multiprocess: Add echoipc RPC method and test

Add simple interfaces::Echo IPC interface with one method that just takes and
returns a string, to test multiprocess framework and provide an example of how
it can be used to spawn and call between processes.

src/Makefile.am

@@ -159,6 +159,7 @@ BITCOIN_CORE_H = \
   init.h \
   init/common.h \
   interfaces/chain.h \
+  interfaces/echo.h \
   interfaces/handler.h \
   interfaces/init.h \
   interfaces/ipc.h \
@@ -563,6 +564,7 @@ libbitcoin_util_a_SOURCES = \
   compat/glibcxx_sanity.cpp \
   compat/strnlen.cpp \
   fs.cpp \
+  interfaces/echo.cpp \
   interfaces/handler.cpp \
   interfaces/init.cpp \
   logging.cpp \
@@ -815,6 +817,7 @@ if HARDEN
 endif
 
 libbitcoin_ipc_mpgen_input = \
+  ipc/capnp/echo.capnp \
   ipc/capnp/init.capnp
 EXTRA_DIST += $(libbitcoin_ipc_mpgen_input)
 %.capnp:

src/init/bitcoin-node.cpp

@@ -2,6 +2,7 @@
 // Distributed under the MIT software license, see the accompanying
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
+#include <interfaces/echo.h>
 #include <interfaces/init.h>
 #include <interfaces/ipc.h>
 #include <node/context.h>
@@ -21,6 +22,7 @@ public:
     {
         m_node.init = this;
     }
+    std::unique_ptr<interfaces::Echo> makeEcho() override { return interfaces::MakeEcho(); }
     interfaces::Ipc* ipc() override { return m_ipc.get(); }
     NodeContext& m_node;
     std::unique_ptr<interfaces::Ipc> m_ipc;

src/interfaces/echo.cpp

@@ -0,0 +1,18 @@
+// 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.
+
+#include <interfaces/echo.h>
+
+#include <memory>
+
+namespace interfaces {
+namespace {
+class EchoImpl : public Echo
+{
+public:
+    std::string echo(const std::string& echo) override { return echo; }
+};
+} // namespace
+std::unique_ptr<Echo> MakeEcho() { return std::make_unique<EchoImpl>(); }
+} // namespace interfaces

src/interfaces/echo.h

@@ -0,0 +1,26 @@
+// 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_INTERFACES_ECHO_H
+#define BITCOIN_INTERFACES_ECHO_H
+
+#include <memory>
+#include <string>
+
+namespace interfaces {
+//! Simple string echoing interface for testing.
+class Echo
+{
+public:
+    virtual ~Echo() {}
+
+    //! Echo provided string.
+    virtual std::string echo(const std::string& echo) = 0;
+};
+
+//! Return implementation of Echo interface.
+std::unique_ptr<Echo> MakeEcho();
+} // namespace interfaces
+
+#endif // BITCOIN_INTERFACES_ECHO_H

src/interfaces/init.cpp

@@ -3,6 +3,7 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <interfaces/chain.h>
+#include <interfaces/echo.h>
 #include <interfaces/init.h>
 #include <interfaces/node.h>
 #include <interfaces/wallet.h>
@@ -11,5 +12,6 @@ namespace interfaces {
 std::unique_ptr<Node> Init::makeNode() { return {}; }
 std::unique_ptr<Chain> Init::makeChain() { return {}; }
 std::unique_ptr<WalletClient> Init::makeWalletClient(Chain& chain) { return {}; }
+std::unique_ptr<Echo> Init::makeEcho() { return {}; }
 Ipc* Init::ipc() { return nullptr; }
 } // namespace interfaces

src/interfaces/init.h

@@ -11,6 +11,7 @@ struct NodeContext;
 
 namespace interfaces {
 class Chain;
+class Echo;
 class Ipc;
 class Node;
 class WalletClient;
@@ -29,6 +30,7 @@ public:
     virtual std::unique_ptr<Node> makeNode();
     virtual std::unique_ptr<Chain> makeChain();
     virtual std::unique_ptr<WalletClient> makeWalletClient(Chain& chain);
+    virtual std::unique_ptr<Echo> makeEcho();
     virtual Ipc* ipc();
 };
 

src/ipc/capnp/echo.capnp

@@ -0,0 +1,17 @@
+# 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.
+
+@0x888b4f7f51e691f7;
+
+using Cxx = import "/capnp/c++.capnp";
+$Cxx.namespace("ipc::capnp::messages");
+
+using Proxy = import "/mp/proxy.capnp";
+$Proxy.include("interfaces/echo.h");
+$Proxy.include("ipc/capnp/echo.capnp.h");
+
+interface Echo $Proxy.wrap("interfaces::Echo") {
+    destroy @0 (context :Proxy.Context) -> ();
+    echo @1 (context :Proxy.Context, echo: Text) -> (result :Text);
+}

src/ipc/capnp/init-types.h

@@ -4,4 +4,7 @@
 
 #ifndef BITCOIN_IPC_CAPNP_INIT_TYPES_H
 #define BITCOIN_IPC_CAPNP_INIT_TYPES_H
+
+#include <ipc/capnp/echo.capnp.proxy-types.h>
+
 #endif // BITCOIN_IPC_CAPNP_INIT_TYPES_H

src/ipc/capnp/init.capnp

@@ -8,9 +8,13 @@ using Cxx = import "/capnp/c++.capnp";
 $Cxx.namespace("ipc::capnp::messages");
 
 using Proxy = import "/mp/proxy.capnp";
+$Proxy.include("interfaces/echo.h");
 $Proxy.include("interfaces/init.h");
 $Proxy.includeTypes("ipc/capnp/init-types.h");
 
+using Echo = import "echo.capnp";
+
 interface Init $Proxy.wrap("interfaces::Init") {
     construct @0 (threadMap: Proxy.ThreadMap) -> (threadMap :Proxy.ThreadMap);
+    makeEcho @1 (context :Proxy.Context) -> (result :Echo.Echo);
 }

src/rpc/misc.cpp

@@ -7,6 +7,9 @@
 #include <index/blockfilterindex.h>
 #include <index/txindex.h>
 #include <interfaces/chain.h>
+#include <interfaces/echo.h>
+#include <interfaces/init.h>
+#include <interfaces/ipc.h>
 #include <key_io.h>
 #include <node/context.h>
 #include <outputtype.h>
@@ -644,6 +647,43 @@ static RPCHelpMan echo(const std::string& name)
 static RPCHelpMan echo() { return echo("echo"); }
 static RPCHelpMan echojson() { return echo("echojson"); }
 
+static RPCHelpMan echoipc()
+{
+    return RPCHelpMan{
+        "echoipc",
+        "\nEcho back the input argument, passing it through a spawned process in a multiprocess build.\n"
+        "This command is for testing.\n",
+        {{"arg", RPCArg::Type::STR, RPCArg::Optional::NO, "The string to echo",}},
+        RPCResult{RPCResult::Type::STR, "echo", "The echoed string."},
+        RPCExamples{HelpExampleCli("echo", "\"Hello world\"") +
+                    HelpExampleRpc("echo", "\"Hello world\"")},
+        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue {
+            std::unique_ptr<interfaces::Echo> echo;
+            if (interfaces::Ipc* ipc = Assert(EnsureAnyNodeContext(request.context).init)->ipc()) {
+                // Spawn a new bitcoin-node process and call makeEcho to get a
+                // client pointer to a interfaces::Echo instance running in
+                // that process. This is just for testing. A slightly more
+                // realistic test spawning a different executable instead of
+                // the same executable would add a new bitcoin-echo executable,
+                // and spawn bitcoin-echo below instead of bitcoin-node. But
+                // using bitcoin-node avoids the need to build and install a
+                // new executable just for this one test.
+                auto init = ipc->spawnProcess("bitcoin-node");
+                echo = init->makeEcho();
+                ipc->addCleanup(*echo, [init = init.release()] { delete init; });
+            } else {
+                // IPC support is not available because this is a bitcoind
+                // process not a bitcoind-node process, so just create a local
+                // interfaces::Echo object and return it so the `echoipc` RPC
+                // method will work, and the python test calling `echoipc`
+                // can expect the same result.
+                echo = interfaces::MakeEcho();
+            }
+            return echo->echo(request.params[0].get_str());
+        },
+    };
+}
+
 static UniValue SummaryToJSON(const IndexSummary&& summary, std::string index_name)
 {
     UniValue ret_summary(UniValue::VOBJ);
@@ -719,6 +759,7 @@ static const CRPCCommand commands[] =
     { "hidden",             &mockscheduler,           },
     { "hidden",             &echo,                    },
     { "hidden",             &echojson,                },
+    { "hidden",             &echoipc,                 },
 };
 // clang-format on
     for (const auto& c : commands) {

test/functional/rpc_misc.py

@@ -61,6 +61,9 @@ class RpcMiscTest(BitcoinTestFramework):
         node.logging(include=['qt'])
         assert_equal(node.logging()['qt'], True)
 
+        self.log.info("test echoipc (testing spawned process in multiprocess build)")
+        assert_equal(node.echoipc("hello"), "hello")
+
         self.log.info("test getindexinfo")
         # Without any indices running the RPC returns an empty object
         assert_equal(node.getindexinfo(), {})