lib/model: Fix accounting error in rescan with multiple subs (fixes #3028)

When doing prefix scans in the database, "foo" should not be considered
a prefix of "foo2". Instead, it should match "foo" exactly and also
strings with the prefix "foo/". This is more restrictive than what the
standard leveldb prefix scan does so we add some code to enforce it.

Also exposes the initialScanCompleted on the rwfolder for testing, and
change it to be a channel (so we can wait for it from another
goroutine). Otherwise we can't be sure when the initial scan has
completed, and we need to wait for that or it might pick up changes
we're doing at an unexpected time.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3067

lib/model/rwfolder.go

@@ -100,6 +100,8 @@ type rwFolder struct {
 
 	errors    map[string]string // path -> error string
 	errorsMut sync.Mutex
+
+	initialScanCompleted chan (struct{}) // exposed for testing
 }
 
 func newRWFolder(model *Model, cfg config.FolderConfiguration, ver versioner.Versioner) service {
@@ -135,6 +137,8 @@ func newRWFolder(model *Model, cfg config.FolderConfiguration, ver versioner.Ver
 		remoteIndex: make(chan struct{}, 1), // This needs to be 1-buffered so that we queue a notification if we're busy doing a pull when it comes.
 
 		errorsMut: sync.NewMutex(),
+
+		initialScanCompleted: make(chan struct{}),
 	}
 
 	f.configureCopiersAndPullers(cfg)
@@ -186,9 +190,6 @@ func (f *rwFolder) Serve() {
 	var prevVer int64
 	var prevIgnoreHash string
 
-	// We don't start pulling files until a scan has been completed.
-	initialScanCompleted := false
-
 	for {
 		select {
 		case <-f.stop:
@@ -200,7 +201,10 @@ func (f *rwFolder) Serve() {
 			l.Debugln(f, "remote index updated, rescheduling pull")
 
 		case <-f.pullTimer.C:
-			if !initialScanCompleted {
+			select {
+			case <-f.initialScanCompleted:
+			default:
+				// We don't start pulling files until a scan has been completed.
 				l.Debugln(f, "skip (initial)")
 				f.pullTimer.Reset(f.sleep)
 				continue
@@ -297,9 +301,11 @@ func (f *rwFolder) Serve() {
 			if err != nil {
 				continue
 			}
-			if !initialScanCompleted {
+			select {
+			case <-f.initialScanCompleted:
+			default:
 				l.Infoln("Completed initial scan (rw) of folder", f.folderID)
-				initialScanCompleted = true
+				close(f.initialScanCompleted)
 			}
 
 		case req := <-f.scan.now: