From f62b017cc7eeb91279b1c3c633fac82a97e0a89f Mon Sep 17 00:00:00 2001
From: Joseph Doherty <dohejw01@gmail.com>
Date: Wed, 19 Aug 2026 14:49:31 -0400
Subject: [PATCH] fix(localdb): fault live sync session on cap-prune; keepalive
 + newest-wins sync guard (0.3.2, ScadaBridge arch-review P4-1 + P4-3)

---
 ZB.MOM.WW.LocalDb/Directory.Build.props       |   2 +-
 ZB.MOM.WW.LocalDb/README.md                   |  27 ++-
 .../Internal/OplogStore.cs                    |  23 ++-
 .../Internal/SyncSession.cs                   |  39 ++++
 .../LocalDbOplogGapException.cs               |  23 +++
 .../LocalDbSyncService.cs                     | 117 ++++++++++-
 .../MaintenanceBackgroundService.cs           |   2 +-
 .../ReplicationOptions.cs                     |  28 +++
 .../ReplicationOptionsValidator.cs            |   4 +
 .../SyncBackgroundService.cs                  |  36 +++-
 .../GrpcAdapterTests.cs                       |  18 +-
 .../SnapshotResyncTests.cs                    |  95 ++++++++-
 .../SyncStreamGuardTests.cs                   | 181 ++++++++++++++++++
 13 files changed, 574 insertions(+), 21 deletions(-)
 create mode 100644 ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/LocalDbOplogGapException.cs
 create mode 100644 ZB.MOM.WW.LocalDb/tests/ZB.MOM.WW.LocalDb.Tests/SyncStreamGuardTests.cs

diff --git a/ZB.MOM.WW.LocalDb/Directory.Build.props b/ZB.MOM.WW.LocalDb/Directory.Build.props
index 63cf118..210aa41 100644
--- a/ZB.MOM.WW.LocalDb/Directory.Build.props
+++ b/ZB.MOM.WW.LocalDb/Directory.Build.props
@@ -5,7 +5,7 @@
     <Nullable>enable</Nullable>
     <ImplicitUsings>enable</ImplicitUsings>
     <LangVersion>latest</LangVersion>
-    <Version>0.3.1</Version>
+    <Version>0.3.2</Version>
     <ManagePackageVersionsCentrally>true</ManagePackageVersionsCentrally>
     <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
     <!-- NU1510 (package-pruning advice): the Replication and Tests projects carry a
diff --git a/ZB.MOM.WW.LocalDb/README.md b/ZB.MOM.WW.LocalDb/README.md
index 1e2a3c4..9a868f1 100644
--- a/ZB.MOM.WW.LocalDb/README.md
+++ b/ZB.MOM.WW.LocalDb/README.md
@@ -10,7 +10,7 @@ Change capture uses SQLite triggers writing a single HLC-stamped oplog inside th
 consumer's own write transaction (crash-consistent, non-bypassable). Convergence is
 last-writer-wins over a hybrid logical clock. All timestamps are UTC.
 
-## Packages (all `0.3.0`)
+## Packages (all `0.3.2`)
 
 | Package | Contents |
 |---|---|
@@ -20,6 +20,31 @@ last-writer-wins over a hybrid logical clock. All timestamps are UTC.
 
 > Status: **published to the Gitea feed; adopted by ScadaBridge (Phases 1+2) and OtOpcUa (Phase 1).**
 >
+> - `0.3.2` (2026-08-19) — two Medium findings from the ScadaBridge adversarial architecture review
+>   (P4-1, P4-3). Wire-compatible with every earlier peer: no proto change, no `lib_schema_version`
+>   bump, no on-disk schema change — the keepalive is transport-level and the rest is local policy.
+>   - **A live session now ends itself when a cap prune eats deltas it has not sent.** The pump reads
+>     `seq > sentThruSeq`, which resumes SILENTLY above a pruned hole; the peer acks the later seqs
+>     and the pair is durably divergent for the whole life of that session. `needs_snapshot` (set in
+>     the prune's own transaction) only heals a RECONNECT, and nothing forced one — so on a stable
+>     LAN with a slow-but-alive peer the divergence was unbounded. Every pump iteration now compares
+>     the oplog horizon (`MIN(seq)`, one b-tree seek) against its sent watermark and throws
+>     `LocalDbOplogGapException` when deltas it still owes are gone. The horizon, not the flag,
+>     because the flag is also raised for reasons that do not invalidate the running session.
+>   - **The sync stream has HTTP/2 keepalive, and the server's single-stream guard is newest-wins.**
+>     Neither end ever timed out on silence (a converged pair is legitimately quiet), so a hard crash
+>     or partition of the dialling node left the passive node's handler parked in `MoveNext` on a
+>     half-open stream — holding the guard, reporting itself connected, and rejecting the rebooted
+>     peer's reconnects with `AlreadyExists`, indefinitely on an idle pair. The initiator now dials
+>     with `KeepAlivePingDelay` / `KeepAlivePingTimeout` (new options, 60s/20s, `Always` policy so
+>     idle connections ping too), and a fresh authenticated `Sync` cancels the incumbent and waits
+>     for it to unwind before starting — the guard serializes sessions, it was never a fence, and an
+>     authenticated same-key dialler is by definition the peer. An incumbent that will not yield
+>     within the takeover timeout still gets `AlreadyExists`, because two live sessions over one
+>     database is worse than a refused dial.
+> - `0.3.1` (2026-08-18) — a faulting session flushes its control lane before cancelling its writer,
+>   so a locally-detected handshake mismatch actually reaches the peer instead of leaving it to
+>   reconnect-loop against a generic disconnect.
 > - `0.3.0` (2026-08-15) — **the snapshot-resync flag is per-table.** On-disk bookkeeping schema
 >   goes to **v3** (new `__localdb_snapshot_state`, upgraded in place on open); wire-compatible with
 >   `0.1.x`/`0.2.x` peers, and `lib_schema_version` deliberately stays `1` because it is compared
diff --git a/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/Internal/OplogStore.cs b/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/Internal/OplogStore.cs
index f110bfd..b8802e8 100644
--- a/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/Internal/OplogStore.cs
+++ b/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/Internal/OplogStore.cs
@@ -24,8 +24,9 @@ internal sealed class OplogStore(ILocalDb db, ReplicationOptions options, Func<D
 
     /// <summary>
     /// Reads up to <paramref name="maxBatch"/> oplog entries with <c>seq &gt; afterSeq</c> in ascending
-    /// seq order. Caller contract: check <c>needs_snapshot</c> before trusting incremental reads —
-    /// after cap-pruning, reads silently resume past a gap.
+    /// seq order. Caller contract: this read resumes SILENTLY past a cap-pruned gap, so a caller
+    /// streaming incrementally must first confirm its position is still below the horizon — see
+    /// <see cref="GetMinSeqAsync"/>, which <c>SyncSession</c>'s pump checks on every iteration.
     /// </summary>
     public Task<IReadOnlyList<OplogEntryRecord>> ReadBatchAboveAsync(long afterSeq, int maxBatch, CancellationToken ct = default) =>
         db.QueryAsync(
@@ -148,6 +149,24 @@ internal sealed class OplogStore(ILocalDb db, ReplicationOptions options, Func<D
         return rows[0];
     }
 
+    /// <summary>
+    /// The oldest oplog seq still present — the delta horizon — or 0 when the oplog is empty.
+    /// </summary>
+    /// <remarks>
+    /// One b-tree seek (<c>seq</c> is the rowid and SQLite optimizes MIN into a seek), which is what
+    /// makes it affordable for the pump to check on every flush interval. Compared against a
+    /// session's sent watermark it answers the only question the pump has: is everything I have not
+    /// yet sent still here? Ack-pruning deletes <c>seq &lt;= last_acked_seq</c> and the peer can only
+    /// ack what it received, so in a healthy session the horizon never climbs past
+    /// <c>sentThruSeq + 1</c>; a higher one means a cap prune ate unsent deltas.
+    /// </remarks>
+    public async Task<long> GetMinSeqAsync(CancellationToken ct = default)
+    {
+        var rows = await db.QueryAsync(
+            "SELECT COALESCE(MIN(seq), 0) FROM __localdb_oplog", static r => r.GetInt64(0), null, ct);
+        return rows[0];
+    }
+
     /// <summary>The highest oplog seq present, or 0 when the oplog is empty.</summary>
     public async Task<long> GetMaxSeqAsync(CancellationToken ct = default)
     {
diff --git a/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/Internal/SyncSession.cs b/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/Internal/SyncSession.cs
index dc4ae33..d195d25 100644
--- a/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/Internal/SyncSession.cs
+++ b/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/Internal/SyncSession.cs
@@ -356,9 +356,14 @@ internal sealed class SyncSession
         while (true)
         {
             ct.ThrowIfCancellationRequested();
+            // BEFORE the read, because the read itself cannot tell a pruned gap from an idle oplog.
+            ThrowIfPruneGap(await _store.GetMinSeqAsync(ct));
             var batch = await _store.ReadBatchAboveAsync(_sentThruSeq, _options.MaxBatchSize, ct);
             if (batch.Count > 0)
             {
+                // The exact guard on the rows about to go out, for a prune that landed between the
+                // probe and the read. Free (the seq is already in hand) and strictly tighter.
+                ThrowIfPruneGap(batch[0].Seq);
                 var index = 0;
                 while (index < batch.Count)
                 {
@@ -390,6 +395,40 @@ internal sealed class SyncSession
         }
     }
 
+    /// <summary>
+    /// Faults the session when the oplog horizon has climbed above this session's read position —
+    /// i.e. a cap prune deleted deltas between <see cref="_sentThruSeq"/> and
+    /// <paramref name="oldestAvailableSeq"/>. An empty oplog (0) is not a gap.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// <b>Why a live session has to die for this.</b> <c>EnforceCapsAsync</c> sets
+    /// <c>needs_snapshot</c> in the same transaction as the prune, so a RECONNECT heals — the
+    /// handshake serves a full snapshot instead of deltas. Nothing, however, made the live session
+    /// notice: the pump reads <c>seq &gt; _sentThruSeq</c>, which silently resumes above the hole, and
+    /// the peer acks those later seqs as if the pair were converged. On a stable LAN with a slow but
+    /// alive peer that session can run for days, and the pair is durably divergent for all of it —
+    /// the flag that would fix it is only ever read at handshake time. Tearing down is what turns a
+    /// persisted "needs snapshot" into an actual snapshot.
+    /// </para>
+    /// <para>
+    /// <b>Why the horizon, not the flag.</b> <c>needs_snapshot</c> is also set for reasons that do
+    /// not invalidate THIS session's position (and a full snapshot this session just streamed clears
+    /// it only when the peer's <c>SnapshotAck</c> lands, which is after the pump has started).
+    /// Comparing the horizon against the sent watermark asks the precise question instead, so a
+    /// session is faulted when — and only when — rows it still owes the peer are gone.
+    /// </para>
+    /// </remarks>
+    private void ThrowIfPruneGap(long oldestAvailableSeq)
+    {
+        if (oldestAvailableSeq == 0 || oldestAvailableSeq <= _sentThruSeq + 1)
+            return;
+        throw new LocalDbOplogGapException(
+            $"Oplog deltas above the sent watermark {_sentThruSeq} were pruned (oldest surviving seq " +
+            $"{oldestAvailableSeq}): this session can no longer stream incrementally without skipping " +
+            "them. Ending the session so the reconnect handshake serves a snapshot resync.");
+    }
+
     /// <summary>
     /// Serialized bytes one entry contributes to its batch message: the entry's own encoding plus
     /// the repeated field's tag and length prefix, as protobuf itself computes them.
diff --git a/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/LocalDbOplogGapException.cs b/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/LocalDbOplogGapException.cs
new file mode 100644
index 0000000..627f603
--- /dev/null
+++ b/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/LocalDbOplogGapException.cs
@@ -0,0 +1,23 @@
+namespace ZB.MOM.WW.LocalDb.Replication;
+
+/// <summary>
+/// Thrown by a live sync session when the oplog horizon has moved ABOVE the position that session
+/// is streaming from: a cap prune deleted deltas it had not yet sent, so every later delta it could
+/// ship would land on the far side of a hole the peer will never receive.
+/// </summary>
+/// <remarks>
+/// This is not a transport failure and not a data error — it is the session discovering that its own
+/// incremental position has become unsound. Nothing can repair it in-session (the deleted rows are
+/// gone), so the session tears itself down; the same prune set <c>needs_snapshot</c>, which makes the
+/// reconnect handshake serve a snapshot instead of deltas. Catching this and continuing would
+/// reintroduce exactly the silent divergence it exists to stop.
+/// </remarks>
+public sealed class LocalDbOplogGapException : Exception
+{
+    /// <summary>Creates the exception with a message describing the gap.</summary>
+    public LocalDbOplogGapException(string message) : base(message) { }
+
+    /// <summary>Creates the exception with a message and the underlying cause.</summary>
+    public LocalDbOplogGapException(string message, Exception innerException)
+        : base(message, innerException) { }
+}
diff --git a/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/LocalDbSyncService.cs b/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/LocalDbSyncService.cs
index 2ef5898..42710fa 100644
--- a/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/LocalDbSyncService.cs
+++ b/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/LocalDbSyncService.cs
@@ -21,8 +21,26 @@ public sealed class LocalDbSyncService : LocalDbSync.LocalDbSyncBase
     private readonly LocalDbMetrics? _metrics;
     private readonly SyncStatus? _status;
 
-    // 0 = idle, 1 = a stream is active. Interlocked-guarded across concurrent calls (singleton service).
-    private int _active;
+    // The stream currently holding the guard, or null when idle. Guarded by _gate (singleton service).
+    private readonly object _gate = new();
+    private Registration? _active;
+
+    /// <summary>
+    /// How long a displacing call waits for the incumbent it evicted to actually finish before
+    /// giving up and refusing itself. Test seam; the default is generous because eviction only has
+    /// to outrun the operator, not the network.
+    /// </summary>
+    internal TimeSpan TakeoverTimeout { get; set; } = TimeSpan.FromSeconds(10);
+
+    /// <summary>One in-flight <c>Sync</c> handler: its cancellation handle plus a completion signal
+    /// a displacing handler can await, so two sessions never run against the database at once.</summary>
+    private sealed class Registration(CancellationTokenSource cts)
+    {
+        public CancellationTokenSource Cts { get; } = cts;
+
+        public TaskCompletionSource Finished { get; } =
+            new(TaskCreationOptions.RunContinuationsAsynchronously);
+    }
 
     /// <summary>
     /// Creates the passive sync service. <paramref name="status"/> is taken as the public
@@ -41,21 +59,47 @@ public sealed class LocalDbSyncService : LocalDbSync.LocalDbSyncBase
         _status = status as SyncStatus;
     }
 
+    /// <summary>
+    /// Accepts one inbound sync stream, evicting any stream already holding the guard.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// <b>Newest wins.</b> The guard exists to SERIALIZE sessions — a node syncs with exactly one
+    /// peer, and two sessions against one database would fight over the same watermarks — not to
+    /// fence out an impostor; authentication is the interceptor's job, and an authenticated caller
+    /// on this key IS the peer by definition. Refusing the newcomer therefore protected nothing and
+    /// cost everything in the case that actually happens: the dialling node crashes or is
+    /// partitioned, this handler stays parked forever inside <c>MoveNext</c> on a half-open stream
+    /// that will never produce another byte, and every reconnect from the rebooted peer is rejected
+    /// <c>AlreadyExists</c> — potentially for as long as the pair stays idle, while this side keeps
+    /// reporting itself connected. The newest dialler is the one with a live link, so it takes over.
+    /// </para>
+    /// <para>
+    /// Eviction is cancel-then-await, not cancel-and-go: the incumbent's cancellation unwinds its
+    /// session and reader pump, and the newcomer waits for that to complete before starting its own,
+    /// so the "one session at a time" property the guard was built for still holds. An incumbent
+    /// that cannot be evicted within <see cref="TakeoverTimeout"/> is the one case where the old
+    /// answer is still right — the newcomer refuses itself rather than double-running.
+    /// </para>
+    /// </remarks>
     public override async Task Sync(
         IAsyncStreamReader<SyncMessage> requestStream,
         IServerStreamWriter<SyncMessage> responseStream,
         ServerCallContext context)
     {
-        if (Interlocked.CompareExchange(ref _active, 1, 0) != 0)
-            throw new RpcException(new Status(StatusCode.AlreadyExists, "a sync stream is already active"));
+        // The linked source exists so the reader pump is deterministically torn down on EVERY exit
+        // path — including a session fault for a local (non-transport) reason, where the call token
+        // never fires and the pump would otherwise keep reading a stream whose handler has exited.
+        // It is also the handle a later call cancels to displace this one.
+        using var linkedCts = CancellationTokenSource.CreateLinkedTokenSource(context.CancellationToken);
+        var mine = new Registration(linkedCts);
 
         try
         {
-            // The linked source exists so the reader pump is deterministically torn down on EVERY
-            // exit path — including a session fault for a local (non-transport) reason, where the
-            // call token never fires and the pump would otherwise keep reading a stream whose
-            // handler has exited.
-            using var linkedCts = CancellationTokenSource.CreateLinkedTokenSource(context.CancellationToken);
+            // Inside the try so the finally below always releases the guard — including when the
+            // takeover itself is abandoned because THIS caller gave up mid-handover.
+            await TakeOverAsync(mine, context.CancellationToken);
+
             var (duplex, readerTask) = GrpcSyncDuplex.Create(requestStream, responseStream, linkedCts.Token);
             var session = SyncSessionFactory.Create(_db, _options, _loggerFactory, _metrics, _status);
             _status?.SessionStarted();
@@ -81,13 +125,66 @@ public sealed class LocalDbSyncService : LocalDbSync.LocalDbSyncBase
         {
             // Client disconnect / server shutdown: the normal end of a long-lived streaming RPC.
         }
+        catch (OperationCanceledException)
+        {
+            // Displaced by a newer stream: an orderly handover, not a failure of this node.
+            _logger.LogInformation("Sync stream ended: displaced by a newer stream from the peer.");
+        }
         catch (LocalDbSchemaMismatchException ex)
         {
             throw new RpcException(new Status(StatusCode.FailedPrecondition, ex.Message));
         }
         finally
         {
-            Interlocked.Exchange(ref _active, 0);
+            lock (_gate)
+            {
+                if (ReferenceEquals(_active, mine))
+                    _active = null;
+            }
+            mine.Finished.TrySetResult();
+        }
+    }
+
+    /// <summary>
+    /// Claims the single-stream guard for <paramref name="mine"/>, cancelling and awaiting whatever
+    /// held it. Throws <c>AlreadyExists</c> only when the incumbent refuses to end.
+    /// </summary>
+    private async Task TakeOverAsync(Registration mine, CancellationToken ct)
+    {
+        Registration? displaced;
+        lock (_gate)
+        {
+            displaced = _active;
+            _active = mine;
+        }
+        if (displaced is null)
+            return;
+
+        _logger.LogWarning(
+            "A new sync stream arrived while one was already active; displacing the incumbent (newest wins).");
+        try
+        {
+            // The incumbent may already be disposing its own source between its finally and here.
+            displaced.Cts.Cancel();
+        }
+        catch (ObjectDisposedException) { }
+
+        try
+        {
+            await displaced.Finished.Task.WaitAsync(TakeoverTimeout, ct);
+        }
+        catch (TimeoutException)
+        {
+            // Could not evict it: two live sessions over one database is worse than a refused dial,
+            // so fall back to the historical answer — and hand the guard back to the incumbent.
+            lock (_gate)
+            {
+                if (ReferenceEquals(_active, mine))
+                    _active = displaced;
+            }
+            throw new RpcException(new Status(
+                StatusCode.AlreadyExists,
+                "a sync stream is already active and did not yield within the takeover timeout"));
         }
     }
 }
diff --git a/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/MaintenanceBackgroundService.cs b/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/MaintenanceBackgroundService.cs
index f098cfb..870fe96 100644
--- a/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/MaintenanceBackgroundService.cs
+++ b/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/MaintenanceBackgroundService.cs
@@ -54,7 +54,7 @@ public sealed class MaintenanceBackgroundService : BackgroundService
                 {
                     _snapshotFlagWarned = true;
                     _logger.LogWarning(
-                        "Oplog backlog/age caps exceeded: pruned to the ceiling and flagged needs_snapshot — the peer must snapshot-resync. Not re-logged until a clean tick.");
+                        "Oplog backlog/age caps exceeded: pruned to the ceiling and flagged needs_snapshot — the peer must snapshot-resync. A session streaming from below the new horizon ends itself on its next pump tick so the reconnect serves that snapshot. Not re-logged until a clean tick.");
                 }
                 else if (!snapshotFlagged)
                 {
diff --git a/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/ReplicationOptions.cs b/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/ReplicationOptions.cs
index 26feb5e..ebd5446 100644
--- a/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/ReplicationOptions.cs
+++ b/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/ReplicationOptions.cs
@@ -64,6 +64,34 @@ public sealed class ReplicationOptions
     /// <summary>Upper bound on the exponential reconnect backoff.</summary>
     public TimeSpan ReconnectBackoffMax { get; set; } = TimeSpan.FromSeconds(60);
 
+    /// <summary>
+    /// How long the initiator's HTTP/2 connection may sit without a keepalive PING before one is
+    /// sent. Default 60s.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// A sync stream is long-lived and, on a converged pair, almost entirely silent — which is
+    /// exactly the shape a half-open TCP connection hides behind. Without pings, a hard crash or a
+    /// partition of one node leaves the other holding a stream that will never produce another byte
+    /// and never fail either: the initiator sits in a session it believes is healthy, and the
+    /// passive node's handler sits inside its single-stream guard, refusing the reconnect that would
+    /// heal the pair. Pings turn that into a detected failure on both ends.
+    /// </para>
+    /// <para>
+    /// Sent with <c>KeepAlivePingPolicy.Always</c>, so they flow on an idle connection too — an
+    /// active-streams-only policy would not fire in precisely the silent-pair case above. Safe over
+    /// h2c: a PING is an HTTP/2 frame, nothing to do with TLS.
+    /// </para>
+    /// </remarks>
+    public TimeSpan KeepAlivePingDelay { get; set; } = TimeSpan.FromSeconds(60);
+
+    /// <summary>
+    /// How long the initiator waits for a keepalive PING ack before tearing the connection down.
+    /// Default 20s. Detection therefore lands within roughly
+    /// <see cref="KeepAlivePingDelay"/> + this.
+    /// </summary>
+    public TimeSpan KeepAlivePingTimeout { get; set; } = TimeSpan.FromSeconds(20);
+
     /// <summary>How often the maintenance service enforces the oplog caps and prunes expired tombstones.</summary>
     public TimeSpan MaintenanceInterval { get; set; } = TimeSpan.FromSeconds(60);
 
diff --git a/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/ReplicationOptionsValidator.cs b/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/ReplicationOptionsValidator.cs
index 0f8709e..625c745 100644
--- a/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/ReplicationOptionsValidator.cs
+++ b/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/ReplicationOptionsValidator.cs
@@ -40,6 +40,10 @@ public sealed class ReplicationOptionsValidator : IValidateOptions<ReplicationOp
             failures.Add($"LocalDb:Replication:ReconnectBackoffMax must be greater than zero; got {options.ReconnectBackoffMax}.");
         else if (options.ReconnectBackoffMax > ReconnectBackoffCeiling)
             failures.Add($"LocalDb:Replication:ReconnectBackoffMax must be at most {ReconnectBackoffCeiling}; got {options.ReconnectBackoffMax}.");
+        if (options.KeepAlivePingDelay <= TimeSpan.Zero)
+            failures.Add($"LocalDb:Replication:KeepAlivePingDelay must be greater than zero; got {options.KeepAlivePingDelay}.");
+        if (options.KeepAlivePingTimeout <= TimeSpan.Zero)
+            failures.Add($"LocalDb:Replication:KeepAlivePingTimeout must be greater than zero; got {options.KeepAlivePingTimeout}.");
         if (options.MaintenanceInterval <= TimeSpan.Zero)
             failures.Add($"LocalDb:Replication:MaintenanceInterval must be greater than zero; got {options.MaintenanceInterval}.");
         if (options.MaxHlcDriftAhead <= TimeSpan.Zero)
diff --git a/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/SyncBackgroundService.cs b/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/SyncBackgroundService.cs
index 8c3938f..eeef8f1 100644
--- a/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/SyncBackgroundService.cs
+++ b/ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/SyncBackgroundService.cs
@@ -1,3 +1,4 @@
+using System.Net.Http;
 using Grpc.Core;
 using Grpc.Net.Client;
 using Microsoft.Extensions.Hosting;
@@ -76,7 +77,7 @@ public sealed class SyncBackgroundService : BackgroundService
             {
                 var channel = ChannelFactory is { } factory
                     ? factory()
-                    : ownedChannel = GrpcChannel.ForAddress(_options.PeerAddress);
+                    : ownedChannel = DialPeer();
 
                 var client = new LocalDbSync.LocalDbSyncClient(channel);
                 var headers = new Metadata();
@@ -152,6 +153,39 @@ public sealed class SyncBackgroundService : BackgroundService
         }
     }
 
+    /// <summary>
+    /// Dials the peer over a channel that sends HTTP/2 keepalive PINGs, so a half-open link is
+    /// detected instead of being waited on forever.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// The bare <c>GrpcChannel.ForAddress(address)</c> this replaces had NO keepalive of any kind,
+    /// and neither end of a sync stream ever times out on silence by design (a converged pair is
+    /// legitimately quiet). A hard crash or partition of the peer therefore left this node inside a
+    /// session that could never make progress and could never fail — and its own half of that stream
+    /// pinned on the peer, where the single-stream guard turns it into a refusal of the very
+    /// reconnect that heals the pair.
+    /// </para>
+    /// <para>
+    /// <c>DisposeHttpClient</c> hands the handler's lifetime to the channel, which this loop already
+    /// disposes on every exit path.
+    /// </para>
+    /// </remarks>
+    private GrpcChannel DialPeer() =>
+        GrpcChannel.ForAddress(_options.PeerAddress, new GrpcChannelOptions
+        {
+            HttpHandler = new SocketsHttpHandler
+            {
+                KeepAlivePingDelay = _options.KeepAlivePingDelay,
+                KeepAlivePingTimeout = _options.KeepAlivePingTimeout,
+                // Always, not WithActiveRequests: the case being closed is a SILENT stream on a dead
+                // link, and an active-requests policy stops pinging once the peer stops responding.
+                KeepAlivePingPolicy = HttpKeepAlivePingPolicy.Always,
+                EnableMultipleHttp2Connections = true,
+            },
+            DisposeHttpClient = true,
+        });
+
     private static bool IsSchemaMismatch(Exception? ex) =>
         ex is LocalDbSchemaMismatchException
         || (ex is RpcException rpc && rpc.StatusCode == StatusCode.FailedPrecondition);
diff --git a/ZB.MOM.WW.LocalDb/tests/ZB.MOM.WW.LocalDb.Tests/GrpcAdapterTests.cs b/ZB.MOM.WW.LocalDb/tests/ZB.MOM.WW.LocalDb.Tests/GrpcAdapterTests.cs
index 41381d7..4a191c2 100644
--- a/ZB.MOM.WW.LocalDb/tests/ZB.MOM.WW.LocalDb.Tests/GrpcAdapterTests.cs
+++ b/ZB.MOM.WW.LocalDb/tests/ZB.MOM.WW.LocalDb.Tests/GrpcAdapterTests.cs
@@ -70,8 +70,12 @@ public sealed class GrpcAdapterTests : IAsyncLifetime
     }
 
     [Fact]
-    public async Task SecondConcurrentStream_Rejected()
+    public async Task SecondConcurrentStream_DisplacesTheIncumbent()
     {
+        // Newest wins (0.3.2). The guard serializes sessions; it was never a fence — the auth
+        // interceptor is, and an authenticated same-key dialler IS the peer. Refusing the newcomer
+        // bought nothing and cost the pair its only route back from a half-open link, where the
+        // incumbent handler can hold the guard indefinitely on a dead stream.
         var serverHost = await BuildServerAsync(NewDbPath(), apiKey: null, sink: null);
         var serverDb = serverHost.Services.GetRequiredService<ILocalDb>();
         var handler = serverHost.GetTestServer().CreateHandler();
@@ -85,11 +89,15 @@ public sealed class GrpcAdapterTests : IAsyncLifetime
 
         // Convergence proves the first stream's server handler is active and holding the single-stream guard.
         await WaitForAsync(async () => (await ReadOrders(serverDb)).Count == 1, RunTimeout);
+        var attemptsBefore = bg.ConnectionAttempts;
 
         var client = new LocalDbSync.LocalDbSyncClient(ChannelOver(handler));
         using var call2 = client.Sync(cancellationToken: cts.Token);
-        var ex = await Assert.ThrowsAsync<RpcException>(() => call2.ResponseStream.MoveNext(cts.Token));
-        Assert.Equal(StatusCode.AlreadyExists, ex.StatusCode);
+        Assert.True(await call2.ResponseStream.MoveNext(cts.Token));
+        Assert.Equal(SyncMessage.MsgOneofCase.Handshake, call2.ResponseStream.Current.MsgCase);
+
+        // The incumbent really ended: the displaced client notices and dials again.
+        await WaitForAsync(() => Task.FromResult(bg.ConnectionAttempts > attemptsBefore), RunTimeout);
 
         await bg.StopAsync(CancellationToken.None);
     }
@@ -262,9 +270,13 @@ public sealed class GrpcAdapterTests : IAsyncLifetime
             TombstoneRetention = TimeSpan.Zero,
             ReconnectBackoffMax = TimeSpan.Zero,
             MaxHlcDriftAhead = TimeSpan.Zero,
+            KeepAlivePingDelay = TimeSpan.Zero,
+            KeepAlivePingTimeout = TimeSpan.Zero,
         });
 
         Assert.True(result.Failed);
+        Assert.Contains(result.Failures, f => f.Contains("KeepAlivePingDelay", StringComparison.Ordinal));
+        Assert.Contains(result.Failures, f => f.Contains("KeepAlivePingTimeout", StringComparison.Ordinal));
         Assert.Contains(result.Failures, f => f.Contains("FlushInterval", StringComparison.Ordinal));
         Assert.Contains(result.Failures, f => f.Contains("MaxBatchSize", StringComparison.Ordinal));
         Assert.Contains(result.Failures, f => f.Contains("MaxOplogRows", StringComparison.Ordinal));
diff --git a/ZB.MOM.WW.LocalDb/tests/ZB.MOM.WW.LocalDb.Tests/SnapshotResyncTests.cs b/ZB.MOM.WW.LocalDb/tests/ZB.MOM.WW.LocalDb.Tests/SnapshotResyncTests.cs
index 1860cdf..6fff574 100644
--- a/ZB.MOM.WW.LocalDb/tests/ZB.MOM.WW.LocalDb.Tests/SnapshotResyncTests.cs
+++ b/ZB.MOM.WW.LocalDb/tests/ZB.MOM.WW.LocalDb.Tests/SnapshotResyncTests.cs
@@ -125,11 +125,15 @@ public sealed class SnapshotResyncTests : IDisposable
     // Completes the two-phase handshake against a real session: the peer never requests a snapshot.
     private static async Task DriveHandshakeAsync(
         SqliteLocalDb sessionDb, ChannelWriter<SyncMessage> toSession, ChannelReader<SyncMessage> fromSession,
-        CancellationToken ct, bool supportsPartialSnapshot = false)
+        CancellationToken ct, bool supportsPartialSnapshot = false, long lastAppliedRemoteSeq = 0)
     {
         await NextOfCaseAsync(fromSession, SyncMessage.MsgOneofCase.Handshake, ct);
         await toSession.WriteAsync(
-            new SyncMessage { Handshake = MatchingHandshake(sessionDb, "peer", supportsPartialSnapshot: supportsPartialSnapshot) }, ct);
+            new SyncMessage
+            {
+                Handshake = MatchingHandshake(
+                    sessionDb, "peer", lastAppliedRemoteSeq, supportsPartialSnapshot),
+            }, ct);
         await NextOfCaseAsync(fromSession, SyncMessage.MsgOneofCase.HandshakeAck, ct);
         await toSession.WriteAsync(new SyncMessage { HandshakeAck = new HandshakeAck { NodeId = "peer", SnapshotRequired = false } }, ct);
     }
@@ -373,6 +377,93 @@ public sealed class SnapshotResyncTests : IDisposable
         await SwallowAsync(runB);
     }
 
+    [Fact]
+    public async Task CapPrune_IntoALiveSessionsUnsentRange_FaultsIt_AndTheReconnectSnapshots()
+    {
+        // P4-1. The pump reads `seq > sentThruSeq`, which resumes SILENTLY above a hole a cap prune
+        // punched — so it keeps shipping later deltas, the peer acks them, and the pair is durably
+        // divergent. `needs_snapshot` (set in the prune's own transaction) heals only a RECONNECT,
+        // and nothing forced one: on a stable LAN with a slow-but-alive peer the session, and the
+        // divergence, could last for days. Faulting is what turns the persisted flag into a snapshot.
+        var a = await NewSideAsync();
+        for (var i = 1; i <= 6; i++)
+            await a.Db.ExecuteAsync("INSERT INTO orders (id, sku, qty) VALUES (@id, 'A', @id)", new { id = i });
+
+        var (duplex, toSession, fromSession) = ScriptedPeer();
+        using var cts = new CancellationTokenSource(RunTimeout);
+        var run = a.Session.RunAsync(duplex, cts.Token);
+
+        // The peer holds everything so far: no snapshot is owed, and the pump idles at seq 6.
+        await DriveHandshakeAsync(a.Db, toSession, fromSession, cts.Token, lastAppliedRemoteSeq: 6);
+
+        // Write three more rows and prune all but the newest, in ONE transaction — which is what a
+        // cap prune is, and what makes this deterministic: the pump's reader can only ever see the
+        // before state (nothing above seq 6) or the after state (seqs 7 and 8 already gone). It can
+        // never catch rows 7/8 in between and legitimately advance past them.
+        await using (var tx = await a.Db.BeginTransactionAsync(cts.Token))
+        {
+            await tx.ExecuteAsync(
+                "INSERT INTO orders (id, sku, qty) VALUES (7,'PRUNED',7),(8,'PRUNED',8),(9,'KEPT',9)", null, cts.Token);
+            await tx.ExecuteAsync("UPDATE __localdb_peer_state SET needs_snapshot = 1 WHERE id = 1", null, cts.Token);
+            await tx.ExecuteAsync(
+                "DELETE FROM __localdb_oplog WHERE seq < (SELECT MAX(seq) FROM __localdb_oplog)", null, cts.Token);
+            await tx.CommitAsync(cts.Token);
+        }
+
+        var fault = await Assert.ThrowsAsync<LocalDbOplogGapException>(() => run);
+        Assert.Contains("6", fault.Message, StringComparison.Ordinal);
+
+        // And it faulted BEFORE shipping anything from above the hole: a delta for seq 9 would have
+        // moved the peer's watermark past rows 7 and 8, which is the divergence itself.
+        while (fromSession.TryRead(out var stray))
+            Assert.NotEqual(SyncMessage.MsgOneofCase.DeltaBatch, stray.MsgCase);
+
+        // The reconnect is what heals: the prune's needs_snapshot makes the handshake owe a full
+        // snapshot, which carries the pruned rows' current state to a real peer.
+        var a2 = NewSessionOver(a.Db);
+        var b = await NewSideAsync();
+        var (da, db) = DuplexPair();
+        var run2A = a2.Session.RunAsync(da, cts.Token);
+        var run2B = b.Session.RunAsync(db, cts.Token);
+
+        await WaitForAsync(async () => (await ReadOrders(b.Db)).Count == 9, RunTimeout);
+        Assert.Equal(await ReadOrders(a.Db), await ReadOrders(b.Db));
+
+        cts.Cancel();
+        await SwallowAsync(run2A);
+        await SwallowAsync(run2B);
+    }
+
+    [Fact]
+    public async Task AckPruningToEmpty_DoesNotFaultALiveSession()
+    {
+        // The other half of the horizon check: ordinary ack-pruning deletes seq <= last_acked_seq,
+        // which by construction sits at or below the sent watermark, so a converged pair that has
+        // pruned its whole oplog must keep streaming. A gap test written against `needs_snapshot`,
+        // or one that treated any risen horizon as a fault, would kill healthy sessions here.
+        var a = await NewSideAsync();
+        var b = await NewSideAsync();
+        for (var i = 1; i <= 4; i++)
+            await a.Db.ExecuteAsync("INSERT INTO orders (id, sku, qty) VALUES (@id, 'A', @id)", new { id = i });
+
+        var (da, db) = DuplexPair();
+        using var cts = new CancellationTokenSource(RunTimeout);
+        var runA = a.Session.RunAsync(da, cts.Token);
+        var runB = b.Session.RunAsync(db, cts.Token);
+
+        await WaitForAsync(async () => (await ReadOrders(b.Db)).Count == 4, RunTimeout);
+        await WaitForAsync(async () => await OplogCount(a.Db) == 0, RunTimeout);
+
+        // Still live, still incremental: a post-prune write reaches B as an ordinary delta.
+        await a.Db.ExecuteAsync("INSERT INTO orders (id, sku, qty) VALUES (5, 'POST', 5)");
+        await WaitForAsync(async () => (await ReadOrders(b.Db)).Any(r => r.Id == 5), RunTimeout);
+        Assert.False(runA.IsFaulted, runA.Exception?.ToString());
+
+        cts.Cancel();
+        await SwallowAsync(runA);
+        await SwallowAsync(runB);
+    }
+
     [Fact]
     public async Task Snapshot_MergesNotTruncates()
     {
diff --git a/ZB.MOM.WW.LocalDb/tests/ZB.MOM.WW.LocalDb.Tests/SyncStreamGuardTests.cs b/ZB.MOM.WW.LocalDb/tests/ZB.MOM.WW.LocalDb.Tests/SyncStreamGuardTests.cs
new file mode 100644
index 0000000..30a9d69
--- /dev/null
+++ b/ZB.MOM.WW.LocalDb/tests/ZB.MOM.WW.LocalDb.Tests/SyncStreamGuardTests.cs
@@ -0,0 +1,181 @@
+using System.Threading.Channels;
+using Grpc.Core;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
+using ZB.MOM.WW.LocalDb.Contracts;
+using ZB.MOM.WW.LocalDb.Replication;
+
+namespace ZB.MOM.WW.LocalDb.Tests;
+
+/// <summary>
+/// The passive service's single-stream guard under a HALF-OPEN link: the dialling node is gone, but
+/// nothing on this side knows it, so the handler sits inside <c>MoveNext</c> forever holding the
+/// guard. Through 0.3.1 that made the pair unrecoverable without a restart — every reconnect from
+/// the rebooted peer was refused <c>AlreadyExists</c>, indefinitely on an idle pair, while this node
+/// went on reporting itself connected. The guard is there to serialize sessions, not to fence out a
+/// caller the auth interceptor already vouched for, so the newest dialler wins.
+/// </summary>
+public sealed class SyncStreamGuardTests : IDisposable
+{
+    private static readonly TimeSpan Timeout = TimeSpan.FromSeconds(10);
+
+    private readonly List<string> _paths = [];
+    private readonly List<IDisposable> _disposables = [];
+
+    public void Dispose()
+    {
+        foreach (var d in _disposables)
+            d.Dispose();
+        Microsoft.Data.Sqlite.SqliteConnection.ClearAllPools();
+        foreach (var p in _paths)
+        {
+            if (File.Exists(p)) File.Delete(p);
+            if (File.Exists(p + "-wal")) File.Delete(p + "-wal");
+            if (File.Exists(p + "-shm")) File.Delete(p + "-shm");
+        }
+    }
+
+    private LocalDbSyncService NewService()
+    {
+        var path = Path.Combine(Path.GetTempPath(), Guid.NewGuid() + ".db");
+        _paths.Add(path);
+        var db = new Internal.SqliteLocalDb(new LocalDbOptions { Path = path });
+        _disposables.Add(db);
+        db.ExecuteAsync("CREATE TABLE orders (id INTEGER PRIMARY KEY, sku TEXT)").GetAwaiter().GetResult();
+        db.RegisterReplicated("orders");
+
+        return new LocalDbSyncService(
+            db,
+            Options.Create(new ReplicationOptions { FlushInterval = TimeSpan.FromMilliseconds(20) }),
+            NullLoggerFactory.Instance);
+    }
+
+    private static ServerCallContext ContextFor(CancellationToken ct) => new FakeCallContext(ct);
+
+    [Fact]
+    public async Task WedgedIncumbent_IsDisplacedByANewStream()
+    {
+        var service = NewService();
+        // Eviction is normally generous; the test only needs it to not be the thing under test.
+        service.TakeoverTimeout = Timeout;
+
+        // The incumbent's peer crashed: its stream will never deliver another message, and neither
+        // end times out on silence — a converged pair is legitimately quiet. The handler parks in
+        // the handshake's first read, holding the guard.
+        using var incumbentCts = new CancellationTokenSource();
+        var incumbentOut = new RecordingWriter();
+        var incumbent = service.Sync(new SilentReader(), incumbentOut, ContextFor(incumbentCts.Token));
+
+        // It really is running and really did claim the guard (its handshake is on the wire).
+        await incumbentOut.NextAsync(Timeout);
+        Assert.False(incumbent.IsCompleted);
+
+        // The rebooted peer dials again. Pre-fix this threw AlreadyExists and the pair stayed wedged.
+        using var newcomerCts = new CancellationTokenSource();
+        var newcomerOut = new RecordingWriter();
+        var newcomer = service.Sync(new SilentReader(), newcomerOut, ContextFor(newcomerCts.Token));
+
+        // The incumbent is torn down — gracefully, not as a fault of this node.
+        await incumbent.WaitAsync(Timeout);
+        // ...and the new stream is genuinely served: its own handshake goes out.
+        var handshake = await newcomerOut.NextAsync(Timeout);
+        Assert.Equal(SyncMessage.MsgOneofCase.Handshake, handshake.MsgCase);
+        Assert.False(newcomer.IsCompleted);
+
+        newcomerCts.Cancel();
+        await SwallowAsync(newcomer);
+    }
+
+    [Fact]
+    public async Task DisplacedIncumbent_ReleasesTheGuard_SoAThirdStreamIsAlsoServed()
+    {
+        // Takeover must leave the guard in a state that can be taken over AGAIN — a handover that
+        // leaked the slot would simply move the wedge one reconnect down the line.
+        var service = NewService();
+        service.TakeoverTimeout = Timeout;
+
+        using var firstCts = new CancellationTokenSource();
+        var firstOut = new RecordingWriter();
+        var first = service.Sync(new SilentReader(), firstOut, ContextFor(firstCts.Token));
+        await firstOut.NextAsync(Timeout);
+
+        using var secondCts = new CancellationTokenSource();
+        var secondOut = new RecordingWriter();
+        var second = service.Sync(new SilentReader(), secondOut, ContextFor(secondCts.Token));
+        await first.WaitAsync(Timeout);
+        await secondOut.NextAsync(Timeout);
+
+        using var thirdCts = new CancellationTokenSource();
+        var thirdOut = new RecordingWriter();
+        var third = service.Sync(new SilentReader(), thirdOut, ContextFor(thirdCts.Token));
+        await second.WaitAsync(Timeout);
+        Assert.Equal(SyncMessage.MsgOneofCase.Handshake, (await thirdOut.NextAsync(Timeout)).MsgCase);
+
+        thirdCts.Cancel();
+        await SwallowAsync(third);
+    }
+
+    private static async Task SwallowAsync(Task task)
+    {
+        try { await task.WaitAsync(Timeout); }
+        catch (OperationCanceledException) { }
+        catch (RpcException) { }
+    }
+
+    /// <summary>
+    /// The minimum <see cref="ServerCallContext"/> the handler actually reads: its cancellation
+    /// token. (Grpc.Core.Api ships no test double, and the ASP.NET Core one is not reachable outside
+    /// a live call.)
+    /// </summary>
+    private sealed class FakeCallContext(CancellationToken ct) : ServerCallContext
+    {
+        protected override string MethodCore => "/localdb_sync.v1.LocalDbSync/Sync";
+        protected override string HostCore => "test";
+        protected override string PeerCore => "test";
+        protected override DateTime DeadlineCore => DateTime.MaxValue;
+        protected override Metadata RequestHeadersCore => [];
+        protected override CancellationToken CancellationTokenCore => ct;
+        protected override Metadata ResponseTrailersCore { get; } = [];
+        protected override Status StatusCore { get; set; }
+        protected override WriteOptions? WriteOptionsCore { get; set; }
+        protected override AuthContext AuthContextCore => new(null, new Dictionary<string, List<AuthProperty>>());
+
+        protected override ContextPropagationToken CreatePropagationTokenCore(ContextPropagationOptions? options) =>
+            throw new NotSupportedException();
+
+        protected override Task WriteResponseHeadersAsyncCore(Metadata responseHeaders) => Task.CompletedTask;
+    }
+
+    /// <summary>A stream that never delivers anything and never ends — a half-open link.</summary>
+    private sealed class SilentReader : IAsyncStreamReader<SyncMessage>
+    {
+        public SyncMessage Current => throw new InvalidOperationException("no message");
+
+        public async Task<bool> MoveNext(CancellationToken cancellationToken)
+        {
+            await Task.Delay(System.Threading.Timeout.Infinite, cancellationToken);
+            return false;
+        }
+    }
+
+    private sealed class RecordingWriter : IServerStreamWriter<SyncMessage>
+    {
+        private readonly Channel<SyncMessage> _written = Channel.CreateUnbounded<SyncMessage>();
+
+        public WriteOptions? WriteOptions { get; set; }
+
+        public Task WriteAsync(SyncMessage message) => WriteAsync(message, CancellationToken.None);
+
+        public Task WriteAsync(SyncMessage message, CancellationToken cancellationToken)
+        {
+            _written.Writer.TryWrite(message);
+            return Task.CompletedTask;
+        }
+
+        public async Task<SyncMessage> NextAsync(TimeSpan timeout)
+        {
+            using var cts = new CancellationTokenSource(timeout);
+            return await _written.Reader.ReadAsync(cts.Token);
+        }
+    }
+}