code: function cb (err) {
      if (cb.done) return
      cb.done = true
      if (err) {
        log.warn('install', 'got an error, rolling back install')
        // roll-back the install if anything went wrong
        gyp.commands.remove([ release.versionDir ], function () {
          callback(err)
        })
      } else {
        callback(null, release.version)
      }
    }
docs: ensure no double-callbacks happen