Add ipc::Context and ipc::capnp::Context structs

These are currently empty structs but they will be used to pass some
function and object pointers from bitcoin application code to IPC hooks
that run, for example, when a remote object is created or destroyed, or
a new process is created.

src/Makefile.am

@@ -840,9 +840,11 @@ EXTRA_DIST += $(libbitcoin_ipc_mpgen_input)
 if BUILD_MULTIPROCESS
 LIBBITCOIN_IPC=libbitcoin_ipc.a
 libbitcoin_ipc_a_SOURCES = \
+  ipc/capnp/context.h \
   ipc/capnp/init-types.h \
   ipc/capnp/protocol.cpp \
   ipc/capnp/protocol.h \
+  ipc/context.h \
   ipc/exception.h \
   ipc/interfaces.cpp \
   ipc/process.cpp \

src/interfaces/ipc.h

@@ -9,6 +9,10 @@
 #include <memory>
 #include <typeindex>
 
+namespace ipc {
+struct Context;
+} // namespace ipc
+
 namespace interfaces {
 class Init;
 
@@ -58,6 +62,9 @@ class Ipc
         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.

src/ipc/capnp/context.h

@@ -0,0 +1,23 @@
+// 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_IPC_CAPNP_CONTEXT_H
+#define BITCOIN_IPC_CAPNP_CONTEXT_H
+
+#include <ipc/context.h>
+
+namespace ipc {
+namespace capnp {
+//! Cap'n Proto context struct. Generally the parent ipc::Context struct should
+//! be used instead of this struct to give all IPC protocols access to
+//! application state, so there aren't unnecessary differences between IPC
+//! protocols. But this specialized struct can be used to pass capnp-specific
+//! function and object types to capnp hooks.
+struct Context : ipc::Context
+{
+};
+} // namespace capnp
+} // namespace ipc
+
+#endif // BITCOIN_IPC_CAPNP_CONTEXT_H

src/ipc/capnp/protocol.cpp

@@ -3,6 +3,7 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <interfaces/init.h>
+#include <ipc/capnp/context.h>
 #include <ipc/capnp/init.capnp.h>
 #include <ipc/capnp/init.capnp.proxy.h>
 #include <ipc/capnp/protocol.h>
@@ -54,7 +55,7 @@ class CapnpProtocol : public Protocol
     {
         assert(!m_loop);
         mp::g_thread_context.thread_name = mp::ThreadName(exe_name);
-        m_loop.emplace(exe_name, &IpcLogFn, nullptr);
+        m_loop.emplace(exe_name, &IpcLogFn, &m_context);
         mp::ServeStream<messages::Init>(*m_loop, fd, init);
         m_loop->loop();
         m_loop.reset();
@@ -63,13 +64,14 @@ class CapnpProtocol : public Protocol
     {
         mp::ProxyTypeRegister::types().at(type)(iface).cleanup.emplace_back(std::move(cleanup));
     }
+    Context& context() override { return m_context; }
     void startLoop(const char* exe_name)
     {
         if (m_loop) return;
         std::promise<void> promise;
         m_loop_thread = std::thread([&] {
             util::ThreadRename("capnp-loop");
-            m_loop.emplace(exe_name, &IpcLogFn, nullptr);
+            m_loop.emplace(exe_name, &IpcLogFn, &m_context);
             {
                 std::unique_lock<std::mutex> lock(m_loop->m_mutex);
                 m_loop->addClient(lock);
@@ -80,6 +82,7 @@ class CapnpProtocol : public Protocol
         });
         promise.get_future().wait();
     }
+    Context m_context;
     std::thread m_loop_thread;
     std::optional<mp::EventLoop> m_loop;
 };

src/ipc/context.h

@@ -0,0 +1,19 @@
+// 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_IPC_CONTEXT_H
+#define BITCOIN_IPC_CONTEXT_H
+
+namespace ipc {
+//! Context struct used to give IPC protocol implementations or implementation
+//! hooks access to application state, in case they need to run extra code that
+//! isn't needed within a single process, like code copying global state from an
+//! existing process to a new process when it's initialized, or code dealing
+//! with shared objects that are created or destroyed remotely.
+struct Context
+{
+};
+} // namespace ipc
+
+#endif // BITCOIN_IPC_CONTEXT_H

src/ipc/interfaces.cpp

@@ -60,6 +60,7 @@ class IpcImpl : public interfaces::Ipc
     {
         m_protocol->addCleanup(type, iface, std::move(cleanup));
     }
+    Context& context() override { return m_protocol->context(); }
     const char* m_exe_name;
     const char* m_process_argv0;
     interfaces::Init& m_init;

src/ipc/protocol.h

@@ -12,6 +12,8 @@
 #include <typeindex>
 
 namespace ipc {
+struct Context;
+
 //! IPC protocol interface for calling IPC methods over sockets.
 //!
 //! There may be different implementations of this interface for different IPC
@@ -33,6 +35,9 @@ class Protocol
     //! 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;
+
+    //! Context accessor.
+    virtual Context& context() = 0;
 };
 } // namespace ipc