capture_checkin returns a check-in id even when the check-in was not sent #7845

How do you use Sentry?

Sentry Saas (sentry.io)

Version

2.71.0 (same on 2.48.0)

Steps to Reproduce

import sentry_sdk
from sentry_sdk.crons import capture_checkin
from sentry_sdk.transport import Transport

# 1. no active client
print(sentry_sdk.capture_event({"message": "x"}))                     # None
print(capture_checkin(monitor_slug="nightly", status="in_progress"))  # '19fe1f76b846410381dd7937594d1ceb'

# 2. check-in dropped by before_send
sent = []

class RecordingTransport(Transport):
    def capture_envelope(self, envelope):
        sent.append(envelope)

sentry_sdk.init(dsn="https://public@example.invalid/1", transport=RecordingTransport,
                before_send=lambda event, hint: None)
print(capture_checkin(monitor_slug="nightly", status="in_progress"))  # '396229524fde41ce9cf4755cae73da0c'
sentry_sdk.flush()
print(len(sent))                                                      # 0

Expected Result

The caller can tell that the check-in didn't go out, the same way capture_event returns None when the event is dropped. For example, capture_checkin returns None in that case.

Actual Result

capture_checkin ignores the return value of capture_event and always returns the generated (or passed-in) check_in_id.

This matters when the check-in is opened in one process and closed in another. We run scheduled Dramatiq jobs: the scheduler opens the in_progress check-in and passes the id to the worker in the message, and the worker opens its own check-in only when no id arrives. When the scheduler has no active client, the worker still gets an id, skips opening its own, and closes a check-in that never reached Sentry. We now check sentry_sdk.get_client().is_active() before calling capture_checkin, but that doesn't cover a check-in dropped in before_send. The Celery Beat integration hands the id over the same way (sentry-monitor-check-in-id).

A possible change: return None from capture_checkin when capture_event returns None, so the return type becomes Optional[str]. monitor and the Celery Beat integration keep working, because capture_checkin(check_in_id=None) generates a fresh id. One existing test would change: test_capture_checkin_sdk_not_initialized asserts that the passed-in id comes back when the SDK is not initialized. I have this change with tests ready locally (tests/test_crons.py and the Celery Beat crons tests pass, mypy is clean) and can open a PR if the approach is OK with you.