_create_protective_aliases scanned every image_record's tagger_predictions JSON
(unindexed full scan, ~59k rows) to find the categories a merged-away tag's name
was predicted under. That scan ran inside the merge transaction AFTER it had
locked series_page — on a large library it held that lock for minutes and is what
blocked migration 0040 (and starved the standardization task into its 40-min
timeout).
The scan was redundant: the tagger's tag_to_category map is one-to-one (a name has
exactly one category) and a tag's kind is set from that category when created, so
kind already IS the tagger's category for the name. The scan only ever rediscovered
the kind. Build the single protective alias from src_kind directly — no scan, no
lock-holding slow step in the merge.
Rewrote test_alias_per_observed_prediction_category (which encoded the
can't-actually-happen one-name-two-categories case) → test_protective_alias_uses_tag_kind.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Reverses the advisory-lock approach (7309d1d) — it treated a replica race that
wasn't the cause and added a new indefinite-hang mode (a sibling/stale migrator
holding the xact lock).
Real cause of the 0040 hang (operator-diagnosed 2026-06-07): web has always been
a single replica. The migration's ALTER series_page queued behind a concurrent
tag-merge that held a series_page lock for minutes — _do_merge repoints
series_page then runs _create_protective_aliases, an unindexed full scan of
image_record (JSON column, ~59k rows). Migrations ran with no lock_timeout, so
the DDL hung indefinitely and silently.
Fix: SET lock_timeout (default 30s, env-overridable) on the migration connection
before alembic's transaction. A blocked DDL now fails fast with 'canceling
statement due to lock timeout'; the entrypoint exits non-zero so the deploy
retries / surfaces loudly instead of wedging. General protection for every
future migration. (The slow _create_protective_aliases scan — the actual lock
holder — is the separate perf fix still under discussion.)
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
bvandeusen