[test] Add FileSystemBackupDescriptor resource counting tests

Verify filesystem backups count resources from __contents__.xml only,
including nested collection descriptors, without parsing binary payloads.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: duncdrum

exist-core/src/test/java/org/exist/backup/FileSystemBackupDescriptorTest.java

@@ -0,0 +1,103 @@
+/*
+ * eXist-db Open Source Native XML Database
+ * Copyright (C) 2001 The eXist-db Authors
+ *
+ * info@exist-db.org
+ * http://www.exist-db.org
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+ */
+package org.exist.backup;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+import static java.nio.charset.StandardCharsets.UTF_8;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class FileSystemBackupDescriptorTest {
+
+    private static final String CONTENTS_XML = """
+            <?xml version="1.0" encoding="UTF-8"?>
+            <exist:collection xmlns:exist="http://exist.sourceforge.net/NS/exist">
+                <exist:resource name="doc1.xml" type="XMLResource" filename="doc1.xml"/>
+                <exist:resource name="test.binary" type="BinaryResource" filename="test.binary"/>
+            </exist:collection>
+            """;
+
+    private static final String NESTED_CONTENTS_XML = """
+            <?xml version="1.0" encoding="UTF-8"?>
+            <exist:collection xmlns:exist="http://exist.sourceforge.net/NS/exist">
+                <exist:resource name="nested.xml" type="XMLResource" filename="nested.xml"/>
+            </exist:collection>
+            """;
+
+    @Test
+    void countsResourcesFromContentsXmlOnly(@TempDir final Path tempDir) throws Exception {
+        final Path backupRoot = tempDir.resolve("backup");
+        final Path dbDir = backupRoot.resolve("db");
+        Files.createDirectories(dbDir);
+
+        Files.writeString(dbDir.resolve(BackupDescriptor.COLLECTION_DESCRIPTOR), CONTENTS_XML, UTF_8);
+        Files.writeString(dbDir.resolve("doc1.xml"), "<test/>", UTF_8);
+        Files.write(dbDir.resolve("test.binary"), "test".getBytes(UTF_8));
+
+        final FileSystemBackupDescriptor descriptor = new FileSystemBackupDescriptor(
+                backupRoot,
+                dbDir.resolve(BackupDescriptor.COLLECTION_DESCRIPTOR));
+
+        assertEquals(2, descriptor.getNumberOfFiles());
+    }
+
+    @Test
+    void countsNestedCollectionDescriptorsInSubtree(@TempDir final Path tempDir) throws Exception {
+        final Path backupRoot = tempDir.resolve("backup");
+        final Path dbDir = backupRoot.resolve("db");
+        final Path nestedDir = dbDir.resolve("nested");
+        Files.createDirectories(nestedDir);
+
+        Files.writeString(dbDir.resolve(BackupDescriptor.COLLECTION_DESCRIPTOR), CONTENTS_XML, UTF_8);
+        Files.writeString(nestedDir.resolve(BackupDescriptor.COLLECTION_DESCRIPTOR), NESTED_CONTENTS_XML, UTF_8);
+        Files.writeString(dbDir.resolve("doc1.xml"), "<test/>", UTF_8);
+        Files.write(dbDir.resolve("test.binary"), "test".getBytes(UTF_8));
+        Files.writeString(nestedDir.resolve("nested.xml"), "<nested/>", UTF_8);
+
+        final FileSystemBackupDescriptor descriptor = new FileSystemBackupDescriptor(
+                backupRoot,
+                dbDir.resolve(BackupDescriptor.COLLECTION_DESCRIPTOR));
+
+        assertEquals(3, descriptor.getNumberOfFiles());
+    }
+
+    @Test
+    void collectionDescriptorCountsOwnSubtreeOnly(@TempDir final Path tempDir) throws Exception {
+        final Path backupRoot = tempDir.resolve("backup");
+        final Path dbDir = backupRoot.resolve("db");
+        final Path nestedDir = dbDir.resolve("nested");
+        Files.createDirectories(nestedDir);
+
+        Files.writeString(dbDir.resolve(BackupDescriptor.COLLECTION_DESCRIPTOR), CONTENTS_XML, UTF_8);
+        Files.writeString(nestedDir.resolve(BackupDescriptor.COLLECTION_DESCRIPTOR), NESTED_CONTENTS_XML, UTF_8);
+
+        final FileSystemBackupDescriptor nestedDescriptor = new FileSystemBackupDescriptor(
+                backupRoot,
+                nestedDir.resolve(BackupDescriptor.COLLECTION_DESCRIPTOR));
+
+        assertEquals(1, nestedDescriptor.getNumberOfFiles());
+    }
+}