Follow-up on top of #65554 (@the3asic):
- Lower FTS5 'usermerge' to its minimum of 2 (persisted in the config
shadow table, applied once per SessionDB instance). Without this a
positive-rank 'merge' skips any level holding fewer than 4 segments
(SQLite FTS5 §6.8), so the fragmented-index case the cadence targets
never converges.
- Run up to _FTS_MERGE_COMMANDS_PER_PASS (4) bounded merge commands per
index per cadence, stopping early on the documented no-progress
signal (total_changes delta < 2). Each command is its own implicit
transaction, so the write lock is released between commands.
- Skip a missing messages_fts instead of raising: the chunked
optimize-storage rebuild legitimately drops + backfills FTS tables
while writers keep running; warning every 1000 writes for the whole
backfill window would be noise, and optimize_fts() has always
treated missing tables as skippable.
- Replace traced-SQL shape assertions with behavioral tests: real
fragmented-index convergence (automerge suppressed, 60 segments) and
a 3-segment below-default-usermerge compaction test that fails
against a bare positive-rank merge (sabotage-verified).
Validation on a sqlite3.backup() copy of a real 10.7 GB production
state.db (1.49M messages, 1.3 GB + 2.9 GB FTS shadow tables):
worst per-command write-lock hold 41.8 ms (was 9.2 s / 18.1 s per
index with 'optimize'), search results byte-identical, fts5
integrity-check and PRAGMA integrity_check clean, steady-state pass
0.0 ms.
Teknium
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