docs(index): LocalDb 0.3.3 published — FK-parent registration guard + atomic per-entry apply (ScadaBridge arch-review P4-7); ScadaBridge pins 0.3.3

Three review follow-ups on 122940c. Still 0.3.3 — unpublished, so no further
version bump. No behaviour change to any succeeding path.

- The per-entry poison handler's recovery work (ROLLBACK TO + the dead-letter
  INSERT) was unguarded: a wedged connection, an I/O error, or a dead-letter
  INSERT that faults would propagate the SECOND exception and silently discard
  the FIRST — the one that says why the entry was poison at all. The pair is now
  wrapped; a recovery failure throws an AggregateException whose message names
  the entry (table + seq) and the recovery fault, and whose InnerException is the
  ORIGINAL entry fault, with the recovery fault carried alongside it at
  InnerExceptions[1] with its own stack. The outcome is unchanged and still
  fail-closed: nothing commits, and disposing the batch transaction unwinds it.
  OperationCanceledException is deliberately excluded from the wrap so a
  shutdown still surfaces as a cancellation rather than as a fault. No upstream
  caller catches SqliteException from the apply path (checked), so widening the
  escaping type is safe.

  Injected cheaply after all: a RAISE(ABORT) trigger on __localdb_dead_letter,
  stacked on the existing row-version poison trigger, produces exactly the
  two-fault case with no production test seam. The test asserts both causes are
  carried AND that the batch committed nothing — the entry that applied cleanly
  before the poison one is gone and the peer watermark never moved.

- Register_SelfReferencingCascade_Throws pins the README's "or from itself"
  claim: a table that is its own cascading parent (parent_id REFERENCES
  nodes(id) ON DELETE CASCADE) is refused. The guard needed no special case —
  it reads every foreign key in the database — but the promise is now tested.

- Comment on why EnterEntryAsync sits outside the try: a SAVEPOINT that failed
  opened nothing, so a handler ROLLBACK TO would name a savepoint that does not
  exist and fault in turn.

Both new tests were run against 122940c first and fail there: the self-reference
case throws nothing, and the stacked-fault case escapes as a bare
SqliteException("dead-letter write failed") with the original cause gone.

Suite: 215/215 green, build 0 warnings / 0 errors.

ZB.MOM.WW.LocalDb/README.md

@@ -14,31 +14,36 @@ last-writer-wins over a hybrid logical clock. All timestamps are UTC.
 
 | Package | Contents |
 |---|---|
-| `ZB.MOM.WW.LocalDb` | Embedded DB host: `ILocalDb` SQL surface (`ExecuteAsync` / `QueryAsync` / transactions), `RegisterReplicated(table[, baselineExistingRows])` (PK validation + trigger install + oplog schema), `DeregisterReplicated(table)` (trigger removal + oplog/ledger prune), `HybridLogicalClock`, `AddZbLocalDb()` DI. |
+| `ZB.MOM.WW.LocalDb` | Embedded DB host: `ILocalDb` SQL surface (`ExecuteAsync` / `QueryAsync` / transactions), `RegisterReplicated(table[, baselineExistingRows])` (PK + BLOB + foreign-key validation, trigger install, oplog schema), `DeregisterReplicated(table)` (trigger removal + oplog/ledger prune), `HybridLogicalClock`, `AddZbLocalDb()` DI. |
 | `ZB.MOM.WW.LocalDb.Contracts` | `localdb_sync.v1` proto + generated gRPC types (packable; a future peer could be non-.NET). |
 | `ZB.MOM.WW.LocalDb.Replication` | gRPC sync engine: passive `LocalDbSyncService`, initiating `SyncBackgroundService`, `MaintenanceBackgroundService`, LWW apply, watermark acks + pruning, snapshot resync, `ISyncStatus`, `LocalDbMetrics`, `AddZbLocalDbReplication()` / `MapZbLocalDbSync()`. |
 
 > Status: **published to the Gitea feed; adopted by ScadaBridge (Phases 1+2) and OtOpcUa (Phase 1).**
 >
-> - `0.3.3` (2026-08-19) — one Low finding from the ScadaBridge adversarial architecture review
->   (P4-7), in two halves. Wire-compatible with every earlier peer: no proto change, no
+> - `0.3.3` (2026-08-20) — two latent apply-path hazards from the ScadaBridge adversarial
+>   architecture review (P4-7). Neither had a live trigger in any current consumer; both are the
+>   kind that surface as silent divergence, not as an error. Wire-compatible: no proto change, no
 >   `lib_schema_version` bump, no on-disk schema change.
->   - **`RegisterReplicated` now refuses a table that another table cascades a DELETE onto.**
->     Applying a remote row uses `INSERT OR REPLACE`, whose implicit DELETE fires `ON DELETE
->     CASCADE` / `SET NULL` / `SET DEFAULT` on every child — while the apply path holds the capture
->     guard raised, so the child rows it touches produce no oplog entry, the peer keeps them, and the
->     pair diverges with nothing reporting a fault. Registration is the only place that is visible,
->     so it is refused there with an actionable `LocalDbRegistrationException` naming the referencing
->     table and its action. `NO ACTION` / `RESTRICT` are still allowed: they mutate no child row (the
->     implicit delete simply fails, which the applier dead-letters — loud, not silent). No current
->     consumer has a foreign key on a replicated table, so this is a guard against a future foot-gun
->     rather than a live fix.
->   - **Applying one entry is atomic across its row write and its ledger write.** They were two
->     statements under the poison catch, so a failure landing between them committed a
->     deleted/replaced row carrying a STALE LIVE ledger version — the shape `SnapshotStreamer` warns
->     about and skips, meaning that key never travels again. A per-entry `SAVEPOINT` makes the pair
->     all-or-nothing while keeping the catch's per-ENTRY granularity: entries applied earlier in the
->     same batch survive, and the poison entry is still dead-lettered and the batch still continues.
+>   - **A replicated table may no longer be the parent of a row-mutating `ON DELETE` foreign key.**
+>     The apply path writes remote rows with `INSERT OR REPLACE`, and REPLACE resolves a PK conflict
+>     by DELETING the existing row first. Connections run `PRAGMA foreign_keys=ON`, so that implicit
+>     delete fires the child's `ON DELETE CASCADE` / `SET NULL` / `SET DEFAULT` — while the capture
+>     guard is raised. The child rows change on the applying node ONLY, produce no oplog entry, and
+>     the pair diverges with nothing able to detect it (LWW compares only rows it was told about).
+>     `RegisterReplicated` now inspects every foreign key in the database and refuses such a table
+>     with a `LocalDbRegistrationException` naming the offending child column and action.
+>     `RESTRICT` / `NO ACTION` remain fine — they abort the statement instead of mutating rows
+>     behind capture's back, which the apply path already handles as per-entry poison. Checked at
+>     registration; SQLite cannot add a foreign key to an existing table, so a child table created
+>     later is caught at the next startup's registration rather than at the DDL.
+>   - **An entry's row write and its ledger write are now atomic with each other.** They are two
+>     statements, and SQLite aborts only the failing one — so a statement-level fault between them
+>     (the case the per-entry poison catch exists for) committed a half-applied entry: a tombstone
+>     whose table row was gone while `__localdb_row_version` still called it live, which the snapshot
+>     streamer can only warn about and skip, so the peer never learned the row died; or an upsert
+>     whose new row carried the OLD version and therefore lost the next comparison it should have
+>     won. The pair is wrapped in a per-entry `SAVEPOINT` that is rolled back before dead-lettering,
+>     so a poison entry leaves both writes or neither and the rest of the batch is untouched.
 > - `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.
@@ -279,18 +284,15 @@ All options are validated at startup (`ZB.MOM.WW.Configuration` validators).
   `RegisterReplicated`, because `json_object` cannot capture it). A *typeless* column can
   still hold a BLOB value at runtime, which then surfaces as a write-time error, not a
   registration error.
-- **Replicated tables must not be cascade-delete foreign-key parents.** If another table
-  references the one you are registering with `ON DELETE CASCADE` (or `SET NULL` /
-  `SET DEFAULT`), `RegisterReplicated` throws. Applying a remote row uses `INSERT OR
-  REPLACE`, whose implicit DELETE fires that referential action while change capture is
-  suppressed — so the child rows it deletes or nulls are never captured, the peer keeps
-  them, and the pair diverges with nothing reporting a fault. Model the parent/child
-  relationship with `ON DELETE NO ACTION` (or `RESTRICT`) and delete children explicitly, or
-  leave the parent unreplicated. `NO ACTION` / `RESTRICT` are allowed because they mutate no
-  child row: the implicit delete just fails, and the applier dead-letters the entry — loud,
-  not silent. The check runs at **registration time**, so a cascading child table created
-  *later* is not caught (SQLite offers no DDL hook); register after schema setup, which is
-  where startup does it anyway.
+- **A replicated table may not be the parent of a row-mutating `ON DELETE` foreign key**
+  (`CASCADE` / `SET NULL` / `SET DEFAULT` from any other table, or from itself). Applying a
+  remote row uses `INSERT OR REPLACE`, whose implicit delete fires that action with change
+  capture suppressed — the child rows would change on the applying node only and never
+  replicate, diverging the pair undetectably. `RegisterReplicated` refuses such a table
+  (`0.3.3`); use `ON DELETE NO ACTION`/`RESTRICT` and delete children explicitly, or leave
+  the parent unreplicated. The check runs at registration and reads the whole database's
+  foreign keys, so a child table created *after* registration is caught at the next startup,
+  not at the moment of the `CREATE TABLE`.
 - **CDC semantics** — rows that existed **before** `RegisterReplicated` are not captured
   and are not snapshotted; only changes after registration are replicated. **Opt out with
   `RegisterReplicated(table, baselineExistingRows: true)`**, which seeds the row-version

ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb.Replication/Internal/LwwApplier.cs

@@ -12,10 +12,9 @@ internal sealed record ApplyResult(long AppliedThruSeq, int Applied, int Discard
 /// Applies a peer's oplog batch under last-writer-wins in one transaction: the applying guard is
 /// raised so capture triggers skip these writes, per-entry LWW resolves against the local
 /// row-version (hlc, then node_id ordinal tie-break), poison rows are dead-lettered while the
-/// batch continues, and the remote watermark advances in the SAME transaction as the data so a
+/// batch continues — each entry's row write and ledger write sharing a savepoint so a dead-lettered
+/// entry leaves neither — and the remote watermark advances in the SAME transaction as the data so a
 /// crash-replay of the batch is a no-op (LWW's strict-greater compare loses on equal versions).
-/// Each entry's row write and its ledger write are additionally scoped by a per-entry SAVEPOINT, so
-/// the pair is all-or-nothing even when the poison catch keeps the batch alive around it.
 /// This never writes <c>__localdb_oplog</c>: the 2-node topology does not re-forward applied deltas.
 /// </summary>
 /// <remarks>
@@ -90,49 +89,59 @@ internal sealed class LwwApplier(SqliteLocalDb db, Func<DateTimeOffset>? utcNow
                 continue;
             }
 
-            // The row mutation and its ledger entry are TWO statements and must land together or
-            // not at all: a failure between them commits a deleted/replaced row carrying a stale
-            // ledger version, which SnapshotStreamer then warns about and skips — so that key never
-            // travels again and the pair is silently, permanently divergent for it. A per-entry
-            // savepoint makes the pair atomic without giving up the poison catch's per-ENTRY
-            // granularity (the surrounding transaction, and every entry already applied in it,
-            // survives a rollback to the savepoint). Cost measured on the path that matters — a
-            // 100 000-row backlog drain — is ~7% (178.6k rows/s → 166.1k rows/s): the two extra
-            // statements are prepared once like the rest and neither one touches the disk, since
-            // an inner savepoint's release just merges into the enclosing transaction.
-            var pairApplied = false;
-            await sql.SavepointAsync(ct);
+            var table = db.ReplicatedTables[entry.TableName];
+
+            // An entry is TWO statements — the row change and its ledger row — and SQLite aborts
+            // only the failing statement, so a fault between them used to commit a half-applied
+            // entry: a tombstone whose table row is gone while __localdb_row_version still calls it
+            // live (which SnapshotStreamer can only warn about and skip, so the peer never learns
+            // the row died), or an upsert whose new row carries the OLD version and therefore loses
+            // the next LWW comparison it should win. A per-entry savepoint makes the pair
+            // all-or-nothing without widening the poison catch to the whole batch.
+            //
+            // Opened OUTSIDE the try on purpose: a SAVEPOINT that failed opened nothing, so the
+            // handler's ROLLBACK TO would name a savepoint that does not exist and fault in turn.
+            await sql.EnterEntryAsync(ct);
             try
             {
-                var table = db.ReplicatedTables[entry.TableName];
                 if (entry.IsTombstone)
                     await sql.DeleteAsync(table, entry.PkJson, ct);
                 else
                     await sql.UpsertAsync(table, entry.RowJson!, ct);
 
                 await sql.WriteRowVersionAsync(entry, utcNow, ct);
-                pairApplied = true;
+                await sql.ReleaseEntryAsync(ct);
+                applied++;
             }
             catch (SqliteException ex)
             {
                 // Deliberately narrow: only statement-level SQLite errors (e.g. json_extract on
                 // malformed row_json) are treated as per-entry poison — SQLite aborts just the
                 // statement, not the transaction, so the batch continues after dead-lettering.
-                // Any other exception propagates and rolls back the whole batch (fail-closed).
-                // The rollback comes FIRST so the dead-letter row it records is not discarded with
-                // the half-applied pair.
-                await sql.RollbackToSavepointAsync(ct);
-                await sql.DeadLetterAsync(entry, ex.Message, utcNow, ct);
-                deadLettered++;
-            }
+                // Any other exception propagates with the savepoint still open, and disposing the
+                // outer transaction unwinds the whole batch (fail-closed).
+                try
+                {
+                    await sql.RollbackEntryAsync(ct);
+                    await sql.DeadLetterAsync(entry, ex.Message, utcNow, ct);
+                }
+                catch (Exception recoveryFailure) when (recoveryFailure is not OperationCanceledException)
+                {
+                    // Recovering from the poison entry itself failed — a wedged connection, an I/O
+                    // error, a dead-letter INSERT that faults. The outcome is unchanged and still
+                    // fail-closed (nothing commits; disposing the batch transaction unwinds it),
+                    // but reporting only the recovery failure would throw away the ORIGINAL cause,
+                    // which is the one that says WHY this entry is poison. Both are carried out:
+                    // the original as InnerException (AggregateException surfaces the first inner
+                    // there), the recovery failure alongside it with its own stack intact.
+                    throw new AggregateException(
+                        $"Failed to dead-letter a poison oplog entry for table '{entry.TableName}' " +
+                        $"(seq {entry.Seq}): {recoveryFailure.Message}. The entry's own fault is the " +
+                        "inner exception. The batch is not committed.",
+                        ex, recoveryFailure);
+                }
 
-            // Released outside the catch on purpose: a RELEASE that itself fails means the
-            // transaction is no longer sound, and there is no savepoint state left to reason about,
-            // so it propagates and rolls the whole batch back rather than being swallowed as poison.
-            if (pairApplied)
-            {
-                await sql.ReleaseSavepointAsync(ct);
-                applied++;
+                deadLettered++;
             }
         }
 
@@ -164,47 +173,21 @@ internal sealed class LwwApplier(SqliteLocalDb db, Func<DateTimeOffset>? utcNow
         private readonly Dictionary<string, SqliteCommand> _deletes = new(StringComparer.Ordinal);
         private readonly Dictionary<string, SqliteCommand> _upserts = new(StringComparer.Ordinal);
 
+        /// <summary>Name of the per-entry savepoint. Fixed: only one is ever open at a time.</summary>
+        private const string EntrySavepoint = "zb_lww_entry";
+
         private SqliteCommand? _applying;
+        private SqliteCommand? _enterEntry;
+        private SqliteCommand? _releaseEntry;
+        private SqliteCommand? _rollbackEntry;
         private SqliteCommand? _lookup;
         private SqliteCommand? _rowVersion;
         private SqliteCommand? _deadLetter;
         private SqliteCommand? _peerState;
-        private SqliteCommand? _savepoint;
-        private SqliteCommand? _releaseSavepoint;
-        private SqliteCommand? _rollbackToSavepoint;
-
-        /// <summary>The per-entry savepoint's name. Reused for every entry — one is open at a time.</summary>
-        private const string SavepointName = "__localdb_apply_entry";
 
         /// <summary>How many distinct statements this set has prepared.</summary>
         public int StatementCount => _owned.Count;
 
-        /// <summary>Opens the per-entry savepoint that scopes one entry's row + ledger writes.</summary>
-        public Task SavepointAsync(CancellationToken ct)
-        {
-            var cmd = _savepoint ??= Create("SAVEPOINT " + SavepointName);
-            return cmd.ExecuteNonQueryAsync(ct);
-        }
-
-        /// <summary>Commits the entry's savepoint into the surrounding batch transaction.</summary>
-        public Task ReleaseSavepointAsync(CancellationToken ct)
-        {
-            var cmd = _releaseSavepoint ??= Create("RELEASE " + SavepointName);
-            return cmd.ExecuteNonQueryAsync(ct);
-        }
-
-        /// <summary>
-        /// Undoes everything the entry did and closes its savepoint. ROLLBACK TO leaves the savepoint
-        /// OPEN, so the RELEASE is required — without it the names would nest one deeper per poison
-        /// entry and the next rollback would unwind more than one entry's work.
-        /// </summary>
-        public async Task RollbackToSavepointAsync(CancellationToken ct)
-        {
-            var cmd = _rollbackToSavepoint ??= Create("ROLLBACK TO " + SavepointName);
-            await cmd.ExecuteNonQueryAsync(ct);
-            await ReleaseSavepointAsync(ct);
-        }
-
         public Task SetApplyingAsync(int value, CancellationToken ct)
         {
             var cmd = _applying ??= Create(
@@ -213,6 +196,32 @@ internal sealed class LwwApplier(SqliteLocalDb db, Func<DateTimeOffset>? utcNow
             return cmd.ExecuteNonQueryAsync(ct);
         }
 
+        /// <summary>Opens the per-entry savepoint inside the batch transaction.</summary>
+        public Task EnterEntryAsync(CancellationToken ct)
+        {
+            var cmd = _enterEntry ??= Create($"SAVEPOINT {EntrySavepoint}");
+            return cmd.ExecuteNonQueryAsync(ct);
+        }
+
+        /// <summary>Commits the entry's work into the batch transaction (pops the savepoint).</summary>
+        public Task ReleaseEntryAsync(CancellationToken ct)
+        {
+            var cmd = _releaseEntry ??= Create($"RELEASE {EntrySavepoint}");
+            return cmd.ExecuteNonQueryAsync(ct);
+        }
+
+        /// <summary>
+        /// Undoes everything the entry did and pops the savepoint, leaving the batch transaction
+        /// open and usable. Two statements, because ROLLBACK TO rewinds to the savepoint but leaves
+        /// it ON the stack — without the RELEASE, every dead-lettered entry would nest another one.
+        /// </summary>
+        public async Task RollbackEntryAsync(CancellationToken ct)
+        {
+            var cmd = _rollbackEntry ??= Create($"ROLLBACK TO {EntrySavepoint}");
+            await cmd.ExecuteNonQueryAsync(ct);
+            await ReleaseEntryAsync(ct);
+        }
+
         public async Task<(long Hlc, string NodeId)?> ReadLocalVersionAsync(
             string tableName, string pkJson, CancellationToken ct)
         {

ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb/ILocalDb.cs

@@ -38,17 +38,19 @@ public interface ILocalDb
     /// schema digest, and installs the three AFTER triggers that capture every change into the
     /// oplog + row-version tables within the same transaction as the consumer's write. Idempotent
     /// (re-registering regenerates the triggers). Throws <see cref="LocalDbRegistrationException"/>
-    /// if the name is unsafe, the table is missing, it has no primary key, it has a BLOB column, or
-    /// another table references it with a row-mutating <c>ON DELETE</c> action.
+    /// if the name is unsafe, the table is missing, it has no primary key, or it is the parent of a
+    /// foreign key whose <c>ON DELETE</c> action mutates the child (<c>CASCADE</c> / <c>SET NULL</c> /
+    /// <c>SET DEFAULT</c>) — see the remarks for why that last one cannot be replicated.
     /// </summary>
     /// <remarks>
-    /// <b>Replicated tables must not be cascade-delete foreign-key parents.</b> Applying a remote row
-    /// uses <c>INSERT OR REPLACE</c>, whose implicit DELETE fires <c>ON DELETE CASCADE</c> /
-    /// <c>SET NULL</c> / <c>SET DEFAULT</c> on referencing tables while change capture is suppressed
-    /// — the child rows it touches are never captured, so the peer keeps them and the pair diverges
-    /// silently. Such a table is refused here, because registration is the only point at which the
-    /// problem is still visible. <c>NO ACTION</c> / <c>RESTRICT</c> are allowed: they mutate no child
-    /// row, so the implicit delete simply fails and the applier dead-letters the entry.
+    /// <b>Replicated tables may not be the parent of a cascading foreign key.</b> The apply path
+    /// writes remote rows with <c>INSERT OR REPLACE</c>; REPLACE deletes the conflicting row first,
+    /// and with <c>PRAGMA foreign_keys=ON</c> that delete fires the child's <c>ON DELETE</c> action
+    /// while change capture is suppressed. The child rows would be mutated on the applying node only,
+    /// never captured and never shipped, diverging the pair with nothing able to detect it. The check
+    /// runs at registration and inspects the whole database's foreign keys; SQLite cannot add a
+    /// foreign key to an existing table, so a later-created child table is caught on the next
+    /// registration (i.e. the next startup), not at the moment of the DDL.
     /// </remarks>
     ReplicatedTable RegisterReplicated(string tableName);
 

ZB.MOM.WW.LocalDb/src/ZB.MOM.WW.LocalDb/Internal/SqliteLocalDb.cs

@@ -199,7 +199,7 @@ internal sealed class SqliteLocalDb : ILocalDb, IDisposable
                 throw new LocalDbRegistrationException(
                     $"Cannot register table '{tableName}': replication requires an explicit primary key.");
 
-            EnsureNoRowMutatingDeleteAction(tableName);
+            EnsureNoInboundActionOnDelete(tableName);
 
             var allColumns = columns.Select(c => (c.Name, c.Type)).ToList();
             var digest = ReplicatedTable.ComputeDigest(tableName, pkColumns, allColumns);
@@ -296,86 +296,65 @@ internal sealed class SqliteLocalDb : ILocalDb, IDisposable
     }
 
     /// <summary>
-    /// Refuses to register <paramref name="tableName"/> when another table references it with an
-    /// <c>ON DELETE</c> action that MUTATES the referencing rows (<c>CASCADE</c>, <c>SET NULL</c>,
-    /// <c>SET DEFAULT</c>).
+    /// Refuses to register a table that is the PARENT of a foreign key whose <c>ON DELETE</c> action
+    /// mutates the child (<c>CASCADE</c>, <c>SET NULL</c>, <c>SET DEFAULT</c>).
     /// </summary>
     /// <remarks>
     /// <para>
-    /// <b>Why this is a replication correctness rule and not a schema opinion.</b> Applying a peer's
-    /// row runs <c>INSERT OR REPLACE</c>, and REPLACE resolves a primary-key conflict by DELETING the
-    /// existing row first. Connections run with <c>PRAGMA foreign_keys=ON</c>, so that implicit delete
-    /// fires the referential action on every child table — while the apply path holds the capture
-    /// guard raised, precisely so its own writes are not re-captured. The child rows are therefore
-    /// deleted (or nulled) locally and produce NO oplog entry: the peer never hears about them, keeps
-    /// them, and the pair is durably divergent with nothing anywhere reporting a fault.
+    /// <b>Why this is a replication hazard, not a schema opinion.</b> The apply path writes a remote
+    /// row with <c>INSERT OR REPLACE</c>, and REPLACE resolves a primary-key conflict by DELETING the
+    /// existing row before inserting the new one. Connections run <c>PRAGMA foreign_keys=ON</c>, and
+    /// that implicit delete fires foreign-key actions (verified behaviour, not inference). It runs
+    /// with the capture guard raised, so the resulting child-row deletes/nulls are invisible to the
+    /// oplog: they happen on this node and are never shipped. The peer, applying the same parent row,
+    /// keeps its children — and the pair has silently diverged with nothing to detect it, since LWW
+    /// only ever compares rows it was told about.
     /// </para>
     /// <para>
-    /// Refusing at registration is the only place the problem is visible. At apply time the damage is
-    /// already done and indistinguishable from a normal write, and no runtime signal exists to detect
-    /// it. <b>NO ACTION / RESTRICT are deliberately allowed</b>: they mutate no child row — the
-    /// implicit delete simply fails, which the applier surfaces as a dead-lettered entry. Loud, not
-    /// silent, so there is nothing to guard.
+    /// <b>Checked at registration only.</b> SQLite cannot add a foreign key to an existing table, so
+    /// a new offender can only arrive as a child table created (or a parent table rebuilt) after this
+    /// call. Consumers register their tables at startup, after schema creation/migration, which is
+    /// where the check belongs; a schema change that adds a cascading child to an already-registered
+    /// parent is caught on the next boot, not at the moment of the DDL.
     /// </para>
     /// <para>
-    /// The check is registration-time, so a cascading child table CREATED LATER is not caught (SQLite
-    /// has no DDL hook to catch it on). Registration normally runs at startup after schema setup,
-    /// which is where the constraint belongs.
+    /// <c>RESTRICT</c> / <c>NO ACTION</c> are deliberately allowed: they abort the offending statement
+    /// with a constraint error rather than mutating rows behind capture's back, which the apply path
+    /// already handles as per-entry poison.
     /// </para>
     /// </remarks>
-    private void EnsureNoRowMutatingDeleteAction(string tableName)
+    private void EnsureNoInboundActionOnDelete(string tableName)
     {
-        var candidates = new List<string>();
+        var offenders = new List<string>();
         using (var cmd = _master.CreateCommand())
         {
-            // Every user table is a candidate CHILD; the internal sqlite_* tables carry no
-            // foreign keys. The registered table itself is included: a self-referencing cascade
-            // is the same hazard.
-            cmd.CommandText = "SELECT name FROM sqlite_master WHERE type = 'table' AND name NOT LIKE 'sqlite_%'";
+            // pragma_foreign_key_list is the table-valued form of PRAGMA foreign_key_list, so the
+            // whole database is one query: fk."table" is the PARENT this child points at.
+            cmd.CommandText =
+                "SELECT m.name, fk.\"from\", fk.\"on_delete\" " +
+                "FROM sqlite_master m JOIN pragma_foreign_key_list(m.name) fk " +
+                "WHERE m.type = 'table' AND fk.\"table\" = $t COLLATE NOCASE " +
+                "AND UPPER(fk.\"on_delete\") IN ('CASCADE', 'SET NULL', 'SET DEFAULT') " +
+                "ORDER BY m.name, fk.\"from\"";
+            cmd.Parameters.AddWithValue("$t", tableName);
             using var reader = cmd.ExecuteReader();
             while (reader.Read())
-                candidates.Add(reader.GetString(0));
+                offenders.Add(
+                    $"{reader.GetString(0)}.{reader.GetString(1)} ON DELETE {reader.GetString(2).ToUpperInvariant()}");
         }
 
-        foreach (var child in candidates)
-        {
-            using var cmd = _master.CreateCommand();
-            cmd.CommandText = $"PRAGMA foreign_key_list({TriggerSqlGenerator.Ident(child)})";
-            using var reader = cmd.ExecuteReader();
-            while (reader.Read())
-            {
-                // foreign_key_list cols: 0=id, 1=seq, 2=table (the PARENT), 3=from, 4=to,
-                // 5=on_update, 6=on_delete, 7=match.
-                var parent = reader.IsDBNull(2) ? "" : reader.GetString(2);
-                // SQLite identifiers are case-insensitive, and the REFERENCES clause need not
-                // match the CREATE TABLE spelling.
-                if (!string.Equals(parent, tableName, StringComparison.OrdinalIgnoreCase))
-                    continue;
+        if (offenders.Count == 0)
+            return;
 
-                var onDelete = reader.IsDBNull(6) ? "" : reader.GetString(6);
-                if (!RowMutatingDeleteActions.Contains(onDelete))
-                    continue;
-
-                throw new LocalDbRegistrationException(
-                    $"Cannot register table '{tableName}': table '{child}' has a foreign key referencing it " +
-                    $"with ON DELETE {onDelete}. Replicated tables must not be cascade-delete foreign-key " +
-                    "parents: applying a remote row uses INSERT OR REPLACE, whose implicit DELETE fires the " +
-                    "referential action while change capture is suppressed — the child rows it touches are " +
-                    "never captured, so the peer never learns of them and the two nodes diverge silently. " +
-                    $"Drop the referential action on '{child}' (ON DELETE NO ACTION, removing children " +
-                    "explicitly instead), or do not replicate this table.");
-            }
-        }
+        throw new LocalDbRegistrationException(
+            $"Cannot register table '{tableName}': it is the parent of a foreign key with a row-mutating " +
+            $"ON DELETE action ({string.Join(", ", offenders)}). Replication applies remote rows with " +
+            "INSERT OR REPLACE, whose implicit delete fires that action with change capture suppressed — " +
+            "the child rows would be changed on this node only and never replicated, diverging the pair " +
+            "undetectably. Use ON DELETE NO ACTION/RESTRICT and delete children explicitly, or leave this " +
+            "table unreplicated.");
     }
 
-    /// <summary>
-    /// The <c>ON DELETE</c> actions that mutate referencing rows, as SQLite spells them in
-    /// <c>PRAGMA foreign_key_list</c>. NO ACTION / RESTRICT are absent on purpose — see
-    /// <see cref="EnsureNoRowMutatingDeleteAction"/>.
-    /// </summary>
-    private static readonly HashSet<string> RowMutatingDeleteActions =
-        new(["CASCADE", "SET NULL", "SET DEFAULT"], StringComparer.OrdinalIgnoreCase);
-
     /// <summary>
     /// Seeds <c>__localdb_row_version</c> at the baseline floor for every row of
     /// <paramref name="table"/> that has no ledger entry, and flags a snapshot resync if it seeded

ZB.MOM.WW.LocalDb/tests/ZB.MOM.WW.LocalDb.Tests/LwwApplierTests.cs

@@ -428,10 +428,9 @@ public sealed class LwwApplierTests : IDisposable
 
         Assert.Equal(202, result.Applied);
         // Six table-independent statements (applying guard, version lookup, row-version write,
-        // peer state, plus the per-entry savepoint's SAVEPOINT and RELEASE), an upsert per table
-        // touched, and a delete for the one table tombstoned. The savepoint's ROLLBACK TO and the
-        // dead-letter insert are absent because this batch has no poison entry — statements are
-        // built on first use, so the count tracks the statement KINDS the batch actually ran.
+        // peer state, and the per-entry savepoint's enter/release), plus an upsert per table
+        // touched and a delete for the one table tombstoned. The savepoint's rollback statement is
+        // built on first use like the rest, and nothing in this batch is poison.
         Assert.Equal(9, applier.LastBatchStatementCount);
 
         // ...and the batch is still correct, statement reuse notwithstanding.
@@ -465,70 +464,118 @@ public sealed class LwwApplierTests : IDisposable
         Assert.Equal(("LAST", (long?)5), await ReadOrder(db, 5));
     }
 
+    /// <summary>
+    /// Arms a BEFORE INSERT trigger on the row-version ledger that aborts the write for one key —
+    /// a genuine statement-level SQLite fault (SQLITE_CONSTRAINT), landing exactly between an
+    /// entry's table write and its ledger write, with no production test seam.
+    /// </summary>
+    private static Task PoisonRowVersionWriteFor(SqliteLocalDb db, string pkJson) =>
+        db.ExecuteAsync(
+            "CREATE TRIGGER poison_rv BEFORE INSERT ON __localdb_row_version " +
+            $"WHEN NEW.pk_json = '{pkJson}' BEGIN SELECT RAISE(ABORT, 'ledger write failed'); END");
+
     [Fact]
-    public async Task Apply_LedgerWriteFails_LeavesNeitherTheDeleteNorTheLedgerEntry()
+    public async Task Apply_TombstoneWhoseLedgerWriteFails_LeavesRowAndLedgerUntouched()
     {
-        // The hazard (P4-7): applying a tombstone is TWO statements — delete the row, then write the
-        // ledger's tombstone version — and the poison catch commits whatever the first one already
-        // did. A failure landing between them therefore committed a DELETED ROW carrying a STALE LIVE
-        // ledger entry, which is exactly the shape SnapshotStreamer warns about and skips: the key
-        // then never travels again and the pair is divergent for that row, permanently and silently.
-        //
-        // The failure is injected with a trigger that aborts the ledger write (in production the
-        // realistic causes are disk-full / IO error / corruption — none of them reproducible here),
-        // and RAISE(ABORT) aborts only the statement, so the applier's narrow SqliteException catch
-        // treats it as per-entry poison exactly as it would a real one.
+        // The hazard: DELETE and the ledger write are two statements, and SQLite aborts only the
+        // failing one. Without a per-entry savepoint the row is gone while __localdb_row_version
+        // still calls it live — SnapshotStreamer can then only warn and skip it, so the peer never
+        // learns the row died and the pair diverges. Both or neither.
         using var db = await NewOrdersDb();
         await db.ExecuteAsync("INSERT INTO orders (id, sku, qty) VALUES (1, 'LOCAL', 5)");
         var localHlc = await RowVersionHlc(db, Pk1);
-
-        await db.ExecuteAsync(
-            "CREATE TRIGGER poison_ledger BEFORE INSERT ON __localdb_row_version " +
-            "WHEN NEW.is_tombstone = 1 BEGIN SELECT RAISE(ABORT, 'injected ledger failure'); END");
+        await PoisonRowVersionWriteFor(db, Pk1);
 
         var result = await new LwwApplier(db).ApplyBatchAsync(
             [Entry(90, Pk1, null, localHlc + 1000, "remote-node", tombstone: true)], default);
 
-        // The entry is poison, so it is dead-lettered and the batch continues — unchanged semantics.
-        Assert.Equal(1, result.DeadLettered);
         Assert.Equal(0, result.Applied);
-
-        // BOTH or NEITHER. Neither, here: the row must still be present, because its ledger entry
-        // still says "live at the local version".
+        Assert.Equal(1, result.DeadLettered);
         Assert.Equal(("LOCAL", (long?)5), await ReadOrder(db, 1));
         var rv = await db.QueryAsync(
             "SELECT hlc, is_tombstone FROM __localdb_row_version WHERE table_name = 'orders' AND pk_json = @pk",
             x => (Hlc: x.GetInt64(0), Tomb: x.GetInt64(1)), new { pk = Pk1 });
-        Assert.Single(rv);
         Assert.Equal(localHlc, rv[0].Hlc);
         Assert.Equal(0, rv[0].Tomb);
     }
 
     [Fact]
-    public async Task Apply_LedgerWriteFails_MidBatch_LeavesTheOtherEntriesApplied()
+    public async Task Apply_UpsertWhoseLedgerWriteFails_LeavesRowAndLedgerUntouched()
     {
-        // The atomicity must be per-ENTRY, not per-batch: rolling back to the savepoint may not
-        // discard the rows applied before the poison one, nor block the ones after it.
+        // Same split, other direction: the new row would sit in the table carrying the OLD version,
+        // so it would lose the very next LWW comparison it should win.
         using var db = await NewOrdersDb();
-        await db.ExecuteAsync("INSERT INTO orders (id, sku, qty) VALUES (2, 'DOOMED', 2)");
-        var doomedHlc = await RowVersionHlc(db, "{\"id\":2}");
+        await db.ExecuteAsync("INSERT INTO orders (id, sku, qty) VALUES (1, 'LOCAL', 5)");
+        var localHlc = await RowVersionHlc(db, Pk1);
+        await PoisonRowVersionWriteFor(db, Pk1);
 
-        await db.ExecuteAsync(
-            "CREATE TRIGGER poison_ledger BEFORE INSERT ON __localdb_row_version " +
-            "WHEN NEW.is_tombstone = 1 BEGIN SELECT RAISE(ABORT, 'injected ledger failure'); END");
+        var result = await new LwwApplier(db).ApplyBatchAsync(
+            [Entry(91, Pk1, "{\"id\":1,\"sku\":\"REMOTE\",\"qty\":99}", localHlc + 1000, "remote-node")], default);
+
+        Assert.Equal(0, result.Applied);
+        Assert.Equal(1, result.DeadLettered);
+        Assert.Equal(("LOCAL", (long?)5), await ReadOrder(db, 1));
+        Assert.Equal(localHlc, await RowVersionHlc(db, Pk1));
+    }
+
+    [Fact]
+    public async Task Apply_LedgerWriteFailureMidBatch_DoesNotUndoTheRestOfTheBatch()
+    {
+        // The savepoint must rewind ONE entry, not the batch: entries either side of the poison
+        // still apply, and the batch still commits.
+        using var db = await NewOrdersDb();
+        await db.ExecuteAsync("INSERT INTO orders (id, sku, qty) VALUES (1, 'LOCAL', 5)");
+        var localHlc = await RowVersionHlc(db, Pk1);
+        await PoisonRowVersionWriteFor(db, Pk1);
 
         var result = await new LwwApplier(db).ApplyBatchAsync(
         [
-            Entry(91, Pk1, "{\"id\":1,\"sku\":\"BEFORE\",\"qty\":1}", doomedHlc + 1, "remote-node"),
-            Entry(92, "{\"id\":2}", null, doomedHlc + 1000, "remote-node", tombstone: true),
-            Entry(93, "{\"id\":3}", "{\"id\":3,\"sku\":\"AFTER\",\"qty\":3}", doomedHlc + 3, "remote-node"),
+            Entry(92, "{\"id\":2}", "{\"id\":2,\"sku\":\"BEFORE\",\"qty\":2}", localHlc + 1000, "remote-node"),
+            Entry(93, Pk1, null, localHlc + 1001, "remote-node", tombstone: true),
+            Entry(94, "{\"id\":3}", "{\"id\":3,\"sku\":\"AFTER\",\"qty\":3}", localHlc + 1002, "remote-node"),
         ], default);
 
         Assert.Equal(2, result.Applied);
         Assert.Equal(1, result.DeadLettered);
-        Assert.Equal(("BEFORE", (long?)1), await ReadOrder(db, 1));
-        Assert.Equal(("DOOMED", (long?)2), await ReadOrder(db, 2));
+        Assert.Equal(94, result.AppliedThruSeq);
+        Assert.Equal(("BEFORE", (long?)2), await ReadOrder(db, 2));
         Assert.Equal(("AFTER", (long?)3), await ReadOrder(db, 3));
+        Assert.Equal(("LOCAL", (long?)5), await ReadOrder(db, 1));
+    }
+
+    [Fact]
+    public async Task Apply_DeadLetterFailure_ReportsBothCausesAndCommitsNothing()
+    {
+        // Two faults stacked: the entry is poison (its ledger write aborts) AND recovering from it
+        // fails (the dead-letter INSERT aborts too). Reporting only the second would discard the
+        // one that says WHY the entry was poison, leaving an operator with "dead-letter failed"
+        // and no cause.
+        using var db = await NewOrdersDb();
+        await db.ExecuteAsync("INSERT INTO orders (id, sku, qty) VALUES (1, 'LOCAL', 5)");
+        var localHlc = await RowVersionHlc(db, Pk1);
+        await PoisonRowVersionWriteFor(db, Pk1);
+        await db.ExecuteAsync(
+            "CREATE TRIGGER poison_dl BEFORE INSERT ON __localdb_dead_letter " +
+            "BEGIN SELECT RAISE(ABORT, 'dead-letter write failed'); END");
+
+        var ex = await Assert.ThrowsAsync<AggregateException>(() => new LwwApplier(db).ApplyBatchAsync(
+        [
+            Entry(95, "{\"id\":2}", "{\"id\":2,\"sku\":\"BEFORE\",\"qty\":2}", localHlc + 1000, "remote-node"),
+            Entry(96, Pk1, null, localHlc + 1001, "remote-node", tombstone: true),
+        ], default));
+
+        // The entry's own fault first (InnerException), the recovery fault carried alongside it.
+        Assert.Contains("ledger write failed", ex.InnerException!.Message);
+        Assert.Contains("dead-letter write failed", ex.InnerExceptions[1].Message);
+        Assert.Contains("orders", ex.Message);
+        Assert.Contains("96", ex.Message);
+
+        // Fail-closed: the batch never commits, so even the entry that applied cleanly is gone.
+        Assert.Null(await ReadOrder(db, 2));
+        Assert.Equal(("LOCAL", (long?)5), await ReadOrder(db, 1));
+        var watermark = await db.QueryAsync(
+            "SELECT last_applied_remote_seq FROM __localdb_peer_state WHERE id = 1", x => x.GetInt64(0));
+        Assert.Equal(0, watermark[0]);
     }
 
     [Fact]

ZB.MOM.WW.LocalDb/tests/ZB.MOM.WW.LocalDb.Tests/RegistrationTests.cs

@@ -70,75 +70,71 @@ public sealed class RegistrationTests : IDisposable
         Assert.Contains("BLOB", ex.Message);
     }
 
-    // ---- Cascading-delete foreign-key parents (P4-7) --------------------------------------
-
     [Fact]
-    public async Task Register_CascadeDeleteFkParent_Throws()
+    public async Task Register_CascadeDeleteParent_Throws()
     {
-        // The hazard: applying a remote row uses INSERT OR REPLACE, whose implicit DELETE fires
-        // ON DELETE CASCADE while the capture guard is RAISED — so the cascaded child rows are
-        // deleted locally and never captured, and the peer keeps them forever.
+        // The apply path writes remote rows with INSERT OR REPLACE, whose implicit delete fires
+        // ON DELETE CASCADE with capture suppressed — the child rows would die on this node only.
         using var db = Create();
         await CreateOrders(db);
         await db.ExecuteAsync(
-            "CREATE TABLE order_lines (id INTEGER PRIMARY KEY, order_id INTEGER " +
-            "REFERENCES orders(id) ON DELETE CASCADE)");
+            "CREATE TABLE order_lines (id INTEGER PRIMARY KEY, " +
+            "order_id INTEGER REFERENCES orders(id) ON DELETE CASCADE, sku TEXT)");
 
         var ex = Assert.Throws<LocalDbRegistrationException>(() => db.RegisterReplicated("orders"));
         Assert.Contains("orders", ex.Message);
-        Assert.Contains("order_lines", ex.Message);
+        Assert.Contains("order_lines.order_id", ex.Message);
         Assert.Contains("ON DELETE CASCADE", ex.Message);
 
-        // Refused means refused: no triggers installed, nothing in the registry.
+        // Refused, not half-registered.
         Assert.Empty(await Triggers(db));
         Assert.False(db.ReplicatedTables.ContainsKey("orders"));
     }
 
     [Fact]
-    public async Task Register_CascadeDeleteFkChild_IsAllowed()
+    public async Task Register_CascadeDeleteParent_IsWhatWouldActuallyDiverge()
     {
-        // Only the PARENT is refused. The child side is safe: replacing a child row cascades
-        // nothing, and the child's own deletes are captured normally.
+        // Proves the guard is guarding something real: with the parent NOT registered, applying a
+        // replace to it silently wipes child rows that produce no oplog entry.
         using var db = Create();
         await CreateOrders(db);
         await db.ExecuteAsync(
-            "CREATE TABLE order_lines (id INTEGER PRIMARY KEY, order_id INTEGER " +
-            "REFERENCES orders(id) ON DELETE CASCADE)");
+            "CREATE TABLE order_lines (id INTEGER PRIMARY KEY, " +
+            "order_id INTEGER REFERENCES orders(id) ON DELETE CASCADE, sku TEXT)");
+        await db.ExecuteAsync("INSERT INTO orders (id, sku, qty) VALUES (1, 'A', 1)");
+        await db.ExecuteAsync("INSERT INTO order_lines (id, order_id, sku) VALUES (10, 1, 'L')");
 
-        db.RegisterReplicated("order_lines");
+        await db.ExecuteAsync("INSERT OR REPLACE INTO orders (id, sku, qty) VALUES (1, 'B', 2)");
 
-        Assert.True(db.ReplicatedTables.ContainsKey("order_lines"));
+        var lines = await db.QueryAsync("SELECT COUNT(*) FROM order_lines", r => r.GetInt64(0));
+        Assert.Equal(0, lines[0]);
     }
 
     [Theory]
-    [InlineData("ON DELETE SET NULL", "ON DELETE SET NULL")]
-    [InlineData("ON DELETE SET DEFAULT", "ON DELETE SET DEFAULT")]
-    public async Task Register_OtherRowMutatingDeleteActions_AlsoThrow(string clause, string expected)
+    [InlineData("SET NULL")]
+    [InlineData("SET DEFAULT")]
+    public async Task Register_RowMutatingOnDeleteParent_Throws(string action)
     {
-        // SET NULL / SET DEFAULT mutate child rows on the same implicit DELETE and are captured
-        // just as little — same divergence, so the same refusal.
         using var db = Create();
         await CreateOrders(db);
         await db.ExecuteAsync(
-            "CREATE TABLE order_lines (id INTEGER PRIMARY KEY, order_id INTEGER DEFAULT 0 " +
-            $"REFERENCES orders(id) {clause})");
+            "CREATE TABLE order_lines (id INTEGER PRIMARY KEY, " +
+            $"order_id INTEGER DEFAULT 0 REFERENCES orders(id) ON DELETE {action}, sku TEXT)");
 
         var ex = Assert.Throws<LocalDbRegistrationException>(() => db.RegisterReplicated("orders"));
-        Assert.Contains(expected, ex.Message);
+        Assert.Contains($"ON DELETE {action}", ex.Message);
     }
 
     [Fact]
-    public async Task Register_NonCascadingFkParent_IsAllowed()
+    public async Task Register_NonMutatingOnDeleteParent_IsAllowed()
     {
-        // The default (NO ACTION / RESTRICT) mutates no child row: the implicit DELETE simply
-        // fails the statement, which the applier already surfaces as a dead-lettered entry. Noisy,
-        // not silent — so there is nothing to refuse.
+        // RESTRICT/NO ACTION abort the statement instead of changing rows behind capture's back,
+        // which the apply path already handles as per-entry poison.
         using var db = Create();
         await CreateOrders(db);
         await db.ExecuteAsync(
-            "CREATE TABLE order_lines (id INTEGER PRIMARY KEY, order_id INTEGER REFERENCES orders(id))");
-        await db.ExecuteAsync("CREATE TABLE restricted (id INTEGER PRIMARY KEY, " +
-            "order_id INTEGER REFERENCES orders(id) ON DELETE RESTRICT)");
+            "CREATE TABLE order_lines (id INTEGER PRIMARY KEY, " +
+            "order_id INTEGER REFERENCES orders(id) ON DELETE RESTRICT, sku TEXT)");
 
         db.RegisterReplicated("orders");
 
@@ -146,19 +142,35 @@ public sealed class RegistrationTests : IDisposable
     }
 
     [Fact]
-    public async Task Register_UnrelatedTablesWithCascades_DoNotBlockRegistration()
+    public async Task Register_SelfReferencingCascade_Throws()
     {
-        // The scan must key on the table being registered, not on "the database contains a cascade".
+        // A table can be its own cascading parent: replacing a row would delete that row's
+        // children in the SAME table, uncaptured. The guard reads every foreign key in the
+        // database, so it does not need a special case — but the README promises this, so it is
+        // pinned.
+        using var db = Create();
+        await db.ExecuteAsync(
+            "CREATE TABLE nodes (id INTEGER PRIMARY KEY, " +
+            "parent_id INTEGER REFERENCES nodes(id) ON DELETE CASCADE, name TEXT)");
+
+        var ex = Assert.Throws<LocalDbRegistrationException>(() => db.RegisterReplicated("nodes"));
+        Assert.Contains("nodes.parent_id", ex.Message);
+        Assert.Contains("ON DELETE CASCADE", ex.Message);
+    }
+
+    [Fact]
+    public async Task Register_CascadeDeleteChild_IsAllowed()
+    {
+        // Only the PARENT side is hazardous: replacing a child row deletes no other table's rows.
         using var db = Create();
         await CreateOrders(db);
-        await db.ExecuteAsync("CREATE TABLE widgets (id INTEGER PRIMARY KEY, name TEXT)");
         await db.ExecuteAsync(
-            "CREATE TABLE widget_parts (id INTEGER PRIMARY KEY, widget_id INTEGER " +
-            "REFERENCES widgets(id) ON DELETE CASCADE)");
+            "CREATE TABLE order_lines (id INTEGER PRIMARY KEY, " +
+            "order_id INTEGER REFERENCES orders(id) ON DELETE CASCADE, sku TEXT)");
 
-        db.RegisterReplicated("orders");
+        var table = db.RegisterReplicated("order_lines");
 
-        Assert.True(db.ReplicatedTables.ContainsKey("orders"));
+        Assert.Equal("order_lines", table.Name);
     }
 
     [Fact]