Downloading multiple files from S3 concurrently and consolidated them

Question

Im trying to download multiple files from S3 concurrently ,and consolidate their contents into a bytes buffer.The files are csv formatted. My code seems to work most of time(8 from 10 tries).But there are instances that after i inspected the consolidated buffer, I've got less that what i should be getting(usually no more than 100 rows missing). Total number of records expected is 4802. If run my code sequentially this problem does not appear.But i need to use goroutines for the speed.This is a major requirement on what im trying to do.I have run the go data race inspector with no data races appear , and the error statements that i print never print out.

This is the code i use:

    var pingsBuffer = aws.NewWriteAtBuffer([]byte{}) 
        //range over the contents of the index file
    for _, file := range indexList {
        wg.Add(1)
        go download(key + string(file), pingsBuffer, &wg)
    }
    wg.Wait()

and the download functions (that also consolidates the downloaded files)

func download(key string, buffer *aws.WriteAtBuffer, wg *sync.WaitGroup)  {

defer wg.Done()

awsBuffer := aws.NewWriteAtBuffer([]byte{})

input := &s3.GetObjectInput {
    Bucket: aws.String(defaultLocationRootBucket),
    Key:    aws.String(key),
}

n1, downloadError := downloader.Download(awsBuffer, input)
if downloadError != nil {
    loglib.Log(loglib.LevelError, applicationType, fmt.Sprintf("Failed to download from S3, file(%v) with error : %v.", key, downloadError))
    return
}


lenghts3:= int64(len(buffer.Bytes()))

n2, bufferError := buffer.WriteAt(awsBuffer.Bytes(), lenghts3 )
if bufferError != nil {
    loglib.Log(loglib.LevelError, applicationType, fmt.Sprintf("Failed to write to buffer, the file(%v) downloaded from S3  with error : %v.", key, bufferError))
}

Answer 1

This code:

lenghts3:= int64(len(buffer.Bytes()))

Is a concurrency problem: two routines may get the length at the same time, getting the same start position, and both proceed to write to the buffer with the same start position, stepping on each other's toes.

Since you're already retrieving whole objects in memory and not streaming to the combined buffer, you may as well just send the full contents of each file on a channel, and have a receiver on that channel append each result to a shared byte buffer as they come in, synchronously.