Core.AlarmHistorian-002 — drain loop now honors exponential backoff:
StartDrainLoop arms a self-rescheduling one-shot Timer. RescheduleDrain
sets the next due-time to max(tickInterval, CurrentBackoff) while the
sink is BackingOff, so a historian outage genuinely slows the cadence
down the 1s->2s->5s->15s->60s ladder instead of hammering at the fixed
tick. Class doc-comment updated.
Core.AlarmHistorian-004 — SQLite busy handling: the connection string
is built via SqliteConnectionStringBuilder with DefaultTimeout=5, and a
new OpenConnection helper applies PRAGMA busy_timeout=5000 and
PRAGMA journal_mode=WAL on every open. A concurrent enqueue-vs-drain
file-lock collision now waits the lock out instead of failing fast with
SQLITE_BUSY. All connection open sites switched to the helper.
Core.AlarmHistorian-006 — drain-loop faults are no longer unobserved:
the timer callback (DrainTimerCallback) awaits DrainOnceAsync inside a
try/catch that logs via _logger.Error, records the message into
_lastError, and sets _drainState=BackingOff so a stalled drain is
visible on GetStatus; a finally always re-arms the timer.
Regression tests added to SqliteStoreAndForwardSinkTests:
StartDrainLoop_honors_backoff_and_slows_cadence_under_retry,
StartDrainLoop_keeps_steady_cadence_when_writer_is_healthy,
StartDrainLoop_records_drain_fault_and_keeps_running,
Concurrent_enqueue_and_drain_do_not_throw_sqlite_busy.
findings.md: 002/004/006 marked Resolved; open count 10 -> 7.
Build: clean (0 warnings). Tests: 20/20 passing.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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