Fix liquid falling if in "float" group (#13789)

* Make falling liquid source nodes replace flowing nodes

This makes falling liquid source nodes in group:float replace
flowing nodes on the ground instead of being placed above
the flowing node.

* Make flowing liquids "fall through" for source nodes

This makes liquids in float and falling_node groups fall through
flowing liquid nodes instead of being supported by them in the air.

---------

Co-authored-by: SmallJoker <SmallJoker@users.noreply.github.com>
Co-authored-by: Lars Mueller <appgurulars@gmx.de>

builtin/game/falling.lua

@@ -79,6 +79,9 @@ core.register_entity(":__builtin:falling_node", {
 		-- Cache whether we're supposed to float on water
 		self.floats = core.get_item_group(node.name, "float") ~= 0
 
+		-- Save liquidtype for falling water
+        self.liquidtype = def.liquidtype
+
 		-- Set entity visuals
 		if def.drawtype == "torchlike" or def.drawtype == "signlike" then
 			local textures
@@ -294,9 +297,17 @@ core.register_entity(":__builtin:falling_node", {
 		end
 
 		-- Decide if we're replacing the node or placing on top
+		-- This condition is very similar to the check in core.check_single_for_falling(p)
 		local np = vector.copy(bcp)
-		if bcd and bcd.buildable_to and
-				(not self.floats or bcd.liquidtype == "none") then
+		if bcd and bcd.buildable_to
+				and -- Take "float" group into consideration:
+				(
+					-- Fall through non-liquids
+					not self.floats or bcd.liquidtype == "none" or
+					-- Only let sources fall through flowing liquids
+					(self.floats and self.liquidtype ~= "none" and bcd.liquidtype ~= "source")
+				) then
+
 			core.remove_node(bcp)
 		else
 			np.y = np.y + 1
@@ -307,7 +318,7 @@ core.register_entity(":__builtin:falling_node", {
 		local nd = core.registered_nodes[n2.name]
 		-- If it's not air or liquid, remove node and replace it with
 		-- it's drops
-		if n2.name ~= "air" and (not nd or nd.liquidtype == "none") then
+		if n2.name ~= "air" and (not nd or nd.liquidtype ~= "source") then
 			if nd and nd.buildable_to == false then
 				nd.on_dig(np, n2, nil)
 				-- If it's still there, it might be protected
@@ -555,11 +566,19 @@ function core.check_single_for_falling(p)
 				local success, _ = convert_to_falling_node(p, n)
 				return success
 			end
+			local d_falling = core.registered_nodes[n.name]
+			local do_float = core.get_item_group(n.name, "float") > 0
 			-- Otherwise only if the bottom node is considered "fall through"
 			if not same and
-					(not d_bottom.walkable or d_bottom.buildable_to) and
-					(core.get_item_group(n.name, "float") == 0 or
-					d_bottom.liquidtype == "none") then
+					(not d_bottom.walkable or d_bottom.buildable_to)
+					and -- Take "float" group into consideration:
+					(
+						-- Fall through non-liquids
+						not do_float or d_bottom.liquidtype == "none" or
+						-- Only let sources fall through flowing liquids
+						(do_float and d_falling.liquidtype == "source" and d_bottom.liquidtype ~= "source")
+					) then
+
 				local success, _ = convert_to_falling_node(p, n)
 				return success
 			end

doc/lua_api.md

@@ -2167,6 +2167,8 @@ to games.
   Negative damage values are discarded as no damage.
 * `falling_node`: if there is no walkable block under the node it will fall
 * `float`: the node will not fall through liquids (`liquidtype ~= "none"`)
+     * A liquid source with `groups = {falling_node = 1, float = 1}`
+       will fall through flowing liquids.
 * `level`: Can be used to give an additional sense of progression in the game.
      * A larger level will cause e.g. a weapon of a lower level make much less
        damage, and get worn out much faster, or not be able to get drops

games/devtest/mods/testnodes/liquids.lua

@@ -9,7 +9,7 @@ for d=0, 8 do
 	end
 	minetest.register_node("testnodes:rliquid_"..d, {
 		description = "Test Liquid Source, Range "..d..
-			tt_normal,
+			tt_normal .. "\n" .. "(falling & floating node)",
 		drawtype = "liquid",
 		tiles = {"testnodes_liquidsource_r"..d..".png"},
 		special_tiles = {
@@ -25,6 +25,8 @@ for d=0, 8 do
 		liquid_alternative_flowing = "testnodes:rliquid_flowing_"..d,
 		liquid_alternative_source = "testnodes:rliquid_"..d,
 		liquid_range = d,
+		-- Also use these nodes to test falling, floating liquid source nodes
+		groups = {float = 1, falling_node = 1},
 	})
 
 	minetest.register_node("testnodes:rliquid_flowing_"..d, {