Fix dynamic media race condition

Wait for *all* players to receive the texture before using it.
Previously this would wait for *any* player to receive the texture.
This works fine in singleplayer (or on pre-5.5,
where dynamic_add_media would block until all players have received the texture),
but it may fail otherwise:
Either entity textures won't be available in time,
or, even worse, the HUD texture is unavailable.
Both produce an error in chat for clients.

dynamic_add_media_all.lua

@@ -0,0 +1,63 @@
+local on_leaves = {} -- functions(name)
+local timeout = 5 -- seconds
+xmaps.dynamic_add_media_all = function(path, on_all_received)
+	if not minetest.features.dynamic_add_media_table then
+		-- minetest.dynamic_add_media() blocks in
+		-- Minetest 5.3 and 5.4 until media loads
+		minetest.dynamic_add_media(path, function() end)
+		on_all_received()
+		return
+	end
+	-- minetest.dynamic_add_media() never blocks
+	-- in Minetest 5.5, callback runs *for each client*
+	-- who has received & loaded the media
+	-- all players currently online must still receive it
+	local to_receive = {}
+	for _, player in pairs(minetest.get_connected_players()) do
+		local name = player:get_player_name()
+		-- Only clients with protocol version >= 39 can receive dynamic media.
+		if minetest.get_player_information(name).protocol_version >= 39 then
+			to_receive[name] = true
+		end
+	end
+	local function remove_to_receive(name)
+		to_receive[name] = nil
+		if next(to_receive) == nil then
+			on_leaves[remove_to_receive] = nil
+			on_all_received()
+		end
+	end
+	on_leaves[remove_to_receive] = true
+	minetest.dynamic_add_media(
+		{filepath = path},
+		function(name)
+			assert(name)
+			if not to_receive[name] then
+				minetest.log("warning", ("xmaps: %s received media despite not being connected"):format(name))
+				return
+			end
+			remove_to_receive(name)
+		end
+	)
+	minetest.after(timeout, function()
+		if next(to_receive) ~= nil then
+			local names = {}
+			for name in pairs(to_receive) do
+				table.insert(names, name)
+			end
+			table.sort(names)
+			minetest.log("warning", ("xmaps: sending media to %s timed out"):format(table.concat(names, ", ")))
+		end
+		on_leaves[remove_to_receive] = nil
+		on_all_received()
+	end)
+end
+
+minetest.register_on_leaveplayer(
+	function(player)
+		local player_name = player:get_player_name()
+		for on_leave in pairs(on_leaves) do
+			on_leave(player_name)
+		end
+	end
+)

init.lua

@@ -48,6 +48,8 @@ end
 
 xmaps = {}
 
+dofile(minetest.get_modpath(minetest.get_current_modname()) .. "/dynamic_add_media_all.lua")
+
 xmaps.dark = {} -- key: player name; value: is it dark?
 xmaps.huds = {} -- key: player name; value: player huds
 xmaps.maps = {} -- key: player name; value: map texture
@@ -393,26 +395,12 @@ xmaps.load_map = function(map_id)
 	end
 
 	local filename = xmaps.get_map_filename(map_id)
+	local path = textures_dir .. filename
 
 	if not xmaps.sent[map_id] then
-		if not minetest.features.dynamic_add_media_table then
+		xmaps.dynamic_add_media_all(path, function()
-			-- minetest.dynamic_add_media() blocks in
-			-- Minetest 5.3 and 5.4 until media loads
-			minetest.dynamic_add_media(
-				textures_dir .. filename,
-				function() end
-			)
 			xmaps.load[map_id] = true
-		else
+		end)
-			-- minetest.dynamic_add_media() never blocks
-			-- in Minetest 5.5, callback runs after load
-			minetest.dynamic_add_media(
-				textures_dir .. filename,
-				function()
-					xmaps.load[map_id] = true
-				end
-			)
-		end
 		xmaps.sent[map_id] = true
 	end