Add generic IPC mechanism between Lua envs

src/script/lua_api/CMakeLists.txt

@@ -6,6 +6,7 @@ set(common_SCRIPT_LUA_API_SRCS
 	${CMAKE_CURRENT_SOURCE_DIR}/l_env.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/l_http.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/l_inventory.cpp
+	${CMAKE_CURRENT_SOURCE_DIR}/l_ipc.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/l_item.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/l_itemstackmeta.cpp
 	${CMAKE_CURRENT_SOURCE_DIR}/l_mapgen.cpp

src/script/lua_api/l_ipc.cpp

@@ -0,0 +1,68 @@
+// Minetest
+// SPDX-License-Identifier: LGPL-2.1-or-later
+
+#include "lua_api/l_ipc.h"
+#include "lua_api/l_internal.h"
+#include "common/c_packer.h"
+#include "server.h"
+#include "debug.h"
+
+typedef std::shared_lock<std::shared_mutex> SharedReadLock;
+typedef std::unique_lock<std::shared_mutex> SharedWriteLock;
+
+int ModApiIPC::l_ipc_get(lua_State *L)
+{
+	auto *store = getGameDef(L)->getModIPCStore();
+
+	auto key = readParam<std::string>(L, 1);
+
+	{
+		SharedReadLock autolock(store->mutex);
+		auto it = store->map.find(key);
+		if (it == store->map.end())
+			lua_pushnil(L);
+		else
+			script_unpack(L, it->second.get());
+	}
+	return 1;
+}
+
+int ModApiIPC::l_ipc_set(lua_State *L)
+{
+	auto *store = getGameDef(L)->getModIPCStore();
+
+	auto key = readParam<std::string>(L, 1);
+
+	luaL_checkany(L, 2);
+	std::unique_ptr<PackedValue> pv;
+	if (!lua_isnil(L, 2)) {
+		pv.reset(script_pack(L, 2));
+		if (pv->contains_userdata)
+			throw LuaError("Userdata not allowed");
+	}
+
+	{
+		SharedWriteLock autolock(store->mutex);
+		if (pv)
+			store->map[key] = std::move(pv);
+		else
+			store->map.erase(key); // delete the map value for nil
+	}
+	return 0;
+}
+
+/*
+ * Implementation note:
+ * Iterating over the IPC table is intentionally not supported.
+ * Mods should know what they have set.
+ * This has the nice side effect that mods are able to use a randomly generated key
+ * if they really *really* want to avoid other code touching their data.
+ */
+
+void ModApiIPC::Initialize(lua_State *L, int top)
+{
+	FATAL_ERROR_IF(!getGameDef(L)->getModIPCStore(), "ModIPCStore missing from gamedef");
+
+	API_FCT(ipc_get);
+	API_FCT(ipc_set);
+}

src/script/lua_api/l_ipc.h

@@ -0,0 +1,15 @@
+// Minetest
+// SPDX-License-Identifier: LGPL-2.1-or-later
+
+#pragma once
+
+#include "lua_api/l_base.h"
+
+class ModApiIPC : public ModApiBase {
+private:
+	static int l_ipc_get(lua_State *L);
+	static int l_ipc_set(lua_State *L);
+
+public:
+	static void Initialize(lua_State *L, int top);
+};