Merge bitcoin/bitcoin#22218: multiprocess: Add ipc::Context and ipc::capnp::Context structs

3e33d170cc0a8f386791777f3cc597e2bd0bf2ee Add ipc::Context and ipc::capnp::Context structs (Russell Yanofsky)

Pull request description:

  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.

  ---

  This PR is part of the [process separation project](https://github.com/bitcoin/bitcoin/projects/10). The commit was first part of larger PR #10102.

ACKs for top commit:
  ariard:
    Code Review ACK 3e33d170

Tree-SHA512: fd949fae5f1a973d39cb97f2745821ab2f62b98e166e53bc2801f97dcde988e18faaaaa0ffc2a82c170938b3a18078b6162fa35460e6e7c635e681b3c9e5b0a6

7 files changed, 62 insertions, 2 deletions

diff --git a/src/Makefile.am b/src/Makefile.am
index 407fdf5a8f..a8d6591e98 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -842,9 +842,11 @@ 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 \
diff --git a/src/interfaces/ipc.h b/src/interfaces/ipc.h
index e9e6c78053..963649fc9a 100644
--- a/src/interfaces/ipc.h
+++ b/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 @@ public:
         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.
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