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proxy-client: fix TSan data race in clientDestroy#286

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ryanofsky wants to merge 2 commits into
bitcoin-core:masterfrom
ryanofsky:pr/drace
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proxy-client: fix TSan data race in clientDestroy#286
ryanofsky wants to merge 2 commits into
bitcoin-core:masterfrom
ryanofsky:pr/drace

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@ryanofsky ryanofsky commented Jun 3, 2026

#273 added a new test which exposed a longstanding race condition in clientDestroy logging code, causing the sanitize CI job to now fail with a TSAN error. Fix the race by simplifying the logging code.

Details about the fix are in the commit message. TSAN error looked like

  Write of size 8 at 0x721400005020 by thread T24 (mutexes: write M0):
    #0 std::__1::__function::__func<mp::ProxyClientBase<mp::test::messages::FooCallback, mp::test::FooCallback>::ProxyClientBase(mp::test::messages::FooCallback::Client, mp::Connection*, bool)::{lambda()#1}, void ()>::operator()() /home/runner/work/libmultiprocess/libmultiprocess/include/mp/proxy-io.h:522 (mptest+0x242b20)
    #1 void mp::Unlock<mp::Lock, std::__1::function<void ()>&>(mp::Lock&, std::__1::function<void ()>&) /nix/store/6z6xj37ljxwrwd5w3rs0zyfkrxd48lzl-libcxx-21.1.2-dev/include/c++/v1/__functional/function.h:274 (mptest+0x35582d)
    #2 mp::Connection::~Connection() /home/runner/work/libmultiprocess/libmultiprocess/src/mp/proxy.cpp:150 (mptest+0x34e596)
    ...
    #11 mp::test::TestSetup::TestSetup(bool)::{lambda()#1}::operator()() const /home/runner/work/libmultiprocess/libmultiprocess/test/mp/test/test.cpp:112 (mptest+0x1504fe)

  Previous read of size 8 at 0x721400005020 by thread T26:
    #0 void mp::clientDestroy<mp::ProxyClient<mp::test::messages::FooCallback> >(mp::ProxyClient<mp::test::messages::FooCallback>&) /home/runner/work/libmultiprocess/libmultiprocess/include/mp/proxy-types.h:647 (mptest+0x342d2a)
     ...
    #21 std::__1::__function::__func<mp::ProxyServerBase<mp::test::messages::FooInterface, mp::test::FooImplementation>::~ProxyServerBase()::{lambda()#1}, void ()>::operator()() /nix/store/6z6xj37ljxwrwd5w3rs0zyfkrxd48lzl-libcxx-21.1.2-dev/include/c++/v1/__functional/function.h:174 (mptest+0x347c76)
     ...
    #23 void* std::__1::__thread_proxy[abi:ne210101]<std::__1::tuple<std::__1::unique_ptr<std::__1::__thread_struct, std::__1::default_delete<std::__1::__thread_struct> >, mp::EventLoop::startAsyncThread()::$_0> >(void*) /home/runner/work/libmultiprocess/libmultiprocess/src/mp/proxy.cpp:309 (mptest+0x353ef2)

https://github.com/bitcoin-core/libmultiprocess/actions/runs/26851297744/job/79183796152

@ryanofsky @claude
proxy-client: fix TSan data race in clientDestroy
25bb3e6 
clientDestroy() reads m_context.connection to decide whether to use
MP_LOG vs KJ_LOG, but Connection::~Connection() can set it to null
concurrently from the event loop thread (via the disconnect_cb sync
cleanup callback) while the destructor runs on an async cleanup thread,
causing a TSan-reported data race. The race is exposed by the test added
in commit 90be835 ("test: regression for ~ProxyClient destroy after peer
disconnect").

The KJ_LOG fallback was only needed before commit 315ff53 ("refactor:
Add ProxyContext EventLoop* member"), when logging required going
through connection->m_loop. Since that commit, m_context.loop is a
direct EventLoopRef that is always valid regardless of whether
m_context.connection is null. The KJ_LOG branch is now dead code, so
drop it and the connection check entirely.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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DrahtBot commented Jun 3, 2026 •
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Conflicts

Reviewers, this pull request conflicts with the following ones:

  • #269 (proxy: add local connection limit to ListenConnections by enirox001)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

@DrahtBot DrahtBot mentioned this pull request Jun 3, 2026
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@ryanofsky
mpgen: iwyu changes required by previous commit
039e5ac 
gen.cpp uses IWYU pragma: keep for include which appears to be necessary for
newer versions of capnproto (1.4.0 but not 1.1.0)
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ryanofsky commented Jun 4, 2026 •
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Added 1 commits 25bb3e6 -> 0963634 (pr/drace.1 -> pr/drace.2, compare) to fix IWYU errors https://github.com/bitcoin-core/libmultiprocess/actions/runs/26919582954/job/79416996848?pr=286

Updated 0963634 -> 039e5ac (pr/drace.2 -> pr/drace.3, compare) to fix IWYU errors https://github.com/bitcoin-core/libmultiprocess/actions/runs/26924152551/job/79430685686?pr=286

@ryanofsky ryanofsky mentioned this pull request Jun 4, 2026
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