How to perform AWS S3 Multipart Copy with golang

I was looking at the AWS golang documentation for S3 Copy Object function, it contains the following details for handling large file uploads

However, to copy an object greater >than 5 GB, you must use the multipart upload Upload Part - >Copy API. For more information, see Copy Object Using the REST Multipart Upload API >(https://docs.aws.amazon.com/AmazonS3/latest/dev/CopyingObjctsUsingRESTMPUapi.html).

When I follow that link, it only contains code examples for Java and .Net

Am I missing some documentation/example somewhere showing how to copy an existing large file in S3 using the golang client?

So it took some experimenting but I finally got the multipart copy working

//imports
import (
  "context"
  "strconv"
  "github.com/aws/aws-sdk-go/service/s3"
  log "github.com/sirupsen/logrus"
)

//constant for number of bits in 5 megabyte chunk
const max_part_size = 5 * 1024 * 1024

//helper function to build the string for the range of bits to copy
func buildCopySourceRange(start int64, objectSize int64) string {
    end := start + max_part_size - 1
    if end > objectSize {
        end = objectSize - 1
    }
    startRange := strconv.FormatInt(start, 10)
    stopRange := strconv.FormatInt(end, 10)
    return "bytes=" + startRange + "-" + stopRange
}

//function that starts, perform each part upload, and completes the copy
func MultiPartCopy(sess *session.Session, sourceBucket string, sourceKey string, destBucket string, destKey string) error {
    svc := s3.New(sess)

    ctx, cancelFn := context.WithTimeout(context.TODO(), 10*time.Minute)
    defer cancelFn()

    //struct for starting a multipart upload
    startInput := s3.CreateMultipartUploadInput{
        Bucket: &destBucket,
        Key:    &destKey,
    }

    //send command to start copy and get the upload id as it is needed later
    var uploadId string
    createOutput, err := svc.CreateMultipartUploadWithContext(ctx, &startInput)
    if err != nil {
        return err
    }
    if createOutput != nil {
        if createOutput.UploadId != nil {
            uploadId = *createOutput.UploadId
        }
    }
    if uploadId == "" {
        return errors.New("No upload id found in start upload request")
    }

    var i int64
    var partNumber int64 = 1
    copySource := "/" + sourceBucket + "/" + sourceKey
    parts := make([]*s3.CompletedPart, 0)
    numUploads := fileSize / max_part_size
    log.Infof("Will attempt upload in %d number of parts to %s", numUploads, destKey)
    for i = 0; i < fileSize; i += max_part_size {
        copyRange := buildCopySourceRange(i, fileSize)
        partInput := s3.UploadPartCopyInput{
            Bucket:          &destBucket,
            CopySource:      &copySource,
            CopySourceRange: &copyRange,
            Key:             &destKey,
            PartNumber:      &partNumber,
            UploadId:        &uploadId,
        }
        log.Debugf("Attempting to upload part %d range: %s", partNumber, copyRange)
        partResp, err := svc.UploadPartCopy(&partInput)

        if err != nil {
            log.Error("Attempting to abort upload")
            abortIn := s3.AbortMultipartUploadInput{
                UploadId: &uploadId,
            }
            //ignoring any errors with aborting the copy
            svc.AbortMultipartUploadRequest(&abortIn)
            return fmt.Errorf("Error uploading part %d : %w", partNumber, err)
        }

        //copy etag and part number from response as it is needed for completion
        if partResp != nil {
            partNum := partNumber
            etag := strings.Trim(*partResp.CopyPartResult.ETag, "\"")
            cPart := s3.CompletedPart{
                ETag:       &etag,
                PartNumber: &partNum,
            }
            parts = append(parts, &cPart)
            log.Debugf("Successfully upload part %d of %s", partNumber, uploadId)
        }
        partNumber++
        if partNumber%50 == 0 {
            log.Infof("Completed part %d of %d to %s", partNumber, numUploads, destKey)
        }
    }

    //create struct for completing the upload
    mpu := s3.CompletedMultipartUpload{
        Parts: parts,
    }

    //complete actual upload
    //does not actually copy if the complete command is not received
    complete := s3.CompleteMultipartUploadInput{
        Bucket:          &destBucket,
        Key:             &destKey,
        UploadId:        &uploadId,
        MultipartUpload: &mpu,
    }
    compOutput, err := svc.CompleteMultipartUpload(&complete)
    if err != nil {
        return fmt.Errorf("Error completing upload: %w", err)
    }
    if compOutput != nil {
        log.Infof("Successfully copied Bucket: %s Key: %s to Bucket: %s Key: %s", sourceBucket, sourceKey, destBucket, destKey)
    }
    return nil
}