[#11893] fix(lance): Add external table guard to deregisterTable

lance/lance-common/src/main/java/org/apache/gravitino/lance/common/ops/gravitino/GravitinoLanceTableOperations.java

@@ -281,7 +281,21 @@ public DeregisterTableResponse deregisterTable(String tableId, String delimiter)
           "Table not found: " + tableId, CommonUtil.formatCurrentStackTrace(), tableId);
     }
     Map<String, String> properties = t.properties();
-    // TODO Support real deregister API.
+
+    // Verify the table is external before deregistering. For non-external (managed) tables,
+    // dropTable would delete the underlying Lance data, violating the deregister contract
+    // ("It will not delete the underlying lance data"). The TableCatalog interface does not
+    // expose a metadata-only removal path, so we fail fast rather than risk silent data loss.
+    boolean external =
+        Optional.ofNullable(properties.get(Table.PROPERTY_EXTERNAL))
+            .map(Boolean::parseBoolean)
+            .orElse(false);
+    if (!external) {
+      throw new UnsupportedOperationException(
+          "deregisterTable only supports external tables, but table " + tableId + " is managed");
+    }
+
+    // External tables: dropTable removes catalog metadata only, preserving Lance data.
     boolean result = catalog.asTableCatalog().dropTable(tableIdentifier);
     if (!result) {
       throw new TableNotFoundException(

lance/lance-rest-server/src/test/java/org/apache/gravitino/lance/service/rest/TestGravitinoLanceTableOperations.java

@@ -21,11 +21,17 @@
 
 import static org.apache.gravitino.lance.common.utils.LanceConstants.LANCE_TABLE_VERSION;
 
+import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
+import org.apache.gravitino.Catalog;
+import org.apache.gravitino.NameIdentifier;
+import org.apache.gravitino.lance.common.ops.gravitino.GravitinoLanceNamespaceWrapper;
 import org.apache.gravitino.lance.common.ops.gravitino.GravitinoLanceTableAlterHandler.AlterColumnsGravitinoLance;
 import org.apache.gravitino.lance.common.ops.gravitino.GravitinoLanceTableAlterHandler.DropColumns;
+import org.apache.gravitino.lance.common.ops.gravitino.GravitinoLanceTableOperations;
 import org.apache.gravitino.rel.Table;
+import org.apache.gravitino.rel.TableCatalog;
 import org.apache.gravitino.rel.TableChange;
 import org.junit.jupiter.api.Assertions;
 import org.junit.jupiter.api.Test;
@@ -92,4 +98,32 @@ void testAlterColumnsHandlerRejectsUnsupportedFields() {
     Assertions.assertEquals(
         "Only RENAME alteration is supported currently.", exception.getMessage());
   }
+
+  @Test
+  void testDeregisterTableRejectsManagedTable() {
+    // Mock a managed table (no PROPERTY_EXTERNAL=true)
+    Table managedTable = Mockito.mock(Table.class);
+    Mockito.when(managedTable.properties()).thenReturn(new HashMap<>());
+
+    TableCatalog tableCatalog = Mockito.mock(TableCatalog.class);
+    Mockito.when(tableCatalog.loadTable(Mockito.any(NameIdentifier.class)))
+        .thenReturn(managedTable);
+
+    Catalog catalog = Mockito.mock(Catalog.class);
+    Mockito.when(catalog.asTableCatalog()).thenReturn(tableCatalog);
+
+    GravitinoLanceNamespaceWrapper wrapper = Mockito.mock(GravitinoLanceNamespaceWrapper.class);
+    Mockito.when(wrapper.loadAndValidateLakehouseCatalog(Mockito.anyString())).thenReturn(catalog);
+
+    GravitinoLanceTableOperations ops = new GravitinoLanceTableOperations(wrapper);
+
+    UnsupportedOperationException exception =
+        Assertions.assertThrows(
+            UnsupportedOperationException.class,
+            () -> ops.deregisterTable("catalog.schema.table", "."));
+    Assertions.assertTrue(exception.getMessage().contains("only supports external tables"));
+
+    // Verify dropTable was never called — the guard must reject before reaching the catalog layer.
+    Mockito.verify(tableCatalog, Mockito.never()).dropTable(Mockito.any());
+  }
 }