Improve FileUtils.forceDelete() tests on Windows (#791)

* Improve `FileUtils.forceDelete()` tests on Windows

On Windows, the `DeleteFile` Win32 API has a little quirk: it refuses to delete files with the legacy **DOS read-only attribute** set. (Because apparently 99% of Windows users don’t realize that “deleting a file” is actually an operation on the *directory*, not the file itself 😉).

So the usual drill is: clear the read-only flag first, then delete.

* Until JDK 25, `File.delete()` did this for you behind the scenes: it quietly stripped the flag before calling into `DeleteFile`. That meant your file might be left behind (with the flag missing) if the *real* ACLs didn’t allow deletion.
* From JDK 25 onward, `File.delete()` doesn’t touch the flag anymore. If the bit is set, `DeleteFile` fails, end of story.
* `FileUtils.forceDelete()` already knows how to juggle the flag itself, so its behavior didn’t change.

This PR:

* Updates two tests that were (unfairly) comparing `File.delete` with `FileUtils.forceDelete`. With JDK 25, their expectations diverged.
* Adds a new test to confirm that `FileUtils.forceDelete` restores the read-only flag if the actual deletion fails.

> [!WARNING]
> I didn’t develop this on a Windows box, so I couldn’t test it locally. Leaving this as a **draft PR** until CI tells us whether Windows agrees with me.

* fix: always clear read-only bit

* fix: check for `IOException`, not `AccessDeniedException`

* feat: add holder for file and parent attributes

Introduce a helper that snapshots file and parent directory attributes before `setReadOnly` is applied.
If a deletion attempt fails, the holder can restore the original attributes to keep the filesystem state consistent.

* fix: Spotbugs failure

* fix: prefer POSIX to DOS attributes when both are available

On Linux/Unix, both POSIX and DOS attribute views may be supported,
while on Windows only DOS attributes are available.
Check for POSIX first to ensure the correct view is used across platforms.

* fix: revert read-only related changes

Author: ppkarwasz

src/test/java/org/apache/commons/io/FileUtilsTest.java

@@ -52,6 +52,7 @@
 import java.nio.file.Paths;
 import java.nio.file.StandardCopyOption;
 import java.nio.file.attribute.AclFileAttributeView;
+import java.nio.file.attribute.DosFileAttributeView;
 import java.nio.file.attribute.FileTime;
 import java.nio.file.attribute.PosixFilePermission;
 import java.nio.file.attribute.PosixFilePermissions;
@@ -1735,21 +1736,31 @@ void testForceDeleteReadOnlyDirectory() throws Exception {
     void testForceDeleteReadOnlyFile() throws Exception {
         try (TempFile destination = TempFile.create("test-", ".txt")) {
             final File file = destination.toFile();
-            assertTrue(file.setReadOnly());
-            assertTrue(file.canRead());
-            assertFalse(file.canWrite());
-            // sanity check that File.delete() deletes read-only files.
-            assertTrue(file.delete());
+            assertTrue(file.setReadOnly(), "Setting file read-only successful");
+            assertTrue(file.canRead(), "File must be readable");
+            assertFalse(file.canWrite(), "File must not be writable");
+            assertTrue(file.exists(), "File doesn't exist to delete");
+            // Since JDK 25 on Windows, File.delete() refuses to remove files
+            // with the DOS readonly bit set (JDK-8355954).
+            // We clear the bit here for consistency across JDK versions.
+            setDosReadOnly(file.toPath(), false);
+            assertTrue(file.delete(), "File.delete() must delete read-only file");
         }
         try (TempFile destination = TempFile.create("test-", ".txt")) {
             final File file = destination.toFile();
             // real test
-            assertTrue(file.setReadOnly());
-            assertTrue(file.canRead());
-            assertFalse(file.canWrite());
+            assertTrue(file.setReadOnly(), "Setting file read-only successful");
+            assertTrue(file.canRead(), "File must be readable");
+            assertFalse(file.canWrite(), "File must not be writable");
             assertTrue(file.exists(), "File doesn't exist to delete");
             FileUtils.forceDelete(file);
-            assertFalse(file.exists(), "Check deletion");
+            assertFalse(file.exists(), "FileUtils.forceDelete() must delete read-only file");
+        }
+    }
+
+    private static void setDosReadOnly(Path p, boolean readOnly) throws IOException {
+        if (Files.getFileStore(p).supportsFileAttributeView(DosFileAttributeView.class)) {
+            Files.setAttribute(p, "dos:readonly", readOnly, LinkOption.NOFOLLOW_LINKS);
         }
     }
 
@@ -1823,23 +1834,27 @@ void testForceDeleteUnwritableDirectory() throws Exception {
     void testForceDeleteUnwritableFile() throws Exception {
         try (TempFile destination = TempFile.create("test-", ".txt")) {
             final File file = destination.toFile();
-            assertTrue(file.canWrite());
-            assertTrue(file.setWritable(false));
-            assertFalse(file.canWrite());
-            assertTrue(file.canRead());
-            // sanity check that File.delete() deletes unwritable files.
-            assertTrue(file.delete());
+            assertTrue(file.canWrite(), "File must be writable");
+            assertTrue(file.setWritable(false), "Setting file unwritable successful");
+            assertFalse(file.canWrite(), "File must not be writable");
+            assertTrue(file.canRead(), "File must be readable");
+            assertTrue(file.exists(), "File must exist to delete");
+            // Since JDK 25 on Windows, File.delete() refuses to remove files
+            // with the DOS readonly bit set (JDK-8355954).
+            // We clear the bit here for consistency across JDK versions.
+            setDosReadOnly(file.toPath(), false);
+            assertTrue(file.delete(), "File.delete() must delete unwritable file");
         }
         try (TempFile destination = TempFile.create("test-", ".txt")) {
             final File file = destination.toFile();
             // real test
-            assertTrue(file.canWrite());
-            assertTrue(file.setWritable(false));
-            assertFalse(file.canWrite());
-            assertTrue(file.canRead());
-            assertTrue(file.exists(), "File doesn't exist to delete");
+            assertTrue(file.canWrite(), "File must be writable");
+            assertTrue(file.setWritable(false), "Setting file unwritable successful");
+            assertFalse(file.canWrite(), "File must not be writable");
+            assertTrue(file.canRead(), "File must be readable");
+            assertTrue(file.exists(), "File must exist to delete");
             FileUtils.forceDelete(file);
-            assertFalse(file.exists(), "Check deletion");
+            assertFalse(file.exists(), "FileUtils.forceDelete() must delete unwritable file");
         }
     }