multiprocess: Add type conversion code for serializable types

Allow any C++ object that has Serialize and Unserialize methods and can be
serialized to a bitcoin CDataStream to be converted to a capnproto Data field
and passed as arguments or return values to capnproto methods using the Data
type.

Extend IPC unit test to cover this and verify the serialization happens
correctly.

1 files changed, 89 insertions, 0 deletions

diff --git a/src/ipc/capnp/common-types.h b/src/ipc/capnp/common-types.h
new file mode 100644
index 0000000000..d1343c40dd
--- /dev/null
+++ b/src/ipc/capnp/common-types.h
@@ -0,0 +1,89 @@
+// Copyright (c) 2023 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_IPC_CAPNP_COMMON_TYPES_H
+#define BITCOIN_IPC_CAPNP_COMMON_TYPES_H
+
+#include <clientversion.h>
+#include <streams.h>
+
+#include <cstddef>
+#include <mp/proxy-types.h>
+#include <type_traits>
+#include <utility>
+
+namespace ipc {
+namespace capnp {
+//! Use SFINAE to define Serializeable<T> trait which is true if type T has a
+//! Serialize(stream) method, false otherwise.
+template <typename T>
+struct Serializable {
+private:
+    template <typename C>
+    static std::true_type test(decltype(std::declval<C>().Serialize(std::declval<std::nullptr_t&>()))*);
+    template <typename>
+    static std::false_type test(...);
+
+public:
+    static constexpr bool value = decltype(test<T>(nullptr))::value;
+};
+
+//! Use SFINAE to define Unserializeable<T> trait which is true if type T has
+//! an Unserialize(stream) method, false otherwise.
+template <typename T>
+struct Unserializable {
+private:
+    template <typename C>
+    static std::true_type test(decltype(std::declval<C>().Unserialize(std::declval<std::nullptr_t&>()))*);
+    template <typename>
+    static std::false_type test(...);
+
+public:
+    static constexpr bool value = decltype(test<T>(nullptr))::value;
+};
+} // namespace capnp
+} // namespace ipc
+
+//! Functions to serialize / deserialize common bitcoin types.
+namespace mp {
+//! Overload multiprocess library's CustomBuildField hook to allow any
+//! serializable object to be stored in a capnproto Data field or passed to a
+//! canproto interface. Use Priority<1> so this hook has medium priority, and
+//! higher priority hooks could take precedence over this one.
+template <typename LocalType, typename Value, typename Output>
+void CustomBuildField(
+    TypeList<LocalType>, Priority<1>, InvokeContext& invoke_context, Value&& value, Output&& output,
+    // Enable if serializeable and if LocalType is not cv or reference
+    // qualified. If LocalType is cv or reference qualified, it is important to
+    // fall back to lower-priority Priority<0> implementation of this function
+    // that strips cv references, to prevent this CustomBuildField overload from
+    // taking precedence over more narrow overloads for specific LocalTypes.
+    std::enable_if_t<ipc::capnp::Serializable<LocalType>::value &&
+                     std::is_same_v<LocalType, std::remove_cv_t<std::remove_reference_t<LocalType>>>>* enable = nullptr)
+{
+    DataStream stream;
+    value.Serialize(stream);
+    auto result = output.init(stream.size());
+    memcpy(result.begin(), stream.data(), stream.size());
+}
+
+//! Overload multiprocess library's CustomReadField hook to allow any object
+//! with an Unserialize method to be read from a capnproto Data field or
+//! returned from canproto interface. Use Priority<1> so this hook has medium
+//! priority, and higher priority hooks could take precedence over this one.
+template <typename LocalType, typename Input, typename ReadDest>
+decltype(auto)
+CustomReadField(TypeList<LocalType>, Priority<1>, InvokeContext& invoke_context, Input&& input, ReadDest&& read_dest,
+                std::enable_if_t<ipc::capnp::Unserializable<LocalType>::value>* enable = nullptr)
+{
+    return read_dest.update([&](auto& value) {
+        if (!input.has()) return;
+        auto data = input.get();
+        SpanReader stream({data.begin(), data.end()});
+        value.Unserialize(stream);
+    });
+}
+} // namespace mp
+
+#endif // BITCOIN_IPC_CAPNP_COMMON_TYPES_H