C#: HoldsIds is read off the stored form each time a store writes

A record's HoldsIds was decided by its admission and trusted afterwards, so a
document changed after it was committed, or loaded and changed in place,
flushed under the header its admission-time content needed (Soul P7, P10).

HoldsIds now has one owner, the record's stored form, read when asked:
- a plain record: CultElementIds.Holds over its document, a scan that reads
  and refuses nothing;
- a variant: the same scan over each override value, decoded with the codec
  the record was built with (the cache's, or the attached cache's at load);
- a variant built with no codec (a store that loaded it with no cache) cannot
  see its values and counts as holding an id, so a store never writes an
  unmarked header over ids it could not see.

Cut: the admission path that set HoldsIds on plain records, on variants from
their resolved copies, and on flattened records; the internal setter and its
backing flag; IdPlan.HeldIds and IdPlan.HoldsIds. The Mesh typed write reads
Holds off the document it writes. Admission keeps its own id counting
(IdPlan.Count) for IdsInMemoryOnly.

Tests: an id added to a committed or loaded deck after admission marks a
PushAll v3, and taking it away unmarks it; a variant read without a codec
counts as holding ids while the same record read with one does not.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/GameCult.Caching/CultCache.cs

@@ -353,17 +353,22 @@ namespace GameCult.Caching
             string storedAt,
             CultDocumentDescriptor descriptor,
             CultVariantDelta variant,
-            object? resolved)
+            object? resolved,
+            CultCodec? codec)
         {
             Key = key;
             StoredAt = storedAt;
             Descriptor = descriptor;
             Variant = variant;
             _document = resolved;
+            _codec = codec;
         }
 
         private readonly object? _document;
 
+        // What reads a variant's override values; null for a plain record.
+        private readonly CultCodec? _codec;
+
         public CultRecordKey Key { get; }
         public string StoredAt { get; }
         public CultDocumentDescriptor Descriptor { get; }
@@ -382,19 +387,26 @@ namespace GameCult.Caching
         internal bool IdsInMemoryOnly { get; set; }
 
         // True when some element this record persists holds a non-empty id, and the only thing a store reads to mark its header.
-        // An admission decides it (it also counts the ids the admission fills; a variant holds ids when an override value does,
-        // its base's ids being the base's). A record that never passed an admission, one a caller built and handed straight to a
-        // store, reads it off the document it stores: the flag means what the stored form persists, wherever the record came from.
+        // It is read off the stored form each time it is asked, so a store asks it as it writes: a document changed after it was
+        // admitted, or loaded and changed in place, writes the header its content needs. A plain record persists its document; a
+        // variant persists its overrides, so it holds an id when an override value does (its base's ids are the base's). A
+        // variant built without a codec (a store loaded it with no cache attached) cannot see into its values and counts as holding
+        // one, so a store never writes an unmarked header over ids it could not see.
         public bool HoldsIds
         {
-            get => _holdsIds ?? (Variant == null && CultElementIds.Plan(_document, Key.Value, deterministic: false).HeldIds);
-            internal set => _holdsIds = value;
+            get
+            {
+                if (Variant == null)
+                    return CultElementIds.Holds(_document);
+                if (_codec == null)
+                    return true;
+                return Variant.Overrides.Any(entry => CultElementIds.Holds(_codec.Deserialize(
+                    Descriptor.RichMembers.First(member => member.Slot == entry.Path[0].Slot).MemberType, Descriptor.DocumentType, entry.Value)));
+            }
         }
 
-        private bool? _holdsIds;
-
-        internal CultStoredDocument Resolved(object document, CultVariantDelta delta, bool idsInMemoryOnly, bool holdsIds) =>
-            new(Key, StoredAt, Descriptor, delta, document) { IdsInMemoryOnly = idsInMemoryOnly, HoldsIds = holdsIds };
+        internal CultStoredDocument Resolved(object document, CultVariantDelta delta, bool idsInMemoryOnly, CultCodec codec) =>
+            new(Key, StoredAt, Descriptor, delta, document, codec) { IdsInMemoryOnly = idsInMemoryOnly };
     }
 
     public sealed class CultDocumentRegistry
@@ -2330,7 +2342,7 @@ namespace GameCult.Caching
 
         // An unresolved variant: the admitting hold resolves it against its base before anything lands.
         private CultStoredDocument StampVariant(CultRecordKey key, CultDocumentDescriptor descriptor, CultVariantDelta delta) =>
-            new(key, MintStoredAt(PreviousStoredAt(key)), descriptor, delta, null);
+            new(key, MintStoredAt(PreviousStoredAt(key)), descriptor, delta, null, _codec);
 
         // The plain record a Flatten stages: the variant's resolved document, under the variant's own key and schema.
         private CultStoredDocument StampFlatten(CultRecordKey key)
@@ -2476,8 +2488,6 @@ namespace GameCult.Caching
                 .Where(stored => stored.Variant == null)
                 .Select(stored => (Stored: stored, Plan: CultElementIds.Plan(stored.Document, stored.Key.Value, deterministic: source != null)))
                 .ToArray();
-            foreach (var (stored, idPlan) in idPlans)
-                stored.HoldsIds = idPlan.HoldsIds;
             VariantPlan plan;
             CultCommitOutcome outcome;
             try
@@ -2487,10 +2497,6 @@ namespace GameCult.Caching
                 // The one resolution: a variant admitted, a base admitted or evicted, all resolved here against the post-batch
                 // set, before land and before memory. land receives the admitted records with variants resolved.
                 plan = Resolve(admitted, evicted, source);
-                // A variant's own record is resolved into a copy; the record a loading store keeps is the original.
-                var resolvedByKey = plan.Admitted.ToDictionary(resolved => resolved.Key.Value, StringComparer.Ordinal);
-                foreach (var stored in admitted.Where(stored => stored.Variant != null))
-                    stored.HoldsIds = resolvedByKey[stored.Key.Value].HoldsIds;
                 outcome = land(home, plan.Admitted);
             }
             catch
@@ -2889,7 +2895,6 @@ namespace GameCult.Caching
                 var kept = new List<KeyValuePair<int, byte[]>>();
                 var overrides = new List<CultVariantOverride>(delta.Overrides.Count);
                 var minted = false;
-                var holdsIds = false;
                 var seen = new HashSet<int>();
                 foreach (var entry in delta.Overrides)
                 {
@@ -2922,9 +2927,6 @@ namespace GameCult.Caching
                     // decoded copy, and the delta this hold lands carries them.
                     var bytes = entry.Value;
                     var idPlan = CultElementIds.Plan(value, key + "." + slot.ToString(CultureInfo.InvariantCulture), deterministic: source != null);
-                    // What the record persists: a write persists the ids it mints, a load keeps the delta as the store held it, so
-                    // the ids a load mints exist only in memory.
-                    holdsIds |= source == null ? idPlan.HoldsIds : idPlan.HeldIds;
                     if (idPlan.Count > 0)
                     {
                         idPlan.Apply();
@@ -2942,13 +2944,10 @@ namespace GameCult.Caching
 
                 stack.RemoveAt(stack.Count - 1);
                 return done[key] = isVariant
-                    ? variant.Resolved(document, minted ? new CultVariantDelta(delta.BaseKey, overrides) : delta, variant.IdsInMemoryOnly || (minted && source != null), holdsIds)
+                    ? variant.Resolved(document, minted ? new CultVariantDelta(delta.BaseKey, overrides) : delta, variant.IdsInMemoryOnly || (minted && source != null), _codec)
                     : new CultStoredDocument(variant.Key, variant.StoredAt, variant.Descriptor, document)
                     {
-                        Flattens = true,
-                        // The flattened record is the document it stores, resolved against the post-batch base: the flag is read
-                        // off that document, not carried from the variant or from the document the flatten was planned against.
-                        HoldsIds = CultElementIds.Plan(document, key, deterministic: source != null).HoldsIds
+                        Flattens = true
                     };
             }
         }
@@ -3384,7 +3383,8 @@ namespace GameCult.Caching
                     record.StoredAt,
                     descriptor,
                     new CultVariantDelta(record.Variant.BaseKey, record.Variant.Overrides.Where(Has).ToArray()),
-                    null);
+                    null,
+                    Cache?.Codec);
             }
 
             var document = deserializePayload(resolution.Descriptor.DocumentType, record.Payload);

src/GameCult.Caching/CultElementIds.cs

@@ -64,12 +64,6 @@ namespace GameCult.Caching
 
             public int Count => _fills.Count;
 
-            // True when some element under the root already holds a non-empty id: one a store persisted, or one the caller set.
-            public bool HeldIds { get; internal set; }
-
-            // True when some element holds a non-empty id once the plan is applied: one already there, or one this plan fills.
-            public bool HoldsIds => HeldIds || _fills.Count > 0;
-
             internal void Add(object item, Shape shape, string? before, string id) => _fills.Add((item, shape, before, id));
 
             public void Apply()
@@ -206,6 +200,45 @@ namespace GameCult.Caching
             return null;
         }
 
+        // ---- write: what the stored form holds ----
+
+        // True when some element under root holds a non-empty id: the one fact a store marks its header by. It reads the value
+        // as it is now, refuses nothing and changes nothing, so a writer asks it of exactly what it is about to write.
+        internal static bool Holds(object? root)
+        {
+            if (root == null)
+                return false;
+            var type = root.GetType();
+            if (IsLeaf(type))
+                return false;
+            if (root is IDictionary dictionary)
+            {
+                foreach (DictionaryEntry entry in dictionary)
+                {
+                    if (Holds(entry.Value))
+                        return true;
+                }
+
+                return false;
+            }
+
+            if (root is IEnumerable list)
+            {
+                foreach (var item in list)
+                {
+                    if (item != null && IsObjectType(item.GetType()) && ShapeOf(item.GetType()).GetId is { } getId &&
+                        !string.IsNullOrEmpty(getId(item)))
+                        return true;
+                    if (Holds(item))
+                        return true;
+                }
+
+                return false;
+            }
+
+            return IsObjectType(type) && ShapeOf(type).Walk.Any(member => Holds(member.Get(root)));
+        }
+
         // ---- write and load: mint and check ----
 
         // Decides every unset element id under root, and refuses what the rule refuses, without changing anything: a random id
@@ -255,7 +288,6 @@ namespace GameCult.Caching
                     var id = getId(item);
                     if (string.IsNullOrEmpty(id))
                         continue;
-                    plan.HeldIds = true;
                     if (shape.Derive == null && !IsRandomId(id!))
                         throw new CultElementIdException(
                             $"Record {rootPath}: element id '{id}' in the list at {path} is not 12 lowercase hex characters.",

src/GameCult.Mesh/CultMeshSingleFileDocuments.cs

@@ -110,7 +110,7 @@ namespace GameCult.Mesh
                     descriptor.ToCatalogEntry(),
                     storedAt,
                     CultDocumentMessagePackSerialization.SerializeUntyped(document, typeof(TDocument)),
-                    ids.HoldsIds,
+                    CultElementIds.Holds(document),
                     contentKnown: true);
             }
             catch

tests/GameCult.Caching.Tests/ElementIdTests.cs

@@ -731,6 +731,60 @@ namespace GameCult.Caching.Tests
             Assert.That(HeaderOf(path), Is.EqualTo("cultcache.store.v1"), "a whole-store flush decides by what the store now holds");
         }
 
+        // Soul's P7 and P10: a store reads the header off what it writes, when it writes it. A document changed after its admission,
+        // or loaded and changed in place, is marked by what it holds at the flush, in both directions.
+        [TestCase(false)]
+        [TestCase(true)]
+        public void AFlushIsMarkedByWhatADocumentHoldsNowNotWhatItHeldWhenAdmitted(bool loaded)
+        {
+            var path = PathOf(loaded ? "mutated-loaded.cc" : "mutated-admitted.cc");
+            var key = new CultRecordKey("d");
+            using (var seed = Open(path))
+                seed.Commit(batch => batch.Upsert(typeof(IdDeck), EmptyDeck("d"), key));
+            Assert.That(HeaderOf(path), Is.EqualTo("cultcache.store.v1"));
+
+            using var cache = Open(path);
+            var deck = EmptyDeck("d");
+            if (loaded)
+                deck = cache.Get<IdDeck>(key)!;
+            else
+                cache.Commit(batch => batch.Upsert(typeof(IdDeck), deck, key));
+
+            deck.Reels.Add(new IdReel { Id = HexA });
+            cache.BackingStores[0].PushAll();
+            Assert.That(HeaderOf(path), Is.EqualTo("cultcache.store.v3"), "an id added after admission marks the flush");
+
+            deck.Reels.Clear();
+            cache.BackingStores[0].PushAll();
+            Assert.That(HeaderOf(path), Is.EqualTo("cultcache.store.v1"), "the last id taken away after admission unmarks it");
+        }
+
+        // A store with no cache has no codec to read a variant's override values with, so it cannot see whether they hold an id: the
+        // variant counts as holding one. The same record read through a cache is marked by what its values hold.
+        [Test]
+        public void AVariantAStoreCannotReadCountsAsHoldingIds()
+        {
+            var path = PathOf("detached-variant.cc");
+            using (var cache = Open(path))
+            {
+                cache.Commit(batch =>
+                {
+                    batch.Upsert(typeof(IdDeck), EmptyDeck("base"), new CultRecordKey("base"));
+                    batch.UpsertVariant(new CultRecordKey("v"), new CultRecordKey("base"), new[] { cache.Override<IdDeck>(nameof(IdDeck.Name), "v") });
+                });
+            }
+
+            using (var cache = Open(path))
+                cache.BackingStores[0].PushAll();
+            Assert.That(HeaderOf(path), Is.EqualTo("cultcache.store.v2"), "read with a codec, a name override holds no id");
+
+            var blind = new SingleFileMessagePackBackingStore(path);
+            blind.AttachRegistry(Registry);
+            blind.PullAll();
+            blind.PushAll();
+            Assert.That(HeaderOf(path), Is.EqualTo("cultcache.store.v3"), "read without one, it cannot be seen, so it is marked");
+        }
+
         [Test]
         public void ADirectoryStoreIsMarkedByTheIdsItWritesAndAnEmptyDeckDoesNotMarkIt()
         {