Move folder errors to state

The "Invalid" config attribute is retained for errors discovered during
config loading (empty path, duplicate ID). This can only be set or
cleared at config loading time.

Errors discovered during runtime (I/O problems, etc) are now in the
folder state instead. Changes to these are sent as any other folder
state change.

internal/model/folderstate.go

@@ -1,17 +1,8 @@
 // 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/>.
+// 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
 
@@ -28,7 +19,7 @@ const (
 	FolderIdle folderState = iota
 	FolderScanning
 	FolderSyncing
-	FolderCleaning
+	FolderError
 )
 
 func (s folderState) String() string {
@@ -37,10 +28,10 @@ func (s folderState) String() string {
 		return "idle"
 	case FolderScanning:
 		return "scanning"
-	case FolderCleaning:
-		return "cleaning"
 	case FolderSyncing:
 		return "syncing"
+	case FolderError:
+		return "error"
 	default:
 		return "unknown"
 	}
@@ -51,10 +42,16 @@ type stateTracker struct {
 
 	mut     sync.Mutex
 	current folderState
+	err     error
 	changed time.Time
 }
 
+// setState sets the new folder state, for states other than FolderError.
 func (s *stateTracker) setState(newState folderState) {
+	if newState == FolderError {
+		panic("must use setError")
+	}
+
 	s.mut.Lock()
 	if newState != s.current {
 		/* This should hold later...
@@ -74,6 +71,7 @@ func (s *stateTracker) setState(newState folderState) {
 		}
 
 		s.current = newState
+		s.err = nil
 		s.changed = time.Now()
 
 		events.Default.Log(events.StateChanged, eventData)
@@ -81,9 +79,35 @@ func (s *stateTracker) setState(newState folderState) {
 	s.mut.Unlock()
 }
 
-func (s *stateTracker) getState() (current folderState, changed time.Time) {
+// getState returns the current state, the time when it last changed, and the
+// current error or nil.
+func (s *stateTracker) getState() (current folderState, changed time.Time, err error) {
 	s.mut.Lock()
-	current, changed = s.current, s.changed
+	current, changed, err = s.current, s.changed, s.err
 	s.mut.Unlock()
 	return
 }
+
+// setError sets the folder state to FolderError with the specified error.
+func (s *stateTracker) setError(err error) {
+	s.mut.Lock()
+	if s.current != FolderError || s.err.Error() != err.Error() {
+		eventData := map[string]interface{}{
+			"folder": s.folder,
+			"to":     FolderError.String(),
+			"from":   s.current.String(),
+			"error":  err.Error(),
+		}
+
+		if !s.changed.IsZero() {
+			eventData["duration"] = time.Since(s.changed).Seconds()
+		}
+
+		s.current = FolderError
+		s.err = err
+		s.changed = time.Now()
+
+		events.Default.Log(events.StateChanged, eventData)
+	}
+	s.mut.Unlock()
+}