[libromdata] IFst::readdir(): Return const DirEnt* instead of DirEnt*.

This ensures the caller won't modify the DirEnt.

Note that this doesn't match POSIX readdir(), but we don't need to
strictly match it. (We're also not returning `struct dirent*`; we're
returning a custom one.)

src/libromdata/Console/GameCube.cpp

@@ -1550,10 +1550,11 @@ int GameCube::loadFieldData(void)
 			//   - 21.29: IOS version. (21.29 == v5405)
 			IFst::Dir *const dirp = d->updatePartition->opendir("/_sys/");
 			if (dirp) {
-				IFst::DirEnt *dirent;
+				const IFst::DirEnt *dirent;
 				while ((dirent = d->updatePartition->readdir(dirp)) != nullptr) {
-					if (!dirent->name || dirent->type != DT_REG)
+					if (!dirent->name || dirent->type != DT_REG) {
 						continue;
+					}
 
 					// Check for a retail System Menu.
 					if (dirent->name[0] == 'R') {

src/libromdata/disc/GcnFst.cpp

@@ -447,7 +447,7 @@ IFst::Dir *GcnFst::opendir(const char *path)
  * @return IFst::DirEnt*, or nullptr if end of directory or on error.
  * (End of directory does not set lastError; an error does.)
  */
-IFst::DirEnt *GcnFst::readdir(IFst::Dir *dirp)
+const IFst::DirEnt *GcnFst::readdir(IFst::Dir *dirp)
 {
 	assert(dirp != nullptr);
 	assert(dirp->parent == this);

src/libromdata/disc/GcnFst.hpp

@@ -68,7 +68,7 @@ public:
 	 * @return DirEnt*, or nullptr if end of directory or on error.
 	 * (TODO: Add lastError()?)
 	 */
-	DirEnt *readdir(Dir *dirp) final;
+	const DirEnt *readdir(Dir *dirp) final;
 
 	/**
 	 * Close an opened directory.

src/libromdata/disc/GcnPartition.cpp

@@ -2,7 +2,7 @@
  * ROM Properties Page shell extension. (libromdata)                       *
  * GcnPartition.cpp: GameCube partition reader.                            *
  *                                                                         *
- * Copyright (c) 2016-2024 by David Korth.                                 *
+ * Copyright (c) 2016-2025 by David Korth.                                 *
  * SPDX-License-Identifier: GPL-2.0-or-later                               *
  ***************************************************************************/
 
@@ -217,7 +217,7 @@ IFst::Dir *GcnPartition::opendir(const char *path)
  * @return IFst::DirEnt*, or nullptr if end of directory or on error.
  * (TODO: Add lastError()?)
  */
-IFst::DirEnt *GcnPartition::readdir(IFst::Dir *dirp)
+const IFst::DirEnt *GcnPartition::readdir(IFst::Dir *dirp)
 {
 	RP_D(GcnPartition);
 	if (!d->fst) {

src/libromdata/disc/GcnPartition.hpp

@@ -2,7 +2,7 @@
  * ROM Properties Page shell extension. (libromdata)                       *
  * GcnPartition.hpp: GameCube partition reader.                            *
  *                                                                         *
- * Copyright (c) 2016-2023 by David Korth.                                 *
+ * Copyright (c) 2016-2025 by David Korth.                                 *
  * SPDX-License-Identifier: GPL-2.0-or-later                               *
  ***************************************************************************/
 
@@ -107,7 +107,7 @@ public:
 	 * @return IFst::DirEnt, or nullptr if end of directory or on error.
 	 * (TODO: Add lastError()?)
 	 */
-	LibRpBase::IFst::DirEnt *readdir(LibRpBase::IFst::Dir *dirp) final;
+	const LibRpBase::IFst::DirEnt *readdir(LibRpBase::IFst::Dir *dirp) final;
 
 	/**
 	 * Close an opened directory.

src/libromdata/disc/WiiUFst.cpp

@@ -492,7 +492,7 @@ IFst::Dir *WiiUFst::opendir(const char *path)
  * @return IFst::DirEnt*, or nullptr if end of directory or on error.
  * (End of directory does not set lastError; an error does.)
  */
-IFst::DirEnt *WiiUFst::readdir(IFst::Dir *dirp)
+const IFst::DirEnt *WiiUFst::readdir(IFst::Dir *dirp)
 {
 	RP_D(WiiUFst);
 	assert(dirp != nullptr);

src/libromdata/disc/WiiUFst.hpp

@@ -67,7 +67,7 @@ public:
 	 * @return DirEnt*, or nullptr if end of directory or on error.
 	 * (TODO: Add lastError()?)
 	 */
-	DirEnt *readdir(Dir *dirp) final;
+	const DirEnt *readdir(Dir *dirp) final;
 
 	/**
 	 * Close an opened directory.

src/libromdata/tests/disc/FstPrint.cpp

@@ -71,7 +71,7 @@ static int fstPrint(IFst *fst, ostream &os, const string &path,
 	}
 
 	// Read the directory entries.
-	IFst::DirEnt *dirent = fst->readdir(dirp);
+	const IFst::DirEnt *dirent = fst->readdir(dirp);
 	while (dirent != nullptr) {
 		if (!dirent->name || dirent->name[0] == 0) {
 			// Empty name...
@@ -79,8 +79,7 @@ static int fstPrint(IFst *fst, ostream &os, const string &path,
 		}
 
 		// Print the tree lines.
-		for (int i = 0; i < level; i++)
-		{
+		for (int i = 0; i < level; i++) {
 			if (tree_lines[i]) {
 				// Directory tree exists for this segment.
 				os << "\xE2\x94\x82   ";

src/libromdata/tests/disc/GcnFstTest.cpp

@@ -357,7 +357,7 @@ void GcnFstTest::checkNoDuplicateFilenames(const char *subdir)
 	ASSERT_TRUE(dirp != nullptr) <<
 		"Failed to open directory '" << subdir << "'.";
 
-	IFst::DirEnt *dirent = m_fst->readdir(dirp);
+	const IFst::DirEnt *dirent = m_fst->readdir(dirp);
 	while (dirent != nullptr) {
 		// Make sure we haven't seen this filename in
 		// the current subdirectory yet.

src/librpbase/disc/IFst.hpp

@@ -2,7 +2,7 @@
  * ROM Properties Page shell extension. (librpbase)                        *
  * IFst.hpp: File System Table interface.                                  *
  *                                                                         *
- * Copyright (c) 2016-2024 by David Korth.                                 *
+ * Copyright (c) 2016-2025 by David Korth.                                 *
  * SPDX-License-Identifier: GPL-2.0-or-later                               *
  ***************************************************************************/
 
@@ -95,7 +95,7 @@ public:
 	 * @return DirEnt*, or nullptr if end of directory or on error.
 	 * (TODO: Add lastError()?)
 	 */
-	virtual DirEnt *readdir(Dir *dirp) = 0;
+	virtual const DirEnt *readdir(Dir *dirp) = 0;
 
 	/**
 	 * Close an opened directory.

src/librpbase/disc/IPartition.cpp

@@ -2,7 +2,7 @@
  * ROM Properties Page shell extension. (librpbase)                        *
  * IPartition.cpp: Partition reader interface.                             *
  *                                                                         *
- * Copyright (c) 2016-2024 by David Korth.                                 *
+ * Copyright (c) 2016-2025 by David Korth.                                 *
  * SPDX-License-Identifier: GPL-2.0-or-later                               *
  ***************************************************************************/
 
@@ -38,7 +38,7 @@ IFst::Dir *IPartition::opendir(const char *path)
  * @return IFst::DirEnt, or nullptr if end of directory or on error.
  * (TODO: Add lastError()?)
  */
-IFst::DirEnt *IPartition::readdir(IFst::Dir *dirp)
+const IFst::DirEnt *IPartition::readdir(IFst::Dir *dirp)
 {
 	RP_UNUSED(dirp);
 	assert(!"IFst wrapper functions are not implemented for this class!");

src/librpbase/disc/IPartition.hpp

@@ -2,7 +2,7 @@
  * ROM Properties Page shell extension. (librpbase)                        *
  * IPartition.hpp: Partition reader interface.                             *
  *                                                                         *
- * Copyright (c) 2016-2024 by David Korth.                                 *
+ * Copyright (c) 2016-2025 by David Korth.                                 *
  * SPDX-License-Identifier: GPL-2.0-or-later                               *
  ***************************************************************************/
 
@@ -87,7 +87,7 @@ public:
 	 * @return IFst::DirEnt, or nullptr if end of directory or on error.
 	 * (TODO: Add lastError()?)
 	 */
-	virtual IFst::DirEnt *readdir(IFst::Dir *dirp);
+	virtual const IFst::DirEnt *readdir(IFst::Dir *dirp);
 
 	/**
 	 * Close an opened directory.