Support unknown gateway errors and convert at handler layer (#7219)

Different gateway implementations due to different backend
API errors, might return different unsupported errors at
our handler layer. Current code posed a problem for us because
this information was lost and we would convert it to InternalError
in this situation all S3 clients end up retrying the request.

To avoid this unexpected situation implement a way to support
this cleanly such that the underlying information is not lost
which is returned by gateway.

cmd/api-errors.go

@@ -22,6 +22,11 @@ import (
 	"fmt"
 	"net/http"
 
+	"github.com/Azure/azure-sdk-for-go/storage"
+	"github.com/aliyun/aliyun-oss-go-sdk/oss"
+	"google.golang.org/api/googleapi"
+
+	minio "github.com/minio/minio-go"
 	"github.com/minio/minio/cmd/crypto"
 	"github.com/minio/minio/cmd/logger"
 	"github.com/minio/minio/pkg/auth"
@@ -212,7 +217,7 @@ const (
 	ErrHealOverlappingPaths
 	ErrIncorrectContinuationToken
 
-	//S3 Select Errors
+	// S3 Select Errors
 	ErrEmptyRequestBody
 	ErrUnsupportedFunction
 	ErrInvalidExpressionType
@@ -304,9 +309,19 @@ const (
 	ErrAddUserInvalidArgument
 )
 
+type errorCodeMap map[APIErrorCode]APIError
+
+func (e errorCodeMap) ToAPIErr(errCode APIErrorCode) APIError {
+	apiErr, ok := e[errCode]
+	if !ok {
+		return e[ErrInternalError]
+	}
+	return apiErr
+}
+
 // error code to APIError structure, these fields carry respective
 // descriptions for all the error responses.
-var errorCodeResponse = map[APIErrorCode]APIError{
+var errorCodes = errorCodeMap{
 	ErrInvalidCopyDest: {
 		Code:           "InvalidRequest",
 		Description:    "This copy request is illegal because it is trying to copy an object to itself without changing the object's metadata, storage class, website redirect location or encryption attributes.",
@@ -1636,12 +1651,66 @@ func toAPIErrorCode(ctx context.Context, err error) (apiErr APIErrorCode) {
 	return apiErr
 }
 
+var noError = APIError{}
+
+// toAPIError - Converts embedded errors. Convenience
+// function written to handle all cases where we have known types of
+// errors returned by underlying layers.
+func toAPIError(ctx context.Context, err error) APIError {
+	if err == nil {
+		return noError
+	}
+
+	var apiErr = errorCodes.ToAPIErr(toAPIErrorCode(ctx, err))
+	if apiErr.Code == "InternalError" {
+		// If we see an internal error try to interpret
+		// any underlying errors if possible depending on
+		// their internal error types. This code is only
+		// useful with gateway implementations.
+		switch e := err.(type) {
+		case minio.ErrorResponse:
+			apiErr = APIError{
+				Code:           e.Code,
+				Description:    e.Message,
+				HTTPStatusCode: e.StatusCode,
+			}
+		case *googleapi.Error:
+			apiErr = APIError{
+				Code:           "XGCSInternalError",
+				Description:    e.Message,
+				HTTPStatusCode: e.Code,
+			}
+			// GCS may send multiple errors, just pick the first one
+			// since S3 only sends one Error XML response.
+			if len(e.Errors) >= 1 {
+				apiErr.Code = e.Errors[0].Reason
+
+			}
+		case storage.AzureStorageServiceError:
+			apiErr = APIError{
+				Code:           e.Code,
+				Description:    e.Message,
+				HTTPStatusCode: e.StatusCode,
+			}
+		case oss.ServiceError:
+			apiErr = APIError{
+				Code:           e.Code,
+				Description:    e.Message,
+				HTTPStatusCode: e.StatusCode,
+			}
+			// Add more Gateway SDKs here if any in future.
+		}
+	}
+
+	return apiErr
+}
+
 // getAPIError provides API Error for input API error code.
 func getAPIError(code APIErrorCode) APIError {
-	if apiErr, ok := errorCodeResponse[code]; ok {
+	if apiErr, ok := errorCodes[code]; ok {
 		return apiErr
 	}
-	return errorCodeResponse[ErrInternalError]
+	return errorCodes.ToAPIErr(ErrInternalError)
 }
 
 // getErrorResponse gets in standard error and resource value and