fix(io): use atomic writes for .param, JSON, and settings files

[yashhzd]

Summary

All file-writing operations used a simple open("w") → write() pattern. If the application crashes, the OS kills the process, or power is lost mid-write, the target file is left truncated or empty.

This is especially dangerous for:

• .param files — uploading an incomplete parameter set to a flight controller could cause unpredictable behavior
• vehicle_components.json — a corrupted components file breaks all derived parameter computations on next startup
• settings.json — a truncated settings file causes JSONDecodeError, losing user preferences

Changes

Add a safe_write() utility (backend_safe_file_io.py) that writes to a temporary file in the same directory, then atomically replaces the target via os.replace(). The utility also:

• Flushes and fsyncs the file before replacing
• Preserves file permissions and ownership from the original target
• Fsyncs the parent directory after replace for full rename durability
• Cleans up temp files on failure

Applied to the four most critical persistence paths:

• ParDict.export_to_param() — .param files
• FilesystemJSONWithSchema.save_json_data() — JSON data files
• ProgramSettings._set_settings_from_dict() — settings.json
• VehicleComponents.save_component_templates_to_file() — vehicle templates

All save-related tests updated to patch safe_write directly and verify written data content via captured callbacks.

os.replace() is atomic on POSIX and near-atomic on Windows (MoveFileEx with MOVEFILE_REPLACE_EXISTING).

Continuation of #1430 (accidentally closed after force push).
Closes #1428

Signed-off-by: Yash Goel yashhzd@users.noreply.github.com

[Copilot]

Pull request overview

This PR introduces crash-safe, atomic file writes across several critical persistence paths in ArduPilot Methodic Configurator to prevent truncated/corrupted .param, JSON data, and settings.json files after interruptions.

Changes:

• Added a shared safe_write() helper that writes to a same-directory temp file and atomically replaces the target.
• Switched key save paths (.param, JSON-with-schema, settings, vehicle component templates) to use safe_write().
• Updated unit/integration tests to patch safe_write() instead of open() and validate behavior.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
ardupilot_methodic_configurator/backend_safe_file_io.py	New atomic write utility using temp file + os.replace(), fsync, and best-effort metadata preservation.
ardupilot_methodic_configurator/data_model_par_dict.py	.param export now writes via safe_write() callback.
ardupilot_methodic_configurator/backend_filesystem_json_with_schema.py	JSON persistence now uses safe_write() and preserves single trailing newline behavior.
ardupilot_methodic_configurator/backend_filesystem_program_settings.py	Settings persistence now uses safe_write() instead of direct open()/json_dump().
ardupilot_methodic_configurator/backend_filesystem_vehicle_components.py	Vehicle component template saving now uses safe_write().
tests/test_backend_filesystem_json_with_schema.py	Tests updated to patch safe_write() in save paths.
tests/test_backend_filesystem_program_settings.py	Tests updated to patch safe_write() for settings save.
tests/test_backend_filesystem_vehicle_components.py	Tests updated to patch safe_write() and capture written JSON via callback.
tests/unit_backend_filesystem_vehicle_components.py	Unit tests updated to patch safe_write() for error-path coverage.

[amilcarlucas]

The capture_safe_write function is defined 6 times. Can you find a way to define it only once and use it?
You could do it on the backend_safe_file_io.py or on the conftest.py file.

[amilcarlucas]

Thanks, I'll merge it now, it would be great if you could do the changes in a follow up PR

[yashhzd]

The capture_safe_write function is defined 6 times. Can you find a way to define it only once and use it? You could do it on the backend_safe_file_io.py or on the conftest.py file.

thanks for the review! please check out #1444