fix: multipart replication and encrypted etag for sse-s3 (#13171)

Replication was not working properly for encrypted
objects in single PUT object for preserving etag,

We need to make sure to preserve etag such that replication
works properly and not gets into infinite loops of copying
due to ETag mismatches.

cmd/fs-v1-multipart.go

@@ -564,9 +564,6 @@ func (fs *FSObjects) CompleteMultipartUpload(ctx context.Context, bucket string,
 		return oi, toObjectErr(err, bucket, object)
 	}
 
-	// Calculate s3 compatible md5sum for complete multipart.
-	s3MD5 := getCompleteMultipartMD5(parts)
-
 	// ensure that part ETag is canonicalized to strip off extraneous quotes
 	for i := range parts {
 		parts[i].ETag = canonicalizeETag(parts[i].ETag)
@@ -755,7 +752,12 @@ func (fs *FSObjects) CompleteMultipartUpload(ctx context.Context, bucket string,
 	if fsMeta.Meta == nil {
 		fsMeta.Meta = make(map[string]string)
 	}
-	fsMeta.Meta["etag"] = s3MD5
+
+	fsMeta.Meta["etag"] = opts.UserDefined["etag"]
+	if fsMeta.Meta["etag"] == "" {
+		fsMeta.Meta["etag"] = getCompleteMultipartMD5(parts)
+	}
+
 	// Save consolidated actual size.
 	fsMeta.Meta[ReservedMetadataPrefix+"actual-size"] = strconv.FormatInt(objectActualSize, 10)
 	if _, err = fsMeta.WriteTo(metaFile); err != nil {