ipc: Expose an RPC interface over the -ipcbind socket

This allows `bitcoin-cli` to connect to the node via IPC instead TCP to execute
RPC methods in an upcoming commit.

src/interfaces/README.md

@@ -16,4 +16,6 @@ The following interfaces are defined here:
 
 * [`Ipc`](ipc.h) — used by multiprocess code to access `Init` interface across processes. Added in [#19160](https://github.com/bitcoin/bitcoin/pull/19160).
 
+* [`Rpc`](rpc.h) — used by `bitcoin-cli` to be able to call RPC methods over a unix socket instead of TCP.
+
 The interfaces above define boundaries between major components of bitcoin code (node, wallet, and gui), making it possible for them to run in [different processes](../../doc/multiprocess.md), and be tested, developed, and understood independently. These interfaces are not currently designed to be stable or to be used externally.

src/interfaces/init.h

@@ -9,6 +9,7 @@
 #include <interfaces/echo.h>
 #include <interfaces/mining.h>
 #include <interfaces/node.h>
+#include <interfaces/rpc.h>
 #include <interfaces/wallet.h>
 
 #include <memory>
@@ -36,6 +37,7 @@ public:
     virtual std::unique_ptr<Mining> makeMining() { return nullptr; }
     virtual std::unique_ptr<WalletLoader> makeWalletLoader(Chain& chain) { return nullptr; }
     virtual std::unique_ptr<Echo> makeEcho() { return nullptr; }
+    virtual std::unique_ptr<Rpc> makeRpc() { return nullptr; }
     virtual Ipc* ipc() { return nullptr; }
     virtual bool canListenIpc() { return false; }
     virtual const char* exeName() { return nullptr; }

src/interfaces/rpc.h

@@ -0,0 +1,31 @@
+// Copyright (c) 2025 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_RPC_H
+#define BITCOIN_INTERFACES_RPC_H
+
+#include <memory>
+#include <string>
+
+class UniValue;
+
+namespace node {
+struct NodeContext;
+} // namespace node
+
+namespace interfaces {
+//! Interface giving clients ability to emulate HTTP RPC calls.
+class Rpc
+{
+public:
+    virtual ~Rpc() = default;
+    virtual UniValue executeRpc(UniValue request, std::string url, std::string user) = 0;
+};
+
+//! Return implementation of Rpc interface.
+std::unique_ptr<Rpc> MakeRpc(node::NodeContext& node);
+
+} // namespace interfaces
+
+#endif // BITCOIN_INTERFACES_RPC_H