Keep newer local player data over a prefetched copy

An instance with older data can upload the generation a save already holds, and a prefetch of it then overwrote that
save's newer state. A join now drops a prefetched copy unless the ordinary download check says the data is needed.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

plugins/player_sync/module/player_sync.lua

@@ -334,6 +334,19 @@ function player_sync.check_players_in_cutscene()
 	end
 end
 
+-- Whether a join should take the stored data: there is some, and this save hasn't synced the player in that epoch at that
+-- generation or newer with every component synced here now. A player without a record here has nothing to keep.
+local function download_needed(player_record, acquire_response, script_data)
+	return acquire_response.has_data
+		and (
+			not player_record
+			or not player_record.sync
+			or (player_record.epoch or 0) ~= (acquire_response.epoch or 0)
+			or acquire_response.generation > player_record.generation
+			or serialize.newly_synced(player_record.components, script_data.components)
+		)
+end
+
 function player_sync.sync_player(acquire_response)
 	local player = assert(game.get_player(acquire_response.player_name))
 
@@ -356,14 +369,15 @@ function player_sync.sync_player(acquire_response)
 	-- now. The generation alone isn't enough: a controller that crashed before saving it restarts at the last saved
 	-- generation, and another instance's upload can then reuse the same number with other data. Otherwise the copy
 	-- goes, and the join carries on as if it had never come: today's download or failure dialog, never the restart
-	-- path below, which only sees a different generation.
+	-- path below, which only sees a different generation. A copy this save doesn't need goes too, keeping the local
+	-- data: an upload from an instance with older data can take the generation this save already holds.
 	for _, downloads in ipairs({ script_data.finished_downloads, script_data.active_downloads }) do
 		local record = downloads[acquire_response.player_name]
-		if record and record.prefetch and not (
+		if record and record.prefetch and (not (
 			acquire_response.status == "acquired" and record.generation == acquire_response.generation
 			and record.token == acquire_response.token
 			and serialize.same_components(record.components, script_data.components)
-		) then
+		) or not download_needed(script_data.players[acquire_response.player_name], acquire_response, script_data)) then
 			downloads[acquire_response.player_name] = nil
 		end
 	end
@@ -458,15 +472,7 @@ function player_sync.sync_player(acquire_response)
 	end
 
 	local stale = (player_record.epoch or 0) ~= epoch
-	if
-		acquire_response.has_data
-		and (
-			not player_record.sync
-			or stale
-			or acquire_response.generation > player_record.generation
-			or serialize.newly_synced(player_record.components, script_data.components)
-		)
-	then
+	if download_needed(player_record, acquire_response, script_data) then
 		player_sync.initiate_inventory_download(player, player_record, acquire_response.generation, epoch)
 
 	else