/*
 * Copyright 2019 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.
 */

import {
  Resolver,
  ResolverListener,
  registerResolver,
  registerDefaultScheme,
} from './resolver';
import * as dns from 'dns';
import * as util from 'util';
import { extractAndSelectServiceConfig, ServiceConfig } from './service-config';
import { Status } from './constants';
import { StatusObject } from './call-stream';
import { Metadata } from './metadata';
import * as logging from './logging';
import { LogVerbosity } from './constants';
import { SubchannelAddress, TcpSubchannelAddress } from './subchannel';
import { GrpcUri, uriToString, splitHostPort } from './uri-parser';
import { isIPv6, isIPv4 } from 'net';
import { ChannelOptions } from './channel-options';

const TRACER_NAME = 'dns_resolver';

function trace(text: string): void {
  logging.trace(LogVerbosity.DEBUG, TRACER_NAME, text);
}

/**
 * The default TCP port to connect to if not explicitly specified in the target.
 */
const DEFAULT_PORT = 443;

const resolveTxtPromise = util.promisify(dns.resolveTxt);
const dnsLookupPromise = util.promisify(dns.lookup);

/**
 * Merge any number of arrays into a single alternating array
 * @param arrays
 */
function mergeArrays<T>(...arrays: T[][]): T[] {
  const result: T[] = [];
  for (
    let i = 0;
    i <
    Math.max.apply(
      null,
      arrays.map((array) => array.length)
    );
    i++
  ) {
    for (const array of arrays) {
      if (i < array.length) {
        result.push(array[i]);
      }
    }
  }
  return result;
}

/**
 * Resolver implementation that handles DNS names and IP addresses.
 */
class DnsResolver implements Resolver {
  private readonly ipResult: SubchannelAddress[] | null;
  private readonly dnsHostname: string | null;
  private readonly port: number | null;
  private pendingLookupPromise: Promise<dns.LookupAddress[]> | null = null;
  private pendingTxtPromise: Promise<string[][]> | null = null;
  private latestLookupResult: TcpSubchannelAddress[] | null = null;
  private latestServiceConfig: ServiceConfig | null = null;
  private latestServiceConfigError: StatusObject | null = null;
  private percentage: number;
  private defaultResolutionError: StatusObject;
  constructor(
    private target: GrpcUri,
    private listener: ResolverListener,
    channelOptions: ChannelOptions
  ) {
    trace('Resolver constructed for target ' + uriToString(target));
    const hostPort = splitHostPort(target.path);
    if (hostPort === null) {
      this.ipResult = null;
      this.dnsHostname = null;
      this.port = null;
    } else {
      if (isIPv4(hostPort.host) || isIPv6(hostPort.host)) {
        this.ipResult = [
          {
            host: hostPort.host,
            port: hostPort.port ?? DEFAULT_PORT,
          },
        ];
        this.dnsHostname = null;
        this.port = null;
      } else {
        this.ipResult = null;
        this.dnsHostname = hostPort.host;
        this.port = hostPort.port ?? DEFAULT_PORT;
      }
    }
    this.percentage = Math.random() * 100;

    this.defaultResolutionError = {
      code: Status.UNAVAILABLE,
      details: `Name resolution failed for target ${uriToString(this.target)}`,
      metadata: new Metadata(),
    };
  }

  /**
   * If the target is an IP address, just provide that address as a result.
   * Otherwise, initiate A, AAAA, and TXT lookups
   */
  private startResolution() {
    if (this.ipResult !== null) {
      trace('Returning IP address for target ' + uriToString(this.target));
      setImmediate(() => {
        this.listener.onSuccessfulResolution(this.ipResult!, null, null, null, {});
      });
      return;
    }
    if (this.dnsHostname === null) {
      setImmediate(() => {
        this.listener.onError({
          code: Status.UNAVAILABLE,
          details: `Failed to parse DNS address ${uriToString(this.target)}`,
          metadata: new Metadata(),
        });
      });
    } else {
      /* We clear out latestLookupResult here to ensure that it contains the
       * latest result since the last time we started resolving. That way, the
       * TXT resolution handler can use it, but only if it finishes second. We
       * don't clear out any previous service config results because it's
       * better to use a service config that's slightly out of date than to
       * revert to an effectively blank one. */
      this.latestLookupResult = null;
      const hostname: string = this.dnsHostname;
      /* We lookup both address families here and then split them up later
       * because when looking up a single family, dns.lookup outputs an error
       * if the name exists but there are no records for that family, and that
       * error is indistinguishable from other kinds of errors */
      this.pendingLookupPromise = dnsLookupPromise(hostname, { all: true });
      this.pendingLookupPromise.then(
        (addressList) => {
          this.pendingLookupPromise = null;
          const ip4Addresses: dns.LookupAddress[] = addressList.filter(
            (addr) => addr.family === 4
          );
          const ip6Addresses: dns.LookupAddress[] = addressList.filter(
            (addr) => addr.family === 6
          );
          this.latestLookupResult = mergeArrays(
            ip6Addresses,
            ip4Addresses
          ).map((addr) => ({ host: addr.address, port: +this.port! }));
          const allAddressesString: string =
            '[' +
            this.latestLookupResult
              .map((addr) => addr.host + ':' + addr.port)
              .join(',') +
            ']';
          trace(
            'Resolved addresses for target ' +
              uriToString(this.target) +
              ': ' +
              allAddressesString
          );
          if (this.latestLookupResult.length === 0) {
            this.listener.onError(this.defaultResolutionError);
            return;
          }
          /* If the TXT lookup has not yet finished, both of the last two
           * arguments will be null, which is the equivalent of getting an
           * empty TXT response. When the TXT lookup does finish, its handler
           * can update the service config by using the same address list */
          this.listener.onSuccessfulResolution(
            this.latestLookupResult,
            this.latestServiceConfig,
            this.latestServiceConfigError,
            null,
            {}
          );
        },
        (err) => {
          trace(
            'Resolution error for target ' +
              uriToString(this.target) +
              ': ' +
              (err as Error).message
          );
          this.pendingLookupPromise = null;
          this.listener.onError(this.defaultResolutionError);
        }
      );
      /* If there already is a still-pending TXT resolution, we can just use
       * that result when it comes in */
      if (this.pendingTxtPromise === null) {
        /* We handle the TXT query promise differently than the others because
         * the name resolution attempt as a whole is a success even if the TXT
         * lookup fails */
        this.pendingTxtPromise = resolveTxtPromise(hostname);
        this.pendingTxtPromise.then(
          (txtRecord) => {
            this.pendingTxtPromise = null;
            try {
              this.latestServiceConfig = extractAndSelectServiceConfig(
                txtRecord,
                this.percentage
              );
            } catch (err) {
              this.latestServiceConfigError = {
                code: Status.UNAVAILABLE,
                details: 'Parsing service config failed',
                metadata: new Metadata(),
              };
            }
            if (this.latestLookupResult !== null) {
              /* We rely here on the assumption that calling this function with
               * identical parameters will be essentialy idempotent, and calling
               * it with the same address list and a different service config
               * should result in a fast and seamless switchover. */
              this.listener.onSuccessfulResolution(
                this.latestLookupResult,
                this.latestServiceConfig,
                this.latestServiceConfigError,
                null,
                {}
              );
            }
          },
          (err) => {
            /* If TXT lookup fails we should do nothing, which means that we
             * continue to use the result of the most recent successful lookup,
             * or the default null config object if there has never been a
             * successful lookup. We do not set the latestServiceConfigError
             * here because that is specifically used for response validation
             * errors. We still need to handle this error so that it does not
             * bubble up as an unhandled promise rejection. */
          }
        );
      }
    }
  }

  updateResolution() {
    trace('Resolution update requested for target ' + uriToString(this.target));
    if (this.pendingLookupPromise === null) {
      this.startResolution();
    }
  }

  destroy() {
    /* Do nothing. There is not a practical way to cancel in-flight DNS
     * requests, and after this function is called we can expect that
     * updateResolution will not be called again. */
  }

  /**
   * Get the default authority for the given target. For IP targets, that is
   * the IP address. For DNS targets, it is the hostname.
   * @param target
   */
  static getDefaultAuthority(target: GrpcUri): string {
    return target.path;
  }
}

/**
 * Set up the DNS resolver class by registering it as the handler for the
 * "dns:" prefix and as the default resolver.
 */
export function setup(): void {
  registerResolver('dns', DnsResolver);
  registerDefaultScheme('dns');
}

export interface DnsUrl {
  host: string;
  port?: string;
}