gui: New rest endpoint to get errors when web UI is opened

Since #4340 pulls aren't happening every 10s anymore and may be delayed up to 1h.
This means that no folder error event reaches the web UI for a long time, thus no
failed items will show up for a long time. Now errors are populated when the
web UI is opened.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4650
LGTM: AudriusButkevicius

lib/model/model.go

@@ -64,6 +64,7 @@ type service interface {
 	Serve()
 	Stop()
 	CheckHealth() error
+	PullErrors() []FileError
 
 	getState() (folderState, time.Time, error)
 	setState(state folderState)
@@ -119,7 +120,7 @@ var (
 	errDeviceUnknown     = errors.New("unknown device")
 	errDevicePaused      = errors.New("device is paused")
 	errDeviceIgnored     = errors.New("device is ignored")
-	errFolderPaused      = errors.New("folder is paused")
+	ErrFolderPaused      = errors.New("folder is paused")
 	errFolderNotRunning  = errors.New("folder is not running")
 	errFolderMissing     = errors.New("no such folder")
 	errNetworkNotAllowed = errors.New("network not allowed")
@@ -2226,6 +2227,15 @@ func (m *Model) State(folder string) (string, time.Time, error) {
 	return state.String(), changed, err
 }
 
+func (m *Model) PullErrors(folder string) ([]FileError, error) {
+	m.fmut.RLock()
+	defer m.fmut.RUnlock()
+	if err := m.checkFolderRunningLocked(folder); err != nil {
+		return nil, err
+	}
+	return m.folderRunners[folder].PullErrors(), nil
+}
+
 func (m *Model) Override(folder string) {
 	m.fmut.RLock()
 	fs, ok := m.folderFiles[folder]
@@ -2657,7 +2667,7 @@ func (m *Model) checkFolderRunningLocked(folder string) error {
 	if cfg, ok := m.cfg.Folder(folder); !ok {
 		return errFolderMissing
 	} else if cfg.Paused {
-		return errFolderPaused
+		return ErrFolderPaused
 	}
 
 	return errFolderNotRunning

lib/model/model_test.go

@@ -3229,7 +3229,7 @@ func TestPausedFolders(t *testing.T) {
 	}
 	w.Wait()
 
-	if err := m.ScanFolder("default"); err != errFolderPaused {
+	if err := m.ScanFolder("default"); err != ErrFolderPaused {
 		t.Errorf("Expected folder paused error, received: %v", err)
 	}
 

lib/model/rofolder.go

@@ -66,3 +66,7 @@ func (f *sendOnlyFolder) Serve() {
 func (f *sendOnlyFolder) String() string {
 	return fmt.Sprintf("sendOnlyFolder/%s@%p", f.folderID, f)
 }
+
+func (f *sendOnlyFolder) PullErrors() []FileError {
+	return nil
+}

lib/model/rwfolder.go

@@ -286,7 +286,7 @@ func (f *sendReceiveFolder) pull(prevIgnoreHash string) (curIgnoreHash string, s
 			// we're not making it. Probably there are write
 			// errors preventing us. Flag this with a warning and
 			// wait a bit longer before retrying.
-			if folderErrors := f.currentErrors(); len(folderErrors) > 0 {
+			if folderErrors := f.PullErrors(); len(folderErrors) > 0 {
 				events.Default.Log(events.FolderErrors, map[string]interface{}{
 					"folder": f.folderID,
 					"errors": folderErrors,
@@ -1797,11 +1797,11 @@ func (f *sendReceiveFolder) clearErrors() {
 	f.errorsMut.Unlock()
 }
 
-func (f *sendReceiveFolder) currentErrors() []fileError {
+func (f *sendReceiveFolder) PullErrors() []FileError {
 	f.errorsMut.Lock()
-	errors := make([]fileError, 0, len(f.errors))
+	errors := make([]FileError, 0, len(f.errors))
 	for path, err := range f.errors {
-		errors = append(errors, fileError{path, err})
+		errors = append(errors, FileError{path, err})
 	}
 	sort.Sort(fileErrorList(errors))
 	f.errorsMut.Unlock()
@@ -1880,13 +1880,13 @@ func (f *sendReceiveFolder) deleteDir(dir string, ignores *ignore.Matcher, scanC
 	return err
 }
 
-// A []fileError is sent as part of an event and will be JSON serialized.
-type fileError struct {
+// A []FileError is sent as part of an event and will be JSON serialized.
+type FileError struct {
 	Path string `json:"path"`
 	Err  string `json:"error"`
 }
 
-type fileErrorList []fileError
+type fileErrorList []FileError
 
 func (l fileErrorList) Len() int {
 	return len(l)