Preference support for Guests (#72332)

## About The Pull Request

Makes it to where if you connect as a guest preferences don't error out
the ass
Resolves https://github.com/tgstation/tgstation/issues/72268
## Why It's Good For The Game

Easier local debugging, and for servers that let guests join
## Changelog
🆑
server: Preferences now support guests
/🆑

code/datums/json_savefile.dm

@@ -1,6 +1,7 @@
 /**
  * A savefile implementation that handles all data using json.
  * Also saves it using JSON too, fancy.
+ * If you pass in a null path, it simply acts as a memory tree instead, and cannot be saved.
  */
 /datum/json_savefile
 	var/path = ""
@@ -13,7 +14,7 @@ GENERAL_PROTECT_DATUM(/datum/json_savefile)
 /datum/json_savefile/New(path)
 	src.path = path
 	tree = list()
-	if(fexists(path))
+	if(path && fexists(path))
 		load()
 
 /**
@@ -43,7 +44,7 @@ GENERAL_PROTECT_DATUM(/datum/json_savefile)
 	tree?.Cut()
 
 /datum/json_savefile/proc/load()
-	if(!fexists(path))
+	if(!path || !fexists(path))
 		return FALSE
 	try
 		tree = json_decode(rustg_file_read(path))
@@ -53,6 +54,7 @@ GENERAL_PROTECT_DATUM(/datum/json_savefile)
 		return FALSE
 
 /datum/json_savefile/proc/save()
+	if(path)
 		rustg_file_write(json_encode(tree), path)
 
 /// Traverses the entire dir tree of the given savefile and dynamically assembles the tree from it

code/modules/client/preferences.dm

@@ -2,20 +2,26 @@ GLOBAL_LIST_EMPTY(preferences_datums)
 
 /datum/preferences
 	var/client/parent
-	//doohickeys for savefiles
+	/// The path to the general savefile for this datum
 	var/path
-	var/default_slot = 1 //Holder so it doesn't default to slot 1, rather the last one used
+	/// Whether or not we allow saving/loading. Used for guests, if they're enabled
+	var/load_and_save = TRUE
+	/// Ensures that we always load the last used save, QOL
+	var/default_slot = 1
+	/// The maximum number of slots we're allowed to contain
 	var/max_save_slots = 3
 
-	//non-preference stuff
-	var/muted = 0
+	/// Bitflags for communications that are muted
+	var/muted = NONE
+	/// Last IP that this client has connected from
 	var/last_ip
+	/// Last CID that this client has connected from
 	var/last_id
 
-	//game-preferences
-	var/lastchangelog = "" //Saved changlog filesize to detect if there was a change
+	/// Cached changelog size, to detect new changelogs since last join
+	var/lastchangelog = ""
 
-	//Antag preferences
+	/// List of ROLE_X that the client wants to be eligible for
 	var/list/be_special = list() //Special role selection
 
 	/// Custom keybindings. Map of keybind names to keyboard inputs.
@@ -95,9 +101,9 @@ GLOBAL_LIST_EMPTY(preferences_datums)
 		middleware += new middleware_type(src)
 
 	if(IS_CLIENT_OR_MOCK(parent))
-		if(!is_guest_key(parent.key))
+		load_and_save = !is_guest_key(parent.key)
 		load_path(parent.ckey)
-			if(!fexists(path))
+		if(load_and_save && !fexists(path))
 			try_savefile_type_migration()
 		unlock_content = !!parent.IsByondMember()
 		if(unlock_content)

code/modules/client/preferences_savefile.dm

@@ -140,14 +140,14 @@ SAVEFILE UPDATING/VERSIONING - 'Simplified', or rather, more coder-friendly ~Car
 		to_chat(parent, span_danger("[conflicted.category]: [conflicted.full_name] needs updating"))
 
 /datum/preferences/proc/load_path(ckey, filename="preferences.json")
-	if(!ckey)
+	if(!ckey || !load_and_save)
 		return
 	path = "data/player_saves/[ckey[1]]/[ckey]/[filename]"
 
 /datum/preferences/proc/load_savefile()
-	if(!path)
+	if(load_and_save && !path)
 		CRASH("Attempted to load savefile without first loading a path!")
-	savefile = new /datum/json_savefile(path)
+	savefile = new /datum/json_savefile(load_and_save ? path : null)
 
 /datum/preferences/proc/load_preferences()
 	if(!savefile)
@@ -158,7 +158,7 @@ SAVEFILE UPDATING/VERSIONING - 'Simplified', or rather, more coder-friendly ~Car
 			return FALSE
 
 	var/needs_update = save_data_needs_update(savefile.get_entry())
-	if(needs_update == -2) //fatal, can't load any data
+	if(load_and_save && (needs_update == -2)) //fatal, can't load any data
 		var/bacpath = "[path].updatebac" //todo: if the savefile version is higher then the server, check the backup, and give the player a prompt to load the backup
 		if (fexists(bacpath))
 			fdel(bacpath) //only keep 1 version of backup
@@ -263,7 +263,6 @@ SAVEFILE UPDATING/VERSIONING - 'Simplified', or rather, more coder-friendly ~Car
 
 /datum/preferences/proc/load_character(slot)
 	SHOULD_NOT_SLEEP(TRUE)
-
 	if(!slot)
 		slot = default_slot
 	slot = sanitize_integer(slot, 1, max_save_slots, initial(default_slot))
@@ -317,7 +316,6 @@ SAVEFILE UPDATING/VERSIONING - 'Simplified', or rather, more coder-friendly ~Car
 
 /datum/preferences/proc/save_character()
 	SHOULD_NOT_SLEEP(TRUE)
-
 	if(!path)
 		return FALSE
 	var/tree_key = "character[default_slot]"