@@ -9,6 +9,10 @@ const databaseNamespace = require('./shared').databaseNamespace;
 const isTransactionCommand = require('../transactions').isTransactionCommand;
 const applySession = require('../sessions').applySession;
 
+function isClientEncryptionEnabled(server) {
+  return server && server.s && server.s.options && server.s.options.autoEncrypter;
+}
+
 function command(server, ns, cmd, options, callback) {
   if (typeof options === 'function') (callback = options), (options = {});
   options = options || {};
'use strict';

const Query = require('../connection/commands').Query;
const Msg = require('../connection/msg').Msg;
const MongoError = require('../error').MongoError;
const getReadPreference = require('./shared').getReadPreference;
const isSharded = require('./shared').isSharded;
const databaseNamespace = require('./shared').databaseNamespace;
const isTransactionCommand = require('../transactions').isTransactionCommand;
const applySession = require('../sessions').applySession;

function command(server, ns, cmd, options, callback) {
  if (typeof options === 'function') (callback = options), (options = {});
  options = options || {};

  if (cmd == null) {
    return callback(new MongoError(`command ${JSON.stringify(cmd)} does not return a cursor`));
  }

  const bson = server.s.bson;
  const pool = server.s.pool;
  const readPreference = getReadPreference(cmd, options);
  const shouldUseOpMsg = supportsOpMsg(server);
  const session = options.session;

  let clusterTime = server.clusterTime;
  let finalCmd = Object.assign({}, cmd);
  if (hasSessionSupport(server) && session) {
    if (
      session.clusterTime &&
      session.clusterTime.clusterTime.greaterThan(clusterTime.clusterTime)
    ) {
      clusterTime = session.clusterTime;
    }

    const err = applySession(session, finalCmd, options);
    if (err) {
      return callback(err);
    }
  }

  // if we have a known cluster time, gossip it
  if (clusterTime) {
    finalCmd.$clusterTime = clusterTime;
  }

  if (
    isSharded(server) &&
    !shouldUseOpMsg &&
    readPreference &&
    readPreference.preference !== 'primary'
  ) {
    finalCmd = {
      $query: finalCmd,
      $readPreference: readPreference.toJSON()
    };
  }

  const commandOptions = Object.assign(
    {
      command: true,
      numberToSkip: 0,
      numberToReturn: -1,
      checkKeys: false
    },
    options
  );

  // This value is not overridable
  commandOptions.slaveOk = readPreference.slaveOk();

  const cmdNs = `${databaseNamespace(ns)}.$cmd`;
  const message = shouldUseOpMsg
    ? new Msg(bson, cmdNs, finalCmd, commandOptions)
    : new Query(bson, cmdNs, finalCmd, commandOptions);

  const inTransaction = session && (session.inTransaction() || isTransactionCommand(finalCmd));
  const commandResponseHandler = inTransaction
    ? function(err) {
        if (
          !cmd.commitTransaction &&
          err &&
          err instanceof MongoError &&
          err.hasErrorLabel('TransientTransactionError')
        ) {
          session.transaction.unpinServer();
        }

        return callback.apply(null, arguments);
      }
    : callback;

  try {
    pool.write(message, commandOptions, commandResponseHandler);
  } catch (err) {
    commandResponseHandler(err);
  }
}

function hasSessionSupport(topology) {
  if (topology == null) return false;
  if (topology.description) {
    return topology.description.maxWireVersion >= 6;
  }

  return topology.ismaster == null ? false : topology.ismaster.maxWireVersion >= 6;
}

function supportsOpMsg(topologyOrServer) {
  const description = topologyOrServer.ismaster
    ? topologyOrServer.ismaster
    : topologyOrServer.description;

  if (description == null) {
    return false;
  }

  return description.maxWireVersion >= 6 && description.__nodejs_mock_server__ == null;
}

module.exports = command;

This could be follow-on work, but it would be nice to have `server.autoEncrypter` similar to `collection.readConcern` so we don't have to do this long check here.