lib/model: Refactor out folder and folderscan types, simplify somewhat

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3007

lib/model/folderscan.go

@@ -0,0 +1,49 @@
+// Copyright (C) 2016 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at http://mozilla.org/MPL/2.0/.
+
+package model
+
+import (
+	"math/rand"
+	"time"
+)
+
+type rescanRequest struct {
+	subdirs []string
+	err     chan error
+}
+
+// bundle all folder scan activity
+type folderscan struct {
+	interval time.Duration
+	timer    *time.Timer
+	now      chan rescanRequest
+	delay    chan time.Duration
+}
+
+func (s *folderscan) reschedule() {
+	if s.interval == 0 {
+		return
+	}
+	// Sleep a random time between 3/4 and 5/4 of the configured interval.
+	sleepNanos := (s.interval.Nanoseconds()*3 + rand.Int63n(2*s.interval.Nanoseconds())) / 4
+	interval := time.Duration(sleepNanos) * time.Nanosecond
+	l.Debugln(s, "next rescan in", interval)
+	s.timer.Reset(interval)
+}
+
+func (s *folderscan) Scan(subdirs []string) error {
+	req := rescanRequest{
+		subdirs: subdirs,
+		err:     make(chan error),
+	}
+	s.now <- req
+	return <-req.err
+}
+
+func (s *folderscan) Delay(next time.Duration) {
+	s.delay <- next
+}