fix: Default breadcrumbs value for events without breadcrumbs

[alexander-alderman-webb]

Description

The breadcrumbs dictionary is normally initialized in Scope._apply_breadcrumbs_to_event():

sentry-python/sentry_sdk/scope.py

Lines 1379 to 1385 in b11c2f2

	def _apply_breadcrumbs_to_event(self, event, hint, options):
	# type: (Event, Hint, Optional[Dict[str, Any]]) -> None
	event.setdefault("breadcrumbs", {})
	# This check is just for mypy -
	if not isinstance(event["breadcrumbs"], AnnotatedValue):
	event["breadcrumbs"].setdefault("values", [])

However, the method is only invoked if the event is not a span or a check in:

sentry-python/sentry_sdk/scope.py

Lines 1550 to 1552 in b11c2f2

	if not is_transaction and not is_check_in:
	self._apply_breadcrumbs_to_event(event, hint, options)
	self._apply_flags_to_event(event, hint, options)

The user that reported the issue is filtering on a check in. Their suspicion that the AnnotatedValue created a default breadcrumbs value of the wrong type for check ins is correct.

Add the following snippet to tests/test_client.py to test. I didn't push this test because I don't see why we're adding truncated breadcrumbs metadata to spans and check ins in the first place.

@pytest.mark.parametrize(
    "sdk_options, expected_breadcrumbs",
    [({}, DEFAULT_MAX_BREADCRUMBS), ({"max_breadcrumbs": 50}, 50)],
)
def test_max_breadcrumbs_span_option(
    sentry_init, capture_events, sdk_options, expected_breadcrumbs
):
    sentry_init(traces_sample_rate=1.0, **sdk_options)
    events = capture_events()

    for _ in range(1231):
        add_breadcrumb({"type": "sourdough"})

    with start_transaction(name="transaction"):
        pass

    assert "values" in events[0]["breadcrumbs"]

Issues

Closes #4951

Reminders

• Please add tests to validate your changes, and lint your code using tox -e linters.
• Add GH Issue ID & Linear ID (if applicable)
• PR title should use conventional commit style (feat:, fix:, ref:, meta:)
• For external contributors: CONTRIBUTING.md, Sentry SDK development docs, Discord community

[sentrivana]

This is a good quick fix. There's a couple of things at play in the original issue:

• check-ins are actually not supposed to go through before_send at all, that's a bug, see also https://develop.sentry.dev/sdk/expected-features/#before-send-hook
• breadcrumbs afaik don't make sense for anything but error events (though we shouldn't populate the field with an invalid empty value in any case so this PR is a good improvement 👍🏻 )
• there's also the lesser known before_breadcrumb that could be used to sanitize breadcrumbs