Add better error handling for background processing thread startup

app.py

@@ -478,27 +478,60 @@ def upload_files():
     # Process files in background threads
     # Read file data before starting threads to avoid "read of closed file" error
     threads = []
+    started_count = 0
     for file in files:
         if file and file.filename.endswith('.pdf'):
-            # Read file data into memory before starting background thread
-            # This prevents "read of closed file" errors when request context ends
-            file_data = file.read()
-            file.seek(0)  # Reset file pointer for potential reuse
-            filename = file.filename
-            
-            thread = threading.Thread(
-                target=process_file_background,
-                args=(task_id, file_data, filename, extraction_mode)
-            )
-            thread.daemon = True
-            thread.start()
-            threads.append(thread)
+            try:
+                # Read file data into memory before starting background thread
+                # This prevents "read of closed file" errors when request context ends
+                file_data = file.read()
+                file.seek(0)  # Reset file pointer for potential reuse
+                filename = file.filename
+                
+                if not file_data:
+                    logger.warning(f"Empty file data for {filename}")
+                    with _progress_lock:
+                        _progress_tracker[task_id]['results'].append({
+                            'filename': filename,
+                            'error': 'File is empty or could not be read'
+                        })
+                    continue
+                
+                thread = threading.Thread(
+                    target=process_file_background,
+                    args=(task_id, file_data, filename, extraction_mode)
+                )
+                thread.daemon = True
+                thread.start()
+                threads.append(thread)
+                started_count += 1
+                logger.info(f"Started background thread for {filename}")
+            except Exception as e:
+                logger.error(f"Failed to start processing thread for {file.filename}: {e}")
+                import traceback
+                logger.error(traceback.format_exc())
+                with _progress_lock:
+                    _progress_tracker[task_id]['results'].append({
+                        'filename': file.filename,
+                        'error': f'Failed to start processing: {str(e)}'
+                    })
+    
+    # Check if any threads were started
+    if started_count == 0:
+        with _progress_lock:
+            _progress_tracker[task_id]['status'] = 'error'
+            _progress_tracker[task_id]['message'] = 'No files could be processed. Check file format and size.'
+        return jsonify({
+            'error': 'No files could be processed',
+            'task_id': task_id,
+            'message': 'Processing could not be started. Please check your files and try again.'
+        }), 400
     
     # Return task ID immediately
     return jsonify({
         'task_id': task_id,
         'message': 'Processing started',
-        'total_files': len(threads)
+        'total_files': started_count
     })