Add minetest.ipc_swap

doc/lua_api.md

@@ -6897,10 +6897,13 @@ different Lua environments (main, mapgen and async).
   * Write a value to the shared data area.
   * `key`: as above
   * `value`: an arbitrary Lua value, cannot be or contain userdata.
+* `minetest.ipc_swap(key, value)`:
+  * Like `minetest.ipc_set` but returns the value that was at the same key
+    before writing the new one. This is atomic.
 
 Interacting with the shared data will perform an operation comparable to
 (de)serialization.
-Modifying references will not do anything, as in this example:
+Modifying references will not have any effect, as in this example:
 ```lua
 minetest.ipc_set("test:foo", {})
 minetest.ipc_get("test:foo").subkey = "value"

games/devtest/mods/unittests/inside_mapgen_env.lua

@@ -22,8 +22,11 @@ local function do_tests()
 	assert(core.registered_items["unittests:description_test"].on_place == true)
 end
 
--- this is checked from the main env
-core.ipc_set("unittests:mg", { pcall(do_tests) })
+-- first thread to get here runs the tests
+if core.ipc_swap("unittests:mg_once", 1) == nil then
+	-- this is checked from the main env
+	core.ipc_set("unittests:mg", { pcall(do_tests) })
+end
 
 core.register_on_generated(function(vm, pos1, pos2, blockseed)
 	local n = tonumber(core.get_mapgen_setting("chunksize")) * 16 - 1

src/script/lua_api/l_ipc.cpp

@@ -27,26 +27,38 @@ int ModApiIPC::l_ipc_get(lua_State *L)
 	return 1;
 }
 
-int ModApiIPC::l_ipc_set(lua_State *L)
+int ModApiIPC::ipc_put(lua_State *L, bool push_old)
 {
 	auto *store = getGameDef(L)->getModIPCStore();
 
 	auto key = readParam<std::string>(L, 1);
 
+	std::unique_ptr<PackedValue> vnew, vold;
 	luaL_checkany(L, 2);
-	std::unique_ptr<PackedValue> pv;
 	if (!lua_isnil(L, 2)) {
-		pv.reset(script_pack(L, 2));
-		if (pv->contains_userdata)
+		vnew.reset(script_pack(L, 2));
+		if (vnew->contains_userdata)
 			throw LuaError("Userdata not allowed");
 	}
 
 	{
 		SharedWriteLock autolock(store->mutex);
-		if (pv)
-			store->map[key] = std::move(pv);
+		auto it = store->map.find(key);
+		if (it == store->map.end()) {
+			[[maybe_unused]] auto r = store->map.emplace(std::move(key), std::move(vnew));
+			assert(r.second);
+		} else {
+			vold = std::move(it->second);
+			it->second = std::move(vnew);
+		}
+	}
+
+	if (push_old) {
+		if (vold)
+			script_unpack(L, vold.get());
 		else
-			store->map.erase(key); // delete the map value for nil
+			lua_pushnil(L);
+		return 1;
 	}
 	return 0;
 }
@@ -65,4 +77,5 @@ void ModApiIPC::Initialize(lua_State *L, int top)
 
 	API_FCT(ipc_get);
 	API_FCT(ipc_set);
+	API_FCT(ipc_swap);
 }

src/script/lua_api/l_ipc.h

@@ -8,7 +8,9 @@
 class ModApiIPC : public ModApiBase {
 private:
 	static int l_ipc_get(lua_State *L);
-	static int l_ipc_set(lua_State *L);
+	static int ipc_put(lua_State *L, bool push_old);
+	static int l_ipc_set(lua_State *L) { return ipc_put(L, false); }
+	static int l_ipc_swap(lua_State *L) { return ipc_put(L, true); }
 
 public:
 	static void Initialize(lua_State *L, int top);