Bitcoin Core UBSan failure for interfaces::Echo #125

When running ipc_tests.cpp with the UBSan fix from #121, it still fails:

export UBSAN_OPTIONS="suppressions=$(pwd)/test/sanitizer_suppressions/ubsan:print_stacktrace=1:halt_on_error=1:report_error_type=1"
cmake -B build -DWITH_MULTIPROCESS=ON -DSANITIZERS=undefined
cmake --build build

build/src/test/test_bitcoin --run_test=ipc_tests --log_level=all | grep -v disabled
...
test/ipc_test.cpp:143: info: check remote_echo->echo("echo test") == "echo test" has passed
/usr/local/include/mp/proxy.h:95:45: runtime error: downcast of address 0x600001f28700 which does not point to an object of type 'ProxyClient<Interface>' (aka 'ProxyClient<ipc::capnp::messages::Echo>')
0x600001f28700: note: object is of type 'mp::ProxyClientBase<ipc::capnp::messages::Echo, interfaces::Echo>'
...
SUMMARY: UndefinedBehaviorSanitizer: dynamic-type-mismatch /usr/local/include/mp/proxy.h:95:45 in 

CI failure from this was posted in https://github.com/bitcoin/bitcoin/pull/30975#issuecomment-2583641138: https://github.com/bitcoin/bitcoin/actions/runs/12715313812/job/35447236983?pr=30975#step:7:5079

This is helpful because it includes a new stack trace. Based on https://github.com/bitcoin/bitcoin/commit/e35f64041750aef1502b011599a2bddab8efec9c checkout with libmultiprocess version 5b8119284760ef0e945b709cfaad0e5370e3677f:

[test/ipc_/usr/local/include/mp/proxy.h:95:45: runtime error: downcast of address 0x5060000392c0 which does not point to an object of type 'ProxyClient<Interface>' (aka 'ProxyClient<ipc::capnp::messages::Echo>')
0x5060000392c0: note: object is of type 'mp::ProxyClientBase<ipc::capnp::messages::Echo, interfaces::Echo>'
 03 00 00 00  88 14 5d dc 2a 56 00 00  38 14 5d dc 2a 56 00 00  e8 50 03 00 70 50 00 00  e0 50 03 00
              ^~~~~~~~~~~~~~~~~~~~~~~
              vptr for 'mp::ProxyClientBase<ipc::capnp::messages::Echo, interfaces::Echo>'
    [#0](/bitcoin-core-multiprocess/0/) 0x562adaebf03e in mp::ProxyClientBase<ipc::capnp::messages::Echo, interfaces::Echo>::self() /usr/local/include/mp/proxy.h:95:45
    [#1](/bitcoin-core-multiprocess/1/) 0x562adaebf03e in mp::ProxyClientBase<ipc::capnp::messages::Echo, interfaces::Echo>::ProxyClientBase(ipc::capnp::messages::Echo::Client, mp::Connection*, bool)::'lambda0'()::operator()() const /usr/local/include/mp/proxy-io.h:415:9
    [#2](/bitcoin-core-multiprocess/2/) 0x562adaebd9be in std::function<void ()>::operator()() const /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/std_function.h:591:9
    [#3](/bitcoin-core-multiprocess/3/) 0x562adaebd9be in mp::ProxyClientBase<ipc::capnp::messages::Echo, interfaces::Echo>::~ProxyClientBase() /usr/local/include/mp/proxy-io.h:441:9
    [#4](/bitcoin-core-multiprocess/4/) 0x562adaefd6fe in mp::ProxyClient<ipc::capnp::messages::Echo>::~ProxyClient() /home/runner/work/_temp/ci/scratch/build-x86_64-pc-linux-gnu/src/ipc/capnp/echo.capnp.proxy-types.c++:7:57
    [#5](/bitcoin-core-multiprocess/5/) 0x562adadf3b9b in std::default_delete<interfaces::Echo>::operator()(interfaces::Echo*) const /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/unique_ptr.h:99:2
    [#6](/bitcoin-core-multiprocess/6/) 0x562adadf3b9b in std::__uniq_ptr_impl<interfaces::Echo, std::default_delete<interfaces::Echo>>::reset(interfaces::Echo*) /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/unique_ptr.h:211:4
    [#7](/bitcoin-core-multiprocess/7/) 0x562adadf3b9b in std::unique_ptr<interfaces::Echo, std::default_delete<interfaces::Echo>>::reset(interfaces::Echo*) /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/unique_ptr.h:509:7
    [#8](/bitcoin-core-multiprocess/8/) 0x562adadf3b9b in IpcSocketPairTest() /home/runner/work/_temp/ci/scratch/build-x86_64-pc-linux-gnu/src/./test/ipc_test.cpp:144:17
    [#9](/bitcoin-core-multiprocess/9/) 0x562ad9e71dc0 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
    [#10](/bitcoin-core-multiprocess/10/) 0x562ad9e7102f 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
    [#11](/bitcoin-core-multiprocess/11/) 0x562ad89dc93d in boost::function0<void>::operator()() const /usr/include/boost/function/function_template.hpp:771:14
    [#12](/bitcoin-core-multiprocess/12/) 0x562ad8a5d3d8 in boost::detail::forward::operator()() /usr/include/boost/test/impl/execution_monitor.ipp:1395:32
    [#13](/bitcoin-core-multiprocess/13/) 0x562ad8a5d3d8 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
    [#14](/bitcoin-core-multiprocess/14/) 0x562ad8a56ecd in boost::function0<int>::operator()() const /usr/include/boost/function/function_template.hpp:771:14
    [#15](/bitcoin-core-multiprocess/15/) 0x562ad894260c 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
    [#16](/bitcoin-core-multiprocess/16/) 0x562ad894260c in boost::execution_monitor::catch_signals(boost::function<int ()> const&) /usr/include/boost/test/impl/execution_monitor.ipp:910:16
    [#17](/bitcoin-core-multiprocess/17/) 0x562ad8942b1d in boost::execution_monitor::execute(boost::function<int ()> const&) /usr/include/boost/test/impl/execution_monitor.ipp:1308:16
    [#18](/bitcoin-core-multiprocess/18/) 0x562ad893b1a8 in boost::execution_monitor::vexecute(boost::function<void ()> const&) /usr/include/boost/test/impl/execution_monitor.ipp:1404:5
    [#19](/bitcoin-core-multiprocess/19/) 0x562ad893b1a8 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
    [#20](/bitcoin-core-multiprocess/20/) 0x562ad899f195 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
    [#21](/bitcoin-core-multiprocess/21/) 0x562ad899e0a4 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](/bitcoin-core-multiprocess/22/) 0x562ad899e0a4 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
    [#23](/bitcoin-core-multiprocess/23/) 0x562ad89395fb in boost::unit_test::framework::run(unsigned long, bool) /usr/include/boost/test/impl/framework.ipp:1722:29
    [#24](/bitcoin-core-multiprocess/24/) 0x562ad89682c0 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
    [#25](/bitcoin-core-multiprocess/25/) 0x7f80fa67e1c9  (/lib/x86_64-linux-gnu/libc.so.6+0x2a1c9) (BuildId: 6d64b17fbac799e68da7ebd9985ddf9b5cb375e6)
    [#26](/bitcoin-core-multiprocess/26/) 0x7f80fa67e28a in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28a) (BuildId: 6d64b17fbac799e68da7ebd9985ddf9b5cb375e6)
    [#27](/bitcoin-core-multiprocess/27/) 0x562ad88344c4 in _start (/home/runner/work/_temp/ci/scratch/build-x86_64-pc-linux-gnu/src/test/test_bitcoin+0x13414c4) (BuildId: e1386a5013133aba2702db4ec3dc7a50fba57dcb)

SUMMARY: UndefinedBehaviorSanitizer: dynamic-type-mismatch /usr/local/include/mp/proxy.h:95:45 

I didn't figure out what's going on yet, but there are some differences between this stack trace and the last one https://github.com/chaincodelabs/libmultiprocess/pull/121#issuecomment-2512190620, the main one seeming to be that this crash happens in the ProxyClient<messages::Echo> destructor instead of the ProxyClient<messages::Init> constructor like last time

Looking into this more, it is basically the same bug that's fixed in #121 but happening in a different place. In both cases a ProxyClientBase pointer is cast to a ProxyClient pointer at a bad time when the ProxyClient object is only partially constructed.

The fix in #121 works by delaying ProxyClient::construct() calls that were happening in ProxyClientBase constructor to happen later in the ProxyClient constructor instead.

A new fix will be required to move ProxyClient::destroy() calls triggered in the ProxyClientBase destructor to happen earlier in the ProxyClient destructor. It looks like the fix should be similar and not too complicated.

I think #126 should fix this, so could be worth testing. I think I might try to implement a different fix instead because this fix adds more complexity, and maybe it is possible to go in opposite direction, but it's not clear yet.