Cache prefixes (#3150)

* Cache prefixes

 - This works towards #3148
 - Ends up centralizing some logic
   - Including that prefixes should be a reverse sorted list

* handle global prefix attempts at none

* fix prefix set for server

* cache using guild id

redbot/core/bot.py

@@ -36,6 +36,8 @@ from .dev_commands import Dev
 from .events import init_events
 from .global_checks import init_global_checks
 
+from .settings_caches import PrefixManager
+
 from .rpc import RPCMixin
 from .utils import common_filters
 
@@ -124,23 +126,13 @@ class RedBase(commands.GroupMixin, commands.bot.BotBase, RPCMixin):  # pylint: d
 
         self._config.init_custom(SHARED_API_TOKENS, 2)
         self._config.register_custom(SHARED_API_TOKENS)
+        self._prefix_cache = PrefixManager(self._config, cli_flags)
 
-        async def prefix_manager(bot, message):
-            if not cli_flags.prefix:
-                global_prefix = await bot._config.prefix()
-            else:
-                global_prefix = cli_flags.prefix
-            if message.guild is None:
-                return global_prefix
-            server_prefix = await bot._config.guild(message.guild).prefix()
+        async def prefix_manager(bot, message) -> List[str]:
+            prefixes = await self._prefix_cache.get_prefixes(message.guild)
             if cli_flags.mentionable:
-                return (
-                    when_mentioned_or(*server_prefix)(bot, message)
-                    if server_prefix
-                    else when_mentioned_or(*global_prefix)(bot, message)
-                )
-            else:
-                return server_prefix if server_prefix else global_prefix
+                return when_mentioned_or(*prefixes)(bot, message)
+            return prefixes
 
         if "command_prefix" not in kwargs:
             kwargs["command_prefix"] = prefix_manager
@@ -273,15 +265,15 @@ class RedBase(commands.GroupMixin, commands.bot.BotBase, RPCMixin):  # pylint: d
         """
         This checks if a user or member is allowed to run things,
         as considered by Red's whitelist and blacklist.
-        
+
         If given a user object, this function will check the global lists
-        
+
         If given a member, this will additionally check guild lists
-        
+
         If omiting a user or member, you must provide a value for ``who_id``
-        
+
         You may also provide a value for ``guild_id`` in this case
-        
+
         If providing a member by guild and member ids,
         you should supply ``role_ids`` as well
 
@@ -289,7 +281,7 @@ class RedBase(commands.GroupMixin, commands.bot.BotBase, RPCMixin):  # pylint: d
         ----------
         who : Optional[Union[discord.Member, discord.User]]
             The user or member object to check
-        
+
         Other Parameters
         ----------------
         who_id : Optional[int]
@@ -906,7 +898,7 @@ class RedBase(commands.GroupMixin, commands.bot.BotBase, RPCMixin):  # pylint: d
         This should realistically only be used for responding using user provided
         input. (unfortunately, including usernames)
         Manually crafted messages which dont take any user input have no need of this
-        
+
         Returns
         -------
         discord.Message