fix(file-sme): preserve file attachments when updating metadata

Previously, updating a File submodel element via PUT would unconditionally delete the associated file, even when the file path (value) did not change. This commit introduces logic to detect whether the file path has actually changed before deleting the file.

Changes:
- Added path comparison logic using java.nio.file.Path to handle OS-specific path formats.
- Only delete attachments if the path has changed.
- Introduced helper methods for clarity and testability: isFileElement, extractFilePath, getExistingFilePath, pathsDiffer.
- Added robust exception handling and conservative fallback (delete if uncertain).
- Added structured SLF4J logging for traceability.

This resolves unintended data loss during metadata-only updates to File submodel elements.

Author: antesch

basyx.submodelservice/basyx.submodelservice-backend/src/main/java/org/eclipse/digitaltwin/basyx/submodelservice/backend/CrudSubmodelService.java

@@ -38,10 +38,14 @@
 import org.eclipse.digitaltwin.basyx.operation.InvokableOperation;
 import org.eclipse.digitaltwin.basyx.submodelservice.SubmodelService;
 import org.eclipse.digitaltwin.basyx.submodelservice.value.SubmodelElementValue;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 import org.springframework.lang.NonNull;
 
 import java.io.File;
 import java.io.InputStream;
+import java.nio.file.Path;
+import java.nio.file.Paths;
 import java.util.List;
 
 /**
@@ -54,6 +58,7 @@ public class CrudSubmodelService implements SubmodelService {
     private final SubmodelBackend backend;
     private final String submodelId;
     private final SubmodelFileOperations submodelFileOperations;
+    private Logger logger = LoggerFactory.getLogger(CrudSubmodelService.class);
 
     public CrudSubmodelService(SubmodelBackend submodelRepositoryBackend, FileRepository fileRepository, @NonNull Submodel submodel) {
         this(submodelRepositoryBackend, fileRepository, submodel.getId());
@@ -101,13 +106,73 @@ public void createSubmodelElement(String idShortPath, SubmodelElement submodelEl
         backend.createSubmodelElement(submodelId, idShortPath, submodelElement);
     }
 
-    @Override
+   @Override
     public void updateSubmodelElement(String idShortPath, SubmodelElement submodelElement) throws ElementDoesNotExistException {
-        deleteAssociatedFileIfAny(idShortPath);
-
+        if (isFileElement(submodelElement)) {
+            handleFileAttachmentUpdate(submodelId, idShortPath, submodelElement);
+        }
         backend.updateSubmodelElement(submodelId, idShortPath, submodelElement);
     }
 
+    private void handleFileAttachmentUpdate(String submodelId, String idShortPath, SubmodelElement updatedElement) {
+        try {
+            if (hasFilePathChanged(submodelId, idShortPath, updatedElement)) {
+                deleteAssociatedFileIfAny(idShortPath);
+            }
+        } catch (Exception e) {
+            logger.warn("Could not verify file path change for '{}'. Assuming changed. Reason: {}", idShortPath, e.getMessage());
+            deleteAssociatedFileIfAny(idShortPath);
+        }
+    }
+
+    private boolean hasFilePathChanged(String submodelId, String idShortPath, SubmodelElement updatedElement) {
+        if (!(updatedElement instanceof org.eclipse.digitaltwin.aas4j.v3.model.File file)) {
+            logger.warn("Expected File element but got: {}", updatedElement.getClass().getSimpleName());
+            return true;
+        }
+
+        String newPathString = extractFilePath(file);
+        if (newPathString == null) {
+            return true;
+        }
+
+        String oldPathString = getExistingFilePath(submodelId, idShortPath);
+        if (oldPathString == null) {
+            return true;
+        }
+
+        return pathsDiffer(oldPathString, newPathString);
+    }
+
+    private boolean isFileElement(SubmodelElement element) {
+        return element instanceof org.eclipse.digitaltwin.aas4j.v3.model.File;
+    }
+
+    // Extracts the new file path (value) from the updated File element
+    private String extractFilePath(org.eclipse.digitaltwin.aas4j.v3.model.File fileElement) {
+        return fileElement.getValue();
+    }
+
+    private String getExistingFilePath(String submodelId, String idShortPath) {
+        try {
+            return submodelFileOperations.getFile(submodelId, idShortPath).getPath();
+        } catch (Exception e) {
+            logger.warn("Failed to retrieve existing file path for '{}': {}", idShortPath, e.getMessage(), e);
+            return null;
+        }
+    }
+
+    private boolean pathsDiffer(String path1, String path2) {
+        try {
+            Path normalized1 = Paths.get(path1).normalize().toAbsolutePath();
+            Path normalized2 = Paths.get(path2).normalize().toAbsolutePath();
+            return !normalized1.equals(normalized2);
+        } catch (Exception e) {
+            logger.error("Error comparing file paths: {}", e.getMessage(), e);
+            return true; // Consider paths different in case of error
+        }
+    }
+
     @Override
     public void deleteSubmodelElement(String idShortPath) throws ElementDoesNotExistException {
         deleteAssociatedFileIfAny(idShortPath);