unable to read lines of log file - node.js

Question

I have an AWS S3 bucket with a bunch of log files in them. I have a lambda function using node.js 8.10 runtime that reads each line of the log file. This is what I have:
const readline = require('readline');
exports.handler = async (event) => {
try {
let bucket = event.Records[0].s3.bucket.name;
let key = event.Records[0].s3.object.key;
// documentation for this method:
// https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/S3.html#getObject-property
let readStream = S3.getObject({ Bucket: bucket, Key: key }).createReadStream(); // 1
const rl = readline.createInterface({
input: readStream
});
rl.on('line', async (line) => {
console.log(line);
// process line as needed
});
} catch (err) {
console.log(err);
return err;
}
};
In the snippet above I'm printing the line in the log file to the console just for testing, but I don't see any output.
But if I refactor the code like this it works:
const stream = require('stream');
const bufferStream = new stream.PassThrough();
const readline = require('readline');
exports.handler = async (event) => {
try {
// retrieving first record only just for
// testing
let bucket = event.Records[0].s3.bucket.name;
let key = event.Records[0].s3.object.key;
// data.Body is a Buffer
let data = await S3.getObject({ Bucket: bucket, Key: key }).promise();
bufferStream.end(data.Body); // 2
const rl = readline.createInterface({
input: bufferStream
});
rl.on('line', (line) => {
console.log(line);
// process line as needed
});
} catch (err) {
console.log(err);
return err;
}
};
For the line marked 2, the getObject function returns a buffer and is converted to a stream.
Is it possible to do this without using a buffer? My thinking is if the log file is very large it is inefficient to convert a buffer to a stream. I'm wondering if I can use a stream directly like the line marked 1.
EDIT:
I did some more testing and got it to work but without an async lambda function. Here it is:
exports.handler = function (event, context, callback) {
// for testing I'm looking at the first record
let bucket = event.Records[0].s3.bucket.name;
let key = event.Records[0].s3.object.key;
const readStream = S3.getObject({ Bucket: bucket, Key: key }).createReadStream();
const rl = readline.createInterface({
input: readStream,
crlfDelay: Infinity
});
rl.on('line', (line) => {
console.log(line);
});
}
Does anyone know why this refactored code works, but not with async lambda ?