Adds RingBufferFallback — an in-memory drop-oldest ring buffer used by
the upcoming FallbackAuditWriter (Bundle B-T4) when the primary SQLite
writer is throwing. Backed by Channel<AuditEvent> with
BoundedChannelFullMode.DropOldest, fixed capacity (default 1024).
Channel.CreateBounded(DropOldest) does NOT natively signal a drop on
TryWrite, so overflow is detected by comparing Reader.Count before and
after the enqueue: when the buffer is already at capacity and a new
TryWrite succeeds while keeping the count at capacity, exactly one
event was displaced and RingBufferOverflowed is raised (one event per
drop).
Public surface:
- bool TryEnqueue(AuditEvent) — always succeeds unless completed.
- IAsyncEnumerable<AuditEvent> DrainAsync(CancellationToken) — FIFO.
- void Complete() — closes the channel so DrainAsync can finish.
- event Action? RingBufferOverflowed — health counter hook.
Tests (3 new, total 23 -> 26):
- Enqueue_1025_Into_1024Cap_Ring_DropsOldest_AndRaisesOverflowOnce
- DrainAsync_Yields_FIFO_Then_Completes_When_Empty
- TryEnqueue_AllSucceeds_ReturnsTrue
Replaces the B-T1 stub WriteAsync with the production hot-path:
- Bounded Channel<PendingAuditEvent> (BoundedChannelFullMode.Wait, capacity
from options) feeds a background ProcessWriteQueueAsync loop that drains
up to BatchSize events per transaction.
- The loop INSERTs each event with explicit parameter binding (enums and
DateTime stored as text); duplicate EventIds (SqliteException with
ErrorCode 19 SQLITE_CONSTRAINT) are swallowed as first-write-wins per
alog.md §11, and the pending TCS is still completed successfully so
callers see idempotent semantics.
- Site rows force ForwardState = Pending on enqueue when the inbound
event leaves it null — site-side default per the M2 design.
- ReadPendingAsync(limit) returns oldest-first pending rows for the
Bundle D telemetry actor; EventId is the deterministic tiebreaker on
identical OccurredAtUtc timestamps. MarkForwardedAsync(ids) flips a
batch to Forwarded in one UPDATE with a parameterised IN list.
- IAsyncDisposable graceful shutdown: TryComplete the writer, await the
drain (5s budget), then dispose the connection.
Tests (7 new, total 16 -> 23):
- WriteAsync_FreshEvent_PersistsWithForwardStatePending
- WriteAsync_Concurrent_1000Calls_All_Persist_NoExceptions
- WriteAsync_DuplicateEventId_FirstWriteWins_NoException
- WriteAsync_ForcesForwardStatePending_IfNull
- ReadPendingAsync_Returns_OldestFirst_LimitedToN
- MarkForwardedAsync_FlipsRowsToForwarded
- MarkForwardedAsync_NonExistentId_NoThrow
