Return error if previous part is missing (#1017)

Closes #1015.

api/layer/multipart_upload.go

@@ -267,6 +267,11 @@ func (n *layer) uploadPart(ctx context.Context, multipartInfo *data.MultipartInf
 		return nil, fmt.Errorf("getLastPart: %w", err)
 	}
 
+	// The previous part is not uploaded yet.
+	if lastPart == nil {
+		return nil, s3errors.GetAPIError(s3errors.ErrOperationAborted)
+	}
+
 	// try to restore hash state from the last part.
 	// the required interface is guaranteed according to the docs, so just cast without checks.
 	binaryUnmarshaler := multipartHash.(encoding.BinaryUnmarshaler)

api/s3errors/errors.go

@@ -268,6 +268,7 @@ const (
 	ErrEvaluatorBindingDoesNotExist
 	ErrMissingHeaders
 	ErrInvalidColumnIndex
+	ErrOperationAborted
 
 	ErrPostPolicyConditionInvalidFormat
 
@@ -1682,6 +1683,12 @@ var errorCodes = errorCodeMap{
 		Description:    "Part number must be an integer between 1 and 10000, inclusive",
 		HTTPStatusCode: http.StatusBadRequest,
 	},
+	ErrOperationAborted: {
+		ErrCode:        ErrOperationAborted,
+		Code:           "OperationAborted",
+		Description:    "A conflicting conditional operation is currently in progress against this resource. Try again.",
+		HTTPStatusCode: http.StatusConflict,
+	},
 	// Add your error structure here.
 }