rpc: add test-only sendmsgtopeer rpc

This rpc can be used when we want a node to send a message, but
cannot use a python P2P object, for example for testing of low-level
net transport behavior.

src/rpc/client.cpp

@@ -299,6 +299,7 @@ static const CRPCConvertParam vRPCConvertParams[] =
     { "getnodeaddresses", 0, "count"},
     { "addpeeraddress", 1, "port"},
     { "addpeeraddress", 2, "tried"},
+    { "sendmsgtopeer", 0, "peer_id" },
     { "stop", 0, "wait" },
 };
 // clang-format on

src/rpc/net.cpp

@@ -968,6 +968,54 @@ static RPCHelpMan addpeeraddress()
     };
 }
 
+static RPCHelpMan sendmsgtopeer()
+{
+    return RPCHelpMan{
+        "sendmsgtopeer",
+        "Send a p2p message to a peer specified by id.\n"
+        "The message type and body must be provided, the message header will be generated.\n"
+        "This RPC is for testing only.",
+        {
+            {"peer_id", RPCArg::Type::NUM, RPCArg::Optional::NO, "The peer to send the message to."},
+            {"msg_type", RPCArg::Type::STR, RPCArg::Optional::NO, strprintf("The message type (maximum length %i)", CMessageHeader::COMMAND_SIZE)},
+            {"msg", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The serialized message body to send, in hex, without a message header"},
+        },
+        RPCResult{RPCResult::Type::OBJ, "", "", std::vector<RPCResult>{}},
+        RPCExamples{
+            HelpExampleCli("sendmsgtopeer", "0 \"addr\" \"ffffff\"") + HelpExampleRpc("sendmsgtopeer", "0 \"addr\" \"ffffff\"")},
+        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue {
+            const NodeId peer_id{request.params[0].getInt<int64_t>()};
+            const std::string& msg_type{request.params[1].get_str()};
+            if (msg_type.size() > CMessageHeader::COMMAND_SIZE) {
+                throw JSONRPCError(RPC_INVALID_PARAMETER, strprintf("Error: msg_type too long, max length is %i", CMessageHeader::COMMAND_SIZE));
+            }
+            auto msg{TryParseHex<unsigned char>(request.params[2].get_str())};
+            if (!msg.has_value()) {
+                throw JSONRPCError(RPC_INVALID_PARAMETER, "Error parsing input for msg");
+            }
+
+            NodeContext& node = EnsureAnyNodeContext(request.context);
+            CConnman& connman = EnsureConnman(node);
+
+            CSerializedNetMsg msg_ser;
+            msg_ser.data = msg.value();
+            msg_ser.m_type = msg_type;
+
+            bool success = connman.ForNode(peer_id, [&](CNode* node) {
+                connman.PushMessage(node, std::move(msg_ser));
+                return true;
+            });
+
+            if (!success) {
+                throw JSONRPCError(RPC_MISC_ERROR, "Error: Could not send message to peer");
+            }
+
+            UniValue ret{UniValue::VOBJ};
+            return ret;
+        },
+    };
+}
+
 void RegisterNetRPCCommands(CRPCTable& t)
 {
     static const CRPCCommand commands[]{
@@ -986,6 +1034,7 @@ void RegisterNetRPCCommands(CRPCTable& t)
         {"network", &getnodeaddresses},
         {"hidden", &addconnection},
         {"hidden", &addpeeraddress},
+        {"hidden", &sendmsgtopeer},
     };
     for (const auto& c : commands) {
         t.appendCommand(c.name, &c);

src/test/fuzz/rpc.cpp

@@ -158,6 +158,7 @@ const std::vector<std::string> RPC_COMMANDS_SAFE_FOR_FUZZING{
     "reconsiderblock",
     "scanblocks",
     "scantxoutset",
+    "sendmsgtopeer", // when no peers are connected, no p2p message is sent
     "sendrawtransaction",
     "setmocktime",
     "setnetworkactive",