proxy-types: fix UndefinedBehaviorSanitizer: null-pointer-use

Fix UndefinedBehaviorSanitizer: null-pointer-use error binding a null pointer
to an unused reference variable. This error should not cause problems in
practice because the reference is not used, but is technically undefined
behavior. Issue was reported:

- Sjors/bitcoin#90 (comment)
- https://github.com/Sjors/bitcoin/actions/runs/15921104847/job/44907875067?pr=90
- Sjors/bitcoin#90 (comment)
- https://github.com/Sjors/bitcoin/actions/runs/15942739184/job/44973004692?pr=90

and fixed:

- Sjors/bitcoin#90 (comment)

Error looks like:

[ TEST ] test.cpp:197: Call IPC method after client connection is closed
/home/runner/work/_temp/src/ipc/libmultiprocess/include/mp/proxy-types.h:612:40: runtime error: reference binding to null pointer of type 'Connection'
    #0 0x5647ad32fc50 in void mp::clientInvoke<mp::ProxyClient<mp::test::messages::FooInterface>, capnp::Request<mp::test::messages::FooInterface::AddParams, mp::test::messages::FooInterface::AddResults> (mp::test::messages::FooInterface::Client::*)(kj::Maybe<capnp::MessageSize>), mp::ClientParam<mp::Accessor<mp::foo_fields::A, 1>, int>, mp::ClientParam<mp::Accessor<mp::foo_fields::B, 1>, int>, mp::ClientParam<mp::Accessor<mp::foo_fields::Result, 2>, int&>>(mp::ProxyClient<mp::test::messages::FooInterface>&, capnp::Request<mp::test::messages::FooInterface::AddParams, mp::test::messages::FooInterface::AddResults> (mp::test::messages::FooInterface::Client::* const&)(kj::Maybe<capnp::MessageSize>), mp::ClientParam<mp::Accessor<mp::foo_fields::A, 1>, int>&&, mp::ClientParam<mp::Accessor<mp::foo_fields::B, 1>, int>&&, mp::ClientParam<mp::Accessor<mp::foo_fields::Result, 2>, int&>&&) /home/runner/work/_temp/build-asan/src/ipc/libmultiprocess/test/./ipc/libmultiprocess/include/mp/proxy-types.h:612:25
    #1 0x5647ad32b62a in mp::ProxyClient<mp::test::messages::FooInterface>::add(int, int) /home/runner/work/_temp/build-asan/src/ipc/libmultiprocess/test/mp/test/foo.capnp.proxy-client.c++:20:5
    #2 0x5647ad2eb9ef in mp::test::TestCase197::run() /home/runner/work/_temp/build-asan/src/ipc/libmultiprocess/test/./ipc/libmultiprocess/test/mp/test/test.cpp:206:14
    #3 0x7f6aaf7100e1  (/lib/x86_64-linux-gnu/libkj-test-1.0.1.so+0x50e1) (BuildId: 2ff7f524274168e50347a2d6dd423f273ae8d268)
    #4 0x7f6aaf710657  (/lib/x86_64-linux-gnu/libkj-test-1.0.1.so+0x5657) (BuildId: 2ff7f524274168e50347a2d6dd423f273ae8d268)
    #5 0x7f6aaf49f37f in kj::MainBuilder::MainImpl::operator()(kj::StringPtr, kj::ArrayPtr<kj::StringPtr const>) (/lib/x86_64-linux-gnu/libkj-1.0.1.so+0x5537f) (BuildId: 4b52c0e2756bcb53e58e705fcb10bab8f63f24fd)
    #6 0x7f6aaf49a499 in kj::runMainAndExit(kj::ProcessContext&, kj::Function<void (kj::StringPtr, kj::ArrayPtr<kj::StringPtr const>)>&&, int, char**) (/lib/x86_64-linux-gnu/libkj-1.0.1.so+0x50499) (BuildId: 4b52c0e2756bcb53e58e705fcb10bab8f63f24fd)
    #7 0x7f6aaf70efcb in main (/lib/x86_64-linux-gnu/libkj-test-1.0.1.so+0x3fcb) (BuildId: 2ff7f524274168e50347a2d6dd423f273ae8d268)
    #8 0x7f6aaeeb81c9  (/lib/x86_64-linux-gnu/libc.so.6+0x2a1c9) (BuildId: 42c84c92e6f98126b3e2230ebfdead22c235b667)
    #9 0x7f6aaeeb828a in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28a) (BuildId: 42c84c92e6f98126b3e2230ebfdead22c235b667)
    #10 0x5647ad1fd614 in _start (/home/runner/work/_temp/build-asan/src/ipc/libmultiprocess/test/mptest+0x11d614) (BuildId: a172f78701ced3f93ad52c3562f181811a1c98e8)

SUMMARY: UndefinedBehaviorSanitizer: null-pointer-use /home/runner/work/_temp/src/ipc/libmultiprocess/include/mp/proxy-types.h:612:40

Author: ryanofsky

include/mp/proxy-types.h

@@ -609,59 +609,61 @@ void clientInvoke(ProxyClient& proxy_client, const GetRequest& get_request, Fiel
             << "{" << g_thread_context.thread_name
             << "} IPC client first request from current thread, constructing waiter";
     }
-    ClientInvokeContext invoke_context{*proxy_client.m_context.connection, g_thread_context};
+    ThreadContext& thread_context{g_thread_context};
+    std::optional<ClientInvokeContext> invoke_context;
     std::exception_ptr exception;
     std::string kj_exception;
     bool done = false;
     const char* disconnected = nullptr;
     proxy_client.m_context.loop->sync([&]() {
         if (!proxy_client.m_context.connection) {
-            const std::unique_lock<std::mutex> lock(invoke_context.thread_context.waiter->m_mutex);
+            const std::unique_lock<std::mutex> lock(thread_context.waiter->m_mutex);
             done = true;
             disconnected = "IPC client method called after disconnect.";
-            invoke_context.thread_context.waiter->m_cv.notify_all();
+            thread_context.waiter->m_cv.notify_all();
             return;
         }
 
         auto request = (proxy_client.m_client.*get_request)(nullptr);
         using Request = CapRequestTraits<decltype(request)>;
         using FieldList = typename ProxyClientMethodTraits<typename Request::Params>::Fields;
-        IterateFields().handleChain(invoke_context, request, FieldList(), typename FieldObjs::BuildParams{&fields}...);
+        invoke_context.emplace(*proxy_client.m_context.connection, thread_context);
+        IterateFields().handleChain(*invoke_context, request, FieldList(), typename FieldObjs::BuildParams{&fields}...);
         proxy_client.m_context.loop->logPlain()
-            << "{" << invoke_context.thread_context.thread_name << "} IPC client send "
+            << "{" << thread_context.thread_name << "} IPC client send "
             << TypeName<typename Request::Params>() << " " << LogEscape(request.toString());
 
         proxy_client.m_context.loop->m_task_set->add(request.send().then(
             [&](::capnp::Response<typename Request::Results>&& response) {
                 proxy_client.m_context.loop->logPlain()
-                    << "{" << invoke_context.thread_context.thread_name << "} IPC client recv "
+                    << "{" << thread_context.thread_name << "} IPC client recv "
                     << TypeName<typename Request::Results>() << " " << LogEscape(response.toString());
                 try {
                     IterateFields().handleChain(
-                        invoke_context, response, FieldList(), typename FieldObjs::ReadResults{&fields}...);
+                        *invoke_context, response, FieldList(), typename FieldObjs::ReadResults{&fields}...);
                 } catch (...) {
                     exception = std::current_exception();
                 }
-                const std::unique_lock<std::mutex> lock(invoke_context.thread_context.waiter->m_mutex);
+                const std::unique_lock<std::mutex> lock(thread_context.waiter->m_mutex);
                 done = true;
-                invoke_context.thread_context.waiter->m_cv.notify_all();
+                thread_context.waiter->m_cv.notify_all();
             },
             [&](const ::kj::Exception& e) {
                 if (e.getType() == ::kj::Exception::Type::DISCONNECTED) {
                     disconnected = "IPC client method call interrupted by disconnect.";
                 } else {
                     kj_exception = kj::str("kj::Exception: ", e).cStr();
                     proxy_client.m_context.loop->logPlain()
-                        << "{" << invoke_context.thread_context.thread_name << "} IPC client exception " << kj_exception;
+                        << "{" << thread_context.thread_name << "} IPC client exception " << kj_exception;
                 }
-                const std::unique_lock<std::mutex> lock(invoke_context.thread_context.waiter->m_mutex);
+                const std::unique_lock<std::mutex> lock(thread_context.waiter->m_mutex);
                 done = true;
-                invoke_context.thread_context.waiter->m_cv.notify_all();
+                thread_context.waiter->m_cv.notify_all();
             }));
     });
 
-    std::unique_lock<std::mutex> lock(invoke_context.thread_context.waiter->m_mutex);
-    invoke_context.thread_context.waiter->wait(lock, [&done]() { return done; });
+    std::unique_lock<std::mutex> lock(thread_context.waiter->m_mutex);
+    thread_context.waiter->wait(lock, [&done]() { return done; });
     if (exception) std::rethrow_exception(exception);
     if (!kj_exception.empty()) proxy_client.m_context.loop->raise() << kj_exception;
     if (disconnected) proxy_client.m_context.loop->raise() << disconnected;