multiprocess: Add type conversion code for UniValue types

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

src/ipc/capnp/common-types.h

@@ -7,6 +7,7 @@
 
 #include <clientversion.h>
 #include <streams.h>
+#include <univalue.h>
 
 #include <cstddef>
 #include <mp/proxy-types.h>
@@ -84,6 +85,24 @@ CustomReadField(TypeList<LocalType>, Priority<1>, InvokeContext& invoke_context,
         value.Unserialize(stream);
     });
 }
+
+template <typename Value, typename Output>
+void CustomBuildField(TypeList<UniValue>, Priority<1>, InvokeContext& invoke_context, Value&& value, Output&& output)
+{
+    std::string str = value.write();
+    auto result = output.init(str.size());
+    memcpy(result.begin(), str.data(), str.size());
+}
+
+template <typename Input, typename ReadDest>
+decltype(auto) CustomReadField(TypeList<UniValue>, Priority<1>, InvokeContext& invoke_context, Input&& input,
+                               ReadDest&& read_dest)
+{
+    return read_dest.update([&](auto& value) {
+        auto data = input.get();
+        value.read(std::string_view{data.begin(), data.size()});
+    });
+}
 } // namespace mp
 
 #endif // BITCOIN_IPC_CAPNP_COMMON_TYPES_H

src/test/ipc_test.capnp

@@ -14,4 +14,5 @@ $Proxy.includeTypes("ipc/capnp/common-types.h");
 interface FooInterface $Proxy.wrap("FooImplementation") {
     add @0 (a :Int32, b :Int32) -> (result :Int32);
     passOutPoint @1 (arg :Data) -> (result :Data);
+    passUniValue @2 (arg :Text) -> (result :Text);
 }

src/test/ipc_test.cpp

@@ -55,6 +55,12 @@ void IpcTest()
     COutPoint txout2{foo->passOutPoint(txout1)};
     BOOST_CHECK(txout1 == txout2);
 
+    UniValue uni1{UniValue::VOBJ};
+    uni1.pushKV("i", 1);
+    uni1.pushKV("s", "two");
+    UniValue uni2{foo->passUniValue(uni1)};
+    BOOST_CHECK_EQUAL(uni1.write(), uni2.write());
+
     // Test cleanup: disconnect pipe and join thread
     disconnect_client();
     thread.join();

src/test/ipc_test.h

@@ -6,12 +6,14 @@
 #define BITCOIN_TEST_IPC_TEST_H
 
 #include <primitives/transaction.h>
+#include <univalue.h>
 
 class FooImplementation
 {
 public:
     int add(int a, int b) { return a + b; }
     COutPoint passOutPoint(COutPoint o) { return o; }
+    UniValue passUniValue(UniValue v) { return v; }
 };
 
 void IpcTest();