Fix #4260: Synchronize access to fileDialog.data file list (#4279)

This change adds a RWMutex to synchronize access to the fileDialog.data file list.
The refreshDir function clears and rebuilds fileDialog.data, meanwhile, several other functions are indexing into it.
This allowed for scenarios where fileDialog.data was either nil or smaller than expected when indexing into it.

Fixes #4260

dialog/file.go

@@ -7,6 +7,7 @@ import (
 	"path/filepath"
 	"runtime"
 	"strings"
+	"sync"
 
 	"fyne.io/fyne/v2"
 	"fyne.io/fyne/v2/container"
@@ -56,7 +57,9 @@ type fileDialog struct {
 	showHidden       bool
 
 	view viewLayout
-	data []fyne.URI
+
+	data     []fyne.URI
+	dataLock sync.RWMutex
 
 	win        *widget.PopUp
 	selected   fyne.URI
@@ -365,7 +368,9 @@ func (f *fileDialog) loadFavorites() {
 }
 
 func (f *fileDialog) refreshDir(dir fyne.ListableURI) {
+	f.dataLock.Lock()
 	f.data = nil
+	f.dataLock.Unlock()
 
 	files, err := dir.List()
 	if err != nil {
@@ -397,7 +402,10 @@ func (f *fileDialog) refreshDir(dir fyne.ListableURI) {
 			icons = append(icons, file)
 		}
 	}
+
+	f.dataLock.Lock()
 	f.data = icons
+	f.dataLock.Unlock()
 
 	f.files.Refresh()
 	f.filesScroll.Offset = fyne.NewPos(0, 0)
@@ -513,20 +521,26 @@ func (f *fileDialog) setView(view viewLayout) {
 	fyne.CurrentApp().Preferences().SetInt(viewLayoutKey, int(view))
 
 	count := func() int {
+		f.dataLock.RLock()
+		defer f.dataLock.RUnlock()
+
 		return len(f.data)
 	}
 	template := func() fyne.CanvasObject {
 		return f.newFileItem(storage.NewFileURI("./tempfile"), true, false)
 	}
 	update := func(id widget.GridWrapItemID, o fyne.CanvasObject) {
-		dir := f.data[id]
-		parent := id == 0 && len(dir.Path()) < len(f.dir.Path())
-		_, isDir := dir.(fyne.ListableURI)
-		o.(*fileDialogItem).setLocation(dir, isDir || parent, parent)
+		if dir, ok := f.getDataItem(id); ok {
+			parent := id == 0 && len(dir.Path()) < len(f.dir.Path())
+			_, isDir := dir.(fyne.ListableURI)
+			o.(*fileDialogItem).setLocation(dir, isDir || parent, parent)
+		}
 	}
 	choose := func(id int) {
-		f.selectedID = id
-		f.setSelected(f.data[id], id)
+		if file, ok := f.getDataItem(id); ok {
+			f.selectedID = id
+			f.setSelected(file, id)
+		}
 	}
 	if f.view == gridView {
 		grid := widget.NewGridWrap(count, template, update)
@@ -544,6 +558,17 @@ func (f *fileDialog) setView(view viewLayout) {
 	f.filesScroll.Refresh()
 }
 
+func (f *fileDialog) getDataItem(id int) (fyne.URI, bool) {
+	f.dataLock.RLock()
+	defer f.dataLock.RUnlock()
+
+	if id >= len(f.data) {
+		return nil, false
+	}
+
+	return f.data[id], true
+}
+
 // effectiveStartingDir calculates the directory at which the file dialog should
 // open, based on the values of startingDirectory, CWD, home, and any error
 // conditions which occur.