cert_chains_sizes != NULL)
    {
      gpr_free (c->config.pem_cert_chains_sizes);
    }
  if (c->config.pem_root_certs != NULL)
    gpr_free (c->config.pem_root_certs);
}

static int
ssl_has_request_metadata (const grpc_credentials * creds)
{
  return 0;
}

static int
ssl_has_request_metadata_only (const grpc_credentials * creds)
{
  return 0;
}

static grpc_security_status
ssl_create_security_connector (grpc_credentials * creds, const char *target, const grpc_channel_args * args, grpc_credentials * request_metadata_creds, grpc_channel_security_connector ** sc, grpc_channel_args ** new_args)
{
  grpc_ssl_credentials *c = (grpc_ssl_credentials *) creds;
  grpc_security_status status = GRPC_SECURITY_OK;
  size_t i = 0;
  const char *overridden_target_name = NULL;
  grpc_arg arg;

  for (i = 0; args && i < args->num_args; i++)
    {
      grpc_arg *arg = &args->args[i];
      if (strcmp (arg->key, GRPC_SSL_TARGET_NAME_OVERRIDE_ARG) == 0 && arg->type == GRPC_ARG_STRING)
	{
	  overridden_target_name = arg->value.string;
	  break;
	}
    }
  status = grpc_ssl_channel_security_connector_create (request_metadata_creds, &c->config, target, overridden_target_name, sc);
  if (status != GRPC_SECURITY_OK)
    {
      return status;
    }
  arg.type = GRPC_ARG_STRING;
  arg.key = GRPC_ARG_HTTP2_SCHEME;
  arg.value.string = "https";
  *new_args = grpc_channel_args_copy_and_add (args, &arg, 1);
  return status;
}

static grpc_security_status
ssl_server_create_security_connector (grpc_server_credentials * creds, grpc_security_connector ** sc)
{
  grpc_ssl_server_credentials *c = (grpc_ssl_server_credentials *) creds;
  return grpc_ssl_server_security_connector_create (&c->config, sc);
}

static grpc_credentials_vtable ssl_vtable = {
  ssl_destruct, ssl_has_request_metadata, ssl_has_request_metadata_only, NULL,
  ssl_create_security_connector
};

static grpc_server_credentials_vtable ssl_server_vtable = {
  ssl_server_destruct, ssl_server_create_security_connector
};

static void
ssl_copy_key_material (const char *input, unsigned char **output, size_t * output_size)
{
  *output_size = strlen (input);
  *output = gpr_malloc (*output_size);
  memcpy (*output, input, *output_size);
}

static void
ssl_build_config (const char *pem_root_certs, grpc_ssl_pem_key_cert_pair * pem_key_cert_pair, grpc_ssl_config * config)
{
  if (pem_root_certs != NULL)
    {
      ssl_copy_key_material (pem_root_certs, &config->pem_root_certs, &config->pem_root_certs_size);
    }
  if (pem_key_cert_pair != NULL)
    {
      GPR_ASSERT (pem_key_cert_pair->private_key != NULL);
      GPR_ASSERT (pem_key_cert_pair->cert_chain != NULL);
      ssl_copy_key_material (pem_key_cert_pair->private_key, &config->pem_private_key, &config->pem_private_key_size);
      ssl_copy_key_material (pem_key_cert_pair->cert_chain, &config->pem_cert_chain, &config->pem_cert_chain_size);
    }
}

static void
ssl_build_server_config (const char *pem_root_certs, grpc_ssl_pem_key_cert_pair * pem_key_cert_pairs, size_t num_key_cert_pairs, int force_client_auth, grpc_ssl_server_config * config)
{
  size_t i;
  config->force_client_auth = force_client_auth;
  if (pem_root_certs != NULL)
    {
      ssl_copy_key_material (pem_root_certs, &config->pem_root_certs, &config->pem_root_certs_size);
    }
  if (num_key_cert_pairs > 0)
    {
      GPR_ASSERT (pem_key_cert_pairs != NULL);
      config->pem_private_keys = gpr_malloc (num_key_cert_pairs * sizeof (unsigned char *));
      config->pem_cert_chains = gpr_malloc (num_key_cert_pairs * sizeof (unsigned char *));
      config->pem_private_keys_sizes = gpr_malloc (num_key_cert_pairs * sizeof (size_t));
      config->pem_cert_chains_sizes = gpr_malloc (num_key_cert_pairs * sizeof (size_t));
    }
  config->num_key_cert_pairs = num_key_cert_pairs;
  for (i = 0; i < num_key_cert_pairs; i++)
    {
      GPR_ASSERT (pem_key_cert_pairs[i].private_key != NULL);
      GPR_ASSERT (pem_key_cert_pairs[i].cert_chain != NULL);
      ssl_copy_key_material (pem_key_cert_pairs[i].private_key, &config->pem_private_keys[i], &config->pem_private_keys_sizes[i]);
      ssl_copy_key_material (pem_key_cert_pairs[i].cert_chain, &config->pem_cert_chains[i], &config->pem_cert_chains_sizes[i]);
    }
}

grpc_credentials *
grpc_ssl_credentials_create (const char *pem_root_certs, grpc_ssl_pem_key_cert_pair * pem_key_cert_pair, void *reserved)
{
  grpc_ssl_credentials *c = gpr_malloc (sizeof (grpc_ssl_credentials));
  GPR_ASSERT (reserved == NULL);
  memset (c, 0, sizeof (grpc_ssl_credentials));
  c->base.type = GRPC_CREDENTIALS_TYPE_SSL;
  c->base.vtable = &ssl_vtable;
  gpr_ref_init (&c->base.refcount, 1);
  ssl_build_config (pem_root_certs, pem_key_cert_pair, &c->config);
  return &c->base;
}

grpc_server_credentials *
grpc_ssl_server_credentials_create (const char *pem_root_certs, grpc_ssl_pem_key_cert_pair * pem_key_cert_pairs, size_t num_key_cert_pairs, int force_client_auth, void *reserved)
{
  grpc_ssl_server_credentials *c = gpr_malloc (sizeof (grpc_ssl_server_credentials));
  GPR_ASSERT (reserved == NULL);
  memset (c, 0, sizeof (grpc_ssl_server_credentials));
  c->base.type = GRPC_CREDENTIALS_TYPE_SSL;
  gpr_ref_init (&c->base.refcount, 1);
  c->base.vtable = &ssl_server_vtable;
  ssl_build_server_config (pem_root_certs, pem_key_cert_pairs, num_key_cert_pairs, force_client_auth, &c->config);
  return &c->base;
}

/* -- Jwt credentials -- */

static void
jwt_reset_cache (grpc_service_account_jwt_access_credentials * c)
{
  if (c->cached.jwt_md != NULL)
    {
      grpc_credentials_md_store_unref (c->cached.jwt_md);
      c->cached.jwt_md = NULL;
    }
  if (c->cached.service_url != NULL)
    {
      gpr_free (c->cached.service_url);
      c->cached.service_url = NULL;
    }
  c->cached.jwt_expiration = gpr_inf_past (GPR_CLOCK_REALTIME);
}

static void
jwt_destruct (grpc_credentials * creds)
{
  grpc_service_account_jwt_access_credentials *c = (grpc_service_account_jwt_access_credentials *) creds;
  grpc_auth_json_key_destruct (&c->key);
  jwt_reset_cache (c);
  gpr_mu_destroy (&c->cache_mu);
}

static int
jwt_has_request_metadata (const grpc_credentials * creds)
{
  return 1;
}

static int
jwt_has_request_metadata_only (const grpc_credentials * creds)
{
  return 1;
}

static void
jwt_get_request_metadata (grpc_exec_ctx * exec_ctx, grpc_credentials * creds, grpc_pollset * pollset, const char *service_url, grpc_credentials_metadata_cb cb, void *user_data)
{
  grpc_service_account_jwt_access_credentials *c = (grpc_service_account_jwt_access_credentials *) creds;
  gpr_timespec refresh_threshold = gpr_time_from_seconds (GRPC_SECURE_TOKEN_REFRESH_THRESHOLD_SECS, GPR_TIMESPAN);

  /* See if we can return a cached jwt. */
  grpc_credentials_md_store *jwt_md = NULL;
  {
    gpr_mu_lock (&c->cache_mu);
    if (c->cached.service_url != NULL && strcmp (c->cached.service_url, service_url) == 0 && c->cached.jwt_md != NULL && (gpr_time_cmp (gpr_time_sub (c->cached.jwt_expiration, gpr_now (GPR_CLOCK_REALTIME)), refresh_threshold) > 0))
      {
	jwt_md = grpc_credentials_md_store_ref (c->cached.jwt_md);
      }
    gpr_mu_unlock (&c->cache_mu);
  }

  if (jwt_md == NULL)
    {
      char *jwt = NULL;
      /* Generate a new jwt. */
      gpr_mu_lock (&c->cache_mu);
      jwt_reset_cache (c);
      jwt = grpc_jwt_encode_and_sign (&c->key, service_url, c->jwt_lifetime, NULL);
      if (jwt != NULL)
	{
	  char *md_value;
	  gpr_asprintf (&md_value, "Bearer %s", jwt);
	  gpr_free (jwt);
	  c->cached.jwt_expiration = gpr_time_add (gpr_now (GPR_CLOCK_REALTIME), c->jwt_lifetime);
	  c->cached.service_url = gpr_strdup (service_url);
	  c->cached.jwt_md = grpc_credentials_md_store_create (1);
	  grpc_credentials_md_store_add_cstrings (c->cached.jwt_md, GRPC_AUTHORIZATION_METADATA_KEY, md_value);
	  gpr_free (md_value);
	  jwt_md = grpc_credentials_md_store_ref (c->cached.jwt_md);
	}
      gpr_mu_unlock (&c->cache_mu);
    }

  if (jwt_md != NULL)
    {
      cb (exec_ctx, user_data, jwt_md->entries, jwt_md->num_entries, GRPC_CREDENTIALS_OK);
      grpc_credentials_md_store_unref (jwt_md);
    }
  else
    {
      cb (exec_ctx, user_data, NULL, 0, GRPC_CREDENTIALS_ERROR);
    }
}

static grpc_credentials_vtable jwt_vtable = {
  jwt_destruct, jwt_has_request_metadata, jwt_has_request_metadata_only,
  jwt_get_request_metadata, NULL
};

grpc_credentials *
grpc_service_account_jwt_access_credentials_create_from_auth_json_key (grpc_auth_json_key key, gpr_timespec token_lifetime)
{
  grpc_service_account_jwt_access_credentials *c;
  if (!grpc_auth_json_key_is_valid (&key))
    {
      gpr_log (GPR_ERROR, "Invalid input for jwt credentials creation");
      return NULL;
    }
  c = gpr_malloc (sizeof (grpc_service_account_jwt_access_credentials));
  memset (c, 0, sizeof (grpc_service_account_jwt_access_credentials));
  c->base.type = GRPC_CREDENTIALS_TYPE_JWT;
  gpr_ref_init (&c->base.refcount, 1);
  c->base.vtable = &jwt_vtable;
  c->key = key;
  c->jwt_lifetime = token_lifetime;
  gpr_mu_init (&c->cache_mu);
  jwt_reset_cache (c);
  return &c->base;
}

grpc_credentials *
grpc_service_account_jwt_access_credentials_create (const char *json_key, gpr_timespec token_lifetime, void *reserved)
{
  GPR_ASSERT (reserved == NULL);
  return grpc_service_account_jwt_access_credentials_create_from_auth_json_key (grpc_auth_json_key_create_from_string (json_key), token_lifetime);
}

/* -- Oauth2TokenFetcher credentials -- */

static void
oauth2_token_fetcher_destruct (grpc_credentials * creds)
{
  grpc_oauth2_token_fetcher_credentials *c = (grpc_oauth2_token_fetcher_credentials *) creds;
  grpc_credentials_md_store_unref (c->access_token_md);
  gpr_mu_destroy (&c->mu);
  grpc_httpcli_context_destroy (&c->httpcli_context);
}

static int
oauth2_token_fetcher_has_request_metadata (const grpc_credentials * creds)
{
  return 1;
}

static int
oauth2_token_fetcher_has_request_metadata_only (const grpc_credentials * creds)
{
  return 1;
}

grpc_credentials_status
grpc_oauth2_token_fetcher_credentials_parse_server_response (const grpc_httpcli_response * response, grpc_credentials_md_store ** token_md, gpr_timespec * token_lifetime)
{
  char *null_terminated_body = NULL;
  char *new_access_token = NULL;
  grpc_credentials_status status = GRPC_CREDENTIALS_OK;
  grpc_json *json = NULL;

  if (response == NULL)
    {
      gpr_log (GPR_ERROR, "Received NULL response.");
      status = GRPC_CREDENTIALS_ERROR;
      goto end;
    }

  if (response->body_length > 0)
    {
      null_terminated_body = gpr_malloc (response->body_length + 1);
      null_terminated_body[response->body_length] = '\0';
      memcpy (null_terminated_body, response->body, response->body_length);
    }

  if (response->status != 200)
    {
      gpr_log (GPR_ERROR, "Call to http server ended with error %d [%s].", response->status, null_terminated_body != NULL ? null_terminated_body : "");
      status = GRPC_CREDENTIALS_ERROR;
      goto end;
    }
  else
    {
      grpc_json *access_token = NULL;
      grpc_json *token_type = NULL;
      grpc_json *expires_in = NULL;
      grpc_json *ptr;
      json = grpc_json_parse_string (null_terminated_body);
      if (json == NULL)
	{
	  gpr_log (GPR_ERROR, "Could not parse JSON from %s", null_terminated_body);
	  status = GRPC_CREDENTIALS_ERROR;
	  goto end;
	}
      if (json->type != GRPC_JSON_OBJECT)
	{
	  gpr_log (GPR_ERROR, "Response should be a JSON object");
	  status = GRPC_CREDENTIALS_ERROR;
	  goto end;
	}
      for (ptr = json->child; ptr; ptr = ptr->next)
	{
	  if (strcmp (ptr->key, "access_token") == 0)
	    {
	      access_token = ptr;
	    }
	  else if (strcmp (ptr->key, "token_type") == 0)
	    {
	      token_type = ptr;
	    }
	  else if (strcmp (ptr->key, "expires_in") == 0)
	    {
	      expires_in = ptr;
	    }
	}
      if (access_token == NULL || access_token->type != GRPC_JSON_STRING)
	{
	  gpr_log (GPR_ERROR, "Missing or invalid access_token in JSON.");
	  status = GRPC_CREDENTIALS_ERROR;
	  goto end;
	}
      if (token_type == NULL || token_type->type != GRPC_JSON_STRING)
	{
	  gpr_log (GPR_ERROR, "Missing or invalid token_type in JSON.");
	  status = GRPC_CREDENTIALS_ERROR;
	  goto end;
	}
      if (expires_in == NULL || expires_in->type != GRPC_JSON_NUMBER)
	{
	  gpr_log (GPR_ERROR, "Missing or invalid expires_in in JSON.");
	  status = GRPC_CREDENTIALS_ERROR;
	  goto end;
	}
      gpr_asprintf (&new_access_token, "%s %s", token_type->value, access_token->value);
      token_lifetime->tv_sec = strtol (expires_in->value, NULL, 10);
      token_lifetime->tv_nsec = 0;
      token_lifetime->clock_type = GPR_TIMESPAN;
      if (*token_md != NULL)
	grpc_credentials_md_store_unref (*token_md);
      *token_md = grpc_credentials_md_store_create (1);
      grpc_credentials_md_store_add_cstrings (*token_md, GRPC_AUTHORIZATION_METADATA_KEY, new_access_token);
      status = GRPC_CREDENTIALS_OK;
    }

end:
  if (status != GRPC_CREDENTIALS_OK && (*token_md != NULL))
    {
      grpc_credentials_md_store_unref (*token_md);
      *token_md = NULL;
    }
  if (null_terminated_body != NULL)
    gpr_free (null_terminated_body);
  if (new_access_token != NULL)
    gpr_free (new_access_token);
  if (json != NULL)
    grpc_json_destroy (json);
  return status;
}

static void
on_oauth2_token_fetcher_http_response (grpc_exec_ctx * exec_ctx, void *user_data, const grpc_httpcli_response * response)
{
  grpc_credentials_metadata_request *r = (grpc_credentials_metadata_request *) user_data;
  grpc_oauth2_token_fetcher_credentials *c = (grpc_oauth2_token_fetcher_credentials *) r->creds;
  gpr_timespec token_lifetime;
  grpc_credentials_status status;

  gpr_mu_lock (&c->mu);
  status = grpc_oauth2_token_fetcher_credentials_parse_server_response (response, &c->access_token_md, &token_lifetime);
  if (status == GRPC_CREDENTIALS_OK)
    {
      c->token_expiration = gpr_time_add (gpr_now (GPR_CLOCK_REALTIME), token_lifetime);
      r->cb (exec_ctx, r->user_data, c->access_token_md->entries, c->access_token_md->num_entries, status);
    }
  else
    {
      c->token_expiration = gpr_inf_past (GPR_CLOCK_REALTIME);
      r->cb (exec_ctx, r->user_data, NULL, 0, status);
    }
  gpr_mu_unlock (&c->mu);
  grpc_credentials_metadata_request_destroy (r);
}

static void
oauth2_token_fetcher_get_request_metadata (grpc_exec_ctx * exec_ctx, grpc_credentials * creds, grpc_pollset * pollset, const char *service_url, grpc_credentials_metadata_cb cb, void *user_data)
{
  grpc_oauth2_token_fetcher_credentials *c = (grpc_oauth2_token_fetcher_credentials *) creds;
  gpr_timespec refresh_threshold = gpr_time_from_seconds (GRPC_SECURE_TOKEN_REFRESH_THRESHOLD_SECS, GPR_TIMESPAN);
  grpc_credentials_md_store *cached_access_token_md = NULL;
  {
    gpr_mu_lock (&c->mu);
    if (c->access_token_md != NULL && (gpr_time_cmp (gpr_time_sub (c->token_expiration, gpr_now (GPR_CLOCK_REALTIME)), refresh_threshold) > 0))
      {
	cached_access_token_md = grpc_credentials_md_store_ref (c->access_token_md);
      }
    gpr_mu_unlock (&c->mu);
  }
  if (cached_access_token_md != NULL)
    {
      cb (exec_ctx, user_data, cached_access_token_md->entries, cached_access_token_md->num_entries, GRPC_CREDENTIALS_OK);
      grpc_credentials_md_store_unref (cached_access_token_md);
    }
  else
    {
      c->fetch_func (grpc_credentials_metadata_request_create (creds, cb, user_data), &c->httpcli_context, pollset, on_oauth2_token_fetcher_http_response, gpr_time_add (gpr_now (exec_ctx, GPR_CLOCK_REALTIME), refresh_threshold));
    }
}

static void
init_oauth2_token_fetcher (grpc_oauth2_token_fetcher_credentials * c, grpc_fetch_oauth2_func fetch_func)
{
  memset (c, 0, sizeof (grpc_oauth2_token_fetcher_credentials));
  c->base.type = GRPC_CREDENTIALS_TYPE_OAUTH2;
  gpr_ref_init (&c->base.refcount, 1);
  gpr_mu_init (&c->mu);
  c->token_expiration = gpr_inf_past (GPR_CLOCK_REALTIME);
  c->fetch_func = fetch_func;
  grpc_httpcli_context_init (&c->httpcli_context);
}

/* -- GoogleComputeEngine credentials. -- */

static grpc_credentials_vtable compute_engine_vtable = {
  oauth2_token_fetcher_destruct, oauth2_token_fetcher_has_request_metadata,
  oauth2_token_fetcher_has_request_metadata_only,
  oauth2_token_fetcher_get_request_metadata, NULL
};

static void
compute_engine_fetch_oauth2 (grpc_exec_ctx * exec_ctx, grpc_credentials_metadata_request * metadata_req, grpc_httpcli_context * httpcli_context, grpc_pollset * pollset, grpc_httpcli_response_cb response_cb, gpr_timespec deadline)
{
  grpc_httpcli_header header = { "Metadata-Flavor", "Google" };
  grpc_httpcli_request request;
  memset (&request, 0, sizeof (grpc_httpcli_request));
  request.host = GRPC_COMPUTE_ENGINE_METADATA_HOST;
  request.path = GRPC_COMPUTE_ENGINE_METADATA_TOKEN_PATH;
  request.hdr_count = 1;
  request.hdrs = &header;
  grpc_httpcli_get (exec_ctx, httpcli_context, pollset, &request, deadline, response_cb, metadata_req);
}

grpc_credentials *
grpc_google_compute_engine_credentials_create (void *reserved)
{
  grpc_oauth2_token_fetcher_credentials *c = gpr_malloc (sizeof (grpc_oauth2_token_fetcher_credentials));
  GPR_ASSERT (reserved == NULL);
  init_oauth2_token_fetcher (c, compute_engine_fetch_oauth2);
  c->base.vtable = &compute_engine_vtable;
  return &c->base;
}

/* -- GoogleRefreshToken credentials. -- */

static void
refresh_token_destruct (grpc_credentials * creds)
{
  grpc_google_refresh_token_credentials *c = (grpc_google_refresh_token_credentials *) creds;
  grpc_auth_refresh_token_destruct (&c->refresh_token);
  oauth2_token_fetcher_destruct (&c->base.base);
}

static grpc_credentials_vtable refresh_token_vtable = {
  refresh_token_destruct, oauth2_token_fetcher_has_request_metadata,
  oauth2_token_fetcher_has_request_metadata_only,
  oauth2_token_fetcher_get_request_metadata, NULL
};

static void
refresh_token_fetch_oauth2 (grpc_exec_ctx * exec_ctx, grpc_credentials_metadata_request * metadata_req, grpc_httpcli_context * httpcli_context, grpc_pollset * pollset, grpc_httpcli_response_cb response_cb, gpr_timespec deadline)
{
  grpc_google_refresh_token_credentials *c = (grpc_google_refresh_token_credentials *) metadata_req->creds;
  grpc_httpcli_header header = { "Content-Type",
    "application/x-www-form-urlencoded"
  };
  grpc_httpcli_request request;
  char *body = NULL;
  gpr_asprintf (&body, GRPC_REFRESH_TOKEN_POST_BODY_FORMAT_STRING, c->refresh_token.client_id, c->refresh_token.client_secret, c->refresh_token.refresh_token);
  memset (&request, 0, sizeof (grpc_httpcli_request));
  request.host = GRPC_GOOGLE_OAUTH2_SERVICE_HOST;
  request.path = GRPC_GOOGLE_OAUTH2_SERVICE_TOKEN_PATH;
  request.hdr_count = 1;
  request.hdrs = &header;
  request.handshaker = &grpc_httpcli_ssl;
  grpc_httpcli_post (httpcli_context, pollset, &request, body, strlen (exec_ctx, body), deadline, response_cb, metadata_req);
  gpr_free (body);
}

grpc_credentials *
grpc_refresh_token_credentials_create_from_auth_refresh_token (grpc_auth_refresh_token refresh_token)
{
  grpc_google_refresh_token_credentials *c;
  if (!grpc_auth_refresh_token_is_valid (&refresh_token))
    {
      gpr_log (GPR_ERROR, "Invalid input for refresh token credentials creation");
      return NULL;
    }
  c = gpr_malloc (sizeof (grpc_google_refresh_token_credentials));
  memset (c, 0, sizeof (grpc_google_refresh_token_credentials));
  init_oauth2_token_fetcher (&c->base, refresh_token_fetch_oauth2);
  c->base.base.vtable = &refresh_token_vtable;
  c->refresh_token = refresh_token;
  return &c->base.base;
}

grpc_credentials *
grpc_google_refresh_token_credentials_create (const char *json_refresh_token, void *reserved)
{
  GPR_ASSERT (reserved == NULL);
  return grpc_refresh_token_credentials_create_from_auth_refresh_token (grpc_auth_refresh_token_create_from_string (json_refresh_token));
}

/* -- Metadata-only credentials. -- */

static void
md_only_test_destruct (grpc_credentials * creds)
{
  grpc_md_only_test_credentials *c = (grpc_md_only_test_credentials *) creds;
  grpc_credentials_md_store_unref (c->md_store);
}

static int
md_only_test_has_request_metadata (const grpc_credentials * creds)
{
  return 1;
}

static int
md_only_test_has_request_metadata_only (const grpc_credentials * creds)
{
  return 1;
}

static void
on_simulated_token_fetch_done (void *user_data)
{
  grpc_credentials_metadata_request *r = (grpc_credentials_metadata_request *) user_data;
  grpc_md_only_test_credentials *c = (grpc_md_only_test_credentials *) r->creds;
  grpc_exec_ctx exec_ctx = GRPC_EXEC_CTX_INIT;
  r->cb (r->user_data, c->md_store->entries, c->md_store->num_entries, GRPC_CREDENTIALS_OK, &closure_list);
  grpc_credentials_metadata_request_destroy (r);
  grpc_exec_ctx_finish (&exec_ctx);
}

static void
md_only_test_get_request_metadata (grpc_exec_ctx * exec_ctx, grpc_credentials * creds, grpc_pollset * pollset, const char *service_url, grpc_credentials_metadata_cb cb, void *user_data)
{
  grpc_md_only_test_credentials *c = (grpc_md_only_test_credentials *) creds;

  if (c->is_async)
    {
      gpr_thd_id thd_id;
      grpc_credentials_metadata_request *cb_arg = grpc_credentials_metadata_request_create (creds, cb, user_data);
      gpr_thd_new (&thd_id, on_simulated_token_fetch_done, cb_arg, NULL);
    }
  else
    {
      cb (exec_ctx, user_data, c->md_store->entries, 1, GRPC_CREDENTIALS_OK);
    }
}

static grpc_credentials_vtable md_only_test_vtable = {
  md_only_test_destruct, md_only_test_has_request_metadata,
  md_only_test_has_request_metadata_only, md_only_test_get_request_metadata,
  NULL
};

grpc_credentials *
grpc_md_only_test_credentials_create (const char *md_key, const char *md_value, int is_async)
{
  grpc_md_only_test_credentials *c = gpr_malloc (sizeof (grpc_md_only_test_credentials));
  memset (c, 0, sizeof (grpc_md_only_test_credentials));
  c->base.type = GRPC_CREDENTIALS_TYPE_OAUTH2;
  c->base.vtable = &md_only_test_vtable;
  gpr_ref_init (&c->base.refcount, 1);
  c->md_store = grpc_credentials_md_store_create (1);
  grpc_credentials_md_store_add_cstrings (c->md_store, md_key, md_value);
  c->is_async = is_async;
  return &c->base;
}

/* -- Oauth2 Access Token credentials. -- */

static void
access_token_destruct (grpc_credentials * creds)
{
  grpc_access_token_credentials *c = (grpc_access_token_credentials *) creds;
  grpc_credentials_md_store_unref (c->access_token_md);
}

static int
access_token_has_request_metadata (const grpc_credentials * creds)
{
  return 1;
}

static int
access_token_has_request_metadata_only (const grpc_credentials * creds)
{
  return 1;
}

static void
access_token_get_request_metadata (grpc_exec_ctx * exec_ctx, grpc_credentials * creds, grpc_pollset * pollset, const char *service_url, grpc_credentials_metadata_cb cb, void *user_data)
{
  grpc_access_token_credentials *c = (grpc_access_token_credentials *) creds;
  cb (exec_ctx, user_data, c->access_token_md->entries, 1, GRPC_CREDENTIALS_OK);
}

static grpc_credentials_vtable access_token_vtable = {
  access_token_destruct, access_token_has_request_metadata,
  access_token_has_request_metadata_only, access_token_get_request_metadata,
  NULL
};

grpc_credentials *
grpc_access_token_credentials_create (const char *access_token, void *reserved)
{
  grpc_access_token_credentials *c = gpr_malloc (sizeof (grpc_access_token_credentials));
  char *token_md_value;
  GPR_ASSERT (reserved == NULL);
  memset (c, 0, sizeof (grpc_access_token_credentials));
  c->base.type = GRPC_CREDENTIALS_TYPE_OAUTH2;
  c->base.vtable = &access_token_vtable;
  gpr_ref_init (&c->base.refcount, 1);
  c->access_token_md = grpc_credentials_md_store_create (1);
  gpr_asprintf (&token_md_value, "Bearer %s", access_token);
  grpc_credentials_md_store_add_cstrings (c->access_token_md, GRPC_AUTHORIZATION_METADATA_KEY, token_md_value);
  gpr_free (token_md_value);
  return &c->base;
}

/* -- Fake transport security credentials. -- */

static void
fake_transport_security_credentials_destruct (grpc_credentials * creds)
{
  /* Nothing to do here. */
}

static void
fake_transport_security_server_credentials_destruct (grpc_server_credentials * creds)
{
  /* Nothing to do here. */
}

static int
fake_transport_security_has_request_metadata (const grpc_credentials * creds)
{
  return 0;
}

static int
fake_transport_security_has_request_metadata_only (const grpc_credentials * creds)
{
  return 0;
}

static grpc_security_status
fake_transport_security_create_security_connector (grpc_credentials * c, const char *target, const grpc_channel_args * args, grpc_credentials * request_metadata_creds, grpc_channel_security_connector ** sc, grpc_channel_args ** new_args)
{
  *sc = grpc_fake_channel_security_connector_create (request_metadata_creds, 1);
  return GRPC_SECURITY_OK;
}

static grpc_security_status
fake_transport_security_server_create_security_connector (grpc_server_credentials * c, grpc_security_connector ** sc)
{
  *sc = grpc_fake_server_security_connector_create ();
  return GRPC_SECURITY_OK;
}

static grpc_credentials_vtable fake_transport_security_credentials_vtable = {
  fake_transport_security_credentials_destruct,
  fake_transport_security_has_request_metadata,
  fake_transport_security_has_request_metadata_only, NULL,
  fake_transport_security_create_security_connector
};

static grpc_server_credentials_vtable fake_transport_security_server_credentials_vtable = {
  fake_transport_security_server_credentials_destruct,
  fake_transport_security_server_create_security_connector
};

grpc_credentials *
grpc_fake_transport_security_credentials_create (void)
{
  grpc_credentials *c = gpr_malloc (sizeof (grpc_credentials));
  memset (c, 0, sizeof (grpc_credentials));
  c->type = GRPC_CREDENTIALS_TYPE_FAKE_TRANSPORT_SECURITY;
  c->vtable = &fake_transport_security_credentials_vtable;
  gpr_ref_init (&c->refcount, 1);
  return c;
}

grpc_server_credentials *
grpc_fake_transport_security_server_credentials_create (void)
{
  grpc_server_credentials *c = gpr_malloc (sizeof (grpc_server_credentials));
  memset (c, 0, sizeof (grpc_server_credentials));
  c->type = GRPC_CREDENTIALS_TYPE_FAKE_TRANSPORT_SECURITY;
  gpr_ref_init (&c->refcount, 1);
  c->vtable = &fake_transport_security_server_credentials_vtable;
  return c;
}

/* -- Composite credentials. -- */

typedef struct
{
  grpc_composite_credentials *composite_creds;
  size_t creds_index;
  grpc_credentials_md_store *md_elems;
  char *service_url;
  void *user_data;
  grpc_pollset *pollset;
  grpc_credentials_metadata_cb cb;
} grpc_composite_credentials_metadata_context;

static void
composite_destruct (grpc_credentials * creds)
{
  grpc_composite_credentials *c = (grpc_composite_credentials *) creds;
  size_t i;
  for (i = 0; i < c->inner.num_creds; i++)
    {
      grpc_credentials_unref (c->inner.creds_array[i]);
    }
  gpr_free (c->inner.creds_array);
}

static int
composite_has_request_metadata (const grpc_credentials * creds)
{
  const grpc_composite_credentials *c = (const grpc_composite_credentials *) creds;
  size_t i;
  for (i = 0; i < c->inner.num_creds; i++)
    {
      if (grpc_credentials_has_request_metadata (c->inner.creds_array[i]))
	{
	  return 1;
	}
    }
  return 0;
}

static int
composite_has_request_metadata_only (const grpc_credentials * creds)
{
  const grpc_composite_credentials *c = (const grpc_composite_credentials *) creds;
  size_t i;
  for (i = 0; i < c->inner.num_creds; i++)
    {
      if (!grpc_credentials_has_request_metadata_only (c->inner.creds_array[i]))
	{
	  return 0;
	}
    }
  return 1;
}

static void
composite_md_context_destroy (grpc_composite_credentials_metadata_context * ctx)
{
  grpc_credentials_md_store_unref (ctx->md_elems);
  if (ctx->service_url != NULL)
    gpr_free (ctx->service_url);
  gpr_free (ctx);
}

static void
composite_metadata_cb (grpc_exec_ctx * exec_ctx, void *user_data, grpc_credentials_md * md_elems, size_t num_md, grpc_credentials_status status)
{
  grpc_composite_credentials_metadata_context *ctx = (grpc_composite_credentials_metadata_context *) user_data;
  if (status != GRPC_CREDENTIALS_OK)
    {
      ctx->cb (exec_ctx, ctx->user_data, NULL, 0, status);
      return;
    }

  /* Copy the metadata in the context. */
  if (num_md > 0)
    {
      size_t i;
      for (i = 0; i < num_md; i++)
	{
	  grpc_credentials_md_store_add (ctx->md_elems, md_elems[i].key, md_elems[i].value);
	}
    }

  /* See if we need to get some more metadata. */
  while (ctx->creds_index < ctx->composite_creds->inner.num_creds)
    {
      grpc_credentials *inner_creds = ctx->composite_creds->inner.creds_array[ctx->creds_index++];
      if (grpc_credentials_has_request_metadata (inner_creds))
	{
	  grpc_credentials_get_request_metadata (exec_ctx, inner_creds, ctx->pollset, ctx->service_url, composite_metadata_cb, ctx);
	  return;
	}
    }

  /* We're done!. */
  ctx->cb (exec_ctx, ctx->user_data, ctx->md_elems->entries, ctx->md_elems->num_entries, GRPC_CREDENTIALS_OK);
  composite_md_context_destroy (ctx);
  grpc_closure_list_run (closure_list);
}

static void
composite_get_request_metadata (grpc_exec_ctx * exec_ctx, grpc_credentials * creds, grpc_pollset * pollset, const char *service_url, grpc_credentials_metadata_cb cb, void *user_data)
{
  grpc_composite_credentials *c = (grpc_composite_credentials *) creds;
  grpc_composite_credentials_metadata_context *ctx;
  if (!grpc_credentials_has_request_metadata (creds))
    {
      cb (exec_ctx, user_data, NULL, 0, GRPC_CREDENTIALS_OK);
      return;
    }
  ctx = gpr_malloc (sizeof (grpc_composite_credentials_metadata_context));
  memset (ctx, 0, sizeof (grpc_composite_credentials_metadata_context));
  ctx->service_url = gpr_strdup (service_url);
  ctx->user_data = user_data;
  ctx->cb = cb;
  ctx->composite_creds = c;
  ctx->pollset = pollset;
  ctx->md_elems = grpc_credentials_md_store_create (c->inner.num_creds);
  while (ctx->creds_index < c->inner.num_creds)
    {
      grpc_credentials *inner_creds = c->inner.creds_array[ctx->creds_index++];
      if (grpc_credentials_has_request_metadata (inner_creds))
	{
	  grpc_credentials_get_request_metadata (exec_ctx, inner_creds, pollset, service_url, composite_metadata_cb, ctx);
	  return;
	}
    }
  GPR_ASSERT (0);		/* Should have exited before. */
}

static grpc_security_status
composite_create_security_connector (grpc_credentials * creds, const char *target, const grpc_channel_args * args, grpc_credentials * request_metadata_creds, grpc_channel_security_connector ** sc, grpc_channel_args ** new_args)
{
  grpc_composite_credentials *c = (grpc_composite_credentials *) creds;
  if (c->connector_creds == NULL)
    {
      gpr_log (GPR_ERROR, "Cannot create security connector, missing connector credentials.");
      return GRPC_SECURITY_ERROR;
    }
  return grpc_credentials_create_security_connector (c->connector_creds, target, args, creds, sc, new_args);
}

static grpc_credentials_vtable composite_credentials_vtable = {
  composite_destruct, composite_has_request_metadata,
  composite_has_request_metadata_only, composite_get_request_metadata,
  composite_create_security_connector
};

static grpc_credentials_array
get_creds_array (grpc_credentials ** creds_addr)
{
  grpc_credentials_array result;
  grpc_credentials *creds = *creds_addr;
  result.creds_array = creds_addr;
  result.num_creds = 1;
  if (strcmp (creds->type, GRPC_CREDENTIALS_TYPE_COMPOSITE) == 0)
    {
      result = *grpc_composite_credentials_get_credentials (creds);
    }
  return result;
}

grpc_credentials *
grpc_composite_credentials_create (grpc_credentials * creds1, grpc_credentials * creds2, void *reserved)
{
  size_t i;
  size_t creds_array_byte_size;
  grpc_credentials_array creds1_array;
  grpc_credentials_array creds2_array;
  grpc_composite_credentials *c;
  GPR_ASSERT (reserved == NULL);
  GPR_ASSERT (creds1 != NULL);
  GPR_ASSERT (creds2 != NULL);
  c = gpr_malloc (sizeof (grpc_composite_credentials));
  memset (c, 0, sizeof (grpc_composite_credentials));
  c->base.type = GRPC_CREDENTIALS_TYPE_COMPOSITE;
  c->base.vtable = &composite_credentials_vtable;
  gpr_ref_init (&c->base.refcount, 1);
  creds1_array = get_creds_array (&creds1);
  creds2_array = get_creds_array (&creds2);
  c->inner.num_creds = creds1_array.num_creds + creds2_array.num_creds;
  creds_array_byte_size = c->inner.num_creds * sizeof (grpc_credentials *);
  c->inner.creds_array = gpr_malloc (creds_array_byte_size);
  memset (c->inner.creds_array, 0, creds_array_byte_size);
  for (i = 0; i < creds1_array.num_creds; i++)
    {
      grpc_credentials *cur_creds = creds1_array.creds_array[i];
      if (!grpc_credentials_has_request_metadata_only (cur_creds))
	{
	  if (c->connector_creds == NULL)
	    {
	      c->connector_creds = cur_creds;
	    }
	  else
	    {
	      gpr_log (GPR_ERROR, "Cannot compose multiple connector credentials.");
	      goto fail;
	    }
	}
      c->inner.creds_array[i] = grpc_credentials_ref (cur_creds);
    }
  for (i = 0; i < creds2_array.num_creds; i++)
    {
      grpc_credentials *cur_creds = creds2_array.creds_array[i];
      if (!grpc_credentials_has_request_metadata_only (cur_creds))
	{
	  if (c->connector_creds == NULL)
	    {
	      c->connector_creds = cur_creds;
	    }
	  else
	    {
	      gpr_log (GPR_ERROR, "Cannot compose multiple connector credentials.");
	      goto fail;
	    }
	}
      c->inner.creds_array[i + creds1_array.num_creds] = grpc_credentials_ref (cur_creds);
    }
  return &c->base;

fail:
  grpc_credentials_unref (&c->base);
  return NULL;
}

const grpc_credentials_array *
grpc_composite_credentials_get_credentials (grpc_credentials * creds)
{
  const grpc_composite_credentials *c = (const grpc_composite_credentials *) creds;
  GPR_ASSERT (strcmp (creds->type, GRPC_CREDENTIALS_TYPE_COMPOSITE) == 0);
  return &c->inner;
}

grpc_credentials *
grpc_credentials_contains_type (grpc_credentials * creds, const char *type, grpc_credentials ** composite_creds)
{
  size_t i;
  if (strcmp (creds->type, type) == 0)
    {
      if (composite_creds != NULL)
	*composite_creds = NULL;
      return creds;
    }
  else if (strcmp (creds->type, GRPC_CREDENTIALS_TYPE_COMPOSITE) == 0)
    {
      const grpc_credentials_array *inner_creds_array = grpc_composite_credentials_get_credentials (creds);
      for (i = 0; i < inner_creds_array->num_creds; i++)
	{
	  if (strcmp (type, inner_creds_array->creds_array[i]->type) == 0)
	    {
	      if (composite_creds != NULL)
		*composite_creds = creds;
	      return inner_creds_array->creds_array[i];
	    }
	}
    }
  return NULL;
}

/* -- IAM credentials. -- */

static void
iam_destruct (grpc_credentials * creds)
{
  grpc_google_iam_credentials *c = (grpc_google_iam_credentials *) creds;
  grpc_credentials_md_store_unref (c->iam_md);
}

static int
iam_has_request_metadata (const grpc_credentials * creds)
{
  return 1;
}

static int
iam_has_request_metadata_only (const grpc_credentials * creds)
{
  return 1;
}

static void
iam_get_request_metadata (grpc_exec_ctx * exec_ctx, grpc_credentials * creds, grpc_pollset * pollset, const char *service_url, grpc_credentials_metadata_cb cb, void *user_data)
{
  grpc_google_iam_credentials *c = (grpc_google_iam_credentials *) creds;
  cb (exec_ctx, user_data, c->iam_md->entries, c->iam_md->num_entries, GRPC_CREDENTIALS_OK);
}

static grpc_credentials_vtable iam_vtable = {
  iam_destruct, iam_has_request_metadata, iam_has_request_metadata_only,
  iam_get_request_metadata, NULL
};

grpc_credentials *
grpc_google_iam_credentials_create (const char *token, const char *authority_selector, void *reserved)
{
  grpc_google_iam_credentials *c;
  GPR_ASSERT (reserved == NULL);
  GPR_ASSERT (token != NULL);
  GPR_ASSERT (authority_selector != NULL);
  c = gpr_malloc (sizeof (grpc_google_iam_credentials));
  memset (c, 0, sizeof (grpc_google_iam_credentials));
  c->base.type = GRPC_CREDENTIALS_TYPE_IAM;
  c->base.vtable = &iam_vtable;
  gpr_ref_init (&c->base.refcount, 1);
  c->iam_md = grpc_credentials_md_store_create (2);
  grpc_credentials_md_store_add_cstrings (c->iam_md, GRPC_IAM_AUTHORIZATION_TOKEN_METADATA_KEY, token);
  grpc_credentials_md_store_add_cstrings (c->iam_md, GRPC_IAM_AUTHORITY_SELECTOR_METADATA_KEY, authority_selector);
  return &c->base;
}

/* -- Plugin credentials. -- */

typedef struct
{
  void *user_data;
  grpc_credentials_metadata_cb cb;
} grpc_metadata_plugin_request;

static void
plugin_destruct (grpc_credentials * creds)
{
  grpc_plugin_credentials *c = (grpc_plugin_credentials *) creds;
  if (c->plugin.state != NULL && c->plugin.destroy != NULL)
    {
      c->plugin.destroy (c->plugin.state);
    }
}

static int
plugin_has_request_metadata (const grpc_credentials * creds)
{
  return 1;
}

static int
plugin_has_request_metadata_only (const grpc_credentials * creds)
{
  return 1;
}

static void
plugin_md_request_metadata_ready (void *request, const grpc_metadata * md, size_t num_md, grpc_status_code status, const char *error_details)
{
  /* called from application code */
  grpc_exec_ctx exec_ctx = GRPC_EXEC_CTX_INIT;
  grpc_metadata_plugin_request *r = (grpc_metadata_plugin_request *) request;
  if (status != GRPC_STATUS_OK)
    {
      if (error_details != NULL)
	{
	  gpr_log (GPR_ERROR, "Getting metadata from plugin failed with error: %s", error_details);
	}
      r->cb (r->user_data, NULL, 0, GRPC_CREDENTIALS_ERROR, &closure_list);
    }
  else
    {
      size_t i;
      grpc_credentials_md *md_array = NULL;
      if (num_md > 0)
	{
	  md_array = gpr_malloc (num_md * sizeof (grpc_credentials_md));
	  for (i = 0; i < num_md; i++)
	    {
	      md_array[i].key = gpr_slice_from_copied_string (md[i].key);
	      md_array[i].value = gpr_slice_from_copied_buffer (md[i].value, md[i].value_length);
	    }
	}
      r->cb (r->user_data, md_array, num_md, GRPC_CREDENTIALS_OK, &closure_list);
      if (md_array != NULL)
	{
	  for (i = 0; i < num_md; i++)
	    {
	      gpr_slice_unref (md_array[i].key);
	      gpr_slice_unref (md_array[i].value);
	    }
	  gpr_free (md_array);
	}
    }
  gpr_free (r);
}

static void
plugin_get_request_metadata (grpc_exec_ctx * exec_ctx, grpc_credentials * creds, grpc_pollset * pollset, const char *service_url, grpc_credentials_metadata_cb cb, void *user_data)
{
  grpc_plugin_credentials *c = (grpc_plugin_credentials *) creds;
  if (c->plugin.get_metadata != NULL)
    {
      grpc_metadata_plugin_request *request = gpr_malloc (sizeof (*request));
      memset (request, 0, sizeof (*request));
      request->user_data = user_data;
      request->cb = cb;
      c->plugin.get_metadata (c->plugin.state, service_url, plugin_md_request_metadata_ready, request);
    }
  else
    {
      cb (exec_ctx, user_data, NULL, 0, GRPC_CREDENTIALS_OK);
    }
}

static grpc_credentials_vtable plugin_vtable = {
  plugin_destruct, plugin_has_request_metadata,
  plugin_has_request_metadata_only, plugin_get_request_metadata, NULL
};

grpc_credentials *
grpc_metadata_credentials_create_from_plugin (grpc_metadata_credentials_plugin plugin, void *reserved)
{
  grpc_plugin_credentials *c = gpr_malloc (sizeof (*c));
  GPR_ASSERT (reserved == NULL);
  memset (c, 0, sizeof (*c));
  c->base.type = GRPC_CREDENTIALS_TYPE_METADATA_PLUGIN;
  c->base.vtable = &plugin_vtable;
  gpr_ref_init (&c->base.refcount, 1);
  c->plugin = plugin;
  return &c->base;
}