Report an unlistable directory as IOException instead of NullPointerException

src/main/java/org/codehaus/plexus/util/FileUtils.java

@@ -1270,6 +1270,9 @@
         IOException exception = null;
 
         final File[] files = directory.listFiles();
+        if (files == null) {
+            throw new IOException("Could not list directory " + directory.getAbsolutePath());
+        }
         for (final File file : files) {
             try {
                 forceDeleteOnExit(file);
@@ -1762,6 +1765,9 @@
         }
 
         File[] files = sourceDirectory.listFiles();
+        if (files == null) {
+            throw new IOException("Could not list directory " + sourceDirectory.getAbsolutePath());
+        }
 
         String sourcePath = sourceDirectory.getAbsolutePath();
 
@@ -1922,9 +1928,9 @@
                 fileReader = null;
             } finally {
                 //noinspection deprecation
                 IOUtil.close(fileReader);
                 //noinspection deprecation
                 IOUtil.close(fileWriter);
             }
         } else {
             if (isSourceNewerThanDestination(from, to) || overwrite) {

src/test/java/org/codehaus/plexus/util/FileUtilsTest.java

@@ -36,6 +36,7 @@
 import org.junit.jupiter.api.condition.OS;
 
 import static org.junit.jupiter.api.Assertions.*;
+import static org.junit.jupiter.api.Assumptions.assumeTrue;
 
 /**
  * This is used to test FileUtils for correctness.
@@ -679,6 +680,21 @@ void removeExtensionWithPaths() {
         }
     }
 
+    @Test
+    void copyDirectoryStructureFailsWhenSourceCannotBeListed() throws Exception {
+        File from = new File(getTestDirectory(), "unlistable");
+        from.mkdirs();
+        File to = new File(getTestDirectory(), "unlistable-copy");
+        assumeTrue(from.setReadable(false) && from.setExecutable(false) && from.listFiles() == null);
+        try {
+            IOException e = assertThrows(IOException.class, () -> FileUtils.copyDirectoryStructure(from, to));
+            assertTrue(e.getMessage().contains(from.getAbsolutePath()));
+        } finally {
+            from.setExecutable(true);
+            from.setReadable(true);
+        }
+    }
+
     @Test
     void copyDirectoryStructureWithAEmptyDirectoryStructure() throws Exception {
         File from = new File(getTestDirectory(), "from");