cmd/syncthing, lib/db: Abort execution if db version is too high (fixes #4994) (#5022)

lib/db/leveldb_dbinstance.go

@@ -9,6 +9,7 @@ package db
 import (
 	"bytes"
 	"encoding/binary"
+	"fmt"
 	"os"
 	"sort"
 	"strings"
@@ -60,31 +61,32 @@ func Open(file string) (*Instance, error) {
 		// the database and reindexing...
 		l.Infoln("Database corruption detected, unable to recover. Reinitializing...")
 		if err := os.RemoveAll(file); err != nil {
-			return nil, err
+			return nil, errorSuggestion{err, "failed to delete corrupted database"}
 		}
 		db, err = leveldb.OpenFile(file, opts)
 	}
 	if err != nil {
-		return nil, err
+		return nil, errorSuggestion{err, "is another instance of Syncthing running?"}
 	}
 
-	return newDBInstance(db, file), nil
+	return newDBInstance(db, file)
 }
 
 func OpenMemory() *Instance {
 	db, _ := leveldb.Open(storage.NewMemStorage(), nil)
-	return newDBInstance(db, "<memory>")
+	ldb, _ := newDBInstance(db, "<memory>")
+	return ldb
 }
 
-func newDBInstance(db *leveldb.DB, location string) *Instance {
+func newDBInstance(db *leveldb.DB, location string) (*Instance, error) {
 	i := &Instance{
 		DB:       db,
 		location: location,
 	}
 	i.folderIdx = newSmallIndex(i, []byte{KeyTypeFolderIdx})
 	i.deviceIdx = newSmallIndex(i, []byte{KeyTypeDeviceIdx})
-	i.updateSchema()
-	return i
+	err := i.updateSchema()
+	return i, err
 }
 
 // Committed returns the number of items committed to the database since startup
@@ -935,3 +937,12 @@ func resize(k []byte, reqLen int) []byte {
 	}
 	return k[:reqLen]
 }
+
+type errorSuggestion struct {
+	inner      error
+	suggestion string
+}
+
+func (e errorSuggestion) Error() string {
+	return fmt.Sprintf("%s (%s)", e.inner.Error(), e.suggestion)
+}