ProxyClientBase: avoid static_cast to partially constructed object

[ryanofsky]

ProxyClientBase constructor was trying to call ProxyClient::construct() method before ProxyClient object had been fully constructed. This is causing a UBSAN error reported:

• Add multiprocess binaries to release build bitcoin/bitcoin#30975 (comment)
• https://github.com/bitcoin/bitcoin/actions/runs/11970857809/job/33374462331?pr=30975

that looks like:

include/mp/proxy.h:95:45: runtime error: downcast of address 0x50600002bdc0 which does not point to an object of type 'ProxyClient<Interface>' (aka 'ProxyClient<ipc::capnp::messages::Init>')
0x50600002bdc0: note: object is of type 'mp::ProxyClientBase<ipc::capnp::messages::Init, interfaces::Init>'

[ryanofsky]

A more complete stack trace from https://github.com/bitcoin/bitcoin/actions/runs/11970857809/job/33374462331?pr=30975 is:

2024-11-22T10:38:22.317815Z/usr/local/include/mp/proxy.h:95:45: runtime error: downcast of address 0x50600002bdc0 which does not point to an object of type 'ProxyClient<Interface>' (aka 'ProxyClient<ipc::capnp::messages::Init>')
0x50600002bdc0: note: object is of type 'mp::ProxyClientBase<ipc::capnp::messages::Init, interfaces::Init>'
 00 00 00 00  18 04 23 9a 4a 56 00 00  08 03 23 9a 4a 56 00 00  68 17 01 00 b0 50 00 00  60 17 01 00
              ^~~~~~~~~~~~~~~~~~~~~~~
              vptr for 'mp::ProxyClientBase<ipc::capnp::messages::Init, interfaces::Init>'
    #0 0x564a98a7b7fa in mp::ProxyClientBase<ipc::capnp::messages::Init, interfaces::Init>::self() /usr/local/include/mp/proxy.h:95:45
    #1 0x564a98a7b7fa in mp::ProxyClientBase<ipc::capnp::messages::Init, interfaces::Init>::ProxyClientBase(ipc::capnp::messages::Init::Client, mp::Connection*, bool) /usr/local/include/mp/proxy-io.h:435:5
    #2 0x564a98a75cac in mp::ProxyClientCustom<ipc::capnp::messages::Init, interfaces::Init>::ProxyClientCustom(ipc::capnp::messages::Init::Client, mp::Connection*, bool) /usr/local/include/mp/proxy.h:106:45
    #3 0x564a98a75cac in mp::ProxyClient<ipc::capnp::messages::Init>::ProxyClient(ipc::capnp::messages::Init::Client, mp::Connection*, bool) /home/runner/work/_temp/ci/scratch/build-x86_64-pc-linux-gnu/src/ipc/capnp/init.capnp.proxy.h:73:30
    #4 0x564a98a75cac in std::__detail::_MakeUniq<mp::ProxyClient<ipc::capnp::messages::Init>>::__single_object std::make_unique<mp::ProxyClient<ipc::capnp::messages::Init>, ipc::capnp::messages::Init::Client, mp::Connection*, bool>(ipc::capnp::messages::Init::Client&&, mp::Connection*&&, bool&&) /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/unique_ptr.h:1070:34
    #5 0x564a98a74cf6 in std::unique_ptr<mp::ProxyClient<ipc::capnp::messages::Init>, std::default_delete<mp::ProxyClient<ipc::capnp::messages::Init>>> mp::ConnectStream<ipc::capnp::messages::Init>(mp::EventLoop&, int) /usr/local/include/mp/proxy-io.h:577:12
    #6 0x564a98a72c26 in ipc::capnp::(anonymous namespace)::CapnpProtocol::connect(int, char const*) /home/runner/work/_temp/ci/scratch/build-x86_64-pc-linux-gnu/src/ipc/./ipc/capnp/protocol.cpp:54:16
    #7 0x564a989e0d13 in IpcSocketPairTest() /home/runner/work/_temp/ci/scratch/build-x86_64-pc-linux-gnu/src/./test/ipc_test.cpp:141:61
    #8 0x564a97a43380 in ipc_tests::ipc_tests::test_method() /home/runner/work/_temp/ci/scratch/build-x86_64-pc-linux-gnu/src/test/./test/ipc_tests.cpp:15:5
    #9 0x564a97a425ef in ipc_tests::ipc_tests_invoker() /home/runner/work/_temp/ci/scratch/build-x86_64-pc-linux-gnu/src/test/./test/ipc_tests.cpp:12:1
    #10 0x564a965e731d in boost::function0<void>::operator()() const /usr/include/boost/function/function_template.hpp:771:14
    #11 0x564a96667db8 in boost::detail::forward::operator()() /usr/include/boost/test/impl/execution_monitor.ipp:1395:32
    #12 0x564a96667db8 in boost::detail::function::function_obj_invoker0<boost::detail::forward, int>::invoke(boost::detail::function::function_buffer&) /usr/include/boost/function/function_template.hpp:137:18
    #13 0x564a966618ad in boost::function0<int>::operator()() const /usr/include/boost/function/function_template.hpp:771:14
    #14 0x564a9654cfec in int boost::detail::do_invoke<boost::shared_ptr<boost::detail::translator_holder_base>, boost::function<int ()>>(boost::shared_ptr<boost::detail::translator_holder_base> const&, boost::function<int ()> const&) /usr/include/boost/test/impl/execution_monitor.ipp:308:30
    #15 0x564a9654cfec in boost::execution_monitor::catch_signals(boost::function<int ()> const&) /usr/include/boost/test/impl/execution_monitor.ipp:910:16
    #16 0x564a9654d4fd in boost::execution_monitor::execute(boost::function<int ()> const&) /usr/include/boost/test/impl/execution_monitor.ipp:1308:16
    #17 0x564a96545b88 in boost::execution_monitor::vexecute(boost::function<void ()> const&) /usr/include/boost/test/impl/execution_monitor.ipp:1404:5
    #18 0x564a96545b88 in boost::unit_test::unit_test_monitor_t::execute_and_translate(boost::function<void ()> const&, unsigned long) /usr/include/boost/test/impl/unit_test_monitor.ipp:49:9
    #19 0x564a965a9b75 in boost::unit_test::framework::state::execute_test_tree(unsigned long, unsigned long, boost::unit_test::framework::state::random_generator_helper const*) /usr/include/boost/test/impl/framework.ipp:815:44
    #20 0x564a965a8a84 in boost::unit_test::framework::state::execute_test_tree(unsigned long, unsigned long, boost::unit_test::framework::state::random_generator_helper const*) /usr/include/boost/test/impl/framework.ipp:784:58
    #21 0x564a965a8a84 in boost::unit_test::framework::state::execute_test_tree(unsigned long, unsigned long, boost::unit_test::framework::state::random_generator_helper const*) /usr/include/boost/test/impl/framework.ipp:784:58
    #22 0x564a96543fdb in boost::unit_test::framework::run(unsigned long, bool) /usr/include/boost/test/impl/framework.ipp:1722:29
    #23 0x564a96572ca0 in boost::unit_test::unit_test_main(boost::unit_test::test_suite* (*)(int, char**), int, char**) /usr/include/boost/test/impl/unit_test_main.ipp:250:9
    #24 0x7f99693bf1c9  (/lib/x86_64-linux-gnu/libc.so.6+0x2a1c9) (BuildId: 6d64b17fbac799e68da7ebd9985ddf9b5cb375e6)
    #25 0x7f99693bf28a in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28a) (BuildId: 6d64b17fbac799e68da7ebd9985ddf9b5cb375e6)
    #26 0x564a9643eea4 in _start (/home/runner/work/_temp/ci/scratch/build-x86_64-pc-linux-gnu/src/test/test_bitcoin+0x1355ea4) (BuildId: 5aa0be89931561f87bfc4b03f135b8264e316e4e)