multiprocess: Add Ipc and Init interface definitions

src/interfaces/ipc.h

@@ -0,0 +1,48 @@
+// Copyright (c) 2021 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_INTERFACES_IPC_H
+#define BITCOIN_INTERFACES_IPC_H
+
+#include <functional>
+#include <memory>
+#include <typeindex>
+
+namespace interfaces {
+class Init;
+
+//! Interface providing access to interprocess-communication (IPC)
+//! functionality.
+class Ipc
+{
+public:
+    virtual ~Ipc() = default;
+
+    //! Spawn a child process returning pointer to its Init interface.
+    virtual std::unique_ptr<Init> spawnProcess(const char* exe_name) = 0;
+
+    //! If this is a spawned process, block and handle requests from the parent
+    //! process by forwarding them to this process's Init interface, then return
+    //! true. If this is not a spawned child process, return false.
+    virtual bool startSpawnedProcess(int argc, char* argv[], int& exit_status) = 0;
+
+    //! Add cleanup callback to remote interface that will run when the
+    //! interface is deleted.
+    template<typename Interface>
+    void addCleanup(Interface& iface, std::function<void()> cleanup)
+    {
+        addCleanup(typeid(Interface), &iface, std::move(cleanup));
+    }
+
+protected:
+    //! Internal implementation of public addCleanup method (above) as a
+    //! type-erased virtual function, since template functions can't be virtual.
+    virtual void addCleanup(std::type_index type, void* iface, std::function<void()> cleanup) = 0;
+};
+
+//! Return implementation of Ipc interface.
+std::unique_ptr<Ipc> MakeIpc(const char* exe_name, const char* process_argv0, Init& init);
+} // namespace interfaces
+
+#endif // BITCOIN_INTERFACES_IPC_H