How to use io.ReadCloser as Body of LambdaFunctionURLStreamingResponse

[phhphc]

Hi,

I want to use body from output of s3.Client.GetObject as body of LambdaFunctionURLStreamingResponse.
However, out.Body is io.ReadCloser, while LambdaFunctionURLStreamingResponse.Body is io.Reader.
I know that io.ReadCloser is compatible with io.Reader, but this would leave out.Body without being close.
How can I use body from output of s3 GetObject as body of LambdaFunctionURLStreamingResponse?

Below is the simplify version of my code

func handler(
	ctx context.Context,
	req events.LambdaFunctionURLRequest,
) (res *events.LambdaFunctionURLStreamingResponse, err error) {

	var s3Client *s3.Client
	// ...
	out, err := s3Client.GetObject(context.TODO(), &s3.GetObjectInput{
		Bucket: &bucket,
		Key:    &key,
	})
	if err != nil {
		return nil, err
	}

	resp := &events.LambdaFunctionURLStreamingResponse{
		StatusCode: http.StatusOK,
		Headers:    map[string]string{"Content-Type": "application/x-ndjson"},
	}

	// What is the secure way to do this ?
	resp.Body = out.Body

	return resp, nil
}

[alehechka]

The events.LambdaFunctionURLStreamingResponse appears to include a Close() method that you can defer:

aws-lambda-go/events/lambda_function_urls.go

Lines 124 to 129 in 288af9e

	func (r *LambdaFunctionURLStreamingResponse) Close() error {
	if closer, ok := r.Body.(io.ReadCloser); ok {
	return closer.Close()
	}
	return nil
	}

[alehechka]

I tried using defer res.Close() in my own example and it appears to close the stream too early due to the return. I think there's a problem with the implementation of ResponseStreaming here in the go sdk.

When you look at examples of the node.js response streaming you can write multiple times to the responseStream object before ending the stream and see that reflect in the client that's calling it. For the go sdk, you have to return the response stream object with an io.Reader and essentially read all at once.

[phhphc]

I found the answer after delving into the lambda.Start function. The Close method is called automatically after it’s used.

aws-lambda-go/lambda/invoke_loop.go

Lines 85 to 88 in 288af9e

	// if the response needs to be closed (ex: net.Conn, os.File), ensure it's closed before the next invoke to prevent a resource leak
	if response, ok := response.(io.Closer); ok {
	defer response.Close()
	}

The behavior here is quite hidden, and I think it should be documented along with the other comments in the Start function.

aws-lambda-go/lambda/entry.go

Lines 41 to 46 in 288af9e

	//
	// "TOut" may also implement the io.Reader interface.
	// If "TOut" is both json serializable and implements io.Reader, then the json serialization is used.
	func Start(handler interface{}) {
	StartWithOptions(handler)
	}

The events.LambdaFunctionURLStreamingResponse appears to include a Close() method that you can defer:

aws-lambda-go/events/lambda_function_urls.go

Lines 124 to 129 in 288af9e
func (r *LambdaFunctionURLStreamingResponse) Close() error {
if closer, ok := r.Body.(io.ReadCloser); ok {
return closer.Close()
}
return nil
}