lib/db: Improve error message on meta inconsistency (#6751)

lib/db/meta.go

@@ -8,6 +8,7 @@ package db
 
 import (
 	"bytes"
+	"errors"
 	"math/bits"
 	"time"
 
@@ -15,6 +16,8 @@ import (
 	"github.com/syncthing/syncthing/lib/sync"
 )
 
+var errMetaInconsistent = errors.New("inconsistent counts detected")
+
 type countsMap struct {
 	counts  CountsSet
 	indexes map[metaKey]int // device ID + local flags -> index in counts
@@ -104,7 +107,13 @@ func (m *metadataTracker) fromDB(db *Lowlevel, folder []byte) error {
 	if err != nil {
 		return err
 	}
-	return m.Unmarshal(bs)
+	if err = m.Unmarshal(bs); err != nil {
+		return err
+	}
+	if m.counts.Created == 0 {
+		return errMetaInconsistent
+	}
+	return nil
 }
 
 // countsPtr returns a pointer to the corresponding Counts struct, if