Change to_bytes_packed() to be more efficient

capnp/helpers/helpers.pxd

@@ -1,5 +1,7 @@
 from .capnp.includes.capnp_cpp cimport Maybe, DynamicStruct, Request, PyPromise, VoidPromise, PyPromiseArray, RemotePromise, DynamicCapability, InterfaceSchema, EnumSchema, StructSchema, DynamicValue, Capability, RpcSystem, MessageBuilder, MessageReader, TwoPartyVatNetwork, PyRestorer, AnyPointer, DynamicStruct_Builder
 
+from .capnp.includes.schema_cpp cimport ByteArray
+
 from non_circular cimport reraise_kj_exception
 
 from cpython.ref cimport PyObject
@@ -30,3 +32,6 @@ cdef extern from "../helpers/rpcHelper.h":
     Capability.Client restoreHelper(RpcSystem&, AnyPointer.Reader&)
     Capability.Client restoreHelper(RpcSystem&, AnyPointer.Builder&)
     RpcSystem makeRpcClientWithRestorer(TwoPartyVatNetwork&, PyRestorer&)
+
+cdef extern from "../helpers/serialize.h":
+    ByteArray messageToPackedBytes(MessageBuilder &)

capnp/helpers/serialize.h

@@ -0,0 +1,20 @@
+#pragma once
+
+#include "kj/io.h"
+#include "capnp/dynamic.h"
+#include "capnp/serialize-packed.h"
+
+kj::Array< ::capnp::byte> messageToPackedBytes(capnp::MessageBuilder & message)
+{
+    auto segments = message.getSegmentsForOutput();
+    size_t totalSize = segments.size() / 2 + 1;
+
+    for (auto& segment: segments) {
+        totalSize += segment.size();
+    }
+
+    kj::Array<capnp::byte> result = kj::heapArray<capnp::byte>(totalSize * 8);
+    kj::ArrayOutputStream out(result.asPtr());
+    capnp::writePackedMessage(out, message);
+    return heapArray(out.getArray()); // TODO: make this non-copying somehow
+}