Guard tag link against missing vCon (CON-617)

[howethomas]

Problem

The conserver tag link crashes with 'NoneType' object has no attribute 'add_tag' and drops the vCon to the DLQ whenever VconRedis.get_vcon(vcon_uuid) returns None.

get_vcon (common/lib/vcon_redis.py) is typed Optional[vcon.Vcon] and its docstring states None is the documented "halt the chain" contract for conserver links (Redis miss with no storage hit). The tag link had no None guard and called .add_tag straight away.

On BDS this fired ~376k times between 2026-06-07 and 2026-06-09, peaking ~126k per 10 min during a chain-latency incident (p95 processing ~2.5h) — delayed vCons had expired/evicted from Redis by the time the lagging tag link reached them.

Fix

Halt the chain on None, matching the existing tag_router link (conserver/links/tag_router/__init__.py:40 already does if not vcon: return None).

vCon = vcon_redis.get_vcon(vcon_uuid)
if vCon is None:
    logger.warning(f"tag: vCon {vcon_uuid} not found, halting chain")
    return None

Test

Adds test_run_halts_chain_when_vcon_missing mocking get_vcon → None, asserting run(...) returns None and store_vcon is never called. The existing tests never mocked a None return, which is why this stayed green.

All 4 tag tests pass locally.

Linear: CON-617

[Copilot]

Pull request overview

Guards the conserver tag link against missing vCons by honoring the documented VconRedis.get_vcon(...) -> Optional[Vcon] contract (None halts the chain) and adds a regression test to prevent crashes/ DLQ drops on Redis/storage misses.

Changes:

• Add a None guard in links.tag.run(...) to return None (halt chain) when get_vcon misses.
• Log when the vCon is missing instead of crashing with NoneType attribute access.
• Add a regression test asserting the chain halts and store_vcon is not called when get_vcon returns None.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
conserver/links/tag/init.py	Adds a None guard + log, returning None to halt the chain when a vCon can’t be loaded.
conserver/links/tag/test_tag.py	Adds a regression test covering the missing-vCon path.

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