lib/model: Refactor {ro,rw}folder serve loop into just folder (#4939)

The actual pull method (which is really the only thing that differs
between them) is now an interface member which gets overridden by the
subclass.

"Subclass?!" Well, this is dynamic dispatch with overriding, I guess.

lib/model/rofolder.go

@@ -25,47 +25,11 @@ type sendOnlyFolder struct {
 }
 
 func newSendOnlyFolder(model *Model, cfg config.FolderConfiguration, _ versioner.Versioner, _ fs.Filesystem) service {
-	return &sendOnlyFolder{folder: newFolder(model, cfg)}
-}
-
-func (f *sendOnlyFolder) Serve() {
-	l.Debugln(f, "starting")
-	defer l.Debugln(f, "exiting")
-
-	defer func() {
-		f.scan.timer.Stop()
-	}()
-
-	if f.FSWatcherEnabled && f.CheckHealth() == nil {
-		f.startWatch()
-	}
-
-	for {
-		select {
-		case <-f.ctx.Done():
-			return
-
-		case <-f.pullScheduled:
-			f.pull()
-
-		case <-f.restartWatchChan:
-			f.restartWatch()
-
-		case <-f.scan.timer.C:
-			l.Debugln(f, "Scanning subdirectories")
-			f.scanTimerFired()
-
-		case req := <-f.scan.now:
-			req.err <- f.scanSubdirs(req.subdirs)
-
-		case next := <-f.scan.delay:
-			f.scan.timer.Reset(next)
-
-		case fsEvents := <-f.watchChan:
-			l.Debugln(f, "filesystem notification rescan")
-			f.scanSubdirs(fsEvents)
-		}
+	f := &sendOnlyFolder{
+		folder: newFolder(model, cfg),
 	}
+	f.folder.puller = f
+	return f
 }
 
 func (f *sendOnlyFolder) String() string {
@@ -77,12 +41,12 @@ func (f *sendOnlyFolder) PullErrors() []FileError {
 }
 
 // pull checks need for files that only differ by metadata (no changes on disk)
-func (f *sendOnlyFolder) pull() {
+func (f *sendOnlyFolder) pull() bool {
 	select {
 	case <-f.initialScanFinished:
 	default:
 		// Once the initial scan finished, a pull will be scheduled
-		return
+		return false
 	}
 
 	f.model.fmut.RLock()
@@ -133,4 +97,6 @@ func (f *sendOnlyFolder) pull() {
 	if len(batch) > 0 {
 		f.model.updateLocalsFromPulling(f.folderID, batch)
 	}
+
+	return true
 }