ipc: Avoid waiting for clients to disconnect when shutting down

This fixes behavior reported by Antoine Poinsot <darosior@protonmail.com>
https://github.com/bitcoin/bitcoin/pull/29409#issuecomment-2546088852 where if
an IPC client is connected, the node will wait forever for it to disconnect
before exiting.

src/init.cpp

@@ -374,6 +374,12 @@ void Shutdown(NodeContext& node)
         client->stop();
     }
 
+    // If any -ipcbind clients are still connected, disconnect them now so they
+    // do not block shutdown.
+    if (interfaces::Ipc* ipc = node.init->ipc()) {
+        ipc->disconnectIncoming();
+    }
+
 #ifdef ENABLE_ZMQ
     if (g_zmq_notification_interface) {
         if (node.validation_signals) node.validation_signals->UnregisterValidationInterface(g_zmq_notification_interface.get());

src/interfaces/ipc.h

@@ -70,6 +70,9 @@ public:
     //! using provided callback. Throws an exception if there was an error.
     virtual void listenAddress(std::string& address) = 0;
 
+    //! Disconnect any incoming connections that are still connected.
+    virtual void disconnectIncoming() = 0;
+
     //! Add cleanup callback to remote interface that will run when the
     //! interface is deleted.
     template<typename Interface>

src/ipc/capnp/protocol.cpp

@@ -65,9 +65,20 @@ public:
         m_loop.emplace(exe_name, &IpcLogFn, &m_context);
         if (ready_fn) ready_fn();
         mp::ServeStream<messages::Init>(*m_loop, fd, init);
+        m_parent_connection = &m_loop->m_incoming_connections.back();
         m_loop->loop();
         m_loop.reset();
     }
+    void disconnectIncoming() override
+    {
+        if (!m_loop) return;
+        // Delete incoming connections, except the connection to a parent
+        // process (if there is one), since a parent process should be able to
+        // monitor and control this process, even during shutdown.
+        m_loop->sync([&] {
+            m_loop->m_incoming_connections.remove_if([this](mp::Connection& c) { return &c != m_parent_connection; });
+        });
+    }
     void addCleanup(std::type_index type, void* iface, std::function<void()> cleanup) override
     {
         mp::ProxyTypeRegister::types().at(type)(iface).cleanup_fns.emplace_back(std::move(cleanup));
@@ -95,6 +106,8 @@ public:
     //! creation, decrements on destruction. The loop thread exits when the
     //! refcount reaches 0. Other IPC objects also hold their own EventLoopRef.
     std::optional<mp::EventLoopRef> m_loop_ref;
+    //! Connection to parent, if this is a child process spawned by a parent process.
+    mp::Connection* m_parent_connection{nullptr};
 };
 } // namespace
 

src/ipc/interfaces.cpp

@@ -86,6 +86,10 @@ public:
         int fd = m_process->bind(gArgs.GetDataDirNet(), m_exe_name, address);
         m_protocol->listen(fd, m_exe_name, m_init);
     }
+    void disconnectIncoming() override
+    {
+        m_protocol->disconnectIncoming();
+    }
     void addCleanup(std::type_index type, void* iface, std::function<void()> cleanup) override
     {
         m_protocol->addCleanup(type, iface, std::move(cleanup));

src/ipc/protocol.h

@@ -58,6 +58,9 @@ public:
     //! clients and servers independently.
     virtual void serve(int fd, const char* exe_name, interfaces::Init& init, const std::function<void()>& ready_fn = {}) = 0;
 
+    //! Disconnect any incoming connections that are still connected.
+    virtual void disconnectIncoming() = 0;
+
     //! Add cleanup callback to interface that will run when the interface is
     //! deleted.
     virtual void addCleanup(std::type_index type, void* iface, std::function<void()> cleanup) = 0;