fix(crons): Return None from capture_checkin when nothing is sent (#7868)

### Description

`capture_checkin` returned a check-in id even when the check-in wasn't
sent, because it ignored the return value of `capture_event`. That
happens when there is no active client, or when `before_send` drops the
event. A caller that hands the id to another process (as in the issue:
the scheduler opens the check-in, the worker closes it, and opens its
own only when no id arrives) can't tell that the check-in never reached
Sentry.

It now returns `None` in that case, the same way `capture_event` does.
`monitor` and the Celery Beat integration pass the returned id into the
closing `capture_checkin` call, which generates a new id when it gets
`None`, so they behave as before. The return type becomes
`Optional[str]`, so type-checked code that uses the result as a `str`
will now get a mypy error until it handles `None`.
`test_capture_checkin_sdk_not_initialized` expected the passed-in id
back when the SDK isn't initialized and now expects `None`.

The alternative was keeping the `str` return type and adding a separate
way to ask whether the check-in went out, but `None` matches
`capture_event` and needs no new API.

#### Issues

* resolves: #7845
* resolves: PY-2924

Author: HardMax71

sentry_sdk/crons/api.py

@@ -42,7 +42,7 @@ def capture_checkin(
     status: "Optional[str]" = None,
     duration: "Optional[float]" = None,
     monitor_config: "Optional[MonitorConfig]" = None,
-) -> str:
+) -> "Optional[str]":
     check_in_event = _create_check_in_event(
         monitor_slug=monitor_slug,
         check_in_id=check_in_id,
@@ -51,7 +51,8 @@ def capture_checkin(
         monitor_config=monitor_config,
     )
 
-    sentry_sdk.capture_event(check_in_event)
+    if sentry_sdk.capture_event(check_in_event) is None:
+        return None
 
     logger.debug(
         f"[Crons] Captured check-in ({check_in_event.get('check_in_id')}): {check_in_event.get('monitor_slug')} -> {check_in_event.get('status')}"

tests/test_crons.py

@@ -321,7 +321,42 @@ def test_capture_checkin_sdk_not_initialized():
         status=None,
         duration=None,
     )
-    assert check_in_id == "112233"
+    assert check_in_id is None
+
+
+def test_capture_checkin_dropped_by_before_send(sentry_init, capture_envelopes):
+    sentry_init(before_send=lambda event, hint: None)
+    envelopes = capture_envelopes()
+
+    check_in_id = capture_checkin(monitor_slug="abc123", status="in_progress")
+
+    assert check_in_id is None
+    assert envelopes == []
+
+
+def test_context_monitor_exit_successfully_sends_checkin_when_enter_checkin_dropped(
+    sentry_init, capture_envelopes
+):
+    sentry_init(
+        before_send=lambda event, hint: (
+            None if event.get("status") == "in_progress" else event
+        )
+    )
+    envelopes = capture_envelopes()
+
+    monitor = sentry_sdk.monitor(monitor_slug="abc123")
+    with monitor:
+        # Confirms that the check in was dropped in the `__enter__`
+        assert monitor.check_in_id is None
+
+    (envelope,) = envelopes
+    check_in = envelope.items[0].payload.json
+
+    # Confirms that the second check in in `__exit__` works correctly
+    assert check_in["monitor_slug"] == "abc123"
+    assert check_in["check_in_id"]
+    assert check_in["status"] == "ok"
+    assert check_in["duration"] > 0
 
 
 def test_scope_data_in_checkin(sentry_init, capture_envelopes):