wasm abi: impl __call_reducer__ & __describe_module__ using bytes_sin…

…k_write

crates/bindings-csharp/Codegen/Module.cs

@@ -442,10 +442,10 @@ public static void Main() {
                     // Exports only work from the main assembly, so we need to generate forwarding methods.
                     #if EXPERIMENTAL_WASM_AOT
                         [UnmanagedCallersOnly(EntryPoint = "__describe_module__")]
-                        public static SpacetimeDB.Internal.Buffer __describe_module__() => SpacetimeDB.Internal.Module.__describe_module__();
+                        public static void __describe_module__(SpacetimeDB.Internal.BytesSink d) => SpacetimeDB.Internal.Module.__describe_module__(d);
 
                         [UnmanagedCallersOnly(EntryPoint = "__call_reducer__")]
-                        public static SpacetimeDB.Internal.Buffer __call_reducer__(
+                        public static short __call_reducer__(
                             uint id,
                             ulong sender_0,
                             ulong sender_1,
@@ -454,7 +454,8 @@ public static SpacetimeDB.Internal.Buffer __call_reducer__(
                             ulong address_0,
                             ulong address_1,
                             SpacetimeDB.Internal.DateTimeOffsetRepr timestamp,
-                            SpacetimeDB.Internal.Buffer args
+                            SpacetimeDB.Internal.BytesSource args,
+                            SpacetimeDB.Internal.BytesSink error
                         ) => SpacetimeDB.Internal.Module.__call_reducer__(
                             id,
                             sender_0,
@@ -464,7 +465,8 @@ SpacetimeDB.Internal.Buffer args
                             address_0,
                             address_0,
                             timestamp,
-                            args
+                            args,
+                            error
                         );
                     #endif
                     }

crates/bindings-csharp/Runtime/Exceptions.cs

@@ -32,11 +32,16 @@ public class NoSuchBytesException : StdbException
     public override string Message => "The provided bytes source or sink does not exist";
 }
 
+public class NoSpaceException : StdbException
+{
+    public override string Message => "The provided bytes sink has no more room left";
+}
+
 public class UnknownException : StdbException
 {
-    private readonly FFI.CheckedStatus.Errno code;
+    private readonly FFI.Errno code;
 
-    internal UnknownException(FFI.CheckedStatus.Errno code) => this.code = code;
+    internal UnknownException(FFI.Errno code) => this.code = code;
 
     public override string Message => $"SpacetimeDB error code {code}";
 }

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

@@ -16,23 +16,7 @@ public readonly record struct BytesSource(uint Handle)
 // forwarding in the codegen for `__describe_module__` and `__call_reducer__` exports which both
 // use this type.
 [StructLayout(LayoutKind.Sequential)]
-[NativeMarshalling(typeof(Marshaller))]
-public readonly record struct Buffer(uint Handle)
-{
-    public static readonly Buffer INVALID = new(uint.MaxValue);
-
-    // We need custom marshaller for `Buffer` because we return it by value
-    // instead of passing an `out` reference, and C# currently doesn't match
-    // the common Wasm C ABI in that a struct with a single field is supposed
-    // to have the same ABI as the field itself.
-    [CustomMarshaller(typeof(Buffer), MarshalMode.Default, typeof(Marshaller))]
-    internal static class Marshaller
-    {
-        public static Buffer ConvertToManaged(uint buf_handle) => new(buf_handle);
-
-        public static uint ConvertToUnmanaged(Buffer buf) => buf.Handle;
-    }
-}
+public readonly record struct BytesSink(uint Handle) { }
 
 #pragma warning disable IDE1006 // Naming Styles - Not applicable to FFI stuff.
 internal static partial class FFI
@@ -48,19 +32,21 @@ internal static partial class FFI
 #endif
     ;
 
+    public enum Errno : ushort
+    {
+        OK = 0,
+        HOST_CALL_FAILURE = 1,
+        NO_SUCH_TABLE = 4,
+        LOOKUP_NOT_FOUND = 2,
+        NO_SUCH_BYTES = 8,
+        NO_SPACE = 9,
+        BUFFER_TOO_SMALL = 11,
+        UNIQUE_ALREADY_EXISTS = 12,
+    }
+
     [NativeMarshalling(typeof(Marshaller))]
     public struct CheckedStatus
     {
-        public enum Errno : ushort
-        {
-            OK = 0,
-            NO_SUCH_TABLE = 1,
-            LOOKUP_NOT_FOUND = 2,
-            UNIQUE_ALREADY_EXISTS = 3,
-            BUFFER_TOO_SMALL = 4,
-            NO_SUCH_BYTES = 8,
-        }
-
         // This custom marshaller takes care of checking the status code
         // returned from the host and throwing an exception if it's not 0.
         // The only reason it doesn't return `void` is because the C# compiler
@@ -86,6 +72,7 @@ public static CheckedStatus ConvertToManaged(Errno status)
                     Errno.UNIQUE_ALREADY_EXISTS => new UniqueAlreadyExistsException(),
                     Errno.BUFFER_TOO_SMALL => new BufferTooSmallException(),
                     Errno.NO_SUCH_BYTES => new NoSuchBytesException(),
+                    Errno.NO_SPACE => new NoSpaceException(),
                     _ => new UnknownException(status),
                 };
             }
@@ -205,9 +192,13 @@ uint message_len
     public static partial short _bytes_source_read(
         BytesSource source,
         Span<byte> buffer,
-        ref uint buffer_len_ptr
+        ref uint buffer_len
     );
 
     [LibraryImport(StdbNamespace)]
-    public static partial Buffer _buffer_alloc([In] byte[] data, uint data_len);
+    public static partial CheckedStatus _bytes_sink_write(
+        BytesSink sink,
+        Span<byte> buffer,
+        ref uint buffer_len
+    );
 }

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

@@ -252,15 +252,29 @@ private static byte[] Consume(this BytesSource source)
                 // but a module should be prepared for it.
                 case 0:
                     break;
+                case (short)(ushort)FFI.Errno.NO_SUCH_BYTES:
+                    throw new NoSuchBytesException();
                 default:
-                    throw new UnreachableException();
+                    throw new UnknownException((FFI.Errno)(ushort)ret);
             }
         }
     }
 
+    private static void Write(this BytesSink sink, byte[] bytes)
+    {
+        var start = 0U;
+        while (start != bytes.Length)
+        {
+            var written = (uint)bytes.Length;
+            var buffer = bytes.AsSpan((int)start);
+            FFI._bytes_sink_write(sink, buffer, ref written);
+            start += written;
+        }
+    }
+
 #pragma warning disable IDE1006 // Naming Styles - methods below are meant for FFI.
 
-    public static Buffer __describe_module__()
+    public static void __describe_module__(BytesSink description)
     {
         // replace `module` with a temporary internal module that will register RawModuleDefV8, AlgebraicType and other internal types
         // during the RawModuleDefV8.GetSatsTypeInfo() instead of exposing them via user's module.
@@ -269,17 +283,15 @@ public static Buffer __describe_module__()
             // We need this explicit cast here to make `ToBytes` understand the types correctly.
             RawModuleDef versioned = new RawModuleDef.V8BackCompat(moduleDef);
             var moduleBytes = IStructuralReadWrite.ToBytes(new RawModuleDef.BSATN(), versioned);
-            var res = FFI._buffer_alloc(moduleBytes, (uint)moduleBytes.Length);
-            return res;
+            description.Write(moduleBytes);
         }
         catch (Exception e)
         {
             Runtime.Log($"Error while describing the module: {e}", Runtime.LogLevel.Error);
-            return Buffer.INVALID;
         }
     }
 
-    public static Buffer __call_reducer__(
+    public static short __call_reducer__(
         uint id,
         ulong sender_0,
         ulong sender_1,
@@ -288,7 +300,8 @@ public static Buffer __call_reducer__(
         ulong address_0,
         ulong address_1,
         DateTimeOffsetRepr timestamp,
-        BytesSource args
+        BytesSource args,
+        BytesSink error
     )
     {
         // Piece together the sender identity.
@@ -310,14 +323,14 @@ BytesSource args
             {
                 throw new Exception("Unrecognised extra bytes in the reducer arguments");
             }
-            return /* no exception */
-            Buffer.INVALID;
+            return 0; /* no exception */
         }
         catch (Exception e)
         {
             var error_str = e.ToString();
             var error_bytes = System.Text.Encoding.UTF8.GetBytes(error_str);
-            return FFI._buffer_alloc(error_bytes, (uint)error_bytes.Length);
+            error.Write(error_bytes);
+            return (short)(ushort)FFI.Errno.HOST_CALL_FAILURE;
         }
     }
 }

crates/bindings-csharp/Runtime/bindings.c

@@ -18,7 +18,7 @@ OPAQUE_TYPEDEF(TableId, uint32_t);
 OPAQUE_TYPEDEF(ColId, uint16_t);
 OPAQUE_TYPEDEF(IndexType, uint8_t);
 OPAQUE_TYPEDEF(LogLevel, uint8_t);
-OPAQUE_TYPEDEF(Buffer, uint32_t);
+OPAQUE_TYPEDEF(BytesSink, uint32_t);
 OPAQUE_TYPEDEF(BytesSource, uint32_t);
 OPAQUE_TYPEDEF(RowIter, uint32_t);
 
@@ -74,7 +74,8 @@ IMPORT(void, _volatile_nonatomic_schedule_immediate,
        (name, name_len, args, args_len));
 IMPORT(int16_t, _bytes_source_read, (BytesSource source, uint8_t* buffer_ptr, size_t* buffer_len_ptr),
        (source, buffer_ptr, buffer_len_ptr));
-IMPORT(Buffer, _buffer_alloc, (const uint8_t* data, uint32_t len), (data, len));
+IMPORT(uint16_t, _bytes_sink_write, (BytesSink sink, uint8_t* buffer_ptr, size_t* buffer_len_ptr),
+       (sink, buffer_ptr, buffer_len_ptr));
 
 #ifndef EXPERIMENTAL_WASM_AOT
 static MonoClass* ffi_class;
@@ -97,7 +98,7 @@ PREINIT(10, startup) {
          "FFI export class (SpacetimeDB.Internal.Module) not found");
 }
 
-#define EXPORT(ret, name, params, args...)                                    \
+#define EXPORT_WITH_MONO_RES(ret, res_code, name, params, args...)            \
   static MonoMethod* ffi_method_##name;                                       \
   PREINIT(20, find_##name) {                                                  \
     ffi_method_##name = mono_wasm_assembly_find_method(ffi_class, #name, -1); \
@@ -107,20 +108,26 @@ PREINIT(10, startup) {
     MonoObject* res;                                                          \
     mono_wasm_invoke_method_ref(ffi_method_##name, NULL, (void*[]){args},     \
                                 NULL, &res);                                  \
-    return *(ret*)mono_object_unbox(res);                                     \
+    res_code                                                                  \
   }
 
-EXPORT(Buffer, __describe_module__, ());
+#define EXPORT(ret, name, params, args...)                                             \
+  EXPORT_WITH_MONO_RES(ret, return *(ret*)mono_object_unbox(res);, name, params, args) \
 
-EXPORT(Buffer, __call_reducer__,
+#define EXPORT_VOID(name, params, args...)                                    \
+  EXPORT_WITH_MONO_RES(void, return;, name, params, args)                      \
+
+EXPORT_VOID(__describe_module__, (BytesSink description), &description);
+
+EXPORT(int16_t, __call_reducer__,
        (uint32_t id,
         uint64_t sender_0, uint64_t sender_1, uint64_t sender_2, uint64_t sender_3,
         uint64_t address_0, uint64_t address_1,
-        uint64_t timestamp, Buffer args),
+        uint64_t timestamp, BytesSource args, BytesSink error),
        &id,
        &sender_0, &sender_1, &sender_2, &sender_3,
        &address_0, &address_1,
-       &timestamp, &args);
+       &timestamp, &args, &error);
 #endif
 
 // Shims to avoid dependency on WASI in the generated Wasm file.

crates/bindings-csharp/Runtime/build/SpacetimeDB.Runtime.targets

@@ -15,7 +15,7 @@
     <WasmImport Include="$(SpacetimeNamespace)!_iter_next" />
     <WasmImport Include="$(SpacetimeNamespace)!_iter_drop" />
     <WasmImport Include="$(SpacetimeNamespace)!_bytes_source_read" />
-    <WasmImport Include="$(SpacetimeNamespace)!_buffer_alloc" />
+    <WasmImport Include="$(SpacetimeNamespace)!_bytes_sink_write" />
 
     <PackageReference Include="Microsoft.DotNet.ILCompiler.LLVM" Version="8.0.0-*" />
     <PackageReference Include="runtime.win-x64.Microsoft.DotNet.ILCompiler.LLVM" Version="8.0.0-*" />

crates/bindings-macro/src/lib.rs

@@ -377,7 +377,7 @@ fn gen_reducer(original_function: ItemFn, reducer_name: &str) -> syn::Result<Tok
     let register_describer_symbol = format!("__preinit__20_register_describer_{reducer_name}");
 
     let generated_function = quote! {
-        fn __reducer(__ctx: spacetimedb::ReducerContext, __args: &[u8]) -> spacetimedb::sys::Buffer {
+        fn __reducer(__ctx: spacetimedb::ReducerContext, __args: &[u8]) -> spacetimedb::ReducerResult {
             #(spacetimedb::rt::assert_reducer_arg::<#arg_tys>();)*
             #(spacetimedb::rt::assert_reducer_ret::<#ret_ty>();)*
             spacetimedb::rt::invoke_reducer(#func_name, __ctx, __args)