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.

5 files changed, 53 insertions, 2 deletions

diff --git a/src/ipc/capnp/context.h b/src/ipc/capnp/context.h
new file mode 100644
index 0000000000..06e1614494
--- /dev/null
+++ b/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
diff --git a/src/ipc/capnp/protocol.cpp b/src/ipc/capnp/protocol.cpp
index 74c66c899a..37b57a9525 100644
--- a/src/ipc/capnp/protocol.cpp
+++ b/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 @@ public:
     {
         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 @@ public:
     {
         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 @@ public:
         });
         promise.get_future().wait();
     }
+    Context m_context;
     std::thread m_loop_thread;
     std::optional<mp::EventLoop> m_loop;
 };
diff --git a/src/ipc/context.h b/src/ipc/context.h
new file mode 100644
index 0000000000..924d7d7450
--- /dev/null
+++ b/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
diff --git a/src/ipc/interfaces.cpp b/src/ipc/interfaces.cpp
index ad4b78ed81..580590fde9 100644
--- a/src/ipc/interfaces.cpp
+++ b/src/ipc/interfaces.cpp
@@ -60,6 +60,7 @@ public:
     {
         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;
diff --git a/src/ipc/protocol.h b/src/ipc/protocol.h
index af955b0007..4cd892e411 100644
--- a/src/ipc/protocol.h
+++ b/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 @@ public:
     //! 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