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/model.go

@@ -48,8 +48,9 @@ type service interface {
 	Jobs() ([]string, []string) // In progress, Queued
 	BringToFront(string)
 
-	setState(folderState)
-	getState() (folderState, time.Time)
+	setState(state folderState)
+	setError(err error)
+	getState() (folderState, time.Time, error)
 }
 
 type Model struct {
@@ -1083,13 +1084,13 @@ func (m *Model) AddFolder(cfg config.FolderConfiguration) {
 
 func (m *Model) ScanFolders() map[string]error {
 	m.fmut.RLock()
-	var folders = make([]string, 0, len(m.folderCfgs))
+	folders := make([]string, 0, len(m.folderCfgs))
 	for folder := range m.folderCfgs {
 		folders = append(folders, folder)
 	}
 	m.fmut.RUnlock()
 
-	var errors = make(map[string]error, len(m.folderCfgs))
+	errors := make(map[string]error, len(m.folderCfgs))
 	var errorsMut sync.Mutex
 
 	var wg sync.WaitGroup
@@ -1102,11 +1103,15 @@ func (m *Model) ScanFolders() map[string]error {
 				errorsMut.Lock()
 				errors[folder] = err
 				errorsMut.Unlock()
+
 				// Potentially sets the error twice, once in the scanner just
 				// by doing a check, and once here, if the error returned is
 				// the same one as returned by CheckFolderHealth, though
-				// duplicate set is handled by SetFolderError
-				m.cfg.SetFolderError(folder, err)
+				// duplicate set is handled by setError.
+				m.fmut.RLock()
+				srv := m.folderRunners[folder]
+				m.fmut.RUnlock()
+				srv.setError(err)
 			}
 			wg.Done()
 		}()
@@ -1182,13 +1187,13 @@ nextSub:
 	}
 
 	runner.setState(FolderScanning)
-	defer runner.setState(FolderIdle)
-	fchan, err := w.Walk()
 
+	fchan, err := w.Walk()
 	if err != nil {
-		m.cfg.SetFolderError(folder, err)
+		runner.setError(err)
 		return err
 	}
+
 	batchSize := 100
 	batch := make([]protocol.FileInfo, 0, batchSize)
 	for f := range fchan {
@@ -1298,6 +1303,7 @@ nextSub:
 		fs.Update(protocol.LocalDeviceID, batch)
 	}
 
+	runner.setState(FolderIdle)
 	return nil
 }
 
@@ -1340,15 +1346,18 @@ func (m *Model) clusterConfig(device protocol.DeviceID) protocol.ClusterConfigMe
 	return cm
 }
 
-func (m *Model) State(folder string) (string, time.Time) {
+func (m *Model) State(folder string) (string, time.Time, error) {
 	m.fmut.RLock()
 	runner, ok := m.folderRunners[folder]
 	m.fmut.RUnlock()
 	if !ok {
-		return "", time.Time{}
+		// The returned error should be an actual folder error, so returning
+		// errors.New("does not exist") or similar here would be
+		// inappropriate.
+		return "", time.Time{}, nil
 	}
-	state, changed := runner.getState()
-	return state.String(), changed
+	state, changed, err := runner.getState()
+	return state.String(), changed, err
 }
 
 func (m *Model) Override(folder string) {
@@ -1528,7 +1537,7 @@ func (m *Model) BringToFront(folder, file string) {
 func (m *Model) CheckFolderHealth(id string) error {
 	folder, ok := m.cfg.Folders()[id]
 	if !ok {
-		return errors.New("Folder does not exist")
+		return errors.New("folder does not exist")
 	}
 
 	fi, err := os.Stat(folder.Path())
@@ -1538,9 +1547,9 @@ func (m *Model) CheckFolderHealth(id string) error {
 		// that all files have been deleted which might not be the case,
 		// so mark it as invalid instead.
 		if err != nil || !fi.IsDir() {
-			err = errors.New("Folder path missing")
+			err = errors.New("folder path missing")
 		} else if !folder.HasMarker() {
-			err = errors.New("Folder marker missing")
+			err = errors.New("folder marker missing")
 		}
 	} else if os.IsNotExist(err) {
 		// If we don't have any files in the index, and the directory
@@ -1555,35 +1564,21 @@ func (m *Model) CheckFolderHealth(id string) error {
 		err = folder.CreateMarker()
 	}
 
-	if err == nil {
-		if folder.Invalid != "" {
-			l.Infof("Starting folder %q after error %q", folder.ID, folder.Invalid)
-			m.cfg.SetFolderError(id, nil)
+	m.fmut.RLock()
+	runner := m.folderRunners[folder.ID]
+	m.fmut.RUnlock()
+	_, _, oldErr := runner.getState()
+
+	if err != nil {
+		if oldErr != nil && oldErr.Error() != err.Error() {
+			l.Infof("Folder %q error changed: %q -> %q", folder.ID, oldErr, err)
+		} else if oldErr == nil {
+			l.Warnf("Stopping folder %q - %v", folder.ID, err)
 		}
-
-		if folder, ok := m.cfg.Folders()[id]; !ok || folder.Invalid != "" {
-			panic("Unable to unset folder \"" + id + "\" error.")
-		}
-
-		return nil
-	}
-
-	if folder.Invalid == err.Error() {
-		return err
-	}
-
-	// folder is a copy of the original struct, hence Invalid value is
-	// preserved after the set.
-	m.cfg.SetFolderError(id, err)
-
-	if folder.Invalid == "" {
-		l.Warnf("Stopping folder %q - %v", folder.ID, err)
-	} else {
-		l.Infof("Folder %q error changed: %q -> %q", folder.ID, folder.Invalid, err)
-	}
-
-	if folder, ok := m.cfg.Folders()[id]; !ok || folder.Invalid != err.Error() {
-		panic("Unable to set folder \"" + id + "\" error.")
+		runner.setError(err)
+	} else if oldErr != nil {
+		l.Infof("Folder %q error is cleared, restarting", folder.ID)
+		runner.setState(FolderIdle)
 	}
 
 	return err