[stable-3.30] fix: FilesSyncHelper: Shuffle array after null check, else NullPointerException is thrown

app/src/main/java/com/owncloud/android/utils/FilesSyncHelper.java

@@ -72,8 +72,8 @@ public static void walkFileTreeRandomly(Path start, FileVisitor<? super Path> vi
                 FileVisitResult preVisitDirectoryResult = visitor.preVisitDirectory(start, (BasicFileAttributes)null);
                 if (preVisitDirectoryResult == FileVisitResult.CONTINUE) {
                     File[] children = start.toFile().listFiles();
-                    Collections.shuffle(Arrays.asList(children));
                     if (children != null) {
+                        Collections.shuffle(Arrays.asList(children));
                         File[] var5 = children;
                         int var6 = children.length;
 

app/src/test/java/com/owncloud/android/utils/FilesSyncHelperTest.java

@@ -0,0 +1,79 @@
+/*
+ * Nextcloud - Android Client
+ *
+ * SPDX-FileCopyrightText: 2024 Benedek Major <[email protected]>
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+package com.owncloud.android.utils;
+
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import org.lukhnos.nnio.file.FileVisitResult;
+import org.lukhnos.nnio.file.Path;
+import org.lukhnos.nnio.file.Paths;
+import org.lukhnos.nnio.file.SimpleFileVisitor;
+import org.lukhnos.nnio.file.attribute.BasicFileAttributes;
+import org.mockito.Mock;
+import org.mockito.Mockito;
+
+
+import java.io.File;
+import java.io.IOException;
+import java.io.PrintWriter;
+import java.io.StringWriter;
+import java.util.Arrays;
+
+import static com.owncloud.android.utils.FilesSyncHelper.walkFileTreeRandomly;
+import static org.junit.Assert.fail;
+import static org.mockito.Mockito.mock;
+
+
+
+public class FilesSyncHelperTest {
+    @Mock
+    private File mockFile;
+
+    @Mock
+    private Path mockPath;
+
+    @Test
+    public void testWalkFileTreeNullHandling(){
+        mockFile = mock(File.class);
+        mockPath = mock(Path.class);
+        Mockito.when(mockFile.isDirectory()).thenReturn(true);
+        Mockito.when(mockPath.toFile()).thenReturn(mockFile);
+        Mockito.when(mockFile.listFiles()).thenReturn(null);
+        Mockito.when(mockFile.canRead()).thenReturn(true);
+
+        SimpleFileVisitor<Path> visitor = new SimpleFileVisitor<Path>() {
+            @Override
+            public FileVisitResult visitFile(Path path, BasicFileAttributes attrs) {
+                return FileVisitResult.CONTINUE;
+            }
+
+            @Override
+            public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs) {
+                return FileVisitResult.CONTINUE;
+            }
+
+            @Override
+            public FileVisitResult visitFileFailed(Path file, IOException exc) {
+                return FileVisitResult.CONTINUE;
+            }
+        };
+        try{
+            walkFileTreeRandomly(mockPath,visitor);
+        } catch (Exception e){
+            StringWriter writer = new StringWriter();
+            PrintWriter printWriter = new PrintWriter( writer );
+            e.printStackTrace( printWriter );
+            printWriter.flush();
+
+            String stackTrace = writer.toString();
+            fail("walkFileTree throws an exception: \n" + stackTrace);
+        }
+    }
+
+}