test(localdb): pin drift-watermark branch; guard writer loop against completed-lane spin

2026-07-17 22:50:33 -04:00
parent 92ccbac8db
commit 183901e8ac
2 changed files with 105 additions and 6 deletions
@@ -157,6 +157,8 @@ internal sealed class SyncSession
    {
        Task<bool>? controlWait = null;
        Task<bool>? dataWait = null;
+        var controlDone = false;
+        var dataDone = false;
        while (true)
        {
            ct.ThrowIfCancellationRequested();
@@ -165,14 +167,27 @@ internal sealed class SyncSession
                await duplex.Send(message, ct);
                continue;
            }
+            // WaitToReadAsync => false means the lane was COMPLETED and is drained: stop waiting on
+            // it (re-creating its waiter would instantly re-complete false — a 100% CPU spin) and
+            // exit once both lanes are terminal. Nothing completes the lanes today, but Task 12 or
+            // a future refactor must not silently detonate this loop.
+            if (controlDone && dataDone)
+                return;
            // At most one outstanding waiter per lane (SingleReader); a stale already-completed
            // waiter only costs one extra TryRead pass.
-            controlWait ??= control.WaitToReadAsync(ct).AsTask();
-            dataWait ??= data.WaitToReadAsync(ct).AsTask();
-            var completed = await Task.WhenAny(controlWait, dataWait);
-            await completed;
-            if (controlWait.IsCompleted) controlWait = null;
-            if (dataWait.IsCompleted) dataWait = null;
+            if (!controlDone) controlWait ??= control.WaitToReadAsync(ct).AsTask();
+            if (!dataDone) dataWait ??= data.WaitToReadAsync(ct).AsTask();
+            await Task.WhenAny(controlWait ?? dataWait!, dataWait ?? controlWait!);
+            if (controlWait is { IsCompleted: true })
+            {
+                controlDone = !await controlWait;
+                controlWait = null;
+            }
+            if (dataWait is { IsCompleted: true })
+            {
+                dataDone = !await dataWait;
+                dataWait = null;
+            }
        }
    }

@@ -278,6 +293,8 @@ internal sealed class SyncSession
        await controlLane.WriteAsync(new SyncMessage { DeltaAck = new DeltaAck { AppliedThruSeq = ackThru } }, ct);
    }

+    // Not transactional with the watermark update: a crash between them redelivers the batch and
+    // dead-letters the same entry again (at-least-once — a benign duplicate dead-letter row).
    private Task DeadLetterDriftedAsync(
        OplogEntryRecord entry, long physicalMs, long nowMs, DateTimeOffset now, CancellationToken ct) =>
        _db.ExecuteAsync(