Refactor state tracking (...)

Move state tracking into the puller/scanner objects. This is a first
step towards resolving #1391.

Rename Puller and Scanner to roFolder and rwFolder as they have more
duties than just pulling and scanning, and don't need to be exported.

internal/model/folderstate.go

@@ -0,0 +1,89 @@
+// Copyright (C) 2015 The Syncthing Authors.
+//
+// This program is free software: you can redistribute it and/or modify it
+// under the terms of the GNU General Public License as published by the Free
+// Software Foundation, either version 3 of the License, or (at your option)
+// any later version.
+//
+// This program is distributed in the hope that it will be useful, but WITHOUT
+// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+// FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+// more details.
+//
+// You should have received a copy of the GNU General Public License along
+// with this program. If not, see <http://www.gnu.org/licenses/>.
+
+package model
+
+import (
+	"sync"
+	"time"
+
+	"github.com/syncthing/syncthing/internal/events"
+)
+
+type folderState int
+
+const (
+	FolderIdle folderState = iota
+	FolderScanning
+	FolderSyncing
+	FolderCleaning
+)
+
+func (s folderState) String() string {
+	switch s {
+	case FolderIdle:
+		return "idle"
+	case FolderScanning:
+		return "scanning"
+	case FolderCleaning:
+		return "cleaning"
+	case FolderSyncing:
+		return "syncing"
+	default:
+		return "unknown"
+	}
+}
+
+type stateTracker struct {
+	folder string
+
+	mut     sync.Mutex
+	current folderState
+	changed time.Time
+}
+
+func (s *stateTracker) setState(newState folderState) {
+	s.mut.Lock()
+	if newState != s.current {
+		/* This should hold later...
+		if s.current != FolderIdle && (newState == FolderScanning || newState == FolderSyncing) {
+			panic("illegal state transition " + s.current.String() + " -> " + newState.String())
+		}
+		*/
+
+		eventData := map[string]interface{}{
+			"folder": s.folder,
+			"to":     newState.String(),
+			"from":   s.current.String(),
+		}
+
+		if !s.changed.IsZero() {
+			eventData["duration"] = time.Since(s.changed).Seconds()
+		}
+
+		s.current = newState
+		s.changed = time.Now()
+
+		events.Default.Log(events.StateChanged, eventData)
+	}
+	s.mut.Unlock()
+}
+
+func (s *stateTracker) getState() (current folderState, changed time.Time) {
+	s.mut.Lock()
+	current, changed = s.current, s.changed
+	s.mut.Unlock()
+	return
+}