code: function _ensureMinimum() {
      // Nothing to do if draining.
      if (this._draining) {
          return Promise.resolve();
      }
  
      var diff = this._opts.min - this._count + this.availableLength;
      var promises = [];
      for (var i = 0; i < diff; ++i) {
          promises.push(this.acquire(function(client){ return Promise.resolve(); }));
      }
  
      return Promise.all(promises).then(function(){});
  }
docs: Constructs more resources to bring the current count up to the minimum specified in the factory.
  
  @private
  @memberof PromisePool
  
  @return {Promise} A promise to create all the resources needed.