User data isn't completely flushed to disk when backupUsers resolves

[mbarneyjr]

Within my backup process, I'm calling the backupUsers process and expect that the file is complete when the promise returned from that function resolves, however the data I can immediately pull from disk is not complete (the stream has not finished writing all of the data to the file).

I see here (src/index.ts#L61) that there is no promise wrapping the callback pattern used for the stream, so there's no way to wrap this function call to ensure that the stream has finished writing all of the data to disk.

The following example:

const cbr = require('cognito-backup-restore');
const aws = require('aws-sdk');
const fs = require('fs');
const userPoolId = 'us-east-1_abc123';
const cognitoIsp = new aws.CognitoIdentityServiceProvider();
cbr.backupUsers(cognitoIsp, userPoolId, '/tmp').then(() => {
  JSON.parse(fs.readFileSync(`/tmp/${userPoolId}.json`).toString())
});

will yield the error:

(node:13604) UnhandledPromiseRejectionWarning: SyntaxError: Unexpected end of JSON input

Because the final bracket (]) has not been written to disk

[jfstephe]

I'm seeing this too. From what I see it only exports the first record. @rahulpsd18 Any chance of a fix? Happy to do a PR....

This probably means that anyone that's using this in a lambda to export their users to S3 isn't:
a) doing it right
b) checking their backups are able to be restored