lib/model: Don't set ignore bit when it's already set

This adds a metric for "committed items" to the database instance that I
use in the test code, and a couple of tests that ensure that scans that
don't change anything also don't commit anything.

There was a case in the scanner where we set the invalid bit on files
that are ignored, even though they were already ignored and had the
invalid bit set. I had assumed this would result in an extra database
commit, but it was in fact filtered out by the Set... Anyway, I think we
can save some work on not pushing that change to the Set at all.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3298

lib/db/leveldb_dbinstance.go

@@ -13,6 +13,7 @@ import (
 	"path/filepath"
 	"sort"
 	"strings"
+	"sync/atomic"
 
 	"github.com/syncthing/syncthing/lib/osutil"
 	"github.com/syncthing/syncthing/lib/protocol"
@@ -31,6 +32,7 @@ type Instance struct {
 	*leveldb.DB
 	folderIdx *smallIndex
 	deviceIdx *smallIndex
+	committed int64
 }
 
 const (
@@ -91,6 +93,11 @@ func newDBInstance(db *leveldb.DB) *Instance {
 	return i
 }
 
+// Committed returns the number of items committed to the database since startup
+func (db *Instance) Committed() int64 {
+	return atomic.LoadInt64(&db.committed)
+}
+
 func (db *Instance) genericReplace(folder, device []byte, fs []protocol.FileInfo, localSize, globalSize *sizeTracker, deleteFn deletionHandler) int64 {
 	sort.Sort(fileList(fs)) // sort list on name, same as in the database