[C#] Update module bindings to use new View ABI (#4146)

# Description of Changes

This change implements the module bindings changes to Views that where
updated with core in #3685 and the Rust module bindings implementation
in #3819.
Updates the C# module bindings to use new header-first view ABI
(`ViewResultHeader`) and updates the return codes for the
`__call_view__` and `__call_view_anon__` module exports. This is a
prerequsite for `Query` builder support being added to C# modules.

# API and ABI breaking changes

Not breaking. Existing modules will continue to use the old ABI. New
modules will use the new ABI. However previous host versions will not
support modules built using this version of the bindings.

# Expected complexity level and risk

2

# Testing

This is an internal refactor. All existing tests should continue to
pass. All existing tests should continue to pass. The only tests that
needed updating were the C# codegen snapshot tests (`Codegen.Tests`)
because the generated view dispatcher bodies changed (they now prefix
`ViewResultHeader.RowData` before the existing row payload).

Author: rekhoff

crates/bindings-csharp/Codegen.Tests/fixtures/diag/snapshots/Module#FFI.verified.cs

@@ -1162,8 +1162,11 @@ public byte[] Invoke(
                 PublicTable.BSATN
             >.GetListSerializer();
             var listValue = ModuleRegistration.ToListOrEmpty(returnValue);
+            var header = new global::SpacetimeDB.Internal.ViewResultHeader.RowData(default);
+            var headerRW = new global::SpacetimeDB.Internal.ViewResultHeader.BSATN();
             using var output = new System.IO.MemoryStream();
             using var writer = new System.IO.BinaryWriter(output);
+            headerRW.Write(writer, header);
             listSerializer.Write(writer, listValue);
             return output.ToArray();
         }
@@ -1208,8 +1211,11 @@ public byte[] Invoke(
                 PublicTable.BSATN
             >.GetListSerializer();
             var listValue = ModuleRegistration.ToListOrEmpty(returnValue);
+            var header = new global::SpacetimeDB.Internal.ViewResultHeader.RowData(default);
+            var headerRW = new global::SpacetimeDB.Internal.ViewResultHeader.BSATN();
             using var output = new System.IO.MemoryStream();
             using var writer = new System.IO.BinaryWriter(output);
+            headerRW.Write(writer, header);
             listSerializer.Write(writer, listValue);
             return output.ToArray();
         }

crates/bindings-csharp/Codegen.Tests/fixtures/server/snapshots/Module#FFI.verified.cs

@@ -1039,15 +1039,21 @@ public string GenerateDispatcherClass(uint index)
             ? $$$"""
                     var listSerializer = {{{ReturnType.BSATNName}}}.GetListSerializer();
                     var listValue = ModuleRegistration.ToListOrEmpty(returnValue);
+                    var header = new global::SpacetimeDB.Internal.ViewResultHeader.RowData(default);
+                    var headerRW = new global::SpacetimeDB.Internal.ViewResultHeader.BSATN();
                     using var output = new System.IO.MemoryStream();
                     using var writer = new System.IO.BinaryWriter(output);
+                    headerRW.Write(writer, header);
                     listSerializer.Write(writer, listValue);
                     return output.ToArray();
                 """
             : $$$"""
                     {{{ReturnType.BSATNName}}} returnRW = new();
+                    var header = new global::SpacetimeDB.Internal.ViewResultHeader.RowData(default);
+                    var headerRW = new global::SpacetimeDB.Internal.ViewResultHeader.BSATN();
                     using var output = new System.IO.MemoryStream();
                     using var writer = new System.IO.BinaryWriter(output);
+                    headerRW.Write(writer, header);
                     returnRW.Write(writer, returnValue);
                     return output.ToArray();            
                 """;

crates/bindings-csharp/Codegen/Module.cs

@@ -366,6 +366,30 @@ BytesSink resultSink
         }
     }
 
+    /// <summary>
+    /// Called by the host to execute a view when the sender calls the view identified by <paramref name="id" />.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// The sender identity is passed as 4 <see cref="ulong" /> values (<paramref name="sender_0" /> through
+    /// <paramref name="sender_3" />) representing a little-endian <see cref="SpacetimeDB.Identity" />.
+    /// </para>
+    /// <para>
+    /// <paramref name="args" /> is a host-registered <see cref="BytesSource" /> containing the BSATN-encoded
+    /// view arguments. For empty arguments, <paramref name="args" /> will be invalid.
+    /// </para>
+    /// <para>
+    /// The view output is written to <paramref name="rows" />, a host-registered <see cref="BytesSink" />.
+    /// </para>
+    /// <para>
+    /// Note: a previous view ABI wrote the return rows directly to the sink.
+    /// The current ABI writes a BSATN-encoded <see cref="ViewResultHeader" /> first, in order to distinguish
+    /// between views that return row data and views that return queries.
+    /// </para>
+    /// <para>
+    /// The current ABI is identified by returning error code <c>2</c>.
+    /// </para>
+    /// </remarks>
     public static Errno __call_view__(
         uint id,
         ulong sender_0,
@@ -386,7 +410,7 @@ BytesSink rows
             using var reader = new BinaryReader(stream);
             var bytes = viewDispatchers[(int)id].Invoke(reader, ctx);
             rows.Write(bytes);
-            return Errno.OK;
+            return (Errno)2;
         }
         catch (Exception e)
         {
@@ -395,6 +419,26 @@ BytesSink rows
         }
     }
 
+    /// <summary>
+    /// Called by the host to execute an anonymous view.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// <paramref name="args" /> is a host-registered <see cref="BytesSource" /> containing the BSATN-encoded
+    /// view arguments. For empty arguments, <paramref name="args" /> will be invalid.
+    /// </para>
+    /// <para>
+    /// The view output is written to <paramref name="rows" />, a host-registered <see cref="BytesSink" />.
+    /// </para>
+    /// <para>
+    /// Note: a previous view ABI wrote the return rows directly to the sink.
+    /// The current ABI writes a BSATN-encoded <see cref="ViewResultHeader" /> first, in order to distinguish
+    /// between views that return row data and views that return queries.
+    /// </para>
+    /// <para>
+    /// The current ABI is identified by returning error code <c>2</c>.
+    /// </para>
+    /// </remarks>
     public static Errno __call_view_anon__(uint id, BytesSource args, BytesSink rows)
     {
         try
@@ -404,7 +448,7 @@ public static Errno __call_view_anon__(uint id, BytesSource args, BytesSink rows
             using var reader = new BinaryReader(stream);
             var bytes = anonymousViewDispatchers[(int)id].Invoke(reader, ctx);
             rows.Write(bytes);
-            return Errno.OK;
+            return (Errno)2;
         }
         catch (Exception e)
         {

crates/bindings-csharp/Runtime/Internal/Module.cs

@@ -0,0 +1,5 @@
+namespace SpacetimeDB.Internal;
+
+[SpacetimeDB.Type]
+public partial record ViewResultHeader
+    : SpacetimeDB.TaggedEnum<(SpacetimeDB.Unit RowData, string RawSql)>;