Implement client node dig prediction

Dig prediction allows clients to remove dug nodes without
waiting for server acknowledgement.
This patch allows mods to override dig prediction,
it can either be turned off or a different "prediction node"
can be selected.

doc/lua_api.txt

@@ -4240,6 +4240,13 @@ Definition tables
           on ground when the player places the item. Server will always update
           actual result to client in a short moment.
         ]]
+        node_dig_prediction = "air",
+        --[[
+        ^ if "", no prediction is made
+        ^ if "air", node is removed
+        ^ Otherwise should be name of node which the client immediately places
+          upon digging. Server will always update actual result shortly.
+        ]]
         sound = {
             breaks = "default_tool_break", -- tools only
             place = --[[<SimpleSoundSpec>]],

src/game.cpp

@@ -4104,7 +4104,17 @@ void Game::handleDigging(const PointedThing &pointed, const v3s16 &nodepos,
 			    		client->getScript()->on_dignode(nodepos, wasnode)) {
 				return;
 			}
-			client->removeNode(nodepos);
+
+			const ContentFeatures &f = client->ndef()->get(wasnode);
+			if (f.node_dig_prediction == "air") {
+				client->removeNode(nodepos);
+			} else if (!f.node_dig_prediction.empty()) {
+				content_t id;
+				bool found = client->ndef()->getId(f.node_dig_prediction, id);
+				if (found)
+					client->addNode(nodepos, id, true);
+			}
+			// implicit else: no prediction
 		}
 
 		client->interact(2, pointed);

src/nodedef.cpp

@@ -335,6 +335,7 @@ void ContentFeatures::reset()
 	color = video::SColor(0xFFFFFFFF);
 	palette_name = "";
 	palette = NULL;
+	node_dig_prediction = "air";
 }
 
 void ContentFeatures::serialize(std::ostream &os, u16 protocol_version) const
@@ -422,6 +423,8 @@ void ContentFeatures::serialize(std::ostream &os, u16 protocol_version) const
 	// legacy
 	writeU8(os, legacy_facedir_simple);
 	writeU8(os, legacy_wallmounted);
+
+	os << serializeString(node_dig_prediction);
 }
 
 void ContentFeatures::correctAlpha(TileDef *tiles, int length)
@@ -530,6 +533,10 @@ void ContentFeatures::deSerialize(std::istream &is)
 	// read legacy properties
 	legacy_facedir_simple = readU8(is);
 	legacy_wallmounted = readU8(is);
+
+	try {
+		node_dig_prediction = deSerializeString(is);
+	} catch(SerializationError &e) {};
 }
 
 #ifndef SERVER

src/nodedef.h

@@ -324,6 +324,8 @@ struct ContentFeatures
 	// Player cannot build to these (placement prediction disabled)
 	bool rightclickable;
 	u32 damage_per_second;
+	// client dig prediction
+	std::string node_dig_prediction;
 
 	// --- LIQUID PROPERTIES ---
 

src/script/common/c_content.cpp

@@ -737,6 +737,10 @@ ContentFeatures read_content_features(lua_State *L, int index)
 	}
 	lua_pop(L, 1);
 
+	// Node immediately placed by client when node is dug
+	getstringfield(L, index, "node_dig_prediction",
+		f.node_dig_prediction);
+
 	return f;
 }
 
@@ -861,6 +865,8 @@ void push_content_features(lua_State *L, const ContentFeatures &c)
 	lua_setfield(L, -2, "legacy_facedir_simple");
 	lua_pushboolean(L, c.legacy_wallmounted);
 	lua_setfield(L, -2, "legacy_wallmounted");
+	lua_pushstring(L, c.node_dig_prediction.c_str());
+	lua_setfield(L, -2, "node_dig_prediction");
 }
 
 /******************************************************************************/