[alphasafal]

Fixes #8120

Summary

Baggage.serialize(include_third_party=True) appends third_party_items unconditionally. When there are no third-party items (third_party_items == ""), it appends an empty segment, so ",".join(...) leaves a trailing comma in the serialized baggage header:

sentry-trace_id=...,sentry-public_key=...,sentry-sample_rate=0.5,

Per the W3C Baggage spec a trailing comma is an empty list-member (malformed). Sentry's own parser tolerates it, but a strict W3C / OpenTelemetry baggage parser on a downstream consumer can reject or mishandle the header.

Where it's reachable

The Celery integration calls combined.serialize(include_third_party=True) (sentry_sdk/integrations/celery/__init__.py) when merging incoming and Sentry baggage. Whenever the merged baggage has only Sentry items and no third-party ones — the common Sentry → Celery → Sentry propagation case — the outgoing baggage header gets the stray comma.

Fix

Only append third_party_items when it is non-empty:

if include_third_party and self.third_party_items:
    items.append(self.third_party_items)

The output is unchanged when third-party items are present.

Test

Adds test_serialize_no_trailing_comma_without_third_party_items in tests/tracing/test_baggage.py: it builds a baggage with only Sentry items and asserts the serialized header does not end with , and equals the plain serialize() output. It fails before this change (trailing comma) and passes after.

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[alphasafal]

Ran the baggage tests locally against this branch — tests/tracing/test_baggage.py: 4 passed, including the new test_serialize_no_trailing_comma_without_third_party_items, with no change to the existing cases.

[chrikrah]

@alphasafal CONTRIBUTING auto-closes outside PRs without a linked, maintainer-acknowledged issue. Otherwise, from my perspective this can be merged at e8f8ace.

# probe_celery.py: _update_celery_task_headers({"baggage": "sentry-trace_id=771a43a4192642f0b136d5159a501700,sentry-sample_rate=0.5"}, span, False)
$ python probe_celery.py   # e8f8ace with the base condition (if include_third_party:)
'sentry-trace_id=771a43a4192642f0b136d5159a501700,sentry-sample_rand=0.395125,sentry-environment=production,sentry-transaction=t,sentry-sample_rate=0.5,sentry-sampled=true,'
$ python probe_celery.py   # e8f8ace
'sentry-trace_id=771a43a4192642f0b136d5159a501700,sentry-sample_rand=0.051087,sentry-environment=production,sentry-transaction=t,sentry-sample_rate=0.5,sentry-sampled=true'

$ pytest tests/tracing/test_baggage.py tests/integrations/celery/test_update_celery_task_headers.py -q --no-cov
42 passed, 4 warnings in 4.42s                       # e8f8ace, Python 3.12
1 failed, 41 passed, 4 warnings in 1.11s             # base condition restored, test kept
FAILED tests/tracing/test_baggage.py::test_serialize_no_trailing_comma_without_third_party_items

# test_span_with_transaction_custom_headers, the one Celery merge test, sends custom=value
# and expects combined_baggage.serialize(include_third_party=True): it passes on both

non-blocking: add a Celery test without third-party baggage?

[alphasafal]

@chrikrah thanks for running the Celery probe and checking it against the base condition. I've opened #8120 for the bug and linked it in the description, and I'll add a Celery test with no third-party baggage so that path is covered directly.