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:

• ci: build multiprocess on most jobs 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)

[sedited]

lgtm ACK 5b81192

Makes sense to instantiate the custom implementation first before calling construct.

[Sjors]

Testing on Bitcoin Core CI: bitcoin/bitcoin@93f7f30 bitcoin/bitcoin#30975 (comment)

Which fails, but it seems to happen at a different place (at Echo instead of Init), so perhaps it's a completely different issue.

[Sjors]

I reproduced the original failure locally on macOS 15.2, by installing libmultiprocess from master @ abe254b (no depends).

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:133: info: check socketpair(1, 1, 0, fds) == 0 has passed
/usr/local/include/mp/proxy.h:95:45: runtime error: downcast of address 0x600001d74480 which does not point to an object of type 'ProxyClient<Interface>' (aka 'ProxyClient<ipc::capnp::messages::Init>')
0x600001d74480: note: object is of type 'mp::ProxyClientBase<ipc::capnp::messages::Init, interfaces::Init>'
...
SUMMARY: UndefinedBehaviorSanitizer: dynamic-type-mismatch /usr/local/include/mp/proxy.h:95:45 in 

I then installed libmultiprocess from this PR, deleted the Bitcoin Core build dir and did the above again.

The test now makes it line 143, so I'd say the original issue is fixed.

test/ipc_test.cpp:133: info: check socketpair(1, 1, 0, fds) == 0 has passed
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>'

I'll open a new issue for the new failure.