# FastAPI current-status step

Python/FastAPI implementation for step 3. It keeps one in-memory completed result per target behind a single `asyncio.Lock` and emits `current_status_implementation` while preserving all prior contract outputs.

## Integrate with the prior service/checker

Create one coordinator for the FastAPI process and include its router:

```python
coordinator = CurrentStatusCoordinator()
app.include_router(build_current_status_router(coordinator))
```

The prior target-create path must call `await coordinator.register_target(target_id)`. The prior target-delete path must call `await coordinator.delete_target(target_id)`. The scheduler/check path calls:

```python
published = await coordinator.run_check(target_id, target, prior_checker)
```

`prior_checker` is an async callable returning a JSON-compatible mapping. `published=False` means the result became stale. These lifecycle hooks are deliberately explicit so target deletion and status cleanup share the coordinator's critical section.

## REST contract

`GET /targets/{target_id}/status` returns:

* `404` with `{"detail":{"code":"target_not_found","target_id":"..."}}` when the target does not exist.
* `200` with `state="not_checked"` and `result=null` when it exists but has no completed check.
* `200` with `state="checked"` and the latest result otherwise.

Only one `_statuses[target_id]` value can exist. A UUID incarnation prevents an in-flight check from publishing after delete/recreate. A per-incarnation sequence prevents an older overlapping check from replacing a newer-started check. Deletion removes status while holding the same lock.

## Contract transformer

```bash
python -m current_status < input.json > output.json
```

The three input objects are deep-copied unchanged and `current_status_implementation` is appended.

## Tests and exact verification command

Files: `tests/test_api.py`, `tests/test_concurrency.py`, and `tests/test_transform.py`.

```bash
python -m pip install -e '.[test]'
python -m pytest -q
```

The tests audit latest-only retention, REST not-found/no-check/latest responses, deletion cleanup, overlapping-check ordering, delete/recreate resurrection prevention, and output preservation/emission.

## Execution evidence

No shell/test runner is available to the generation call, so this commit does **not** claim that pytest was executed there. The exact reproducible command is above; committed tests are the auditable evidence supplied by this generation step.