Bytes after the value: a payload is one MessagePack value (C#) (#125)

src/GameCult.Caching.MessagePack/CultPayloadDepth.cs

@@ -7,11 +7,12 @@ namespace GameCult.Caching.MessagePack;
 internal enum PayloadDirection { Read, Write }
 
 /// <summary>
-/// The one payload nesting count. Walks the MessagePack headers of a document payload iteratively, counting every array and
+/// The one walk of a payload's shape: its depth and its one value. Walks the MessagePack headers of a document payload iteratively, counting every array and
 /// map (read by a formatter or skipped past it; the outermost array is depth 1) and refusing the first container past
 /// <see cref="CultMessagePackSecurity.MaximumPayloadDepth"/>. Scalars are skipped by length, nothing is allocated per
 /// element, and the count stack is bounded by the limit. A refusal names the type, the depth reached and the limit,
-/// never payload bytes. A payload that ends early is the deserializer's to refuse.
+/// never payload bytes. A payload that ends early is the deserializer's to refuse; bytes after the first value are refused
+/// with their count and never walked.
 /// </summary>
 internal static class CultPayloadDepth
 {
@@ -73,7 +74,13 @@ internal static class CultPayloadDepth
 
             // One element is complete; close every container it was the last element of.
             while (depth > 0 && --remaining[depth - 1] == 0) depth--;
-            if (depth == 0) return;
+            if (depth == 0)
+            {
+                if (position < bytes.Length)
+                    throw new MessagePackSerializationException(
+                        $"{type.FullName} payload has {bytes.Length - position} bytes after its first MessagePack value (refused on {(direction == PayloadDirection.Read ? "read" : "write")}).");
+                return;
+            }
         }
     }
 

src/GameCult.Caching/Contracts/cultcache-persistence-format.md

@@ -403,6 +403,8 @@ runtime enforces the same bound on read and on write, and the refusal names the
 the depth and the limit, never payload bytes. The vectors are in
 `tests/vectors/union-parity` (`bounds-outcomes.msgpack`).
 
+A record's payload is exactly one MessagePack value: every runtime refuses a payload with bytes after its first value wherever it reads a payload under the nesting bound, and on write where a write takes bytes the library did not encode itself (a C# type's own formatter, a Rust raw envelope), without walking those bytes, so a tail past the limit is refused as bytes after the value and not as depth; the refusal names the type and the count of bytes after the value, never the bytes (vectors: `tests/vectors/union-parity/bytes-after-outcomes.msgpack`). A CDN manifest is stored and keyed as a record and follows the same rule (`contracts/cultmesh/content-acceptance-vectors.json`: `trailing_nil`, `trailing_reserved_code` and `trailing_truncated_array` are refused). Streams of values (CultNet frames, a store's record array) are different contracts and are not covered.
+
 ## Variant Records
 
 A variant record is a record whose document is a base record's document with some

src/GameCult.Mesh/CultMeshProviderSessionWire.cs

@@ -284,7 +284,10 @@ namespace GameCult.Mesh
                 throw new ArgumentException($"Expected provider command schema '{CultMeshProviderSessionWireContract.CommandSchema}'.", nameof(document));
             if (!string.Equals(document.PayloadEncoding, "messagepack", StringComparison.Ordinal))
                 throw new ArgumentException("Provider command document must use MessagePack.", nameof(document));
-            var command = MessagePackSerializer.Deserialize<CultMeshProviderCommandWire>(document.Payload, Options);
+            // A raw record's payload is one MessagePack value (ruling trailing-bytes-refuse): a tail is refused by count, never walked.
+            var command = MessagePackSerializer.Deserialize<CultMeshProviderCommandWire>(new ReadOnlyMemory<byte>(document.Payload), Options, out var read);
+            if (read < document.Payload.Length)
+                throw new MessagePackSerializationException($"Provider command document: {document.Payload.Length - read} bytes after its first MessagePack value (refused on read).");
             if (command == null) throw new MessagePackSerializationException("Provider command decoded to null.");
             Validate(command);
             return command;

src/GameCult.Networking/CultNetDocumentRegistry.cs

@@ -641,23 +641,19 @@ namespace GameCult.Networking
             string storedAt,
             CultNetDocumentMessageOptions? options = null)
         {
-            var binding = GetByDocumentType(descriptor.DocumentType) ??
-                          new CultNetDocumentBinding(
-                              descriptor.DocumentType,
-                              descriptor.SchemaId,
-                              value => CultDocumentMessagePackSerialization.SerializeUntyped(value, value.GetType(), _documents),
-                              payload => CultDocumentMessagePackSerialization.DeserializeUntyped(descriptor.DocumentType, payload, _documents));
+            var binding = GetByDocumentType(descriptor.DocumentType);
 
             return new CultNetRawDocumentRecord
             {
-                SchemaId = binding.SchemaId,
+                SchemaId = binding?.SchemaId ?? descriptor.SchemaId,
                 SchemaName = descriptor.SchemaName,
                 SchemaVersion = descriptor.SchemaVersion,
                 SchemaContentHash = descriptor.ContentHash,
                 RecordKey = key.Value,
                 StoredAt = storedAt,
                 PayloadEncoding = "messagepack",
-                Payload = binding.PayloadSerializer(document),
+                Payload = binding?.PayloadSerializer(document) ??
+                          CultDocumentMessagePackSerialization.SerializeUntyped(document, document.GetType(), _documents),
                 SourceRuntimeId = options?.SourceRuntimeId,
                 SourceAgentId = options?.SourceAgentId,
                 SourceRole = options?.SourceRole,