move: add mp/type-data.h

BuildField definition was moved from
https://github.com/bitcoin/bitcoin/blob/0a931a9787b196d7a620863cc143d9319ffd356d/src/ipc/capnp/common-types.h#L137-L162
and tweaked to be more restrictive so it only matches `LocalType`'s that are
convertible to spans and constructible from spans, not just types that are
convertible to spans. This allows adding a matching CustomReadField function
below, which is new.

Having the CustomReadField function fixes a serialization bug in the bitcoin
core mining IPC interface that was reported in
Sjors/bitcoin#71 and
#122

The bug was caused by not having CustomBuildField and CustomReadField paired
together, so a lower priority CustomReadField was chosen that was not
compatible.

Author: ryanofsky

CMakeLists.txt

@@ -48,6 +48,7 @@ set(MP_PUBLIC_HEADERS
   include/mp/type-char.h
   include/mp/type-chrono.h
   include/mp/type-context.h
+  include/mp/type-data.h
   include/mp/type-decay.h
   include/mp/type-exception.h
   include/mp/type-function.h

include/mp/type-data.h

@@ -0,0 +1,46 @@
+// Copyright (c) 2025 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 MP_PROXY_TYPE_DATA_H
+#define MP_PROXY_TYPE_DATA_H
+
+#include <mp/util.h>
+
+namespace mp {
+template <typename T, typename U>
+concept IsSpanOf =
+    std::convertible_to<T, std::span<const U>> &&
+    std::constructible_from<T, const U*, const U*>;
+
+template <typename T>
+concept IsByteSpan =
+    IsSpanOf<T, std::byte> ||
+    IsSpanOf<T, char> ||
+    IsSpanOf<T, unsigned char> ||
+    IsSpanOf<T, signed char>;
+
+//! Generic ::capnp::Data field builder for any C++ type that can be converted
+//! to a span of bytes, like std::vector<char> or std::array<uint8_t>, or custom
+//! blob types like uint256 or PKHash with data() and size() methods pointing to
+//! bytes.
+template <typename LocalType, typename Value, typename Output>
+void CustomBuildField(TypeList<LocalType>, Priority<2>, InvokeContext& invoke_context, Value&& value, Output&& output)
+requires (std::is_same_v<decltype(output.get()), ::capnp::Data::Builder> && IsByteSpan<LocalType>)
+{
+    auto data = std::span{value};
+    auto result = output.init(data.size());
+    memcpy(result.begin(), data.data(), data.size());
+}
+
+template <typename LocalType, typename Input, typename ReadDest>
+decltype(auto) CustomReadField(TypeList<LocalType>, Priority<2>, InvokeContext& invoke_context, Input&& input, ReadDest&& read_dest)
+requires (std::is_same_v<decltype(input.get()), ::capnp::Data::Reader> && IsByteSpan<LocalType>)
+{
+    using ByteType = decltype(std::span{std::declval<LocalType>().begin(), std::declval<LocalType>().end()})::element_type;
+    const kj::byte *begin{input.get().begin()}, *end{input.get().end()};
+    return read_dest.construct(reinterpret_cast<const ByteType*>(begin), reinterpret_cast<const ByteType*>(end));
+}
+} // namespace mp
+
+#endif // MP_PROXY_TYPE_DATA_H