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src/interfaces/ipc.h
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// Copyright (c) 2021-present 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 ipc {
struct Context;
} // namespace ipc

namespace interfaces {
class Init;

//! Interface providing access to interprocess-communication (IPC)
//! functionality. The IPC implementation is responsible for establishing
//! connections between a controlling process and a process being controlled.
//! When a connection is established, the process being controlled returns an
//! interfaces::Init pointer to the controlling process, which the controlling
//! process can use to get access to other interfaces and functionality.
//!
//! When spawning a new process, the steps are:
//!
//! 1. The controlling process calls interfaces::Ipc::spawnProcess(), which
//!    calls ipc::Process::spawn(), which spawns a new process and returns a
//!    socketpair file descriptor for communicating with it.
//!    interfaces::Ipc::spawnProcess() then calls ipc::Protocol::connect()
//!    passing the socketpair descriptor, which returns a local proxy
//!    interfaces::Init implementation calling remote interfaces::Init methods.
//! 2. The spawned process calls interfaces::Ipc::startSpawnProcess(), which
//!    calls ipc::Process::checkSpawned() to read command line arguments and
//!    determine whether it is a spawned process and what socketpair file
//!    descriptor it should use. It then calls ipc::Protocol::serve() to handle
//!    incoming requests from the socketpair and invoke interfaces::Init
//!    interface methods, and exit when the socket is closed.
//! 3. The controlling process calls local proxy interfaces::Init object methods
//!    to make other proxy objects calling other remote interfaces. It can also
//!    destroy the initial interfaces::Init object to close the connection and
//!    shut down the spawned process.
//!
//! When connecting to an existing process, the steps are similar to spawning a
//! new process, except a socket is created instead of a socketpair, and
//! destroying an Init interface doesn't end the process, since there can be
//! multiple connections.
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;

    //! Connect to a socket address and return a pointer to its Init interface.
    //! Returns a non-null pointer if the connection was established, returns
    //! null if address is empty ("") or disabled ("0") or if a connection was
    //! refused but not required ("auto"), and throws an exception if there was
    //! an unexpected error.
    virtual std::unique_ptr<Init> connectAddress(std::string& address) = 0;

    //! Listen on a socket address exposing this process's init interface to
    //! clients. 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>
    void addCleanup(Interface& iface, std::function<void()> cleanup)
    {
        addCleanup(typeid(Interface), &iface, std::move(cleanup));
    }

    //! IPC context struct accessor (see struct definition for more description).
    virtual ipc::Context& context() = 0;

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