Add API for mods to hook liquid transformation events (#11405)

Add API for mods to hook liquid transformation events

Without this API, there is no reliable way for mods to be
notified when liquid transform modifies nodes and mods are
forced to poll for changes.  This allows mods to detect
changes to flowing liquid nodes and liquid renewal using
event-driven logic.

builtin/game/register.lua

@@ -610,6 +610,7 @@ core.registered_on_modchannel_message, core.register_on_modchannel_message = mak
 core.registered_on_player_inventory_actions, core.register_on_player_inventory_action = make_registration()
 core.registered_allow_player_inventory_actions, core.register_allow_player_inventory_action = make_registration()
 core.registered_on_rightclickplayers, core.register_on_rightclickplayer = make_registration()
+core.registered_on_liquid_transformed, core.register_on_liquid_transformed = make_registration()
 
 --
 -- Compatibility for on_mapgen_init()

doc/lua_api.txt

@@ -4859,6 +4859,12 @@ Call these functions only at load time!
     * Called when an incoming mod channel message is received
     * You should have joined some channels to receive events.
     * If message comes from a server mod, `sender` field is an empty string.
+* `minetest.register_on_liquid_transformed(function(post_list, node_list))`
+    * Called after liquid nodes are modified by the engine's liquid transformation
+      process.
+    * `pos_list` is an array of all modified positions.
+    * `node_list` is an array of the old node that was previously at the position
+      with the corresponding index in pos_list.
 
 Setting-related
 ---------------

src/map.cpp

@@ -829,7 +829,7 @@ void Map::transformLiquids(std::map<v3s16, MapBlock*> &modified_blocks,
 		m_transforming_liquid.push_back(iter);
 
 	voxalgo::update_lighting_nodes(this, changed_nodes, modified_blocks);
-
+	env->getScriptIface()->on_liquid_transformed(changed_nodes);
 
 	/* ----------------------------------------------------------------------
 	 * Manage the queue so that it does not grow indefinately

src/script/cpp_api/s_env.cpp

@@ -25,6 +25,8 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "mapgen/mapgen.h"
 #include "lua_api/l_env.h"
 #include "server.h"
+#include "script/common/c_content.h"
+
 
 void ScriptApiEnv::environment_OnGenerated(v3s16 minp, v3s16 maxp,
 	u32 blockseed)
@@ -267,3 +269,35 @@ void ScriptApiEnv::on_emerge_area_completion(
 		luaL_unref(L, LUA_REGISTRYINDEX, state->args_ref);
 	}
 }
+
+void ScriptApiEnv::on_liquid_transformed(
+	const std::vector<std::pair<v3s16, MapNode>> &list)
+{
+	SCRIPTAPI_PRECHECKHEADER
+
+	// Get core.registered_on_liquid_transformed
+	lua_getglobal(L, "core");
+	lua_getfield(L, -1, "registered_on_liquid_transformed");
+	luaL_checktype(L, -1, LUA_TTABLE);
+	lua_remove(L, -2);
+
+	// Skip converting list and calling hook if there are
+	// no registered callbacks.
+	if(lua_objlen(L, -1) < 1) return;
+
+	// Convert the list to a pos array and a node array for lua
+	int index = 1;
+	const NodeDefManager *ndef = getEnv()->getGameDef()->ndef();
+	lua_createtable(L, list.size(), 0);
+	lua_createtable(L, list.size(), 0);
+	for(std::pair<v3s16, MapNode> p : list) {
+		lua_pushnumber(L, index);
+		push_v3s16(L, p.first);
+		lua_rawset(L, -4);
+		lua_pushnumber(L, index++);
+		pushnode(L, p.second, ndef);
+		lua_rawset(L, -3);
+	}
+
+	runCallbacks(2, RUN_CALLBACKS_MODE_FIRST);
+}

src/script/cpp_api/s_env.h

@@ -21,6 +21,8 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 
 #include "cpp_api/s_base.h"
 #include "irr_v3d.h"
+#include "mapnode.h"
+#include <vector>
 
 class ServerEnvironment;
 struct ScriptCallbackState;
@@ -41,5 +43,8 @@ public:
 	void on_emerge_area_completion(v3s16 blockpos, int action,
 		ScriptCallbackState *state);
 
+	// Called after liquid transform changes
+	void on_liquid_transformed(const std::vector<std::pair<v3s16, MapNode>> &list);
+
 	void initializeEnvironment(ServerEnvironment *env);
 };