backend: add IsPermanentError() method to interface

internal/backend/backend.go

@@ -38,7 +38,9 @@ type Backend interface {
 
 	// Load runs fn with a reader that yields the contents of the file at h at the
 	// given offset. If length is larger than zero, only a portion of the file
-	// is read.
+	// is read. If the length is larger than zero and the file is too short to return
+	// the requested length bytes, then an error MUST be returned that is recognized
+	// by IsPermanentError().
 	//
 	// The function fn may be called multiple times during the same Load invocation
 	// and therefore must be idempotent.
@@ -66,6 +68,12 @@ type Backend interface {
 	// for unwrapping it.
 	IsNotExist(err error) bool
 
+	// IsPermanentError returns true if the error can very likely not be resolved
+	// by retrying the operation. Backends should return true if the file is missing,
+	// the requested range does not (completely) exist in the file or the user is
+	// not authorized to perform the requested operation.
+	IsPermanentError(err error) bool
+
 	// Delete removes all data in the backend.
 	Delete(ctx context.Context) error
 }

internal/backend/dryrun/dry_backend.go

@@ -72,6 +72,10 @@ func (be *Backend) IsNotExist(err error) bool {
 	return be.b.IsNotExist(err)
 }
 
+func (be *Backend) IsPermanentError(err error) bool {
+	return be.b.IsPermanentError(err)
+}
+
 func (be *Backend) List(ctx context.Context, t backend.FileType, fn func(backend.FileInfo) error) error {
 	return be.b.List(ctx, t, fn)
 }

internal/backend/mock/backend.go

@@ -13,6 +13,7 @@ import (
 type Backend struct {
 	CloseFn            func() error
 	IsNotExistFn       func(err error) bool
+	IsPermanentErrorFn func(err error) bool
 	SaveFn             func(ctx context.Context, h backend.Handle, rd backend.RewindReader) error
 	OpenReaderFn       func(ctx context.Context, h backend.Handle, length int, offset int64) (io.ReadCloser, error)
 	StatFn             func(ctx context.Context, h backend.Handle) (backend.FileInfo, error)
@@ -83,6 +84,14 @@ func (m *Backend) IsNotExist(err error) bool {
 	return m.IsNotExistFn(err)
 }
 
+func (m *Backend) IsPermanentError(err error) bool {
+	if m.IsPermanentErrorFn == nil {
+		return false
+	}
+
+	return m.IsPermanentErrorFn(err)
+}
+
 // Save data in the backend.
 func (m *Backend) Save(ctx context.Context, h backend.Handle, rd backend.RewindReader) error {
 	if m.SaveFn == nil {