all: Add filesystem notification support

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3986

lib/model/rofolder.go

@@ -34,11 +34,20 @@ func (f *sendOnlyFolder) Serve() {
 		f.scan.timer.Stop()
 	}()
 
+	if f.FSWatcherEnabled {
+		f.startWatcher()
+	}
+
 	for {
 		select {
 		case <-f.ctx.Done():
 			return
 
+		case <-f.ignoresUpdated:
+			if f.FSWatcherEnabled {
+				f.restartWatcher()
+			}
+
 		case <-f.scan.timer.C:
 			l.Debugln(f, "Scanning subdirectories")
 			f.scanTimerFired()
@@ -48,6 +57,10 @@ func (f *sendOnlyFolder) Serve() {
 
 		case next := <-f.scan.delay:
 			f.scan.timer.Reset(next)
+
+		case fsEvents := <-f.watchChan:
+			l.Debugln(f, "filesystem notification rescan")
+			f.scanSubdirs(fsEvents)
 		}
 	}
 }