Implement minetest.ipc_cas()

doc/lua_api.md

@@ -7066,6 +7066,18 @@ minetest.ipc_get("test:foo").subkey = "value" -- WRONG!
 minetest.ipc_get("test:foo") -- returns an empty table
 ```
 
+**Advanced**:
+
+* `minetest.ipc_cas(key, old_value, new_value)`:
+  * Write a value to the shared data area, but only if the previous value
+    equals what was given.
+    This operation is called Compare-and-Swap and can be used to implement
+    synchronization between threads.
+  * `key`: as above
+  * `old_value`: value compared to using `==` (`nil` compares equal for non-existing keys)
+  * `new_value`: value that will be set
+  * returns: true on success, false otherwise
+
 Bans
 ----
 

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_cas("unittests:mg_once", nil, true) 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/common/c_packer.cpp

@@ -507,6 +507,7 @@ PackedValue *script_pack(lua_State *L, int idx)
 
 void script_unpack(lua_State *L, PackedValue *pv)
 {
+	assert(pv);
 	// table that tracks objects for keep_ref / PUSHREF (key = instr index)
 	lua_newtable(L);
 	const int top = lua_gettop(L);

src/script/lua_api/l_ipc.cpp

@@ -10,6 +10,17 @@
 typedef std::shared_lock<std::shared_mutex> SharedReadLock;
 typedef std::unique_lock<std::shared_mutex> SharedWriteLock;
 
+static inline auto read_pv(lua_State *L, int idx)
+{
+	std::unique_ptr<PackedValue> ret;
+	if (!lua_isnil(L, idx)) {
+		ret.reset(script_pack(L, idx));
+		if (ret->contains_userdata)
+			throw LuaError("Userdata not allowed");
+	}
+	return ret;
+}
+
 int ModApiIPC::l_ipc_get(lua_State *L)
 {
 	auto *store = getGameDef(L)->getModIPCStore();
@@ -34,12 +45,7 @@ int ModApiIPC::l_ipc_set(lua_State *L)
 	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");
-	}
+	auto pv = read_pv(L, 2);
 
 	{
 		SharedWriteLock autolock(store->mutex);
@@ -51,6 +57,42 @@ int ModApiIPC::l_ipc_set(lua_State *L)
 	return 0;
 }
 
+int ModApiIPC::l_ipc_cas(lua_State *L)
+{
+	auto *store = getGameDef(L)->getModIPCStore();
+
+	auto key = readParam<std::string>(L, 1);
+
+	luaL_checkany(L, 2);
+	const int idx_old = 2;
+
+	luaL_checkany(L, 3);
+	auto pv_new = read_pv(L, 3);
+
+	bool ok = false;
+	{
+		SharedWriteLock autolock(store->mutex);
+		// unpack and compare old value
+		auto it = store->map.find(key);
+		if (it == store->map.end()) {
+			ok = lua_isnil(L, idx_old);
+		} else {
+			script_unpack(L, it->second.get());
+			ok = lua_equal(L, idx_old, -1);
+			lua_pop(L, 1);
+		}
+		// put new value
+		if (ok) {
+			if (pv_new)
+				store->map[key] = std::move(pv_new);
+			else
+				store->map.erase(key);
+		}
+	}
+	lua_pushboolean(L, ok);
+	return 1;
+}
+
 /*
  * Implementation note:
  * Iterating over the IPC table is intentionally not supported.
@@ -65,4 +107,5 @@ void ModApiIPC::Initialize(lua_State *L, int top)
 
 	API_FCT(ipc_get);
 	API_FCT(ipc_set);
+	API_FCT(ipc_cas);
 }

src/script/lua_api/l_ipc.h

@@ -9,6 +9,7 @@ class ModApiIPC : public ModApiBase {
 private:
 	static int l_ipc_get(lua_State *L);
 	static int l_ipc_set(lua_State *L);
+	static int l_ipc_cas(lua_State *L);
 
 public:
 	static void Initialize(lua_State *L, int top);