mem: stricter handling of out of bounds requests

internal/backend/dryrun/dry_backend_test.go

@@ -96,7 +96,7 @@ func TestDry(t *testing.T) {
 			}
 		case "load":
 			data := ""
-			err = step.be.Load(ctx, handle, 100, 0, func(rd io.Reader) error {
+			err = step.be.Load(ctx, handle, 0, 0, func(rd io.Reader) error {
 				buf, err := io.ReadAll(rd)
 				data = string(buf)
 				return err

internal/backend/mem/mem_backend.go

@@ -43,6 +43,7 @@ func NewFactory() location.Factory {
 }
 
 var errNotFound = fmt.Errorf("not found")
+var errTooSmall = errors.New("access beyond end of file")
 
 const connectionCount = 2
 
@@ -69,6 +70,10 @@ func (be *MemoryBackend) IsNotExist(err error) bool {
 	return errors.Is(err, errNotFound)
 }
 
+func (be *MemoryBackend) IsPermanentError(err error) bool {
+	return be.IsNotExist(err) || errors.Is(err, errTooSmall)
+}
+
 // Save adds new Data to the backend.
 func (be *MemoryBackend) Save(ctx context.Context, h backend.Handle, rd backend.RewindReader) error {
 	be.m.Lock()
@@ -131,12 +136,12 @@ func (be *MemoryBackend) openReader(ctx context.Context, h backend.Handle, lengt
 	}
 
 	buf := be.data[h]
-	if offset > int64(len(buf)) {
-		return nil, errors.New("offset beyond end of file")
+	if offset+int64(length) > int64(len(buf)) {
+		return nil, errTooSmall
 	}
 
 	buf = buf[offset:]
-	if length > 0 && len(buf) > length {
+	if length > 0 {
 		buf = buf[:length]
 	}